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UBC Theses and Dissertations

Some effects of a unique hydroelectric development on the littoral \ benthic community and ecology of… Mylechreest, Peter 1978

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SOME EFFECTS OF A UNIQUE HYDROELECTRIC DEVELOPMENT ON THE LITTORAL BENTHIC COMMUNITY AND ECOLOGY OF TROUT IN A LARGE NEW ZEALAND LAKE BY PETER MYLECHREEST B.A. (1958); M.B., B,. C h i r . (1962) CAMBRIDGE UNIVERSITY, England A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1973 (c} Peter Mylechreest, 1978 MASTER OF SCIENCE IN In presenting th i s thes is in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i lab le for reference and study. I further agree that permission for extensive copying of th i s thesis for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t ion of th is thes is for f inanc ia l gain sha l l not be allowed without my writ ten permission. Depa rtment The Univers i ty of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 6 ABSTRACT A three year study (1974-1977) examined the e f f e c t s of h y d r o e l e c t r i c development on the l i t t o r a l i n v e r t e b r a t e fauna and ecology of brown t r o u t (Salmo t r u t t a L.) and rainbow t r o u t (.Salmo g a i r d n e r i Richardson) i n Lake Waikaremoana on the North I s l a n d of New Zealand. Lake Waikaremoana was formed over 2 000 years ago by a massive l a n d s l i d e , which c r e a t e d a n a t u r a l rock and e a r t h - f i l l dam. I t s development f o r h y d r o e l e c t r i c purposes i n 1946 was unique i n t h a t the lake l e v e l was i n i t i a l l y lowered r a t h e r than being r a i s e d . The most s i g n i f i c a n t morphometric changes f o l l o w i n g hydro-e l e c t r i c development were a d i s p r o p o r t i o n a t e l y g r e a t l o s s of l i t t o r a l area, and the c r e a t i o n of deep winding channels a t the stream mouths e n t e r i n g the l a k e . In r e c e n t years the amplitude of lake l e v e l f l u c t u a t i o n s has not been s i g n i f i c a n t l y g r e a t e r than the amplitude of the n a t u r a l l a k e l e v e l f l u c t u a t i o n s , but t h e i r seasonal p e r i o d i c i t y has been r e v e r s e d . The seasonal changes i n the depth d i s t r i -b u t i o n of the l i t t o r a l i n v e r t e b r a t e fauna are adapted to a f a l l i n g l a k e l e v e l i n summer and a r i s i n g lake l e v e l i n w i n t e r . H y d r o e l e c t r i c drawdown i s now concurrent with an upward migra-t i o n of some animals d u r i n g w i n t e r . The maximum d e n s i t y of animals i n the deep l i t t o r a l i n summer no longer c o i n c i d e s w i t h a f a l l i n g lake l e v e l and i n c r e a s i n g l i g h t p e n e t r a t i o n , but i n s t e a d i s s u b j e c t e d to deeper submergence and reduced water transparency due to summer storage of water. The s m a l l j u v e n i l e and l a r g e , o l d t r o u t are most depend-ent on the shallow l i t t o r a l space, and there i s an i n c r e a s i n g u t i l i z a t i o n o f the l i t t o r a l food r e s o u r c e s w i t h i n c r e a s i n g age and s i z e . The combined e f f e c t s of the morphometric changes and u n n a t u r a l f l u c t u a t i o n s i n lake l e v e l , through r e d u c t i o n of l i t t o r a l space and food r e s o u r c e s , has decreased the c a r r y i n g c a p a c i t y o f the l a k e f o r t r o u t to a degree out of p r o p o r t i o n to the o v e r a l l r e d u c t i o n i n the s u r f a c e area of the l a k e . - i v -TABLE OF CONTENTS Page Abstract i i Table of Contents i v L i s t of Tables v i i L i s t of Figures v i i i Acknowledgements x. I. INTRODUCTION 1... II. STUDY AREA 4 A. LAKE WAIKAREMOANA AND CATCHMENT 4 Geographical location and geology 4 Lake formation 6 The catchment 7 Native f i s h and introduced species.... 9 B. HYDROELECTRIC DEVELOPMENT 10 C. LAKE LEVEL FLUCTUATIONS 13 Amplitude 13 Seasonal p e r i o d i c i t y 15 Major cycle 18 Rate of r i s e and f a l l 18 I I I . MATERIALS AND METHODS 20 A. PHYSICAL 20 Surveys of the lake shore 20 Planimetry . 21 Water transparency and temperature.... 22 Lake l e v e l , power generation and r a i n f a l l 23 - v -Page B. LITTORAL BENTHOS 23 Ekman dredge sampling 23 S t a t i s t i c a l procedure 27 C. ZOOPLANKTON AND LARVAL FISH 27 D. TROUT 29 N e t t i n g programme 29 Stomach a n a l y s i s 31 IV. RESULTS 32 A. PHYSICAL 32 Morphometric changes. 32 Thermal c o n d i t i o n s and water transparency 34 D i s s o l v e d oxygen 36 B. AQUATIC MACROPHYTES 36 C. LITTORAL BENTHOS 39 Sampling v a r i a b i l i t y 39 Depth d i s t r i b u t i o n . . 43 Composition 45 Seasonal changes i n depth d i s t r i b u t i o n 48 Mo l l u s c s 52 Odonata and L e p i d o p t e r a 52 T r i c h o p t e r a 56 Chironomids 58 D. TROUT 58 Composition o f the t o t a l c a t c h 58 S p a t i a l d i s t r i b u t i o n 60 - v i -Page Stomach a n a l y s i s 63 Lim n e t i c food resources 66 Recruitment of j u v e n i l e s t o lake 68 C o n d i t i o n f a c t o r 69 V. ... DISCUSSION 73 A. MORPHOMETRIC CHANGES 73 B. LAKE LEVEL FLUCTUATIONS 74 Seasonal p e r i o d i c i t y 74 E f f e c t s on l i g h t p e n e t r a t i o n and primary producers 78 Major c y c l e 80 Rate of r i s e and f a l l 81 C. LITTORAL INVERTEBRATE FAUNA 82 Q u a n t i t a t i v e l o s s e s 84 Seasonal changes i n depth d i s t r i b u t i o n 85 Q u a l i t a t i v e l o s s e s 88 D. TROUT 89 Species composition 89 S p a t i a l s e g r e g a t i o n 90 E f f e c t s o f h y d r o e l e c t r i c development on a v a i l a b l e space 9 3 Food and c o n d i t i o n f a c t o r 94 E f f e c t s of h y d r o e l e c t r i c development on food r e s o u r c e s 95 VI. SUMMARY 97 V I I . MANAGEMENT IMPLICATIONS 99 V I I I . LITERATURE CITED 100 - v i i -LIST OF TABLES TABLE PAGE I. The Morphometry of Lake Waikaremoana Cat lake l e v e l 6 08 metres a . s . l . ) . . . 8 I I . The seasonal p e r i o d i c i t y o f lake l e v e l f l u c -t u a t i o n s b e f o r e and a f t e r h y d r o e l e c t r i c development 17 I I I . Morphometric changes i n Lake Waikaremoana f o l l o w i n g h y d r o e l e c t r i c development o f the lake 3 3 IV. Sampling v a r i a b i l i t y w i t h i n the drawdown Cat 2-3m), mixed (at 5-6m), and N i t e l l a (at 11-12m) zones a t Hautaruke Bay 40 - v i i i -LIST OF FIGURES F i g u r e Page .1. Lake Waikaremoana and catchment showing the i n f l o w i n g streams . .. 5 2. The o u t l e t of Lake Waikaremoana before and a f t e r h y d r o e l e c t r i c development 12 3. Lake l e v e l f l u c t u a t i o n s i n Lake Waikaremoana from 1931 - 1976. From data p r o v i d e d by the New Zealand E l e c t r i c i t y Department 14 4. The p e r i o d i c i t y of lake l e v e l f l u c t u a t i o n s i n Lake Waikaremoana be f o r e and a f t e r h y d r o e l e c t r i c develop-ment. Mean monthly r a i n f a l l (Onepoto) and mean monthly power g e n e r a t i o n (Kaitawa) 16 5. Lake l e v e l f l u c t u a t i o n s i n Lake Waikaremoana, and s i x monthly r a i n f a l l (.Onepoto) and power g e n e r a t i o n (Kaitawa) 1961-1976 19 6. Lake. Waikaremoana showing s h o r e l i n e geology, l o s t l i t t o r a l , bathymetry and s t a t i o n l o c a t i o n s 25 7. Diagrammatic t r a n s e c t s of., a) the l i t t o r a l zone a t Hautaruke Bay - benthos sampling s t a t i o n and b i the n e t t i n g s t a t i o n s showing the p o s i t i o n i n g of the g i l l n e t s 26 8. The net used f o r sampling l a r v a l f i s h 28 9. Thermal c o n d i t i o n s , water transparency, and lake l e v e l i n Lake Waikaremoana October 1974 - J u l y 1977 35 10. The temperature and d i s s o l v e d oxygen p r o f i l e s i n Lake Waikaremoana towards the end of the p e r i o d of thermal s t r a t i f i c a t i o n 1975 37 11. The depth d i s t r i b u t i o n of t o t a l animals, m o l l u s c s , i n s e c t s and o l i g o c h a e t e s i n the l i t t o r a l zone a t Hautaruke Bay 44 12. The composition of the l i t t o r a l i n v e r t e b r a t e fauna a t Hautaruke Bay 46 13. Seasonal changes i n the depth d i s t r i b u t i o n of t o t a l animals a t Hautaruke Bay 49 - i x -F i g u r e Page 14. Seasonal changes i n the depth d i s t r i b u t i o n o f mol-l u s c s , i n s e c t s and o l i g o c h a e t e s a t Hautaruke Bay 51 15. Seasonal changes i n the depth d i s t r i b u t i o n o f gastropods and b i v a l v e s a t Hautaruke Bay 53 16. Seasonal changes i n the depth d i s t r i b u t i o n of Odonata and L e p i d o p t e r a a t Hautaruke Bay 55 17. Seasonal changes i n the depth d i s t r i b u t i o n o f T r i c h o p t e r a a t Hautaruke Bay 57 18. Seasonal changes.'in the depth d i s t r i b u t i o n o f c h i r o n -omids a t Hautaruke Bay 59 19. The composition of the t o t a l c a t c h of t r o u t i n the g i l l n e t s from both s t a t i o n s , a c c o r d i n g to s p e c i e s , s i z e and m a t u r i t y 61 20. The l e n g t h frequency d i s t r i b u t i o n s of t r o u t caught i n g i l l n e t s i n the drawdown zone, i n the deep l i t t o r a l zone and i n the l i m n e t i c zone, from both s t a t i o n s 62 21. The composition of the d i e t s of brown and rainbow t r o u t i n Lake Waikaremoana 65 22. Seasonal changes i n the s i z e and numbers of l a r v a l smelt and l a r v a l b u l l i e s , and i n the numbers of Daphnia i n Lake Waikaremoana 6 7 23. The l e n g t h frequency d i s t r i b u t i o n s of rainbow t r o u t caught i n g i l l n e t s during each 2-monthly sampling (October 1975 - February 1976) 70 24. The mean c o n d i t i o n f a c t o r of rainbow t r o u t i n r e l a t i o n to s i z e and m a t u r i t y 71 25. The seasonal p e r i o d i c i t y and amplitude of the n a t u r a l f l u c t u a t i o n s i n lake l e v e l i n a)_ Lake B l a s j o n , n o r t h e r n Sweden b) Loch Lomond, Sc o t l a n d and c). Lake Waikaremoana 75 26. The n a t u r a l and r e g u l a t e d l a k e l e v e l f l u c t u a t i o n s i n Lake B l a s j o n and Lake Waikaremoana 77 27. The depth d i s t r i b u t i o n o f the bottom fauna i n Lakes B l a s j o n and A n k a r v a t t n e t , and i n Lake Waikaremoana.... 83 - x -ACKNOWLEDGEMENT I would l i k e t o thank Mr. P.J. B u r s t a l l , Conservator of W i l d l i f e , Rotorua, who was the d r i v i n g f o r c e behind the Waikaremoana study, and the remainder of the s t a f f o f the New Zealand W i l d l i f e S e r v i c e , Rotorua Conservancy, f o r t h e i r support. I am indebted to Dr. M.A. Chapman f o r her h e l p w i t h i d e n t i f i c a t i o n o f the b e n t h i c i n v e r t e b r a t e s ; Dr. P. Coleman, Ms. L. T i e r n e y , and Mr. S. P u l l a n f o r c a r r y i n g out scuba sampling; Mr. R. G i l l f o r h i s a s s i s t a n c e w i t h the n e t t i n g programme; No. 3 Squadron, R.N.Z.A.F. and Squadron Leader D. P a t t e r s o n f o r making p o s s i b l e the a e r i a l survey of the s h o r e l i n e ; the a n g l e r s and a n g l i n g c l u b s o f the Gisborne, Wairoa, Hawkes Bay and Manawatu D i s t r i c t s f o r t h e i r coopera-t i o n ; Mr. J . Irwin o f the New Zealand Oceanographic I n s t i t u t e f o r p r o v i d i n g me w i t h a copy of h i s , a t the time unpublished, bathymetry o f Lake Waikaremoana; Mr. L.S. Dennis o f the T o u r i s t H o t e l C o r p o r a t i o n , Mr. John S c o t t , a r c h i t e c t , and the s t a f f o f the Urewera N a t i o n a l Park f o r help w i t h accomodation and the many problems which a r i s e from l i v i n g and working i n a remote area. T h i s study c o u l d not have been c a r r i e d out without the f i n a n c i a l support from the New Zealand E l e c t r i c i t y Department, and I would l i k e to thank them f o r t h e i r h elp and c o o p e r a t i o n a t a l l times, and f o r p r o v i d i n g me with, e s s e n t i a l data on - x i -l a k e - l e v e l , r a i n f a l l , power g e n e r a t i o n and e n g i n e e r i n g aspects o f the Waikaremoana Power Scheme. I would p a r t i c u l a r l y l i k e to thank my s u p e r v i s o r , Dr. T. G. Northcote, f o r h i s h e l p and p a t i e n c e d u r i n g the past f i v e y e a r s , and Dr. P.A. L a r k i n and Dr. J.D. McPhail f o r t h e i r h e l p f u l c r i t i c i s m and advi c e d u r i n g the p r e p a r a t i o n of t h i s t h e s i s . - 1 -INTRODUCTION The development of a la k e f o r h y d r o e l e c t r i c power genera-t i o n u s u a l l y i n v o l v e s the c o n s t r u c t i o n of a dam, and r a i s i n g o f the lake l e v e l w i t h f l o o d i n g of the l o w - l y i n g surrounding l a n d . But, i n the case of Lake Waikaremoana, the c o n s t r u c t i o n of a dam was not r e q u i r e d . The Lake had been formed by a nat-u r a l rock and e a r t h - f i l l dam as a r e s u l t of a massive l a n d -s l i d e , which o c c u r r e d over 2000 years ago. I t s development f o r h y d r o e l e c t r i c purposes i n 1946 was unique i n t h a t the l a k e l e v e l was i n i t i a l l y lowered r a t h e r than being r a i s e d . In temperate r e g i o n s l a k e s u s u a l l y pass through three r e c o g n i z e d stages f o l l o w i n g h y d r o e l e c t r i c development (Rzoska 1966). The f i r s t o f these stages - the "damming-up e f f e c t " -i s f r e q u e n t l y a s s o c i a t e d w i t h i n c r e a s e d p l a n k t o n i c p r o d u c t i o n (Axelsson 1961, Rodhe 1964) and i n c r e a s e d growth r a t e s of f i s h (Runnstrom 1951, F r o s t 1956, Campbell 1963). T h i s has i n p a r t been a t t r i b u t e d to the r e l e a s e of n u t r i e n t s from sub-merged t e r r e s t r i a l v e g e t a t i o n and s o i l (Rawson, 1958), the i n c r e a s e i n space and consequent r e d u c t i o n i n the p o p u l a t i o n d e n s i t y of f i s h ( Elder 1965) and the a v a i l a b i l i t y t o bottom-f e e d i n g f i s h of submerged t e r r e s t r i a l i n v e r t e b r a t e s (.Campbell 1963, N i l s s o n 1964). But the f i r s t stage i s u s u a l l y o n l y t r a n s i t o r y , l a s t i n g a t b e s t 2-5 years (Campbell 1957, Stube 1958, Rzoska 1966). I t i s f o l l o w e d by the second stage - a g e n e r a l d e p r e s s i o n i n p r o d u c t i v i t y - d u r i n g which the i n o r g a n i c - 2 -sediments u n d e r l y i n g the s o i l s o f the inundated l a n d are 0 eroded and the new l i t t o r a l zones are un s t a b l e (Grimas 1965). T h i s produces an avalanche e f f e c t o f i n o r g a n i c sediments i n t o the deeper l i t t o r a l and p r o f u n d a l zones (.Lindstrom 1973) . As the new l i t t o r a l zones p h y s i c a l l y s t a b i l i z e the lake passes i n t o the t h i r d stage - a gradual recovery i n product-i v i t y . Organic sediments s t a r t to b u i l d up i n the p r o f u n d a l zone, and depending upon the extent and regime of lake l e v e l f l u c t u a t i o n s , r o o t e d a q u a t i c macrophytes may r e c o v e r to some exten t i n the l i t t o r a l zones. However, these h y d r o e l e c t r i c l a k e s seldom r e g a i n t h e i r former p r o d u c t i v i t y CRawson 1958, Rzoska 1966, Lindstrom 1973). In southern Russian r e s e r v o i r s the onset of t h i s recovery may occur a f t e r a 6-10 year p e r i o d of d e p r e s s i o n , and a t higher l a t i t u d e s (>50°N) recovery may be delayed f o r up to 25-30 years (Beckman 1966). Lake Waikaremoana d i d not experience a "damming-up e f f e c t " immediately f o l l o w i n g h y d r o e l e c t r i c development. I t probably entered d i r e c t l y i n t o a d e p r e s s i o n stage, but by now (.some 3 0 years l a t e r ) should be w e l l i n t o the " t h i r d stage" of gradual r e c o v e r y . The i n i t i a l d e t e r i o r a t i o n i n the t r o u t f i s h e r y gave r i s e to i n c r e a s i n g p r e s s u r e from r e g i o n a l a n g l i n g c l u b s on the f i s h e r i e s management a u t h o r i t i e s t o take remedial a c t i o n . T h i s p r e s s u r e was f r e q u e n t l y i n the form of demands f o r the l i b e r a t i o n of l a r g e numbers of h a t c h e r y - r e a r e d t r o u t . The f u t i l i t y o f s t o c k i n g thousands of f r y or f i n g e r l i n g s i n such - 3 -h y d r o e l e c t r i c lakes i s d i s c u s s e d by E l d e r (1965). The New Zealand W i l d l i f e S e r v i c e , who are r e s p o n s i b l e f o r the manage-ment of Lake Waikaremoana, w i s e l y r e s i s t e d these demands and i n s t e a d i n s t i g a t e d a 3 year study (made p o s s i b l e by a grant from the New Zealand E l e c t r i c i t y Department), which was c a r r i e d out between August 1974 and August 1977. The purpose of t h i s study was to g a i n a b e t t e r understanding of the t r o u t f i s h e r y o f Lake Waikaremoana and the impacts of h y d r o e l e c t r i c develop-ment. Some of the f i n d i n g s form the b a s i s of t h i s t h e s i s . P a r t i c u l a r a t t e n t i o n was p a i d to 1. The morphometric changes r e s u l t i n g from the l o w e r i n g of the l a k e l e v e l , and 2. The a l t e r e d seasonal p e r i o d i c i t y of the l a k e l e v e l f l u c t u a t i o n s i n r e l a t i o n to the seasonal changes i n the depth d i s t r i b u t i o n of the l i t t o r a l i n v e r t e b r a t e fauna, and the d i s -t r i b u t i o n and f e e d i n g ecology of the t r o u t . U n f o r t u n a t e l y , as w i t h most such s t u d i e s CGeen 1974), there was l i t t l e a v a i l a b l e data on the ecology of t r o u t or the limnology of Lake Waikaremoana from the p e r i o d b e f o r e h y d r o e l e c t r i c development. T h i s problem was overcome to some degree by Grimas (1961) i n h i s s t u d i e s of the h y d r o e l e c t r i c -r e g u l a t e d Lake B l a s j o n i n n o r t h e r n Sweden by usxng a s i m i l a r , nearby, undeveloped l a k e (Lake Ankarvattnet) as a "before" study. .This approach was c o n s i d e r e d f o r the Lake Waikaremoana study, but although there i s a s m a l l e r undeveloped lake w i t h i n i t s catchment (Lake W a i k a r e i t i ) the d i s s i m i l a r i t i e s were too - 4 -great to use t h i s lake for comparative ("before") studies. Conclusions regarding the ecological significance of the im-pacts of hydroelectric development on Lake Waikaremoana are therefore somewhat speculative i n nature. STUDY AREA LAKE WAIKAREMOANA AND CATCHMENT Geographical location & geology Lake Waikaremoana, with a maximum depth of 248 metres and a surface area of 51 square kilometres, i s the deepest, but not the largest lake on the North Island-of New Zealand. The lake l i e s at an elevation of 610 metres above sea l e v e l i n the Urewera Mountains, at Latitutde 38° 45'S and Longitude 177° 05'E (Figure 1). This area of the North Island of New Zealand l i e s d i r e c t l y over the plate margins of the P a c i f i c and Australian plates, a t e c t o n i c a l l y active subduction zone with moderately frequent earthquakes and an active volcanic b e l t — 100 kms. to the west. Convergence of the plates i s taking place at a rate of 3-7 cms. a year (Minster: et a l 1974). The shoreline and basin of the lake are composed of Tert-i a r y sedimentary rocks, 10-22 m i l l i o n years old (N.Z. Geol. Survey). They consist of alternating strata of sandstone and a sof t , l i g h t grey-coloured s i l t s t o n e (papa). These alternat-ing strata vary from a few centimetres thick to over 3 0 metres thick. They are t i l t e d , dipping at an angle of about 18° i n a southeasterly d i r e c t i o n (Carter 1951). In the northwest FIGURE 1. Lake Waikaremoana and catchment showing the i n f l o w i n g streams. Bars across the l a r g e r streams i n d i c a t e the l o c a t i o n of w a t e r f a l l s impassable to upstream m i g r a t i n g t r o u t . - 6 -corner of the catchment the rocks are o l d e r , w i t h outcroppings of J u r a s s i c greywacke i n the Hopuruahine catchment (N.Z. Geo-l o g i c a l Survey) . An o v e r l a y of pumice (.approximately 3 metres t h i c k ) b l a n k e t s the sedimentary rocks throughout much of the catchment. Lake formation Lake Waikaremoana was formed by a l a n d s l i d e , which broke o f f the Ngamoko Range (Figure 1) and slumped down a g a i n s t the P a n e k i r i b l u f f s o b l i t e r a t i n g a deep narrow gorge, through which the o l d Waikare-Taheke R i v e r had p r e v i o u s l y flowed, thus c r e a t -i n g a massive n a t u r a l rock and e a r t h ^ f i l l dam (Anderson 1948). T h i s damming back of the Waikare-Taheke R i v e r c r e a t e d a lake s i m i l a r i n morphometry to the much more r e c e n t man-made hydro-e l e c t r i c r i v e r - r e s e r v o i r s , although the dam has a much more gradual s l o p e i n t o the lake than the man-made v a r i e t y . The f o r e s t e d v a l l e y s , which were f l o o d e d , now form the lake b a s i n , and many of the t r e e s of t h i s drowned f o r e s t per-s i s t to t h i s day as s t a n d i n g stumps i n the lake-bed. I t was through r a d i o a c t i v e carbon d a t i n g of t h i s drowned f o r e s t t h a t the l a k e ' s age was determined - approximately 220 0 years (.Dr. V.H. J o l l y pers.comm.). The outflow was l a r g e l y through subsurface l e a k s i n the n a t u r a l dam (Figure 2), and o n l y o c c a s i o n a l l y d i d the l a k e l e v e l r i s e s u f f i c i e n t l y to form a s u r f a c e o v e r f l o w (.Figure 3) . For t h i s reason the la k e l e v e l remained e i t h e r r e l a t i v e l y 7 -stable, or else had been r i s i n g only very gradually since the lake was formed, allowing wave erosion to work p e r s i s t e n t l y on the shores over a r e s t r i c t e d v e r t i c a l range. The wave-cut terraces, which were carved from the open papa shores, and the deltas, which were b u i l t at the numerous stream mouths entering the lake, remained submerged, and considerably increased the .. area of shallow l i t t o r a l i n this deep, steep-sided lake. A continuous surface overflow would have caused rapid erosion and cutting down of the l i p of the dam, with a consequent pro-gressive lowering of the lake l e v e l and "loss" of these shallow l i t t o r a l areas (as occurs i n most lakes). Some of the present morphometric c h a r a c t e r i s t i c s are given i n Table 1. The catchment The catchment of Lake Waikaremoana, which i s about 7 times the area of the lake, consists almost e n t i r e l y of undisturbed beech forest and forms a small part of the Urewera National Park. The mean annual r a i n f a l l i s about 200-250 cms. Highway 38 passes through the catchment from the outlet along the eastern shores of the lake to the northern corner of the catchment. At Home Bay (Figure 1) and nearby at Ani-waniwa there are situated a t o u r i s t complex and the Park Head-quarters respectively. These are the only permanent human settlements within the catchment. Trampers, fishermen, hunters and others v i s i t the lake throughout the year, and up to 500-10.00 v i s i t o r s per day stay at these two locations during peak holiday - 8 -Table 1. The Morphometry of le v e l 6 08 metres a AREA OF CATCHMENT: AREA OF LAKE: LENGTH OF SHORELINE: SHORELINE DEVELOPMENT: VOLUME OF LAKE: MAXIMUM DEPTH: MEAN DEPTH: RESIDENCE TIME: Lake Waikaremoana (at lake s . 1. ) . 371 square kilometres 51.4 square kilometres 9 3.2 kilometres 3.67 9 4.76 X 10 cubic metres 248 metres 9 3 metres — 8 years - 9 -times (Christmas and New Year h o l i d a y s ) . Sewage e f f l u e n t from s e p t i c tanks at these settlements passes i n t o the Aniwaniwa stream or seeps d i r e c t l y i n t o the lake i n the v i c i n i t y of Home Bay. N a t i v e f i s h and i n t r o d u c e d s p e c i e s P r i o r to the i n t r o d u c t i o n of t r o u t , the o n l y n a t i v e f i s h o c c u r r i n g i n Lake Waikaremoana, and which are s t i l l p r e s ent, were the l o n g - f i n n e d e e l ( A n g u i l l a d i e f f e n b a c h i i Gray), a b u l l y (Gobiomorphus c o t i d i a n u s ) , and a g a l a x i d (Galaxias b r e v i p i n n i s Giinther) . There may a l s o have been other s p e c i e s of g a l a x i d s (P. B u r s t a l l pers.comm.), but t h e i r presence now i s d o u b t f u l . E e l s can no longer g a i n access to the lake ( s i n c e h y d r o e l e c t r i c development), but a few o l d and very l a r g e specimens are s t i l l p r e s e n t i n the l a k e (up to a t l e a s t 1.5 metres long, 14 kgs. i n weight and 40 years of age). Brown t r o u t (Salmo t r u t t a L.) were i n t r o d u c e d to the l a k e i n 1896 and rainbow t r o u t (Salmo g a i r d n e r i Richardson) i n 1907 ( B u r s t a l l 1975). There are now w e l l - e s t a b l i s h e d l a k e -m i g r a t o r y and stream r e s i d e n t ( a b o v e - f a l l s ) p o p u l a t i o n s of both s p e c i e s . Smelt (Retropinna l a c u s t r i s S t o k e l l ) were i n t r o d u c e d from the Rotorua Lakes i n 194 8 to compensate f o r the a n t i c i p a t e d adverse e f f e c t s of h y d r o e l e c t r i c development on the food r e ^ sources of the t r o u t . The adventive a q u a t i c weed, Elodea canadensis, has been - 10 -present i n Lake Waikaremoana since before 1946 and i s widely-d i s t r i b u t e d throughout the l i t t o r a l areas of the lake. HYDROELECTRIC DEVELOPMENT Before 1946 the Tuai power station, which commenced power generation i n 1929, had operated only on the natural discharge of water from the lake. In 1946 outlet modifications began, which allowed some control over the outflow and lake l e v e l of Lake Waikaremoana, and made feasible the s i t i n g of another power-station (Kaitawa) between the Lake and Tuai. At f i r s t the lake l e v e l was lowered by means of temporary siphons (aided by an exceptionally dry summer). Twin tunnels were driven from the Kaitawa side through the natural rock and e a r t h - f i l l dam, to connect with the intake structure, which was i n s t a l l e d i n an excavation behind a s i l l close to the lake shore. When the intake structure was completed, the s i l l was removed opening the intake to the lake (Figure 2). During maximum power gen-eration at Kaitawa the penstocks deli v e r water at a rate of approximately 30 cu. metres/sec. I n i t i a l l y there was a loss of trout through the intake and down the penstocks, u n t i l a g r i l l (4.8 cms gap) was i n s t a l l e d across the mouth of the intake i n 1959. Between 1948 and 1955 repeated attempts were made to seal the subsurface leaks i n the dam, but t h i s was only p a r t i a l l y successful. As some of the shallower leaks were sealed, cav-i t i e s within the dam collapsed and an extension of the leakage - 11 -area i n t o deeper water occ u r r e d (Figure 2). The leakage was reduced to about 5 cu. metres/sec. (approximately one t h i r d o f the o r i g i n a l f l o w ) . The o l d s u r f a c e overflow channel was lowered 60 cms, and i n 1954 twin c o n c r e t e siphons were i n s t a l l e d beneath the s u r -face o v e r f l o w channel. These can be used to bypass Kaitawa and d e l i v e r water to the Tuai power s t a t i o n i n the event of Kaitawa power s t a t i o n b e i n g c l o s e d down. They were used i n December 1976 w i t h a recorded flow of 35.4 cu. metres/sec. Since 1946 the l a k e l e v e l has been maintained a t a low-ered l e v e l (Figure 3). T h i s has been done to ensure s t a b i l i t y o f the n a t u r a l rock and e a r t h - f i l l dam, and a l s o i t p r o v i d e s a measure o f f l o o d p r o t e c t i o n to downstream areas. T h i s i n i t i a l l o w ering of the lake l e v e l r e s u l t e d i n the "dewatering" of p r e v i o u s l y submerged wave-cut t e r r a c e s along the papa shores and e x t e n s i v e areas of the o l d s h a l l o w l i t t o r a l a t stream mouth d e l t a s . The lake l e v e l i s now i n the v i c i n i t y o f the o l d " d r o p - o f f " and the l i t t o r a l zone has been s h i f t e d downwards onto a steeper average s l o p e . Assuming t h a t there has been no s i g n i f i c a n t change i n water tr a n s p a r e n c y , t h i s w i l l have r e s u l t e d i n a net r e d u c t i o n i n the area of the l i t -t o r a l zone. At the stream mouths deep channels have been carved through the exposed d e l t a s . These " g a u n t l e t s " are under the i n f l u e n c e of l a k e l e v e l f l u c t u a t i o n s , h a ving s w i f t l y f l o w i n g water at low l a k e l e v e l s , and f l o o d i n g back w i t h s l u g g i s h flow TKE-HYDRO ELECTRIC DEVELOPMENT " P O S T - H Y D R O E L E C T R I C DEVELOPMENT FIGURE 2. The ou t l e t of Lake Waikaremoana before and a f t e r h y d r o e l e c t r i c development. - 13 -a t h i g h l a k e l e v e l s . Because o f the p r o x i m i t y o f h i g h water-f a l l s c l o s e to the mouths of the m a j o r i t y of the streams enter-i n g the l a k e (Figure 1), the a c c e s s i b l e l e n g t h of streams a v a i l a b l e t o spawning runs of t r o u t i s g r e a t l y r e s t r i c t e d . These g a u n t l e t s are p o t e n t i a l l y important spawning areas p a r t -i c u l a r l y f o r rainbow t r o u t , whose e a r l y migrant f r y are not as dependent on stream nursery areas as are the brown t r o u t f r y . LAKE LEVEL FLUCTUATIONS The continuous r e c o r d of l a k e l e v e l from 1931 t o 1975 (Figure 3) breaks down i n t o three r e a s o n a b l y d i s t i n c t p e r i o d s : 1. 19 31 - 1946 - Before h y d r o e l e c t r i c development - the n a t u r a l f l u c t u a t i o n s i n lake l e v e l . 2. 1946 - the e a r l y 1960's - Immediately f o l l o w i n g h y d r o e l e c -t r i c development - an e a r l y p e r i o d of e x t e n s i v e f l u c t u a -t i o n s i n l a k e l e v e l . 3. The e a r l y 1960's - 1975 - A r e c e n t p e r i o d of more s t a b i l i z e d f l u c t u a t i o n s i n lake l e v e l . Amplitude During the r e c e n t p o s t - h y d r o e l e c t r i c development p e r i o d the mean annual amplitude of the lake l e v e l f l u c t u a t i o n s was 2.8 metres, which i s not much g r e a t e r than the n a t u r a l mean annual amplitude of 2.5 metres (Figure 3), but the l a k e l e v e l now f l u c t u a t e s about a mean lake l e v e l 4.7 metres below the n a t u r a l mean lake l e v e l . I-fl K 2 0 2 . 0 -E 2 0 i o • L Z O O O . E V E L (FEETfl-S-L-) mi •az '33 '34 as -3t 37 '3» '3i v> \i tu. u u ^ v W v 'so ; 5 i 'n ;n 'it w;» ;i7 ' i i ; s , ;i,c ;n /tx ;t3 ;t» ;ts > > T >t /« 'TO 71 ^ ^ -74. 7s- -7. PRE-HYDROELECTRIC DEVELOPMENT PERIOD ERRLY POST-HYDROELECTRIC D E V E LOPM ENT PERIOD RECENT POST-HYDROELECTRIC D E V E L O P M E N T PERIOD A M P L I T U D E O F M E A N A N N U A L A M P L I T U D E L A K E L E V E L Z - 5 F L U C T U A T I O N S ( M E T R E S ) 5-Z Z - 8 M A X 1 M U M A N N U A L 5-1 A M P L I T U D E M E A N L A K E L E V E L A - S L -6l/f--0 FIGURE 3. Lake level fluctuations in Lake Waikaremoana from 1931 to 1976. From data provided by the New Zealand E l e c t r i c i t y Department. - 15 -During the e a r l y p o s t - h y d r o e l e c t r i c development p e r i o d the mean annual amplitude o f lake l e v e l f l u c t u a t i o n s was 5.2 metres, which i s more than twice the n a t u r a l mean annual amplitude. During t h i s p e r i o d of time Waikaremoana was pro-v i d i n g a much h i g h e r p r o p o r t i o n o f the h y d r o e l e c t r i c power f o r the North I s l a n d than i t does now. The planned hydro-e l e c t r i c o p e r a t i n g range i s now 607.8 - 611.4 metres a . s . l . (amplitude 3.6 metres). Seasonal p e r i o d i c i t y Before h y d r o e l e c t r i c development the l a k e l e v e l tended to r i s e i n wint e r and f a l l i n summer (Table I I ) . Although the mean summer r a i n f a l l (89 cms - October t o March i n c l u s i v e ) i s not very much l e s s than the mean winter r a i n f a l l (113 cms -A p r i l to September i n c l u s i v e ) , i n c r e a s e d e v a p o t r a n s p i r a t i o n from the f o r e s t e d catchment d u r i n g the summer months, and i n -creased e v a p o r a t i o n from the lake s u r f a c e , w i l l c o n t r i b u t e to a f a l l i n g l a k e l e v e l i n summer - wit h an un r e g u l a t e d outflow. The p a r t i a l s e a l i n g of the lea k s i n the dam and the r e g -u l a t e d o u t f l o w now al l o w the New Zealand E l e c t r i c i t y Department to s t o r e water d u r i n g the summer months f o r maximum power gen-e r a t i o n i n the wi n t e r , (Figure 4). T h i s has tended to r e v e r s e the n a t u r a l seasonal p e r i o d i c i t y , so t h a t the lake l e v e l now tends to r i s e i n summer and f a l l i n w i n t e r (Table I I ) . L A K E L E V E L F E E T A . S . L -lozo 2 . 0 1 0 \j 1152. I /*? 5"3 I /95/f. I C??? I /<?S6 I/1??? \l1S8 \ 19 St I / ^ O rTTTT Tl 1 TTTT7T1 1 1 1" ITT Tl 'I r IT I I I 1 1 I 1 I I I I 1 ITT TTTI 1 r 1 T T If 1 1 1 I I T T I I rTTTTTT 2 0 1 0 2 . 0 0 0 f-I ISO zooof I ^ 9 0 M E A N M 0 N T H L Y K fl I N F f l L L ( O N E P O T O ) M E A N M O N T H L Y POWER. <$ENERf\TION FIGURE 4. The p e r i o d i c i t y of lake l e v e l f l uctuations i n Lake Waikaremoana before and a f t e r h y d r o e l e c t r i c deve-lopment. Mean monthly r a i n f a l l at Onepoto (near the lake outlet) 1925-1974, and mean monthly power generation at the Kaitawa power station 1966-1975 - from data provided by the New Zealand E l e c t r i c i t y Department. - 17 -Table II. The seasonal p e r i o d i c i t y of lake l e v e l fluctuations before and after hydroelectric development. Before After C1932 - 1941) (1966 - 1975) LAKE LEVEL INCIDENCE - NUMBER OF YEARS RISING IN SUMMER 0 6 FALLING IN WINTER 1 6 FALLING IN SUMMER 8 0 RISING IN WINTER 7 2 SEASONAL TIMING OF LOW LAKE LEVELS EXTREME LATE SUMMER -EARLY WINTER 7 0 MID WINTER 3 1 LATE WINTER -EARLY SUMMER 0 9 - 18 -The major cycle Before 194 6 the lake l e v e l tended to fluctuate on an annual cycle about a reasonably steady mean annual lake l e v e l . Immediately following hydroelectric development, although an annual cycle i s s t i l l present, a "major cycle" spanning 2-4 years i s superimposed, and t h i s i s s t i l l present, but less marked, in the recent post-hydroelectric development period (Figure 4) . The major cycle i s a consequence of hydroelectric u t i l -i z a t i o n of the lake. The more or less constant (or increas-ing) annual demand for power accentuates the e f f e c t s of dry years between successions of wet years. Dry winters (e.g. 1964, 1969 - Figure 5) predispose to extreme drawdown, and a succession of wetter than average years (e.g. 1970, 1971 -Figure 5) produce a "long term" upward trend i n the lake l e v e l . Rate of r i s e and f a l l With the intake closed, the rate of r i s e i n lake l e v e l for any given inflow w i l l have increased since hydroelectric development due to the p a r t i a l sealing of the leaks, and the t o t a l absence, since 1946, of a surface overflow. The rate of drawdown during maximum power generation at Kaitawa i s , of course, p a r t l y dependent on the inflows, but averages about 3 cms/day, and may increase to about 6 cms/day at low lake l e v e l s and low inflows.• FIGURE 5. Lake l e v e l f l u c t u a t i o n s i n Lake Waikaremoana and s i x monthly r a i n f a l l (Onepoto) and power generation (Kaitawa) 1961 - 1976. The s i x monthly periods are October to March i n c l u s i v e (summer) and A p r i l to September i n c l u s i v e (winter). - 20 -During maximum power generation at Kaitawa the outflow from the lake, 30 cubic metres/sec, i s more than twice the natural outflow, when there was no surface overflow, — 14 cubic metres/sec. (Andejrson 194 8) . MATERIALS AND METHODS PHYSICAL Surveys of the lake shore A e r i a l photographs of the lake taken before and after hydroelectric development (N.Z. A e r i a l Mapping.) were used to determine the positions of the pre-1946 and the 1971 shorelines of the lake, 615.7 metres a . s . l . and 610.5 metres a . s . l . respectively (an elevation difference of 5.2 metres) - and thus define the shallow areas of the pre-1946 lake-bed, which were "dewatered" as a r e s u l t of lowering the lake l e v e l with hydroelectric development i n 19 46. Henceforth t h i s dewatered area of the old lake bed w i l l be referred to as the " l o s t l i t t o r a l " . Its lower boundary i s delineated on sheltered areas of the lake shore by the lower l i m i t s of t e r r e s t r i a l vegetation (which are not e n t i r e l y stable), but i t does not include the shallow areas of the present l i t -t o r a l zone, which are intermittently exposed by hydro-e l e c t r i c drawdown. Work sheets showing the l o s t l i t t o r a l were prepared from the a e r i a l photographs (scale 1:12,667), and, along - 21 -with the relevant sections of the 1971 a e r i a l photograph, were used during an a e r i a l survey of the entire shoreline by RNZAF Iroquois helicopter i n September 1976. Doubtful areas of the l o s t l i t t o r a l on the a e r i a l photograph Ce.g. areas i n shadow or covered by scrub) were checked for accuracy on the work sheets, and the nature of the vegeta-ti o n cover of the l o s t l i t t o r a l was recorded. Two separate shoreline surveys have been c a r r i e d out by boat. The f i r s t , during October to December 1974, was a detailed survey of the present l i t t o r a l zone to record shoreline geology, development of wave cut platforms, sub-strate i n the shallow l i t t o r a l zone, and d i s t r i b u t i o n of aquatic macrophytes. The second survey i n October 1976 was car r i e d out to record the geology of the pre-1946 shoreline (Figure 6). Planimetry The bathymetry of Lake Waikaremoana with isobaths at 20 metre depth i n t e r v a l s was completed by Mr. J . Irwin of the New Zealand Oceanographic Institute i n September 1972 (Irwin 1977). This chart (scale 1:15,840) was used to measure the areas enclosed by the shoreline and isobaths with a polar planimeter, the mean of three readings being taken. A second chart was prepared from the 19 71 a e r i a l photograph and work sheets (.scale 1:12,667). This was used - 22 -to measure the area o f the l o s t l i t t o r a l and l e n g t h o f shore-l i n e b e f o r e and a f t e r h y d r o e l e c t r i c development. The areas o f the l o s t l i t t o r a l and prese n t 0-20 metre depth zone, and the l e n g t h of s h o r e l i n e were measured i n s e c t i o n s i n r e l a t i o n to s h o r e l i n e geology. The area o f the present l i t t o r a l zone was estimated by us i n g a c o r r e c t i o n f a c t o r o f 17/20 on the measurements of the pre s e n t area of the 0-20 metre depth zone. The area of the pre-1946 l i t t o r a l zone was estimated by adding the area of the l o s t l i t t o r a l to (the area o f the pr e s e n t 0-20 metre 17 - 5 2 depth zone X — — ) • Lake volume, mean depth, s h o r e l i n e development and the area of the l i m n e t i c zone before and a f t e r h y d r o e l e c t r i c development were a l s o c a l c u l a t e d from the above measurements. Water transparency and temperature Temperature p r o f i l e s i n the lake and water transparency were recorded a t l e a s t monthly between October 1974 and J u l y 1977 a t two s t a t i o n s - one a t the c e n t r e of the main la k e , and the other i n the Wairaumoana arm (Figure 1). Temperature p r o f i l e s were recorded a t metre i n t e r v a l s down to 55 metres depth with a Y.S.I, temperature/oxygen meter, model 51A. Water transparency was measured w i t h a 20 cm. diameter S e c c h i d i s c . - 23 -Lake l e v e l , power generation and r a i n f a l l The charts of lake l e v e l fluctuations were prepared from da i l y records of the lake l e v e l kept by the New Zealand E l e c t r i c i t y Department. A s t a f f gauge was i n s t a l l e d near the laboratory at Home Bay for recording lake l e v e l on samp-l i n g days. Data on r a i n f a l l at Onepoto (close to the lake outlet) and power generation at the Kaitawa power station were provided by the New Zealand E l e c t r i c i t y Department. LITTORAL BENTHOS Ekman dredge sampling T r i a l Ekman dredge sampling was car r i e d out at scatter-ed l o c a l i t i e s around the lake shore i n conjunction with the shoreline survey. The substrate below the drawdown zone was found to be more or less uniform throughout the l i t t o r a l areas of the lake, consisting of fine papa s i l t , which was i d e a l for Ekman sampling. The substrate i n the drawdown zone was much more variable, and i n many instances (e.g. stones, boulders or bedrock of sandstone and papa) i t was impossible to sample with an Ekman dredge. In some sheltered bays the substrate i n the drawdown zone also consisted of fine papa s i l t , and such was the case at Hautaruke Bay which was chosen as the main sampling station (Figure 6). The transect at Hautaruke Bay was clear of standing or f a l l e n tree stumps, and the shallow l i t t o r a l area contained a r e l a t i v e l y low growth of macrophytes, such that the macrophytes - 24 -and their underlying substrate and roots were obtained i n each Ekman sample. Scuba surveys were carried out along the samp-l i n g transect on three occasions, and during one of these the slope of the l i t t o r a l and lower l i m i t s of the "mixed" and N i t e l l a zones were plotted (Figure 7). A second station close to Home Bay was sampled regularly i n conjunction with the Hautaruke Bay sampling programme to examine the possible e f f e c t s of sewage enrichment, but the results from t h i s second station are not presented here. The l i t t o r a l invertebrate fauna was sampled over a 12 month period (April 1975 - A p r i l 1976) at Hautaruke Bay. Sampling was carried out on a monthly basis when possible (July, October and December 1975 samplings were missed due to other commitments). On each sampling occasion single Ekman dredge (.15 cm X 15 cm ) samples were taken at metre depth in t e r v a l s from 1 down to 9 metres, and at alternate metres between 9 and 2 0 metres, along the transect through the l i t t o r a l zone (Figure 7). The depth i n t e r v a l s sampled were i n r e l a t i o n to a theo-r e t i c a l "zero" lake l e v e l (609.6 metres a . s . l . ) , which was the lower l i m i t of t e r r e s t r i a l vegetation during the sampling period. Samples were washed and sieved through a 0.8 mm bronze wire mesh and sorted l i v e the day they were obtained. In addition to the monthly sampling, 10 r e p l i c a t e s were taken at each of the three depths (2, 5 and 12 metres), on separate occasions, to examine sampling v a r i a b i l i t y i n the OFFSHORE W » ONSHORE MerriNtr STATIONS FIGURE 6. Lake Waikaremoana showing shoreline geology, l o s t l i t t o r a l (black), Bathymetry (20,100 & 200 metre isobaths /HYDROELECTRIC INTAKE from the bathymetry of Lake Waikaremoana, Irwin 1972), and s t a t i o n locations. v o 7o Io 30 40 50 ~~2o 7 0 DISTANCE FROM SHORE - METRES D E P r H •M E T R E S DRAWDOWN ^IZONE 40 60 80-J DEEP LITTORAL L I M N E T I C ~ /2.00 (RED REEF) FIGURE 7 • S"0 D I S T A N C E F R O M S H O R E - M E T R E S Diagrammatic transects of a) the l i t t o r a l zone at Hautaruke Bay - benthos sampling s t a t i o n and, b) the ne t t i n g stations showing the posi t i o n i n g of the g'illnets. - 2 7 -drawdown, mixed and N i t e l l a zones respectively. S t a t i s t i c a l procedure A one-way non-parametric analysis of variance (The Kruskal-Wallis Test) was used to test for s i g n i f i c a n t differences at p<0.05 i n the depth d i s t r i b u t i o n of animals between seasons. Each sample col l e c t e d during the monthly samplings within each season was used as a r e p l i c a t e of the respective depth zone. ZOOPLANKTON AND LARVAL FISH Zooplankton samples were taken monthly between October 1975 and.August 1977 at two stations - the Te Puna and Red Reef offshore netting s i t e s (Figure 6). Two v e r t i c a l hauls from 55 metres depth to the surface were taken at each sampling s i t e on each sampling occasion using a 25 cm diameter Wisconsin nylon net (10 meshes/mm). Two r e p l i c a t e samples of the l a r v a l f i s h were obtained at the same stations on the same occasions between August 1976 and August 19 77. A heavy-rimmed 55 cm diameter net (5.3 meshes/mm) was used, which sampled v e r t i c a l l y from the surface  down to a depth of 75 metres. At t h i s point the handline was allowed to tighten, snaring the net, and t i l t i n g the heavy rim through 90°, and then immediately commencing the retrieve of the closed net to the surface (.Figure 8) . Echosounder runs of approximately 400 metres were made at the offshore netting stations i n conjunction with the sampling - 28 -D I A M E T E R 55 c m s . FIGURE 8. The net used f o r sampling l a r v a l f i s h . - 29 -of the l a r v a l f i s h using a Furuno FG 11 Mark 3 echosounder (.50 khz) at a setting of gain 4. Larval b u l l i e s (Gobiomorphus  cotidianus) are i n a similar size range to Chaoborus larvae and with th e i r g a s - f i l l e d swim-bladders produce deep scattering layers on the echosounder tracings s i m i l a r to those described for Chaoborus larvae (Northcote 1964). Larval smelt (Retropinna  l a c u s t r i s Stokell) do not have a g a s - f i l l e d swim-bladder, and were found to produce no scattering layer on the echosounder on those sampling occasions when they were p l e n t i f u l , but l a r v a l b u l l i e s were absent. Daphnia i n the August 1976 to June 1977 Wisconsin samples were counted i n entirety and together with the remaining Wis-consin samples have been examined and counted by Dr. M.A. Chapman of Waikato University. The l a r v a l f i s h samples were counted i n entirety and t o t a l length measurements were made on each l a r v a l f i s h using a precision c a l i p e r f i t t e d with needle-point extensions to the measuring arms. TROUT Netting programme Samples of trout were obtained by g i l l n e t t i n g at in t e r v a l s of two months from October 1975 to December 1976. Two stations were, used - one i n the main lake, Red Reef, and the other i n Wairaumoana, Te Puna (Figure 6). At each station separate nets were set i n the limnetic zone, i n the deep l i t t o r a l zone, and - 30 -i n the drawdown zone (Figure 71. The offshore net (in the limnetic zone) was set at the surface i n water 80-100 metres deep. This net was 60 metres long and 10 metres deep. An onshore net was set at the surface over the deep l i t -t o r a l i n water 10-15 metres deep; th i s net was 60 metres long and 5 metres deep. Both th i s onshore net and the offshore net contained six 10 metre long panels of d i f f e r e n t mesh si z e s : 2.5, 3.8, 5.1, 7.0, 7.6 and 10.2 cms - monofilament nylon netting. In addition two small nets - 15 metres long and 2.5 metres deep were used i n the drawdown zone, one with a mesh size of 2.5 cms. the other with a mesh size of 5.1 cms. These small nets i n the drawdown zone were not used during the f i r s t two nettings - i . e . October 1975 and December 1975. The nets were set before dusk and l i f t e d a f t e r dawn the next day. The d i s t r i b u t i o n of trout within and between the nets was recorded; species, sex, length, weight, and state of the gonads were recorded. The entire stomach was removed and preserved i n 10% formalin for l a t e r examination, together with a sample of scales. The d i s t i n c t i o n between immature f i s h and "previous spawn-ers" could be made i n most instances by macroscopic examination of the gonads, but where there was any doubt scales were ex-amined l a t e r for spawning marks, which were well developed i n both rainbow and brown trout. - 31 -Stomach analysis The stomach was divided at the p y l o r i c flexure, and only the anterior portion was used for detailed analysis of the contents. The smaller food items were examined and sorted at X 20 - 40 magnification under a dissecting microscope. The volume of the d i f f e r e n t food items was measured to the nearest 0.1 ml by displacement of water i n a graduated glass cylinder. The volume of the p y l o r i c contents was also measured to give the t o t a l volume of the stomach contents. - 32 -RESULTS PHYSICAL Morphometric Changes The i n i t i a l l owering of the lake l e v e l w i t h h y d r o e l e c t r i c development reduced the s u r f a c e area of the l a k e from 54.8 to 51.4 km 2 (Table I I I ) . 3.4 km 2 of the pre-1946 shallow l i t t o r a l area was l o s t . The estimated gain i n l i t t o r a l area due to the pre-1946 s u b l i t t o r a l , which i s now i n c l u d e d i n the euphotic zone and has become the lower e x t r e m i t y of the new l i t t o r a l , 2 was approximately 2.1 km - and t h i s was the extent of the r e d u c t i o n i n the l i m n e t i c area. There has t h e r e f o r e been a d i s p r o p o r t i o n a t e percentage r e d u c t i o n i n the l i t t o r a l area (^ 17% net r e d u c t i o n ) as conv-pared w i t h the l i m n e t i c area (cL 4% r e d u c t i o n ) and t h i s has a l t e r e d the r a t i o of l i t t o r a l t o l i m n e t i c areas from 1 : 5.8 to 1 : 6.7. Almost h a l f of the net r e d u c t i o n i n the l i t t o r a l area was a r e s u l t of the l o s s of p r e v i o u s l y submerged stream-mouth d e l t a s , and to a s l i g h t l y l e s s e r extent the l o s s of wave-cut t e r r a c e s on the exposed papa shores (Figure 6). There has been l i t t l e net r e d u c t i o n i n l i t t o r a l area along mixed shores, and no net r e d u c t i o n along the sandstone shores. The s l i g h t r e d u c t i o n i n s h o r e l i n e development i s due i n p a r t to the l o s s of i s l a n d s , which now form p e n i n s u l a s . There has been l i t t l e change i n the mean depth of the - 33 -Table I I I . Morphometric changes i n Lake Waikaremoana following hydroelectric development of the lake. PRE- POST-HYDROELECTRIC HYDROELECTRIC DEVELOPMENT DEVELOPMENT LOSS SURFACE AREA km2 54.8 51.4 3.4 (6.2%) LENGTH OF SHORELINE (INCLUDING ISLANDS) km 103.5 93.2 10.3 (10%) SHORELINE DEVELOPMENT 3.95 3.67 MEAN DEPTH metres 91.7 92.7 ESTIMATED AREA OF LITTORAL ZONE (0-17 METRES DEPTH) km2 8.1 6.7 1.4 (17%) ESTIMATED AREA OF LIMNETIC ZONE (> 17 METRES DEPTH) km2 46.7 44. 6 2.1 (.4,4%) RATIO OF AREAS OF LITTORAL:LIMNETIC 1 : 5.8 1 : 6.7 - 34 -lake (.in f a c t a s l i g h t i n c r e a s e ) , and t h e r e f o r e no s i g n i f i c a n t change i n the r a t i o of the volumes of e p i l i m n i o n to hypolimn-.. i o n . The 5% r e d u c t i o n i n the lake volume w i l l have had an i n s i g n i f i c a n t e f f e c t on the r e s i d e n c e time of approximately 8 years. Thermal c o n d i t i o n s & water transparency Thermal s t r a t i f i c a t i o n l a s t s f o r about 6 months of the year, December to May (Figure 9). C l i m a t i c v a r i a t i o n between the years 1974 - 1977 produced a p p r e c i a b l e d i f f e r e n c e s between years i n the depth and s t r e n g t h of thermal s t r a t i f i c a t i o n and maximum s u r f a c e temperatures (Figure 9). The c o n s i d e r a b l e depth of the thermocline r e f l e c t s the g e n e r a l l y windy c o n d i -t i o n s of the Waikaremoana c l i m a t e . The thermocline, a p a r t from the f i r s t 1-2 months of the p e r i o d of thermal s t r a t i f i c a t i o n , l i e s below the lower l i m i t o f the l i t t o r a l zone, and so throughout most o f the year the e n t i r e l i t t o r a l zone i s exposed to more or l e s s the same sea-s o n a l changes i n water temperature. Secchi d i s c readings v a r i e d from 5.5 to 17.5 metres wi t h a mean of 11.5 metres. There i s a s i g n i f i c a n t n e g a t i v e c o r -r e l a t i o n (p<0.01) between water transparency (Secchi d i s c readings) and l a k e l e v e l , (Spearman's rank c o r r e l a t i o n c o e f -f i c i e n t , r = -0.61). The g r e a t e s t r e d u c t i o n i n water t r a n s -parency o c c u r r e d immediately a f t e r an e x c e p t i o n a l f l o o d d u r i n g the New Year p e r i o d 1975/76, when heavy sediment loads and much OCT|NOV|DEC TAN|FEB |WflR|flPP, \n*Y | TUN |Tui | flufr |SE?T|OCT | NOV |D£C / 1 7 5 TflN |F EB I^ Ar |TUN [TUL | US [5£PT| OCT | NOV \pjc 11 7 6 W A T E R T R A N S P A R E N C Y SECCHI DISC READINGS THE wew r u R F L O O D |LOWER LIMIT OF THE L l T T O R f t L Tf\N I FEB JMflR | APR I MAY fruN |ruL | / 7 7 .DAMM IN t r -UP THEORET ICAL : ,~ZERO'LAKE LEVEL ^DRAWDOWN FIGURE 9. Thermal conditions, water transparency and lake l e v e l October 1974 - Ju l y 1977, - 36 -f l o a t i n g woody d e b r i s entered the l a k e . The amplitude of lake l e v e l f l u c t u a t i o n s d u r i n g the study p e r i o d (October 1974 - J u l y 1977) was 3.54 metres; 1.60 metres "damming up" above the t h e o r e t i c a l zero l a k e l e v e l (609.6 metres a . s . l . ) and 1.94 metres drawdown below. During each of the 3 years 1975, 1976 and 1977, "damming up" w i t h f l o o d i n g of t e r r e s t r i a l v e g e t a t i o n and s o i l o c c u r r e d d u r i n g l a t e summer or e a r l y w i n t e r , l a s t i n g f o r 2-5 months. Extreme drawdown oc c u r r e d d u r i n g l a t e w i n t e r (August) 1976. D i s s o l v e d oxygen Only one r e l i a b l e d i s s o l v e d oxygen p r o f i l e was obt a i n e d . T h i s was towards the end o f the p e r i o d o f thermal s t r a t i f i c a -t i o n i n l a t e A p r i l 1975 (Figure 10). D i s s o l v e d oxygen i n the hypolimnion down to 55 metres depth was c l o s e to f u l l s a t u r a -t i o n f o r the temperatures observed and the a l t i t u d e (Hutchinson 1957), but a m e t a l i m n i a l oxygen minimum was p r e s e n t . Echo-sounder runs d u r i n g summer showed t h a t the l a r v a l b u l l i e s c oncentrate a t the thermocline d u r i n g the d a y l i g h t hours. AQUATIC MACROPHYTES Rooted a q u a t i c macrophytes extended down to a depth of 17.5 metres a t Hautaruke Bay (Figure 7). T h i s was taken to be the lower l i m i t o f the l i t t o r a l zone and commencement of the s u b l i t t o r a l CMacan 1951). During t r i a l Ekman sampling and scuba surveys a t s e v e r a l other l o c a t i o n s i n the main l a k e , - 37 -TEMPERATURE VEQ-REES CENTIGRADE <? 10 ll IX 13 14- '5" FIGURE 10. The temperature and dissolved oxygen p r o f i l e s i n Lake Waikaremoana 30th A p r i l 1975. - 38 -the lower l i m i t of roo t e d macrophytes was a l s o found to occur a t about 17-18 metres depth, w i t h the e x c e p t i o n o f the l i t -t o r a l areas i n the p r o x i m i t y of the motor camp a t Home Bay, where the lower l i m i t o c c u r r e d a t 15-16 metres depth. Extreme drawdown d u r i n g the l a t e w i n t e r of 19 7 3 (Figure 3) extended 3 metres below the t h e o r e t i c a l zero l a k e l e v e l . In s h e l t e r e d l i t t o r a l areas o f the lake the upper boundary o f continuous macrophyte beds p r o v i d e d a very c l e a r demarcation of the lower l i m i t o f t h i s drawdown, which had been the lowest lake l e v e l s i n c e the wi n t e r o f 1969. On shores exposed to much wave a c t i o n r o o t e d macrophytes were sparse or absent i n l i t t o r a l areas l e s s than ^ 5 metres depth. In the lower "drawdown zone" sparse clumps o f some n a t i v e a q u a t i c macrophytes had s u r v i v e d i n most s h e l t e r e d areas o f the l a k e , but the adventive weed, Elodea canadensis, was not found i n the drawdown zone d u r i n g the s p r i n g o f 1974. During the h i g h e r l a k e l e v e l s o f 1975 and e a r l y 1976 Elodea invaded the lower drawdown zone by means of l a t e r a l v e g e t a t i v e shoots. At Hautaruke Bay extending from the lower l i m i t o f draw-down (3 metres depth) down to 8 metres depth there was a "mixed zone" of N i t e l l a sp., Myriophyllum sp. and Potamogeton  sp. with, a sparse i n t e r m i n g l e d growth o f Elodea canadensis. T h i s mixed zone was t y p i c a l o f most ot h e r areas o f the lake examined, except i n the v i c i n i t y o f Home Bay, where the mixed zone was r e p l a c e d by a dense monoculture o f Elodea canadensis. - 39 -From 8 metres depth down to 17.5 metres depth there was a monoculture o f N i t e l l a sp. forming the " N i t e l l a zone". During the e a r l y summer of 197 5/7 6 t h e r e was a grad u a l exten-s i o n o f N i t e l l a i n t o deeper water, but f o l l o w i n g the sudden r i s e i n lake l e v e l i n January 1976, and reduced water t r a n s -parency, there was a d i e - o f f of the deeper N i t e l l a beds, which was apparent from examination o f the macrophytes o b t a i n e d i n the Ekman samples. During the summer of 1974/75 the emergent s p e c i e s o f a q u a t i c macrophytes (Myriophyllum sp. and Potamogeton s p . ) , i n the shallow l i t t o r a l zone of s h e l t e r e d areas o f the l a k e , p e n e t r a t e d the lake s u r f a c e and s u c c e s s f u l l y flowered, but d u r i n g the summers of 1975/76 and 1976/77 the r i s i n g and maintained lake l e v e l s d u r i n g summer prevented s u c c e s s f u l f l o w e r i n g o f these s p e c i e s . LITTORAL BENTHOS Sampling v a r i a b i l i t y The v a r i a n c e to mean r a t i o , which i s an index o f d i s p e r -s i o n , i s g r e a t e r than one f o r t o t a l animals, t o t a l m o l l u s c s , t o t a l i n s e c t s and o l i g o c h a e t e s a t each of the three depth i n t e r v a l s sampled (Table I V ) . T h i s suggests contagious d i s -t r i b u t i o n s , which are usual- f o r most b e n t h i c i n v e r t e b r a t e s ( E l l i o t t 1971). However, o n l y a t the 11-12 metre depth i n t e r -v a l f o r the t o t a l m o l l u s c s i s t h i s a s i g n i f i c a n t departure 2 from u n i t y (at p<0.05 X Table) i n d i c a t i n g t h a t the remainder Table IV. Sampling v a r i a b i l i t y within the drawdown (at 2-3 m, 27th May '76), mixed (at 5-6 m, 9th June '76), and N i t e l l a (at 11-12 m, 29th March '76) zones at Hautaruke Bay. n = 10 replicates at each depth i n t e r v a l . Depth Interval (metres) Mean Number 95% Confidence Limits Variance • Variance Mean TOTAL ANIMALS 2 - 3 5 - 6 11 - 12 221 405 463 + 24 + 39 + 48 1532 3955 5886 6.9 9.8 12.7 TOTAL MOLLUSCS 2 - 3 5 - 6 11 - 12 71 126 266 + 12 + 9 ± 4 7 37 6 . 191 5660 5.3 1.5 21.3* TOTAL INSECTS 2 - 3 5 - 6 11 - 12 75 244 130 + 16 + 32 + 17 663 2666 711 8.8 10. 9 5.5 OLIGOCHAETES 2 - 3 5 - 6 11 - 12 68 32 66 + 18 + 11 + 11 840 333 336 12. 4 10. 4 5.1 * = s i g n i f i c a n t departure from unity (p<0.05 Table of X ) - 41 -c o u l d be randomly d i s t r i b u t e d . Some o f the problems a s s o c i a t e d w i t h sampling of b e n t h i c i n v e r t e b r a t e s i n l a k e s , and causes of sampling v a r i a b i l i t y are d e s c r i b e d by Northcote (1952). Among those r e l e v a n t t o t h i s study are contagious d i s t r i b u t i o n s i n r e l a t i o n to quadrat s i z e , o p e r a t i o n of the sampling equipment, h e t e r o g e n e i t y of s u b s t r a t e , and v a r i a b i l i t y due to changes i n depth and season. Northcote found the g r e a t e s t v a r i a b i l i t y i n the shallow l i t -t o r a l (0 - 5m.), and a t t r i b u t e d t h i s to the wide v a r i a t i o n i n s u b s t r a t e w i t h i n t h i s depth zone. The s c a l e of contagious d i s t r i b u t i o n s i n r e l a t i o n to the area e n c l o s e d by the sampler i s an unavoidable problem, be-cause i t v a r i e s c o n s i d e r a b l y between d i f f e r e n t taxa of b e n t h i c i n v e r t e b r a t e s , and so the c h o i c e of quadrat s i z e i s u n l i k e l y to be a p p r o p r i a t e f o r a l l taxa ( E l l i o t t 1971). I t appears t h a t a t Hautaruke Bay i n the upper N i t e l l a zone t h i s was a problem w i t h t o t a l m o l l u s c s . At Hautaruke Bay h e t e r o g e n e i t y w i t h i n zones i n the sampl-i n g area was minimal. The i n o r g a n i c s u b s t r a t e was uniform throughout the l i t t o r a l zone. There was l i t t l e gross clumping o f the d i f f e r e n t macrophyte s p e c i e s i n the mixed and drawdown zones, and the d e n s i t y of macrophyte growth was r e l a t i v e l y homogeneous w i t h i n zones, the g r e a t e s t v a r i a b i l i t y i n d e n s i t y o c c u r r i n g w i t h depth i n the N i t e l l a zone. Although throughout the lake the shallow l i t t o r a l areas showed the g r e a t e s t h e t e r o -g e n e i t y w i t h i n the l i t t o r a l zone, w i t h i n the sampling area t h i s - 42 -was not so. The papa s i l t s u b s t r a t e was i d e a l f o r Ekman sampling and f r e e from coarse d e b r i s , which might i n t e r f e r e w i t h c l o s i n g of the jaws. The ch o i c e o f a 0.8 mm mesh was perhaps not i d e a l f o r chironomids and o l i g o c h a e t e s . Many of t h e i r e a r l y i n s t a r s and smal l i n d i v i d u a l s would have been l o s t d u r i n g washing and s i e v i n g o f the samples. P o o l i n g o f the data over the twelve month p e r i o d i n t r o -duces seasonal changes as a cause of v a r i a b i l i t y , and p o o l -i n g the data from d i f f e r e n t depths w i t h i n zones i n t r o d u c e s depth as an a d d i t i o n a l cause of v a r i a b i l i t y . The 95% con-f i d e n c e l i m i t s on the means of 10 r e p l i c a t e s taken on the same day and from the same depth g i v e an i n d i c a t i o n of l e s s v a r i a b i l i t y than c o u l d be expected i n the r e s u l t s o f the sampling programme. - 43 -Depth d i s t r i b u t i o n The mean number of t o t a l animals for the twelve month 2 sampling period i n the upperdrawdown zone i s less than 1000/m (Figure 11) with an order of magnitude increase i n numbers 2 i n the lower drawdown zone {cL 9 000/m ). There i s l i t t l e change i n mean numbers of t o t a l animals throughout the lower drawdown, mixed, and upper N i t e l l a zones (1 - 10 metres), ... but there i s a decrease i n numbers throughout the lower N i t e l l a zone to the s u b l i t t o r a l . Molluscs, insects, and oligochaetes account for > 9 5% of the numbers of the macro-invertebrate fauna (Figure 12). I t i s the molluscs and insects which contribute most to produce the o v e r a l l pattern of depth d i s t r i b u t i o n . Oligochaetes show a maximum density at 1 - 3 metres depth, which, i n the p r o f i l e of t o t a l animals, masks the s l i g h t l y reduced numbers of insects and molluscs i n the lower drawdown zone (JFigure 11) . T O T A L A N I M - A L S OLiq-OCHflETES o u 5000 NUMBER OF A N I M A L S pvt. S Q U A R E M E T R E FIGURE 11. The depth d i s t r i b u t i o n of t o t a l animals, molluscs, insects, and oligochaetes i n the l i t t o r a l zone at Hautaruke Bay. The mean number of ind i v i d u a l s per Ekman sample from each metre depth i n t e r v a l during the 12 month sampling period, expressed as number of animals per square metre. (n.) = sample s i z e s . - 45 -Composition S e v e r a l taxa, which are prominent i n the fauna of North-ern Hemisphere temperate l a k e s are absent (e.g. Chaoborinae) or r a r e (e.g. Ephemeroptera, P l e c o p t e r a , Isopoda, Amphipoda) i n New Zealand Lakes (Marples 1962, F o r s y t h 1975). Lake d w e l l i n g P l e c o p t e r a are absent i n Lake Waikaremoana, as are t r u l y l a k e - d w e l l i n g Ephemeroptera. However, d u r i n g the winter months nymphs of one o f the stream-dwelling may-f l i e s (Zephlebia sp.) do c o l o n i z e the stony shores of the l a k e i n the p r o x i m i t y o f stream mouths, but d i s a p p e a r a f t e r emergence i n l a t e s p r i n g / e a r l y summer. Amphipods, although present w i t h i n the catchment of Lake Waikaremoana, are r a r e i n the i n f l o w i n g streams, and were not found i n the l a k e . In New Zealand s e m i - t e r r e s t r i a l s p e c i e s of Amphipoda are common i n the damp l e a f l i t t e r o f f o r e s t f l o o r s (Dr. M.A. Chapman per s . comm.) and the two specimens found i n the i n f l o w i n g streams may have o r i g i n a t e d from t h i s source. The Odonata and T r i c h o p t e r a are t h e r e f o r e of much g r e a t e r importance i n New Zealand lakes - p a r t i c u l a r l y w i t h r e s p e c t to t r o u t food. - 46 -SLm.lLmr\a.e.a.sp. PhjjSa&thaSp-G^tjK.<wiMS sp. | Fwu.sLa-Sp-SPHAERIIDS Hy'u.citjLipL. Sp-M E T R E s Pycnocznihodes sp-"T>upLui.uljLS sp HYDROPTl LIT>S CERRTOPOqONlDS Pcui.ox<j£th.bia,3p. C H I R O N O M I D S CYCLOPOID COPEPoDS Oi NEMERTERNS OSTRftCOBS CHYBpRIDS | HYDRACRRINA 1 O L I C T O C H A E T E S NEMATODES LEECHES \ PLANARIANS FIGURE 12. The composition of the l i t t o r a l i n v e r t e b r a t e fauna at Hautaruke Bay, Lake Waikaremoana, showing the mean number of i n d i v i d u a l s per square metre f o r each metre depth i n t e r v a l of the l i t t o r a l zone during the 12 month sampling p e r i o d . - 47 -Even those groups of animals, which are p r e s e n t i n New Zealand l a k e s , show a r a t h e r low s p e c i e s d i v e r s i t y . Among the l i t t o r a l s p e c i e s of chironomids i n Lake Waikaremoana th e r e are a t l e a s t 6 s p e c i e s , but probably no more than 10 (Forsyth 1975) - compared with some Scandinavian l a k e s e.g. Lake Ankar-v a t t n e t , Sweden: 56 s p e c i e s of l i t t o r a l chironomids, and the r e g u l a t e d l a k e B l a s j o n : 27 s p e c i e s (Grimas 1961), and a s t o r -age r e s e r v o i r i n England: 57 s p e c i e s of chironomids (Mundie 1957) . The g r e a t e s t d i v e r s i t y i n the l i t t o r a l i n v e r t e b r a t e fauna of Lake Waikaremoana o c c u r r e d i n the shallow l i t t o r a l . Nymph-u l a n i t e n s , o s t r a c o d s , c h y d o r i d s , p l a n a r i a n s , ceratopogonids and a t l e a s t 2 s p e c i e s of chironomids were o n l y found a t depths l e s s than 7 metres (Figure 12). The s p h a e r i i d s and two s p e c i e s of T r i c h o p t e r a , Pycnocent-rodes sp. and P a r o x y e t h i r a hendersoni, occur throughout the l i t t o r a l , but show maxima i n the drawdown and/or s u b l i t t o r a l zones. The other 2 s p e c i e s of T r i c h o p t e r a , the Odonata, t o t a l chironomids and the two abundant s p e c i e s of gastropods, Pota- mopyrgus sp. and Gyraulus sp., a l l more or l e s s show a s i m i l a r depth d i s t r i b u t i o n to the p r o f i l e of t o t a l animals. Two of the l e s s common gastropods, P h y s a s t r a sp. and Simlimnaea sp., occur w i t h i n more l o c a l i z e d depth zones. P h y s a s t r a sp., which was more abundant d u r i n g summer, o c c u r r e d - 48 -largely i n the N i t e l l a zone, and Simlimnaea sp. showed a max-imum density i n the mixed and lower drawdown zones and was more abundant during winter. Only a single koura (.Paranephrops planifrons - the fresh-water crayfish) was taken i n the Ekman samples; they are prob-ably able to take evasive action. They were not infrequently seen during scuba surveys. Seasonal Changes i n depth d i s t r i b u t i o n Total animals The three series of samples taken during May to August incl u s i v e were combined to examine the p r o f i l e of winter depth d i s t r i b u t i o n , and the three series of samples taken during November to February i n c l u s i v e were combined to give the summer p r o f i l e . For s t a t i s t i c a l comparison between summer and winter depth d i s t r i b u t i o n the l i t t o r a l zone was divided into the upper and lower drawdown zone, the mixed zone, and the upper and lower N i t e l l a zone (Figure 13). Sample sizes were too small within the upper drawdown and s u b l i t t o r a l zones to make v a l i d stat-i s t i c a l comparisons between winter and summer i n these depth i n t e r v a l s . There i s l i t t l e difference i n the mean number of t o t a l animals within the entire l i t t o r a l zone between winter and 2 2 summer (6200/m i n winter, 6700/m t i n summer), but there are t mean of means from each 2 metre depth i n t e r v a l of the l i t t o r -a l zone. - 49 -T O T A L A N I M A L S i • 3 D E P T H 5 M 7 E T R E S II (UPPER) R^Awpqw_N_ Z O N E (LOWER) M I X E D Z O N E (UPPER) /3-15 -I7i 11-" Z O N E ( L O W E R ) S U 6 L I T T O R . A U (3) (0 (0 (6) (2-). (3) (4-) (4-) (3) (5) (0 (5) (3) (3) (3) (3) (3) 5 0 oo N U M f l E R O F A N I M A L S /Oer SQUARE M E T R E W I N T E R (MAY-Auq-USr L M . t L u s t v e ) S U M M E R (NOVEMBER-FEBRUARY inclusive,) FIGURE 13. Seasonal changes i n the depth d i s t r i b u t i o n of t o t a l animals at Hautaruke Bay. (it) = sample s i z e s i n each 2 metre depth i n t e r v a l . % = s i g n i f i c a n t d i f f e r e n c e at p<0.05 K r u s k a l - W a l l i s Test between summer and win t e r i n the depth zones i n d i c a t e d by the v e r t i c a l bars. - 50 -s i g n i f i c a n t differences between winter and summer i n the depth d i s t r i b u t i o n . During winter the maximum density of animals occurs i n the lower drawdown and mixed zones with s i g n i f i c a n t l y greater numbers than occurred i n the mixed zone during summer (p<0.05 Kruskal Wallis Test). During summer there i s a more even d i s t r i b u t i o n of animals throughout the l i t t o r a l zone, with a bulge i n numbers i n the N i t e l l a zone. The increased numbers of animals i n the N i t e l l a zone during summer i s only s i g n i f -i c a n t l y greater than winter (at p<0.05 Kruskal Wallis Test) within the lower N i t e l l a zone. Total molluscs, t o t a l insects and oligochaetes There may be some o v e r a l l increase i n the numbers of t o t a l molluscs within the entire l i t t o r a l zone during summer from a 2 2 mean of 2850/m i n winter to 3250/m i n summer, but there ap-pears to be l i t t l e difference between summer and winter i n the numbers of insects and oligochaetes (Figure 14). However, these seasonal comparisons i n o v e r a l l numbers, where there are seasonal changes i n depth d i s t r i b u t i o n , are not s t r i c t l y v a l i d , because of the uneven slope and configuration of the l i t t o r a l (Figures 6 & 7). Both t o t a l molluscs and t o t a l insects show maximum dens-i t i e s i n the mixed zone during winter, with a downward s h i f t during summer. This downward s h i f t i s deeper i n molluscs, which show greater numbers during summer than winter throughout FIGURE 14. Seasonal changes i n the depth d i s t r i b u t i o n of molluscs, i n s e c t s and oligochaetes at Hautaruke Bay. A s t e r i s k i n d i c a t e s a s i g n i f i c a n t d i f f e r e n c e (at p < 0,05, K r u s k a l - W a l l i s Test) between summer and winter i n the depth zones i n d i c a t e d by the v e r t i c a l bars. (n.) = sample s i z e s . - 52 -the N i t e l l a zone, whereas i n s e c t s show an i n c r e a s e i n numbers i n the upper, but not the lower, N i t e l l a zone d u r i n g summer. There i s an i n c r e a s e i n numbers of o l i g o c h a e t e s i n the lower drawdown zone d u r i n g summer, but th e r e i s no s t a t i s t i c -a l l y s i g n i f i c a n t d i f f e r e n c e between summer and w i n t e r i n the depth d i s t r i b u t i o n (at p<0.05 K r u s k a l W a l l i s T e s t ) . Seasonal changes - M o l l u s c s The gastropods, Potamopyrgus antipodarum and Gyraulus sp., both show the same seasonal p a t t e r n s of i n c r e a s e d numbers i n the N i t e l l a zone d u r i n g summer, and i n c r e a s e d numbers i n the mixed zone d u r i n g w i n t e r (Figure 15). These d i f f e r e n c e s be-tween summer and wint e r are s t a t i s t i c a l l y s i g n i f i c a n t (p<0.05 K r u s k a l W a l l i s Test) i n the lower N i t e l l a zone and i n the mixed zone, and f o r Gyraulus sp. i n the lower drawdown zone a l s o . The bivalves show a d i f f e r e n t seasonal p a t t e r n . They have a more or l e s s even d i s t r i b u t i o n throughout the l i t t o r a l zone d u r i n g w i n t e r , but d u r i n g summer they show an i n c r e a s e i n numbers i n the lower drawdown and s u b l i t t o r a l zones, and a decrease i n the mixed•and N i t e l l a zones. These seasonal d i f -f e r e n c e s are not s t a t i s t i c a l l y s i g n i f i c a n t . Seasonal changes - Odonata and Le p i d o p t e r a The two s p e c i e s o f Odonata - the d r a g o n f l y P r o c o r d u l i a  g r a y i and the d a m s e l f l y Xanthocnemis z e a l a n d i c a - show l i t t l e q - A S T K O P O D S B / V A L V E S LO W I N T E R S U M M E R W I N T E R S U M M E R VJINTER S U M M E R o i 3000 N U M B E R O F A N I M A L S />e* S Q U A R E M E T R E FIGURE 15. Seasonal changes i n the depth d i s t r i b u t i o n of gastropods and b i v a l v e s at Hautaruke Bay. A s t e r i s k i n d i c a t e s a s i g n i f i c a n t d i f f e r e n c e (at p < 0.05, K r u s k a l - W a l l i s Test) between summer and winter i n the depth zones i n d i c a t e d by the v e r t i c a l bars. - 54 -d i f f e r e n c e i n t o t a l numbers between summer and win t e r . The s i z e frequency d i s t r i b u t i o n s of the l a r v a e f o l l o w i n g the,, p e r i o d o f emergence of the a d u l t s i n summer suggests t h a t many of the l a r v a e o f these two s p e c i e s take more than one year to reach the f i n a l i n s t a r , and hence t h e r e i s not such a gre a t d e c l i n e i n numbers f o l l o w i n g emergence of the a d u l t s i n summer. Th i s i s not unusual i n Odonata (Macan 1977, Pendergast & Cowley 1966). Both s p e c i e s shown an upward s h i f t i n the maximum d e n s i t -i e s o f l a r v a e d u r i n g the wint e r months (Figure 16). The max-imum d e n s i t y o f P. g r a y i d u r i n g w i n t e r occurs i n the lower drawdown zone, whereas the maximum d e n s i t y of X. z e a l a n d i c a d u r i n g w i n t e r occurs deeper than P. g r a y i - in, the mixed zone. The a d u l t male d r a g o n f l i e s , P. g r a y i , take up s t a t i o n c l o s e to the water's edge, and e x h i b i t t e r r i t o r i a l behaviour, c l a s h i n g w i t h i n t r u d i n g males and s e i z i n g any p a s s i n g female. O v i p o s i t i n g by P. g r a y i occurs d u r i n g November t o January. The a d u l t females d e p o s i t s i n g l e eggs a t the s u r f a c e of the water over the l i t t o r a l zone, which s i n k to the bottom and take 1-2 months to hatch (Armstrong 1958). E a r l y i n s t a r l a r -vae were found w i d e l y d i s p e r s e d throughout the l i t t o r a l zone below 2 metres depth by l a t e summer (February). The a d u l t d a m s e l f l i e s , X. z e a l a n d i c a , d e p o s i t t h e i r eggs on the stems or lea v e s of emergent macrophytes (Pendergast & Cowley 1966). The e a r l y i n s t a r l a r v a e were found down to 19 metres depth by l a t e summer (January), but the g r e a t e s t O D O N A T A LEPIDO P T E R A cn W I N T E R S U M M E R W I N T E R S U M M E R W I N T E R S U M M E R O 5oo i i — i i—t i NUMBER OF ANIMALS piK S q f f l R E M E T R E FIGURE 16. Seasonal changes i n the depth d i s t r i b u t i o n of Odonata and Lepidoptera at Hautaruke Bay. A s t e r i s k i n d i c a t e s a s i g n i f i c a n t d i f f e r e n c e (at p < 0 . 0 5 , K r u s k a l - W a l l i s Test) between summer and wi n t e r i n the depth zones i n d i c a t e d by the v e r t i c a l bars. - 56 -numbers occurred between 5 and 10 metres depth. The aquatic larvae of the small moth, Nymphula nitens, take a single year to complete t h e i r l i f e cycle. They are confined to the shallow l i t t o r a l , and t h e i r numbers are temp-o r a r i l y reduced following emergence of the adults i n summer (Figure 16). Seasonal changes - Trichoptera The large caddis, T r i p l e c t i d e s sp., which takes a single year to complete i t s l i f e cycle, shows a decrease i n numbers in a l l zones during the summer months following emergence of the adults (Figure 17). Pupae were found attached to aquatic macrophytes during December, January and February at depths ranging from 1 to 14 metres, with a maximum at 5 metres depth. The small sandgrain-cased caddis, Pycnocentrodes sp., shows no s t a t i s t i c a l l y s i g n i f i c a n t difference i n numbers or depth d i s t r i b u t i o n between summer and winter, although the greatest numbers were found i n the drawdown and lower N i t e l l a zones with a paucity, p a r t i c u l a r l y during summer, i n the mid-l i t t o r a l . Pupae were not found during Ekman sampling. The finding of the pupae (attached to aquatic macrophytes) throughout the year suggests that the small h y d r o p t i l i d s , Paroxyethira t i l l y a r d i and Paroxyethira hendersoni, pass through at least 2 generations i n a year. The winter genera-tion of P. t i l l y a r d i i s concentrated mainly i n the mixed and lower drawdown zones with a maximum number of pupae found i n T R I C H O P T E R A PijcnjOpeM-btodzs sp- P. kje.ndexsonL U l W I N T E R S U M M E R WINTER SUMMER WINTER S U M M E R WINTER SUMMER 500 N U M B E R OF A N I M A L S pen S Q U A R E M E T R E FIGURE 17. Seasonal changes i n the depth d i s t r i b u t i o n of Trichoptera at Hautaruke Bay. As t e r i s k indicates a s i g n i f i c a n t difference (at p < 0.05, Kruskal-Wallis Test) between summer and winter i n the depth zones indicated by the v e r t i c a l bars. - 58 -October a t 4 - 6 metres depth. The summer g e n e r a t i o n i s con-c e n t r a t e d mainly i n the N i t e l l a zone w i t h maximum numbers of pupae o c c u r r i n g i n January between 9 t o 15 metres depth. There i s no s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e i n the numbers or depth d i s t r i b u t i o n of P. hendersoni between summer and wi n t e r , although the g r e a t e s t numbers were found i n the drawdown zone i n wi n t e r , and i n the s u b l i t t o r a l i n summer, with a p a u c i t y of numbers i n the m i d - l i t t o r a l . Pupae were found a t depths ranging from 1 to 2 0 metres. Seasonal changes - chironomids There i s a s l i g h t downward s h i f t i n the depth d i s t r i b u -t i o n o f Chironominae i n summer compared wi t h w i n t e r , but the only s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e i s a g r e a t e r number i n the lower N i t e l l a zone d u r i n g w i n t e r (Figure 18). The Tanypodinae show no s i g n i f i c a n t d i f f e r e n c e i n numbers or depth d i s t r i b u t i o n between summer and win t e r . The u n i d e n t i f i e d group of chironomids i n c l u d e a t l e a s t 4 s p e c i e s w i t h r e p r e s e n t a t i v e s of the O r t h o c l a d i n a e . There i s an o v e r a l l decrease i n numbers d u r i n g summer, but t h i s i s not s t a t i s t i c a l l y s i g n i f i c a n t . TROUT Composition of the t o t a l c a t c h The s i z e , m a t u r i t y and s p e c i e s composition of the t o t a l c a t c h o f t r o u t i n the g i l l n e t s d u r i n g the p e r i o d February 1976 C H I R O N O M I D S W I N T E R S U M M E R WINTER. SUMMER WINTER SUMMER O zooo (a I I N U M B E R O F A N I M A L S pvi S Q U A R E M E T R E FIGURE 18. Seasonal changes i n the depth d i s t r i b u t i o n of chironomids at Hautaruke Bay. A s t e r i s k i n d i c a t e s a s i g n i f i c a n t d i f f e r e n c e (at p < 0 . 0 5 , K r u s k a l - W a l l i s Test) between summer and win t e r i n the depth zones i n d i c a t e d by the v e r t i c a l bars. - 60 -to December 1976 i n c l u s i v e (Figure 19) shows that 82% of the t o t a l catch were rainbow trout; 18% were brown trout. 80% of the rainbow trout were immature whereas only 53% of the brown trout were immature. The mature brown trout have a s i g n i f i -cantly greater mean length (56.5 cms. , 95% C L . + 1.4) than the mature rainbow trout (mean length 51.8 cms , 95% C L . + 0.9) . Spatial d i s t r i b u t i o n The length frequency d i s t r i b u t i o n s of trout caught i n the drawdown, deep l i t t o r a l and limnetic nets are shown in Figure 20. Considering f i r s t the rainbow trout: - i n the drawdown zone the catch consisted of small juveniles (< 26 cms.) and large, mostly mature, f i s h , with only a small proportion of intermediate-sized f i s h . In the limnetic zone the catch con-s i s t e d largely of intermediate-sized juveniles. Over the deep l i t t o r a l zone the catch shows a bimodal length-frequency d i s t r i b u t i o n of intermediate-sized juveniles, very few small juveniles (.< 26 cms ), and a group of larger, mostly mature f i s h . There i s no s i g n i f i c a n t difference i n size between the mature rainbow trout caught i n the deep l i t t o r a l and i n the drawdown zones. The d i s t r i b u t i o n of brown trout i n the limnetic, deep l i t t o r a l , and drawdown nets shows a similar, but less c l e a r l y defined, pattern according to size and maturity (N.B. smaller sample s i z e s ) , but i n addition there i s a s i g n i f i c a n t difference - 61 -LENGTH CMS -F I G U R E 19. The composition of the t o t a l catch of t r o u t i n the g i l l n e t s (February 1976 - December 1976) from both s t a t i o n s , according to s p e c i e s , s i z e and m a t u r i t y . (rv) = sample s i z e s . '5-1 RAINBOW TROUT llf. \i ZL Zi 30 31* 38 42. 4t 50 5^ . S"S '5i 10-i 7 to 5 H DRAWDOWN ZONE (78 ) 2 0 10 BROWN TROUT LIMNETI C (33) 3 0 3£j_ 38 ( i 50 5Jf 58 t i U 10 DEEP LITTORAL (ZO) 3 0 34- 38 L2. 4 i 50 54- 58 DRAWDOWN ZONE (*0 FIGURE 20. The percentage length composition i n 2 cm. groupings of trout caught i n g i l l n e t s i n the drawdown zone, i n the deep l i t t o r a l zone and i n the limnetic zone, from both stations (February 1976 -December 1976 i n c l u s i v e ) . Immature f i s h (black); mature f i s h (cross-hatched). (^) = sample sizes to - 63 -i n s i z e between the mature brown t r o u t caught i n the deep l i t -t o r a l and i n the drawdown zones. The mean l e n g t h of mature brown t r o u t caught i n the drawdown zone, 57.5 cms. , i s s i g -n i f i c a n t l y g r e a t e r than the mean l e n g t h of mature brown t r o u t caught i n the deep l i t t o r a l zone, 55.1 cms (p<0.01 K r u s k a l W a l l i s T e s t ) . Stomach a n a l y s i s The food items recorded d u r i n g stomach a n a l y s i s were c a t -e g o r i z e d as: 1. L i m n e t i c food, which c o n s i s t e d of Daphnia c a r i n a t a , l a r v a l smelt, l a r v a l b u l l i e s Cand an i n s i g n i f i c a n t q u a n t i t y by volume of chironomid pupae), a l l of which are s m a l l (<3 cms i n l e n g t h ) . 2. L i t t o r a l food, which c o n s i s t e d of l i t t o r a l i n v e r t e -b r a t e s r a n g i n g i n s i z e up to d r a g o n f l y l a r v a e of =^2.5 cms , and koura (the freshwater c r a y f i s h , Paranephrops p l a n i f r o n s ) up to c± 12 cms- l e n g t h , subadult and a d u l t b u l l i e s up to <=: 10 cms l e n g t h , and i n the case o f rainbow t r o u t , the shoots of a q u a t i c macrophytes, which may o n l y have been taken i n c i d e n t -a l l y along w i t h an i n v e r t e b r a t e prey item. 3. Subadult and a d u l t smelt, t r e a t e d as a separate c a t e -gory, because, although they were observed i n s c h o o l s i n the l i t t o r a l areas from October to A p r i l , J o l l y (19671 found t h a t i n Lakes Taupo and Rotorua they remain p a r t i a l l y p e l a g i c i n behaviour and f e e d i n g . - 64 -4. T e r r e s t r i a l insects - largely Coleoptera and cicadas. 5. Frogs - Hyla aurea. 6. Debris - other than aquatic macrophytes. The smaller juvenile rainbow trout contained lar g e l y limnetic food i n th e i r stomachs (Figure 21), and Daphnia were an important food item p a r t i c u l a r l y during spring and early summer. Daphnia were found i n the stomachs of immature rainbow trout of up to 54 cms, length, but Daphnia were not found i n the stomachs of any brown trout. T e r r e s t r i a l insects formed a s i g n i f i c a n t component of the diet of rainbow trout during summer (November to February i n -cl u s i v e ) , and, o v e r a l l , formed 10 - 20% of t h e i r d i e t , but they contributed i n s i g n i f i c a n t l y to the d i e t of brown trout. Rainbow trout contained an increasing amount of debris i n t h e i r stomachs with increasing age and size, but debris formed a n e g l i g i b l e proportion of the stomach contents of brown trout. Subadult and adult smelt were an important food item for both rainbow and brown trout. They formed an increasing pro-portion of the di e t of rainbow trout with increasing age and size, but they formed an even greater proportion of the di e t of brown trout; 50--. 70% i n the immature brown trout, and 30 -4 0% i n the previous spawner brown trout, i n which b u l l i e s be-came the most important food item. Brown trout were more piscivorous than rainbow trout. R A I N B O W T R O U T I I M M A T U R E F ; s H | PREVIOUS I+- ! 1+ 2+ SPAWNERS L I TTOR.AL FOOD ( p o S T L A R V A u ) I 2 31 T E R R E S T R I A L . I N S E C T S DEBRIS 3 R 0 WN T R O U T I I M M A T U R E F I S H I F ' " S T V £ « R SecONJ) Y E A R //V L A K E //V L A K E LIMNETIC FOOD u L I T T O R A L FOOP i • I SMELT ( P O S T L A R V A L ) T E R R E S T R I A L I N S E C T S PREV/OUS SPA WAVERS L FIGURE 21. The composition of the diet s of brown and rainbow trout i n Lake Waikaremoana. (February 1976 - December 1976). - 66 -Frogs were o n l y found i n the stomachs of l a r g e brown t r o u t (.> 55 cms length) between November and January i n c l u s i v e . The o n l y l i t t o r a l i n v e r t e b r a t e , which c o n t r i b u t e d s i g n i f -i c a n t l y by volume or occurrence to the d i e t of brown t r o u t , was the l a t e i n s t a r l a r v a e of the d r a g o n f l y , P r o c o r d u l i a g r a y i . Rainbow t r o u t c o n t a i n e d a g r e a t e r v a r i e t y of l i t t o r a l i n v e r t e -b r a t e s ; they a l s o fed upon the l a t e i n s t a r l a r v a e of P. g r a y i , but took e a r l y i n s t a r l a r v a e as w e l l . T r i p l e c t i d e s l a r v a e were an important item i n the d i e t of l a r g e r rainbow t r o u t , but were not found i n any brown t r o u t stomachs. Damselfly l a r v a e , Xanthocnemis z e a l a n d i c a , formed a s i g n i f i c a n t propor-t i o n by occurrence, but not volume, i n the d i e t of rainbow t r o u t . There was an i n c r e a s i n g percentage o f l i t t o r a l food, and a d e c r e a s i n g percentage of l i m n e t i c food, i n the d i e t w i t h i n -c r e a s i n g age and s i z e i n both brown t r o u t and rainbow t r o u t . The l i m n e t i c food resources Seasonal changes i n the abundance of Daphnia c a r i n a t a , l a r v a l smelt and l a r v a l b u l l i e s , and seasonal changes i n the s i z e of the l a r v a l f i s h are shown i n F i g u r e 22. Throughout much of the year one or more of these prey items i s abundant, but they are a l l s m a l l ; Daphnia up to 4 mms l e n g t h , l a r v a l b u l l i e s up to d 19 mms l e n g t h , and l a r v a l smelt r e a c h i n g a s i z e o f 25 - 30 mms by l a t e w i n t e r / e a r l y s p r i n g . During the p e r i o d October to December 1976 i n c l u s i v e , - 67 -30 L Z5 E N 10 <T T IS H 10 vnmS. 5-0-I i :: I 5 5 LARVAL FISH |Aufr | S E P T | Q C T I N O V | D E C T « N I F E B | M A R [HPR I MAY |TUN |TUL | /<?77 (W-l-r) (Sttx) IM U M B E A O F I N D I V / D 1/ A L S P £ R 3 Q Ll fl R E M E T ft £ 4 q 30 40 20 10,000 /O/JOO L A R V A L S M E L T | A u q - [ S E P T I O C T | N O V I D E C | T A N | F E B ) M A R | fl P R | M A Y | J U N | J u L ("Ix) LARVAL BULLIES l — l A up |s£>r | oc*r~| NOV | DEC | J~AN | FE B | MAR |BPR |MA» | JUN |JUC LAPHNIA cL) i AUG|SEPT|ocr | NO v [DEC I TAN | FE 8 | M AR | APPT| MAY*TUNT FIGURE 22. Seasonal changes i n a) the size of l a r v a l smelt (•) and l a r v a l b u l l i e s (°), and i n the numbers of b) l a r v a l smelt c) l a r v a l b u l l i e s & d) Daphnia carinata (August 1976 - Ju l y 1977). C i r c l e s are the mean of 4 samples per month (2 from each stat i o n ) and the v e r t i c a l bars represent 957. confidence l i m i t s . - 68 -when e i t h e r numbers or s i z e o f l a r v a l f i s h were low, Daphnia p a r t i a l l y f i l l e d the gap i n t h i s supply of l i m n e t i c food. T e r r e s t r i a l i n s e c t s , p a r t i c u l a r l y the Manuka b e e t l e , Pyronota  f e s t i v a , were a l s o becoming i n c r e a s i n g l y a v a i l a b l e a t the lake s u r f a c e a t t h i s time. There were two peaks i n the numbers of l a r v a l f i s h , one i n February/March 19 77 and the- other i n June 197 7. Recruitment of j u v e n i l e s to the lake J u v e n i l e rainbow t r o u t entered the lake from the nursery streams mostly as autumn immigrants or s p r i n g immigrants. A high p r o p o r t i o n o f the s m a l l j u v e n i l e t r o u t caught i n the ...... drawdown zone.in A p r i l and October had r e t a i n e d t h e i r p a r r marks up to the time of capture i n the g i l l n e t s , s u g g e s t i n g r e c e n t e n t r y from the nursery streams. Few smal l j u v e n i l e s were captured i n the drawdown zone i n February or August. The d e v a s t a t i n g f l o o d o f New Year 1975/76 undoubtedly had a s i g n i f i c a n t e f f e c t on the l e v e l of r e c r u i t m e n t of juven-r i l e t r o u t to the lake d u r i n g the subsequent year. The spawning streams i n the n o r t h e a s t corner of the catchment were completely devoid of f i s h f o l l o w i n g the f l o o d , but the streams r a p i d l y recovered t h e i r i n v e r t e b r a t e fauna, and the spawning runs of t r o u t entered the streams as u s u a l d u r i n g the win t e r of 1976. The f o l l o w i n g s p r i n g e l e c t r o f i s h i n g of one of these spawning streams r e v e a l e d the presence of a s i n g l e year c l a s s of t r o u t CO + f r y ) . T h e i r growth and seasonal t i m i n g of e m i g r a t i o n - 69 -from the stream was followed by e l e c t r o f i s h i n g at 1-2 monthly i n t e r v a l s , and i t was established that the bulk of the autumn immigrants to the lake were the faster-growing 0 + juveniles. The slower growing 0 + juveniles overwinter i n the stream and form the bulk of the spring immigrants as 1 + juveniles, (Fig-ure 23) . This produces a bimodal length frequency d i s t r i b u t i o n i n the 1 + juveniles during t h e i r f i r s t year i n the lake (see February 1976, and October and December 1976, Figure 23), but by the end of th e i r second year they merge to form a single s i z e - c l a s s . Condition factor The small juvenile rainbow trout have a high mean condi-tion factor when leaving the nursery streams, but during t h e i r f i r s t 2-4 months i n the lake, when they were inhabiting the drawdown zone, there was a s i g n i f i c a n t drop i n t h e i r mean condition factor (Figure 2 4). When they moved out into the limnetic zone (at a size of approximately 26 cms, length) the mean condition factor increased and remained high u n t i l they reached a size of approximately 3 8 cms. and then there was a gradual f a l l i n the mean condition factor with increasing size. In "previous spawner" rainbow trout the mean condition factor was s i g n i f i c a n t l y lower than i n immature rainbow trout of a comparable s i z e . - 70 -P R E V I O U S S P A W N E R.S N U M 6 E A O F F I S H FIGURE 23. Length frequency d i s t r i b u t i o n s of rainbow t r o u t caught i n nets during each 2 monthly sampling. ^  = no nets set i n drawdown zone. Trout which entered the lake as j u v e n i l e immigrants i n autumn = A, i n s p r i n g = S -• = immature f i s h w i t h r i p e n i n g gonads. 52 50 C O N D I T I O N F A C T O R 4s J 4^ 44 42-4.0-38-36 -| % 32-30->(37) >(37) • ( t o ) f ( Z l ) T (4) 5(zo) i /g-0- 22.-0- 26-0- 3o-o- 34.0- 38-0- 4i-o- 4.6.0- 5b-o- 54.-0-2(-c? Z5<? _<\.<\ 33-? 37-? ^'-f 45-<? \<?.<? 53-7 57? L£Nq-T"H cms FIGURE 24. The mean condition factor of rainbow trout i n r e l a t i o n to size and maturity (February 1976 - December 1976). (n.) = sample sizes. V e r t i c a l bars represent 95% confidence l i m i t s © =immature f i s h X =previous spawners - 72 -After the small juvenile rainbow trout moved out into the limnetic zone t h e i r growth rate was rapid up to a size of approximately 44 cms. There was an increase i n modal length of ^ 3 cms /month i n the 1 + spring immigrant juveniles between February 1976 and June 1976, (Figure 23). Above a 44 eras, the growth rate slowed down. Approximately half of the rainbow trout matured at 2 . • years old; the remainder continuing as immature f i s h into t h e i r t h i r d year are subjected to intense angling pressure i n October and November, which depletes the numbers of rainbow trout maturing at 3 years old. After reaching maturity there i s l i t t l e increase i n length i n rainbow trout, and with the f a l l i n condition factor, very l i t t l e increase i n weight. - 73 -DISCUSSION Before h y d r o e l e c t r i c development Lake Waikaremoana was v i r t u a l l y a s t a b i l i z e d r i v e r " r e s e r v o i r " of much g r e a t e r age than any of the now more f a m i l i a r man-made h y d r o e l e c t r i c r i v e r r e s e r v o i r s . There were n a t u r a l f l u c t u a t i o n s i n lake l e v e l w i t h a mean annual amplitude of 2.5 metres. The morphometric changes f o l l o w i n g h y d r o e l e c t r i c develop-ment i n 1946, due to the i n i t i a l lowering of the lake l e v e l , were i n e f f e c t the r e v e r s e of those d e s c r i b e d b e f o r e and a f t e r impoundment of the l a k e s o f the Campbell R i v e r drainage area i n B.C. (McMynn and L a r k i n 1953), and Loch Garry, S c o t l a n d (Campbell 196 3). However, the c o n t i n u i n g impacts may be more s i m i l a r i n t h a t h y d r o e l e c t r i c u t i l i z a t i o n of the l a k e s imposes a regime of lake l e v e l f l u c t u a t i o n s , which i s u n n a t u r a l i n p e r i o d i c i t y , i f not i n amplitude as w e l l . MORPHOMETRIC CHANGES The most s i g n i f i c a n t morphometric changes i n Lake Waik-aremoana are those a f f e c t i n g the l i t t o r a l area and the lower reaches of streams e n t e r i n g the l a k e . The c± 17% r e d u c t i o n i n the l i t t o r a l area, w i t h a d i s p r o -p o r t i o n a t e l y g r e a t l o s s of shallow l i t t o r a l (.< 5 metres deep) , w i l l have reduced the p o t e n t i a l t o t a l p r o d u c t i o n of the l i t -t o r a l zone of the l a k e by a s i m i l a r , i f not g r e a t e r , degree of magnitude thus d i m i n i s h i n g both the l i t t o r a l l i v i n g space and - 74 -the l i t t o r a l food r e s o u r c e s f o r the t r o u t to a f a r g r e a t e r e x t e n t than the r e d u c t i o n i n the l i m n e t i c area ( d 4% l o s s ) . The dependence o f the t r o u t a t c e r t a i n stages of t h e i r l i f e on the space and/or food resources of the l i t t o r a l zone, and d u r i n g c e r t a i n seasons o f the year, when l i m n e t i c food r e -sources are scarce ( f o r t u n a t e l y o n l y s h o r t , t r a n s i e n t p e r i o d s ) , makes the r e l a t i v e l y s m a l l area of the l i t t o r a l zone a l i m i t i n g f a c t o r i n the c a r r y i n g c a p a c i t y of the l a k e f o r t r o u t . Although the t o t a l l e n g t h of t r i b u t a r y streams a c c e s s i b l e to spawning runs of t r o u t from the lake has been i n c r e a s e d by the c r e a t i o n o f g a u n t l e t s , these are of q u e s t i o n a b l e v a l u e -b e i n g under the i n f l u e n c e of lake l e v e l f l u c t u a t i o n s (McMynn and L a r k i n 195 3). The redds of t r o u t spawning i n the lower reaches d u r i n g l a t e w i n t e r , when lake l e v e l s are low, are l i a b l e to i n u n d a t i o n by r i s i n g l a k e l e v e l s d u r i n g i n c u b a t i o n of the eggs i n e a r l y s p r i n g . The s l u g g i s h flow w i l l p r e d i s p o s e to s i l t a t i o n and reduce the p e r c o l a t i o n of water through the g r a v e l s of the redds. LAKE LEVEL FLUCTUATIONS Seasonal p e r i o d i c i t y Even though there may be c o n s i d e r a b l e d i f f e r e n c e s i n the seasonal p e r i o d i c i t y of n a t u r a l (unregulated) l a k e s between temperate and s u b a r c t i c c l i m a t e s (Figure 25), f o l l o w i n g hydro-e l e c t r i c development they are l i k e l y to f o l l o w a s i m i l a r p a t -t e r n r e l a t e d to summer storage of water and maximum power gen-- 75 M £ T R £ S L A K E BLASTON a -) LOCH L O M O N L c L f l K E . W f l l K f l R E M O f l N f l ° V a ^ l i o ' i i ' / a . ' / 'a ' 3 U ' s 1 1 1 1 8 1 1 110' 11 '/a] 1 1 z ' 3 I -V5' fe ' M O N T H S FIGURE 25. Seasonal p e r i o d i c i t y of natural f l u c t u a t i o n s i n lake l e v e l a) Lake Blasjon, subarctic Sweden (Grimas 1961) b) Loch Lomond, Scotland ( a f t e r Slack 1957) c) Lake Waikaremoana (a & b from Elder 1975) - 76 -eration during winter (Figure 26). This w i l l tend to produce or accentuate, a r i s i n g and maintained lake l e v e l i n summer, and a f a l l i n g lake l e v e l i n winter. This regulated pattern i s closer to the natural period-i c i t y of lake l e v e l fluctuations i n a r c t i c or continental climates, where winter p r e c i p i t a t i o n i s i n the form of snow-f a l l , and freezing temperatures delay the runoff to the lakes u n t i l the spring thaw. The delayed runoff may be prolonged into summer i n high alpine and g l a c i a l melt catchments (e.g. L i l l o o e t Lake, B.C.). The natural fluctuations i n lake l e v e l i n Lake Biasjon following the spring runoff show a sim i l a r summer pattern to the natural fluctuations i n Lake Waikaremoana - i . e . a high lake l e v e l i n early summer and f a l l i n g through late summer. The major difference between the natural fluctuations i n these two lakes i s i n t h e i r winter levels - f a l l i n g i n Lake Blasjon; r i s i n g i n Lake Waikaremoana. The major change i n the seasonal p e r i o d i c i t y i n Lake Blasjon following hydroelectric development i s due to summer storage maintaining a high l e v e l throughout the summer, a l -though t h i s i s perhaps overshadowed by the extensive increase i n amplitude (Figure 26). In Lake Waikaremoana the seasonal p e r i o d i c i t y has been t o t a l l y reversed following hydroelectric development, with l i t t l e change i n amplitude. 43fci 435-M 434' E T R 433-E s 432-431 -43o-(,14 -(,13 -M E &I2. -T R E til -S Cio -Gof -6og -I T 3 2 . I 1 9 3 3 | / ? 3 4 FIGURE 26. The natural and regulated lake l e v e l f l u c t u a t i o n s i n Lake Blasjon ( a f t e r Grimas 1961) and i n Lake Waikaremoana. The shaded areas represent the extent of submergence of the weed beds during summer. S = summer; W = winter. - 78 -E f f e c t s on l i g h t p e n e t r a t i o n and primary producers Quennerstedt C1958) d e s c r i b e s the e f f e c t s of water l e v e l f l u c t u a t i o n s on lake v e g e t a t i o n i n Scandinavian l a k e s , and d i s c u s s e s the e f f e c t s o f summer storage. The prolonged deep submergence of the a q u a t i c macrophyte beds d u r i n g t h e i r veg-e t a t i o n a l p e r i o d produces an upward displacement of the lower boundary of roo t e d macrophytes and a l s o of attached algae. I f a r i s i n g lake l e v e l i n summer causes a r e d u c t i o n i n water transparency as w e l l , then t h i s e f f e c t w i l l be i n c r e a s e d . Grimas (1962) d e s c r i b e s a r e d u c t i o n i n water transparency due to shore e r o s i o n i n Lake B l a s j o n f o l l o w i n g i n c r e a s e d water l e v e l f l u c t u a t i o n s . In Lake Waikaremoana i n t e n s e wave e r o s i o n o f the papa shores accompanies a r i s i n g l a k e l e v e l i n summer. Papa bedrock i n the drawdown zone cr a c k s and crumbles when exposed above the water l e v e l d u r i n g summer. A r i s i n g l a k e l e v e l and wave a c t i o n work upon t h i s sunbaked papa t o produce heavy papa s i l t l o a d s , which are c a r r i e d out i n t o the lake i n the deeper, r e v e r s e , o f f s h o r e c u r r e n t s . F o l l o w i n g s p r i n g and summer storms there are profound t r a n s i e n t e f f e c t s on water transparency i n the l i t t o r a l zones along exposed papa shores, and l e s s e r , more prolonged e f f e c t s throughout the whole lake due to wind-induced e p i l i m n i a l c u r r e n t s . With a f a l l i n g l a k e l e v e l the exposed papa has the t e x t u r e of smooth co n c r e t e and wave a c t i o n produces r e l a t i v e l y l i t t l e suspended papa s i l t u n l e s s extreme drawdown reaches down to the f i n e r sediments - 79 -of the lower drawdown zone. Shore e r o s i o n may be o n l y p a r t l y r e s p o n s i b l e f o r the s i g n i f i c a n t n e g ative c o r r e l a t i o n between lake l e v e l and water transparency i n Lake Waikaremoana, be-cause there are, of course, other f a c t o r s , such as suspended sediment i n p u t s d u r i n g f l o o d s and a l g a l blooms, which reduce water transparency. Floods are a s s o c i a t e d w i t h a r i s i n g lake l e v e l . A l g a l blooms, although p r i m a r i l y r e l a t e d t o seasonal changes i n l i g h t , temperature, and c i r c u l a t i o n of the l a k e , may a l s o be a s s o c i a t e d w i t h a r i s i n g l a k e l e v e l , i f i t i s s u f f i c i e n t t o produce a damming-up e f f e c t through f l o o d i n g of t e r r e s t r i a l v e g e t a t i o n and s o i l ( M i t c h e l l 19 75). A r i s i n g lake l e v e l i n l a t e summer may prevent emergent s p e c i e s of a q u a t i c macrophytes from p e n e t r a t i n g the l a k e s u r -face and f l o w e r i n g . T h i s o c c u r r e d i n two out of the three summers d u r i n g the study p e r i o d , and may weaken any competi-t i v e advantage t h a t these s p e c i e s have over Elodea canadensis. There are o n l y female p l a n t s of Elodea i n New Zealand ( F i s h , p e r s . comm.) and i t s growth and s p r e a d . i s e n t i r e l y v e g e t a t i v e . Winter drawdown exposes the upper l i t t o r a l zone not o n l y to d e s s i c a t i o n , but a l s o t o . f r e e z i n g . T h i s commonly e l i m i n a t e s a q u a t i c macrophytes from the drawdown zone (.Quennerstedt 1958, ' Grimas 1961), but a t Lake Waikaremoana, where the f r e e z i n g i s l e s s severe or prolonged than i n northern S c a n d i n a v i a , some hardy s p e c i e s of n a t i v e a q u a t i c macrophytes Ce.g. Myriophyllum sp.) s u r v i v e . T h e i r h i g h e r t o l e r a n c e to d e s s i c a t i o n and/or - 80 -freezing gives these native species some competitive advantage over the adventive Elodea canadensis i n the drawdown zone. High winter lake l e v e l s provide a measure of protection to the weedbeds of the shallower l i t t o r a l from the disruptive wave action of winter storms. The seasonal timing of th i s protection i s altered by hydroelectric development. The major cycle The major cycle i n lake l e v e l fluctuations, with i t s p e r i o d i c i t y i n the order of 2 - 5 years (Figure 3), w i l l pro-duce a s h i f t i n g l i t t o r a l zone. When thi s i s showing an upward trend i n the order of 1 - 2 metres depth/year te.g. recent post-hydroelectric development period - 1970 - 1971 - Figure 5) there w i l l tend to be an upward displacement of the lower boundary of aquatic macrophytes and a recolonization of the lower drawdown zone. Periods of extreme drawdown (e.g. winters of 1969 and 1973, Figure 4) w i l l function as a "reset" by eliminating macrophytes i n the lower drawdown zone, and i f the lake l e v e l remains lower than usual during summer, by allowing some extension of macrophytes into deeper water. A complex of factors related to lake l e v e l fluctuations w i l l a f f e c t the composition of the macrophyte beds i n the drawdown zone - e.g. e f f e c t s on substrate, resistance of macrophyte species to exposure, dessication and freezing, wave action, and t h e i r a b i l i t y and rate of recolonization. The major cycle combined with the altered seasonal per^, - 81 -i o d i c i t y has s i g n i f i c a n t impacts on the trout spawning areas in the gauntlets. During a low phase of the major cycle ex-tensive areas of suitable gravels become available to the spawning trout. From records dating back to 1931 of the aver-age length and weight of rainbow trout caught by anglers i n Lake Waikaremoana (New Zealand W i l d l i f e Service - unpublished data) there i s evidence of fluctuations i n population numbers of the rainbow trout, which seem to be correlated with the major cycle of lake l e v e l fluctuations. It appears that the amplitude and period of a "cycle" i n population numbers of rainbow trout may have increased following hydroelectric dev-elopment. The rate of r i s e and f a l l i n lake l e v e l The a b i l i t y of aquatic invertebrates to follow changes i n lake l e v e l and avoid the danger of stranding varies with the species. (Moon 1935, Hynes 1961) - and probably varies also with the time of year and water temperature. In Lake Waikaremoana effects of lake l e v e l fluctuations on the habitat and food of l i t t o r a l invertebrates (substrate, macrophytes, and attached algae) are probably more important than the dangers of stranding. The maximum rate of drawdown i n Lake Waikaremoana (^6 cms/day) i s well within the l i m i t s of the rate of drawdown (15 cms/day), which was set to provide some protection for the l i t t o r a l fauna of Llyn Tegid - a hydroelectric lake i n North - 82 -Wales (Hunt and Jones 19 72). LITTORAL INVERTEBRATE FAUNA The mean number of animals per square metre immediately below the drawdown l i m i t i n Lake Waikaremoana (c± 9000 i n d i v -i d u a l s per square metre) i s comparable to the d e n s i t y of animals i n the upper 2 metres of the unregulated, n o r t h e r n Swedish, Lake An k a r v a t t n e t ( d 10,000 i n d i v i d u a l s per square metre) - see F i g u r e 27. The samples i n Lake Ankarvattnet (and Lake Blasjon) were taken d u r i n g summer and autumn (June to October i n c l u s i v e ) w i t h an Ekman dredge and s i e v e d through a 0.6 mm mesh (Grimas 1961). The 0.8 mm mesh, which was used i n Lake Waikaremoana, would not have r e t a i n e d many of the s m a l l e r i n d i v i d u a l s counted i n the A n k a r v a t t n e t and B l a s j o n samples. Water transparency i n the Swedish lakes (Secchi d i s c readings 9.5 - 13.5 metres) was s i m i l a r to Lake Waikaremoana. The t h e r m o c l i n e , d u r i n g the s h o r t p e r i o d of thermal s t r a t i f i -c a t i o n (.< 4 months) i n the Swedish lakes o c c u r r e d a t a depth o f approximately 3 metres, whereas i n Lake Waikaremoana the thermocline l a y a t or below 15 metres, and t h i s may account f o r the g r e a t e r d e n s i t y o f animals i n the deeper l i t t o r a l o f Lake Waikaremoana (JFigure 27). The depth of thermal s t r a t i -f i c a t i o n may t h e r e f o r e have a s i g n i f i c a n t i n f l u e n c e on the q u a n t i t a t i v e v u l n e r a b i l i t y of the l i t t o r a l i n v e r t e b r a t e s t o a given amplitude of drawdown. L A K E fl N K ft R VATTN E T L A K E BLBSTON L A K E Wfl I Kf lREMOflNR 0 SOOO i i i • -i i NUMBER OF ANIMALS pvt. SQCflRe METRE FIGURE 27. The depth d i s t r i b u t i o n of the bottom fauna i n Lake B l a s j o n and Ankarvattnet ( a f t e r Grimas 1961) and i n Lake Waikaremoana. - 84 -Q u a n t i t a t i v e l o s s e s Grimas a t t r i b u t e s the " i n v e r t e d bathymetric d i s t r i b u t i o n of animals" i n the drawdown zone of Lake B l a s j o n (.Figure 27) to the e f f e c t s of win t e r drawdown. By superimposing the B l a s j o n and Ank a r v a t t n e t p r o f i l e s of the q u a n t i t a t i v e depth d i s t r i b u t i o n o f the bottom fauna, he estimated the q u a n t i t a t i v e l o s s e s due to h y d r o e l e c t r i c u t i l i z a t i o n of Lake B l a s j o n a t about 70% i n the drawdown zone, and 2 5% below the drawdown l i m i t . Grimas a t t r i b u t e s these l o s s e s i n the drawdown zone t o d e s t r u c t i o n o f food and h a b i t a t ( p a r t i c u l a r l y the e l i m i n a t i o n of macrophytes), more than t o d i r e c t m o r t a l i t y o f l i t t o r a l i n -v e r t e b r a t e s due to s t r a n d i n g . Below the drawdown l i m i t he a t t r i b u t e s the l o s s e s to an a l t e r e d temperature regime i n the sediments r e s u l t i n g from abnormal c o o l i n g d u r i n g extreme win t e r drawdown (an e f f e c t which extends w e l l below the drawdown l i m i t as deep as the upper p r o f u n d a l zone) and a l s o t o i n o r g a n i c s i l t a t i o n o r i g i n a t i n g from e r o s i o n i n the drawdown zone, which i s e s p e c i a l l y d e t r i m e n t a l t o f i l t e r - f e e d e r s such as s p h a e r i i d s . The d e s t r u c t i o n o f h a b i t a t i n the shallow l i t t o r a l of Lake Waikaremoana due to the s m a l l e r amplitude of win t e r draw-down was not as e x t e n s i v e , c o o l i n g o f the l i t t o r a l sediments i s u n l i k e l y to be as extreme, but i n o r g a n i c s i l t a t i o n from e r o s i o n i n the drawdown zone may be more i n t e n s e than i n Lake B l a s j o n . F i l t e r f eeders are not n u m e r i c a l l y prominent i n the Waikaremoana l i t t o r a l fauna. - 85 -Seasonal changes i n depth d i s t r i b u t i o n Seasonal changes i n the depth d i s t r i b u t i o n of the l i t t o r a l i n v e r t e b r a t e s are l i k e l y to be due to a combination of f a c t o r s ; d i f f e r e n t i a l m o r t a l i t y and r e p r o d u c t i v e r a t e s between zones, seasonal m i g r a t i o n s between zones, and f o r some taxa (e.g. chironomids and o l i g o c h a e t e s ) , growth to a s u f f i c i e n t s i z e t o be r e t a i n e d by a 0.8 mm mesh. During s p r i n g and summer i n c r e a s i n g l i g h t p e n e t r a t i o n (which would be enhanced by a f a l l i n g lake l e v e l ) , and r i s i n g water temperatures, probably cause a bloom of a t t a c h e d algae i n the deep l i t t o r a l . The deep summer ge n e r a t i o n of Paroxye-t h i r a t i l l y a r d i and the summer i n c r e a s e i n numbers of g a s t r o -pods i n the N i t e l l a zone are probably a s s o c i a t e d w i t h t h i s . The gastropods i n the N i t e l l a zone may respond w i t h i n c r e a s e d r e p r o d u c t i v e r a t e s to an i n c r e a s e i n food supply and r i s i n g temperature, but the decrease i n numbers i n the mixed zone i n summer i s suggestive o f a downward m i g r a t i o n . During w i n t e r the i n c r e a s e d numbers of Potamopyrgus  antipodarum i n the mixed zone may be l a r g e l y due to an upward m i g r a t i o n . They showed a s t r o n g p o s i t i v e p h o t o t a x i s d u r i n g s o r t i n g of the l i v e samples i n l a t e summer ( M a r c h / A p r i l ) , which was not so apparent i n Gyraulus sp. The i n c r e a s e d numbers of Odonata i n the upper l i t t o r a l d u r i n g w i n t e r c o u l d be e x p l a i n e d by an upward m i g r a t i o n of l a t e i n s t a r l a r v a e d u r i n g w i n t e r i n p r e p a r a t i o n f o r emergence d u r i n g the f o l l o w i n g s p r i n g and e a r l y summer. Macan (.1977) - 86 -d e s c r i b e s an i n c r e a s e i n numbers i n shallow water d u r i n g w i n t e r of Odonata l a r v a e i n t h e i r second year. The d r a g o n f l y , P r o c o r d u l i a g r a y i , emerges from l a t e Oct-ober to December. The l a t e i n s t a r l a r v a e are slow-moving when c r a w l i n g through dense weed beds, although they can t r a v e l much f a s t e r by " j e t - p r o p u l s i o n " across a weed-free s u b s t r a t e , such as the upper drawdown zone. The d a m s e l f l y , Xanthocnemis  z e a l a n d i c a , emerges about a month l a t e r ; t h e i r f i n a l i n s t a r l a r v a e are powerful swimmers and can r a p i d l y t r a v e r s e the weedbeds on t h e i r f i n a l emergence m i g r a t i o n . During w i n t e r the maximum c o n c e n t r a t i o n of l a r v a e of X. z e a l a n d i c a (.in the mixed zone) occurs deeper than the maximum c o n c e n t r a t i o n of the e a r l i e r emerging, slower-moving l a r v a e of P. g r a y i (Figure 16). The e a r l y i n s t a r l a r v a e of Odonata appear to be w i d e l y d i s p e r s e d throughout the depth o f the l i t t o r a l by l a t e summer. Th i s probably r e s u l t s from the p a t t e r n of o v i p o s i t i n g by P. g r a y i , but f o r X. z e a l a n d i c a there must be some d i s p e r s a l mech-anism i n t o the deeper l i t t o r a l f o r the e a r l y i n s t a r l a r v a e ,'. from the shallow l i t t o r a l areas where the eggs are d e p o s i t e d . Macan (19 77) d e s c r i b e s such a d i s p e r s a l of newly-hatched zygopteran l a r v a e , which appeared to be i n f l u e n c e d by the d i r -e c t i o n of wind-induced c u r r e n t s . The t e r r i t o r i a l behaviour noted p r e v i o u s l y of a d u l t male P. g r a y i probably ensures some l a t e r a l d i s p e r s a l of o v i p o s i t i n g i n the l i t t o r a l areas. The b i v a l v e s show q u i t e a d i f f e r e n t p a t t e r n of seasonal changes i n depth d i s t r i b u t i o n . Being f i l t e r - f e e d e r s they might - 87 -be expected to f l o u r i s h b e t t e r i n a r e l a t i v e l y weed-free sub-s t r a t e - p a r t i c u l a r l y i n summer d u r i n g the maximal v e g e t a t i o n a l p e r i o d of the weedbeds, but d u r i n g w i n t e r the weed beds may p r o v i d e g r e a t e r p r o t e c t i o n from p r e d a t i o n . S i m i l a r e c o l o g i c a l requirements of the l a r v a e of the T r i c h o p t e r a n s , Pycnocentrodes  sp. and P a r o x y e t h i r a hendersoni, may account f o r t h e i r s i m i l a r p a t t e r n of d i s t r i b u t i o n . Those i n s e c t s , which take a s i n g l e year to complete t h e i r l i f e c y c l e (e.g. T r i p l e c t i d e s sp. and Nymphula n i t e n s ) show a temporary d e c l i n e i n numbers f o l l o w i n g emergence of the a d u l t s i n summer, and b e f o r e the next g e n e r a t i o n o f l a r v a e appear. F o l l o w i n g h y d r o e l e c t r i c development the r i s i n g lake l e v e l i n summer and reduced water transparency w i l l probably reduce the primary p r o d u c t i o n of both at t a c h e d algae and macrophytes i n the deeper l i t t o r a l , and i f the r i s i n g lake l e v e l i s ex- . treme, a d i e - o f f of the deeper weed beds i n summer w i l l pro-duce an u n s e a s o n a l l y e a r l y d e t r i t u s food c h a i n . The s i g n i f -i c a n t i n c r e a s e i n the l a r v a e of Chironominae i n the lower N i t e l l a zone (Figure 18), which had developed by w i n t e r 1976, may have been r e l a t e d to such an event. A r i s i n g lake l e v e l i n summer may i n t e r f e r e w i t h o v i -p o s i t i n g by a d u l t i n s e c t s , p a r t i c u l a r l y those s p e c i e s , such as Xanthocnemis z e a l a n d i c a , which may be dependent on emergent a q u a t i c v e g e t a t i o n . Species which d e p o s i t t h e i r eggs i n t o shallow water may do so i n t o an unfavourable h a b i t a t . A d u l t - 88 -females o f P r o c o r d u l i a g r a y i are a t t r a c t e d by the dark green of u n d e r l y i n g weed beds d u r i n g o v i p o s i t i n g a t the s u r f a c e of deeper water (Armstrong 1958); the lake l e v e l d u r i n g t h i s p e r i o d (December to March i n c l u s i v e ) may a f f e c t the d i s t r i -b u t i o n o f t h e i r eggs i n the l i t t o r a l . During the f i n a l emergence m i g r a t i o n o f f i n a l i n s t a r l a r v a e o f P r o c o r d u l i a g r a y i they are extremely v u l n e r a b l e t o p r e d a t i o n by l a r g e t r o u t c r u i s i n g the drawdown zone. On b r i g h t sunny days ( l a t e October to December i n c l u s i v e ) they leave the shallower weed beds and t r a v e r s e the weed-free drawdown zone to crawl out onto the lake shore. A r i s i n g l a k e l e v e l i n summer i n c r e a s e s the d i s t a n c e they have to t r a v e l a c r o s s t h i s weed f r e e zone, i n c r e a s i n g t h e i r a v a i l a b i l i t y to the t r o u t . Tree stumps spanning the drawdown zone p r o v i d e v a l u -able escape routes from p r e d a t i o n . An upward m i g r a t i o n o f gastropods and the l a t e i n s t a r l a r v a e of P r o c o r d u l i a g r a y i and Xanthocnemis z e a l a n d i c a d u r i n g w i n t e r would normally c o i n c i d e w i t h a r i s i n g lake l e v e l . F o l -lowing h y d r o e l e c t r i c development the f a l l i n g lake l e v e l i n win t e r , poses a g r e a t e r t h r e a t of s t r a n d i n g , than would occur w i t h a f a l l i n g lake l e v e l i n summer. P. g r a y i would appear to be p a r t i c u l a r l y v u l n e r a b l e i n t h i s r e s p e c t (Figure 16). Q u a l i t a t i v e l o s s e s Maximum s p e c i e s d i v e r s i t y i n the l i t t o r a l zone occurs i n the shallow water wi t h i t s g r e a t e r v a r i a t i o n i n s u b s t r a t e and - 89 -macrophyte species. I t i s the shallow water fauna, which i s most vulnerable to destruction by hydroelectric drawdown (Aass 1958).. Fluctuating lake l e v e l s tend to produce more uniformity of substrate i n the drawdown zone, as well as the elimination of some or a l l species of aquatic macrophytes, thus reducing the likelihood of any short term recovery of a high species d i v e r s i t y i n t h i s zone. Grimas demonstrated the e f f e c t s of hydroelectric develop-ment of Lake Blasjon on the species d i v e r s i t y of the l i t t o r a l fauna by comparison with Lake Ankarvattnet. He records 56 species of chironomids i n the l i t t o r a l zone of Lake Ankarvat-tnet and only.27 species, i n Lake Blasjon. He describes a peaked and intermittent pattern of emergence of the fewer species of chironomids i n Lake Blasjon. This reduces t h e i r a v a i l a b i l i t y to trout as a predictable and u t i l i z a b l e source of food (Nilsson 1961). The species d i v e r s i t y i n Lake Waikaremoana may have been reduced by hydroelectric development - perhaps not so much by the amplitude of lake l e v e l fluctuations, as by some of the consequences of the altered seasonal p e r i o d i c i t y . New Zealand lakes, with a low species d i v e r s i t y anyway, can i l l afford to lose even one or two species. TROUT Species composition The percent composition of rainbow trout and brown trout - 90 -caught i n the g i l l n e t s may not t r u l y r e p r e s e n t the composition i n the l a k e . Trapping of the spawning runs of t r o u t i n the Waiotukapuna stream (one of the major spawning streams i n the system) d u r i n g the w i n t e r s of 1971, 1972 & 1973 r e v e a l e d a s p e c i e s composition v a r y i n g from 62 to 7 6% brown t r o u t (Ewing and Gibbs 1973) - compared wi t h 18% brown t r o u t i n the n e t t i n g programme (1976). However, as the spawning runs c o n s i s t o n l y of mature f i s h , and the r a t i o of immature to mature f i s h i n the l a k e d i f f e r s by a f a c t o r of 4 between the s p e c i e s , t h i s d i s c r e p a n c y can be p a r t l y accounted f o r . But i n s p i t e of t h i s , the f i g u r e s would suggest t h a t rainbow t r o u t were more v u l n e r a b l e to capture i n the g i l l n e t s than brown t r o u t . Daytime o b s e r v a t i o n s suggest higher f o r a g i n g v e l o c i t i e s i n rainbow t r o u t than brown t r o u t . I f they remain more a c t i v e than brown t r o u t through the n i g h t , t h i s may i n c r e a s e t h e i r chances of encountering a net. S p a t i a l s e g r e g a t i o n The s i z e dominance of mature brown t r o u t over mature r a i n -bow t r o u t i s probably o f importance i n i n t e r s p e c i f i c i n t e r -a c t i o n s i n the l i t t o r a l zone of the l a k e . Both i n t r a and , i n t e r - s p e c i f i c i n t e r a c t i o n s i n brown and rainbow t r o u t can be c l e a r l y observed i n the. shallow water of the drawdown zone. During the daytime i n s p r i n g and e a r l y summer l a r g e brown t r o u t dominate the drawdown zone. I n d i v i d u a l f i s h c r u i s e a r e g u l a r beat along 50 - 100 metres of s h o r e l i n e and p a r t i c u -- 91 -l a r l y d u r i n g September and October a g g r e s s i v e encounters be-tween brown t r o u t can be observed, and i n t e r s p e c i f i c encounters between brown t r o u t and the o c c a s i o n a l rainbow t r o u t s t r a y i n g i n t o the drawdown zone from the deeper l i t t o r a l (a more f r e q -uent occurrence on the steeper s h o r e s ) . T h i s undoubtedly hastens post spawning d i s p e r s a l around the lake shore. The s i g n i f i c a n t d i f f e r e n c e i n s i z e between brown t r o u t caught i n the nets i n the drawdown zone and i n the deep l i t t o r -a l zone i s probably r e l a t e d to daytime i n t r a s p e c i f i c i n t e r -a c t i o n s , and supports the d i r e c t o b s e r v a t i o n s of t e r r i t o r i a l behaviour and dominance of the l a r g e r f i s h . At n i g h t t i m e , as evidenced by f l y - f i s h i n g from the shore a f t e r dark, and the r e s u l t s of the n e t t i n g programme, the rainbow t r o u t move onto the shallows of the drawdown zone. At n i g h t they are u n l i k e l y to be a f f e c t e d by s i z e - r e l a t e d i n t e r a c t i o n s , and they showed no s i g n i f i c a n t d i f f e r e n c e i n s i z e between those caught i n the deep l i t t o r a l zone and i n the drawdown zone. L a t e r i n summer, i f s u r f a c e temperatures r i s e to approx-i m a t e l y 19°C, brown t r o u t tend to leave the g e n t l y s l o p i n g shores and rainbow t r o u t move i n d u r i n g the day time. Rainbow t r o u t appear to t o l e r a t e h i g h e r temperatures. The brown t r o u t move to c o o l e r stream mouths or steeper shores, where they emerge from the depths and c r u i s e a s h o r t e r beat i n the draw-down zone before descending to the depths again - to reappear 15 to 2 0 minutes l a t e r . - 92 -A p a r a d o x i c a l s i t u a t i o n has been observed i n some s h e l t -ered bays where brown t r o u t have been seen to c o n s t a n t l y c r u i s e the lower drawdown zone j u s t above dense beds of Elodea, when water temperatures have been as h i g h as 22°C. Photosyn-t h e t i c oxygen s u p e r s a t u r a t i o n may r a i s e the temperature t o l e r -ance of brown t r o u t , i f i t becomes w e l l developed i n the calm waters of s h e l t e r e d bays. At n i g h t t i m e the brown t r o u t may leave these warmer shallows f o r deeper water i f the oxygen l e v e l s f a l l . The behaviour of these l a r g e r brown t r o u t has been des-c r i b e d a t some l e n g t h , because the r e s u l t s of the n e t t i n g programme (where the f i s h were caught a t dusk, and dawn, and through the night) d i d not show any s i g n i f i c a n t s p a t i a l seg-r e g a t i o n between brown and rainbow t r o u t i n the l i t t o r a l zone of the l a k e - and y e t d i r e c t o b s e r v a t i o n d u r i n g the daytime and the r e s u l t s of daytime a n g l i n g show a very c l e a r p i c t u r e of s p a t i a l s e g r e g a t i o n d u r i n g s p r i n g and e a r l y summer; the brown t r o u t i n the drawdown zone and the rainbow t r o u t over the weedbeds of the deeper l i t t o r a l . I n t e r a c t i o n s between l a r g e brown t r o u t and s m a l l j u v e n i l e t r o u t i n the drawdown zone have been observed, which gave a s t r o n g impression of p r e d a t o r y i n t e n t , but no evidence of p r e d a t i o n by l a r g e t r o u t on s m a l l j u v e n i l e t r o u t was o b t a i n e d . T h i s i s perhaps more l i k e l y to occur i n the stream mouth gaunt-l e t s . T i l s e y d e s c r i b e s p r e d a t i o n by brown t r o u t on rainbow t r o u t j u v e n i l e s i n Lake Eucumbene, A u s t r a l i a (.Tilsey 1970) . - 93 -Effects of hydroelectric development on available space The lowering of the lake l e v e l i n 1946 greatly reduced the area of shallow l i t t o r a l , and the altered seasonal period-i c i t y of lake l e v e l fluctuations has a profound e f f e c t on the a v a i l a b i l i t y of the reduced area of weed-free shallows. The length of shoreline was also reduced 10% loss - Table I I I ) . For the larger brown trout length of shoreline i s probably a more relevant s p a t i a l consideration, although the width of the weed-free shallow l i t t o r a l has an important e f f e c t on the a v a i l a b i l i t y of food ( p a r t i c u l a r l y emerging dragonfly larvae). During the period of extreme drawdown the mature trout are mostly i n the spawning streams. For the small juvenile trout, during t h e i r f i r s t few months in the lake, changes i n t h i s available space may be more important, and hydroelectric dev-elopment may have altered the selective pressures for seasonal timing of the immigration of juvenile trout to the lake. Even before hydroelectric development the limnetic space and food resources were probably not f u l l y u t i l i z e d by trout. The increase i n r a t i o of limnetic : l i t t o r a l areas from 5.8:1 to 6.7:1 w i l l have accentuated the r e l a t i v e u n d e r u t i l i z a t i o n of the limnetic area. E p i l i m n i a l temperatures i n the lake during summer vary from year to year within approximately 2°C. above or below the c r i t i c a l tempeature for brown trout (ci 19°C). The depth of the thermocline i s such, that a cold water refuge may l i e - 94 -some d i s t a n c e from the shore. C l i m a t i c v a r i a t i o n from year to year undoubtedly has a more important e f f e c t than hydro-e l e c t r i c development on the thermal c o n d i t i o n s i n the l a k e , but a s l i g h t warming e f f e c t due to s e a l i n g of the shallower leaks i n the dam and summer storage may have a c r i t i c a l e f f e c t on the brown t r o u t d u r i n g some y e a r s . Food & c o n d i t i o n f a c t o r Competition f o r food between the s m a l l j u v e n i l e and l a r g e t r o u t appears to be unimportant; not onl y are the s m a l l j u v -e n i l e s t a k i n g s m a l l e r food items, but i t i s mostly of l i m n e t i c o r i g i n , and t h e i r f a l l i n g c o n d i t i o n f a c t o r w h i l e l i v i n g i n the drawdown zone suggests t h a t t h i s supply food i s e i t h e r not abundant or e l s e not c o n s i s t e n t l y a v a i l a b l e . A v a i l a b i l i t y o f " l i m n e t i c food" i n the drawdown zone probably depends upon onshore wind a t n i g h t , when l a r v a l f i s h or Daphnia are c l o s e to the s u r f a c e and l i a b l e to l a t e r a l displacement by s u r f a c e c u r r e n t s . There i s probably an e r r a t i c replenishment of t h i s source of food to the drawdown zone, and a c o n s i d e r a b l e v a r i a -t i o n between exposed and s h e l t e r e d , and steep and g e n t l y s l o p i n g , l i t t o r a l areas. A f t e r the j u v e n i l e t r o u t have moved out i n t o the ample space of the l i m n e t i c zone w i t h i t s abundant and more or l e s s continuous supply of food the r i s e i n t h e i r c o n d i t i o n f a c t o r and r a p i d growth r a t e r e f l e c t s the ease w i t h which they can - 95 -f i l l t h e i r stomachs, but these food items are s m a l l , and when the rainbow t r o u t reach a s i z e of 38 cms. t h e i r c o n d i t i o n f a c t o r s t a r t s to f a l l , and t h e r e a f t e r t h e i r growth r a t e de-c l i n e s . I t i s probable t h a t a t about t h i s stage the r e l a t i o n -s h i p o f f i s h s i z e to food s i z e (.LindstrSm 1955) becomes c r i t -i c a l , and the l a r g e r t r o u t are compelled to t u r n more towards the l i m i t e d area of the l i t t o r a l zone f o r l a r g e r food items. Changes i n c o n d i t i o n f a c t o r i n the l a r g e r t r o u t are confounded by changes i n body form a s s o c i a t e d w i t h m a t u r i t y , ..the r i p e n i n g gonads and s t r e s s e s of spawning, but the slow and incomplete r e c o v e r y of c o n d i t i o n a f t e r spawning i n the rainbow t r o u t i s probably p a r t l y due to t h e i r g r e a t e r dependence on the l i m i t e d space and food resources of the l i t t o r a l zone, and i n t e r a c t i o n s w i t h the l a r g e r brown t r o u t . E f f e c t s of h y d r o e l e c t r i c development on food r e s o u r c e s I t would be d i f f i c u l t to q u a n t i f y the l o s s of p r o d u c t i o n of l i t t o r a l i n v e r t e b r a t e s r e s u l t i n g from the r e d u c t i o n i n the area of the l i t t o r a l zone, and the c o n t i n u i n g impacts of lake l e v e l f l u c t u a t i o n s . L i t t o r a l i n v e r t e b r a t e s make t h e i r most important d i r e c t c o n t r i b u t i o n to the d i e t of the t r o u t d u r i n g s p r i n g , when the biomass of i n s e c t l a r v a e and pupae i s a t i t s peak, and d u r i n g e a r l y summer, when they become most a v a i l a b l e d u r i n g their, emergence p e r i o d s . However, the importance of l i t t o r a l i n v e r t e b r a t e s i n f,ood cha,ins through b u l l y and smelt may be more s i g n i f i c a n t , and has not been s t u d i e d . - 96 -The i n t r o d u c t i o n of smelt has not o n l y p r o v i d e d a v a l u a b l e l i m n e t i c food r e s o u r c e , a v a i l a b l e f o r a longer p e r i o d of the year than e i t h e r l a r v a l b u l l i e s or Daphnia, but i t a l s o c y c l e s more o f the " s u r p l u s " l i m n e t i c p r o d u c t i o n i n t o the l i t t o r a l areas through the onshore movement of p o s t - l a r v a l smelt. Smelt form a g r e a t e r p r o p o r t i o n of the d i e t o f brown t r o u t than r a i n -bow t r o u t , and t h e i r i n t r o d u c t i o n may t h e r e f o r e have b e n e f i t e d the brown t r o u t more. The g a t h e r i n g c a p a c i t y of a lake f o r t e r r e s t r i a l i n s e c t s i s a f f e c t e d by the steepness of i t s shores (JSIorlin 1964) . The low e r i n g of the lake l e v e l with h y d r o e l e c t r i c development and topography of the l o s t l i t t o r a l may have reduced the a v a i l a b i l -i t y o f t e r r e s t r i a l i n s e c t s to the t r o u t , and t h i s would have a g r e a t e r impact on the rainbow t r o u t . The l o s t l i t t o r a l pro-v i d e s e x t e n s i v e areas of f r o g h a b i t a t , and f r o g s are important food items to the l a r g e r brown t r o u t . The e f f e c t s o f morphometric changes and f l u c t u a t i n g l a k e l e v e l s on the spawning o f b u l l i e s and the e f f e c t s o f f l u c t u a t -i n g lake l e v e l s on the spawning of smelt were not s t u d i e d . J o l l y (.1967) d e s c r i b e s a continuous spawning p e r i o d f o r smelt from November to l a t e A p r i l i n Lake Taupo, and from mid-Sept-ember to the end of June i n Lake Rotorua. In Lake Waikaremoana the apparent gap i n the r e c r u i t m e n t of l a r v a l f i s h to the l i m n e t i c zone i n the l a t e summer/early win t e r of 19 76/77 f o l -lowed a p e r i o d of h i g h l a k e l e v e l s , s u g g e s t i n g t h a t e x c e s s i v e summer storage may be d e t r i m e n t a l to spawning of smelt and - 97 -perhaps a l s o b u l l i e s . S t u d i e s of the ecology o f smelt and b u l l i e s was suggested as the most u s e f u l area f o r c o n t i n u i n g r e s e a r c h a t Lake Waikaremoana. SUMMARY 1. The : morphometric changes were v i r t u a l l y the r e v e r s e of those u s u a l l y o c c u r r i n g with h y d r o l e c t r i c development of a l a k e , because the lake l e v e l was i n i t i a l l y lowered r a t h e r than being r a i s e d . There was a d i s p r o p o r t i o n a t e l y g r e a t l o s s o f l i t t o r a l area. 2. F l u c t u a t i n g lake l e v e l s are now u n n a t u r a l due more to t h e i r a l t e r e d p e r i o d i c i t y than t h e i r amplitude. The seasonal p e r i o d -i c i t y has been r e v e r s e d and a "major c y c l e " spanning s e v e r a l years has been superimposed on the annual p e r i o d i c i t y . 3. There was no i n i t i a l damming-up e f f e c t , but a r e c u r r i n g , t r a n s i e n t damming-up e f f e c t now occurs due to summer storage of water, the topography of the o l d , now grass-covered, wave-cut t e r r a c e s and stream-mouth d e l t a s , and the major c y c l e of lake l e v e l f l u c t u a t i o n s . 4. Q u a n t i t a t i v e l o s s e s i n the l i t t o r a l i n v e r t e b r a t e fauna can be assumed due to the r e d u c t i o n i n the t o t a l area of the l i t -t o r a l . F u r t h e r l o s s e s i n l i t t o r a l p r o d u c t i o n occur as a r e -s u l t of lake l e v e l f l u c t u a t i o n s , p a r t i c u l a r l y due to summer submergence of the weed-beds and reduced water transparency. - 98 -5. The l i t t o r a l invertebrate fauna i s adapted to a f a l l i n g lake l e v e l i n summer, and a r i s i n g lake l e v e l i n winter. Any reduction i n the species d i v e r s i t y of the l i t t o r a l fauna, which may have occurred following hydroelectric development, i s l i k e l y to be related more to the altered p e r i o d i c i t y rather than the amplitude of lake l e v e l fluctuations. New Zealand lakes with t h e i r low species d i v e r s i t y can i l l a f ford to lose even a few species. 6. The small juvenile trout and old large trout are most de-pendent on the space and/or food resources of the l i t t o r a l zone. Because of t h e i r requirements, the carrying capacity of the lake for trout has been reduced out of proportion to the reduction i n the t o t a l area of the lake. 7. Rainbow trout dominate numerically, but brown trout dom-inate i n s i z e . Rainbow trout have probably been more adversely affected by the changes following hydroelectric development than brown trout. Brown trout appear to have benefited more from the introduction of smelt. 8. The morphometric changes i n the lake are now l a r g e l y of " h i s t o r i c a l " and academic inter e s t , but f l u c t u a t i n g lake l e v e l s , being a continuing impact, are of more concern to Management. A more detailed understanding of the ecology of the l i t t o r a l invertebrates and native f i s h i s needed before the significance of the altered p e r i o d i c i t y of the lake l e v e l fluctuations can be f u l l y appreciated. However, i t i s probably safe to assume that any measures to restore the seasonal p e r i o d i c i t y back - 99 -towards the n a t u r a l s i t u a t i o n would be d e s i r a b l e . MANAGEMENT IMPLICATIONS A country which depends l a r g e l y on h y d r o e l e c t r i c power i s committed t o maximum h y d r o e l e c t r i c power g e n e r a t i o n d u r i n g the wint e r months. The e c o l o g i c a l impacts of a summer s t o r a g e / winter drawdown seasonal p e r i o d i c i t y i n the l e v e l s of t h e i r h y d r o e l e c t r i c lakes are unavoidable. I f there was an o p t i m a l balance between h y d r o e l e c t r i c power and thermal (.or nuclear) power, the h y d r o e l e c t r i c power co u l d be used more d u r i n g the summer months, e s p e c i a l l y i f the thermal power s t a t i o n s were c l o s e d down f o r maintenance d u r i n g summer. Thermal power s t a t i o n s operate most e f f i c i e n t l y a t con-tinuous h i g h output, and c o u l d be used to meet a g r e a t e r pro-p o r t i o n o f the w i n t e r power demands. T h i s would a l l o w the lake l e v e l f l u c t u a t i o n s i n e x i s t i n g h y d r o e l e c t r i c l a k e s t o r e v e r t to a more n a t u r a l seasonal p e r i o d i c i t y and amplitude. A l t e r n a t i v e s to h y d r o e l e c t r i c power would not o n l y save remaining r i v e r s and la k e s from development, but c o u l d a l s o reduce the c o n t i n u i n g harmful impacts on a l r e a d y developed h y d r o e l e c t r i c lakes and r i v e r r e s e r v o i r s . - 100 -LITERATURE CITED Aass, P. 1958. The e f f e c t s o f impoundment on i n l a n d f i s h e r i e s . I n t e r n a t . Union f o r Co n s e r v a t i o n of Nature and N a t u r a l Resources. Seventh T e c h n i c a l Meeting, Athens, Greece. 9 pp. Anderson, G.P. 1948. Waikaremoana - The problem of lake con-t r o l Proceedings N.Z.I.E. Armstrong, J.S. 1958. The bree d i n g h a b i t s o f the C o r d u l i i d a e (Odonata) i n the Taupo d i s t r i c t of New Zealand. Trans. Roy. Soc. N.Z. 85: 275-282. Axelsson, J . 1961. Zooplankton and impoundment of two la k e s i n n o r t h e r n Sweden (Ransaren and K u l t s j o n ) . Rep. I n s t . Freshw. Res., Drottningholm. 42: 84-168. Beckman, W.C. 1966. In d i s c u s s i o n - The b i o l o g y o f r e s e r v o i r s i n the U.S.S.R. Proc. Symp. I n s t . B i o l . 15: 154. B u r s t a l l , P.J. 1975. In "New Zealand Lakes". Auckland U n i v e r s i t y P r e s s . E d i t o r s , V.H. J o l l y and J.M.A. Brown. Sport f i s h e r i e s . Ch. 22: 308 - 318. Campbell, R.N. 1957. The e f f e c t o f f l o o d i n g on the growth r a t e of brown t r o u t i n Loch Tummel. S c i . Invest. Freshwat. F i s h . Scot. 14: 7 pp. . 1963. 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