UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

The effect of trout predation on the abundance and production of stream insects Griffiths, Ronald W. 1981

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Notice for Google Chrome users:
If you are having trouble viewing or searching the PDF with Google Chrome, please download it here instead.

Item Metadata

Download

Media
831-UBC_1981_A6_7 G76.pdf [ 3.92MB ]
Metadata
JSON: 831-1.0095228.json
JSON-LD: 831-1.0095228-ld.json
RDF/XML (Pretty): 831-1.0095228-rdf.xml
RDF/JSON: 831-1.0095228-rdf.json
Turtle: 831-1.0095228-turtle.txt
N-Triples: 831-1.0095228-rdf-ntriples.txt
Original Record: 831-1.0095228-source.json
Full Text
831-1.0095228-fulltext.txt
Citation
831-1.0095228.ris

Full Text

THE EFFECT OF TROUT PREDATION ON THE ABUNDANCE AND PRODUCTION OF STREAM INSECTS B. Sc., U n i v e r s i t y Of Western O n t a r i o , London, O n t a r i o , 1978 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 30 J u l y 1981 by RONALD W. GRIFFITHS MASTER OF SCIENCE i n Ronald W. G r i f f i t h s , 1981 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date r>T?_<; • to /new i i ABSTRACT Trout were int r o d u c e d i n t o a flow-through enclosure c o n s t r u c t e d i n the headwaters of a small f i s h l e s s stream to examine the e f f e c t s of t r o u t p r e d a t i o n on the d e n s i t y , standing crop and production of l o t i c i n s e c t p o p u l a t i o n s . F o l l o w i n g the t r o u t i n t r o d u c t i o n , the d e n s i t y and standing crop of 3 of the 9 i n s e c t s p e c i e s examined decreased i n the experimental stream study s e c t i o n (enclosure) while the d e n s i t y and standing crop of 2 i n s e c t s p e c i e s i n c r e a s e d i n the experimental stream s e c t i o n compared with the c o n t r o l stream s e c t i o n . Data on the food h a b i t s of the t r o u t i n d i c a t e d that t r o u t p r e d a t i o n had reduced the d e n s i t y and standing crop of these i n s e c t p o p u l a t i o n s i n the experimental stream s e c t i o n . Competitive r e l e a s e was suggested as the reason f o r the i n c r e a s e i n d e n s i t y and standing crop of the i n s e c t p o p u l a t i o n s i n the experimental stream s e c t i o n . The r o l e of t r o u t p r e d a t i o n i n s t r u c t u r i n g stream i n s e c t communities i s b r i e f l y d i s c u s s e d . P r o d u c t i o n estimates of l o t i c s p e c i e s i n the experimental stream s e c t i o n were not s i g n i f i c a n t l y d i f f e r e n t from those i n the c o n t r o l s e c t i o n . Trout consumed only 0.4 times the mean standing crop or 9-10% of the p r o d u c t i o n of prey s p e c i e s . However, t r o u t were thought to be i n t e n s i v e l y g r a z i n g the a v a i l a b l e food supply. i i i TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES v LIST OF. FIGURES v i i ACKNOWLEDGEMENTS ix I n t r o d u c t i o n 1 D e s c r i p t i o n of Study Area 3 M a t e r i a l s and Methods 7 Experimental Design 7 Sampling Methods 8 Benthos 8 D r i f t 9 Stomach Contents 10 Trout Abundance • ••• 10 Methods of A n a l y s i s 11 Benthos 11 L i f e H i s t o r i e s of Insect Species 12 Insect Density 13 Insect Standing Crop 14 Insect Production 15 D r i f t 16 Trout Abundance 17 Trout Stomach Contents 17 Food Consumption by Trout 18 Sampling V a r i a b i l i t y '. 20 i v L i f e H i s t o r i e s of Insect Species 21 S t o n e f l i e s 21 M a y f l i e s . . . . 31 C a d d i s f l i e s 41 R i f f l e B e e t l e s 46 R e s u l t s 52 Trout Abundance 52 Food Hab i t s of Trout 56 Food Consumption by Trout 58 Insect Density and Standing Crop 61 Insect Production 77 D r i f t 77 D i s c u s s i o n 84 E f f e c t of Trout P r e d a t i o n on the Abundance of L o t i c P o p u l a t i o n s 84 Trout P r e d a t i o n and Insect P r o d u c t i o n 89 P r e d a t i o n and Community S t r u c t u r e 92 Appendix 1 93 Insect Density C o r r e c t i o n F a c t o r s 93 Appendix 2 97 R e l a t i o n s h i p between Head Width and Dry Weight of Aquatic I n s e c t s 97 Appendix 3 98 Insect Taxa L i s t f o r Spring Creek 98 L i t e r a t u r e C i t e d 101 V LIST OF TABLES Table 1. Monthly minimum and maximum water temperatures from June to December, 1979 6 Table 2. Head-capsule widths of l a r v a l i n s t a r s of Glossosoma peniturn from S p r i n g Creek 41 Table 3. Head-capsule widths of l a r v a l i n s t a r s of H e t e r l i m n i u s koebelei from S p r i n g Creek 51 Table 4. Density and standing crop of c u t t h r o a t t r o u t i n Sp r i n g Creek at Road F-30 55 Table 5. Mean weight of the t r o u t in the experimental stream s e c t i o n 56 Table 6. Occurrence of benthic i n s e c t s i n the t r o u t stomach contents 57 Table 7. The range of head-capsule widths of benthic i n s e c t s i n the benthos and t r o u t stomachs 59 Table 8. Mean number and dry weight of N. c i n c t i p e s , P. d e b i l i s and C. sp_.A per t r o u t stomach 75 Table 9. D i f f e r e n c e in mean d e n s i t y and standing crop of N. c i n c t i p e s , P. debi 1 i s and C. sp_.A between stream s e c t i o n s 76 Table 10. Estimates of p r o d u c t i o n of l o t i c s p e c i e s i n the c o n t r o l and experimental stream s e c t i o n s of Spring Creek 78 Table A1.1. Number of i n d i v i d u a l s of H. koeb e l e i and G. peniturn c o l l e c t e d in each c o l l e c t i n g net of a double bag sampler 93 Table A1.2. Density correction factors for Nemoura, Triznaka and Paraleptophlebia 94 Table A2.1. Values of regression constants from functional regressions of dry weight on head-capsule width 97 LIST OF FIGURES Fi g u r e 1 . L o c a t i o n of study area 4 Fi g u r e 2. Siz e - f r e q u e n c y histograms of Nemoura c a l i f o r n i c a and Nemoura c i n c t i p e s nymphs from May, 1979 to March 1980 , 22 Fi g u r e 3. Size- f r e q u e n c y histograms of T r i z n a k a d i v e r s a nymphs from May, 1979 to March, 1980 26 Fi g u r e 4. Siz e - f r e q u e n c y histograms of P a r a l e p t o p h l e b i a sg.A and P a r a l e p t o p h l e b i a d e b i l i s nymphs from May, 1979 to March, 1980 32 Fi g u r e 5. Siz e - f r e q u e n c y histograms of Cinygmula sp_.A and Cinygmula sp_.B nymphs from May, 1979 to March, 1980 ... 36 Fi g u r e 6. Size- f r e q u e n c y histograms of Glossosoma peniturn l a r v a from May, 1979 to March, 1980 42 Fi g u r e 7. Siz e - f r e q u e n c y histograms of H e t e r l i m n i u s k o e b e l e i l a r v a from May, 1979 to March, 1980 47 Fi g u r e 8. Frequency histogram of fork lengths of c u t t h r o a t t r o u t from Spring Creek at Road F-30, August 1979 53 Fi g u r e 9. R e l a t i o n s h i p between d r i f t r a t e of prey and the mean number of prey per t r o u t stomach 60 Fi g u r e 10. Mean d e n s i t i e s and standing crops of N. c a l i f o r n i c a , T. d i v e r s a , P. sp.A and G. peniturn ... 62 Fi g u r e 11. Mean d e n s i t i e s and standing crops of N. c i n c t i p e s , P. d e b i l i s and C. sp.A 68 Fi g u r e 12. Mean d e n s i t i e s and standing crops of v i i i H. koeb e l e i and C. srj.B 72 F i g u r e 13. T o t a l number of i n d i v i d u a l s d r i f t i n g from the c o n t r o l and experimental stream s e c t i o n s 80 F i g u r e A1.1. P r o p o r t i o n of i n d i v i d u a l s r e t a i n e d i n the 0.471 mm c o l l e c t i n g net of the double bag sampler 95 ix ACKNOWLEDGEMENTS I wish to thank my supervisor, Dr. T.G. Northcote and my committee members Drs. G.G.E. Scudder, J.D. McPhail, C.J. Walters and Mr. P.A. Slaney for their help and c r i t i c i s m . Linda Berg provided invaluable f i e l d assistance, both night and day. Personnel of the Bioscience Data Centre were extremely helpful and patient, especially Dave Zitten. Special thanks go to Tom Johnston, whose ideas and comments contributed to t h i s study. I am grateful to Dr.- G.B. Wiggins of the Royal Ontario Museum for confirming the i d e n t i f i c a t i o n s of caddisfly larva. F i n a l l y , I wish to thank my wife, Beverly, for her patience, understanding and valuable assistance throughout this study. 1 INTRODUCTION L i t t l e i s known about the factors that determine the abundance and d i s t r i b u t i o n of l o t i c insect populations (Fox 1977). Studies in l e n t i c and i n t e r t i d a l systems have found predation and competition to be important influences of the population dynamics of aquatic species ( N e i l l and Peacock 1980; Northcote et a l . 1978; Menge 1976; Paine 1974; Connell 1970; Hall et a l . 1970). Stream studies though have been generally concerned with the effect of ab i o t i c factors on the d i s t r i b u t i o n of l o t i c species (Corkum et a l . 1977; Rabeni and Minshall 1977; Hynes 1970, chapter 11). Thus the r e l a t i v e roles of predation and competition in influencing stream community structure are unclear. Numerous feeding studies have shown trout to be important predators of invertebrates in cool water streams (Allan 1981; G r i f f i t h 1974; Narver 1972; E l l i o t t 1967). Allan (1975) suggested that trout predation may be an important mechanism determining the abundance of benthic invertebrate populations in jLotic environments. He noted that benthic insect densities were two to six times greater in the trout-free headwaters of a stream then in adjacent downstream areas inhabited by cutthroat trout (Salmo c l a r k i Richardson). S i m i l a r i l y , Straskraba (1965) found that the amphipod, Rivulogammarus fossarum, abundant in the headwaters, was scarce in downstream reaches of a stream inhabited by brown trout, Salmo tr u t t a Linnaeus. However, Waters (1972) noted that i f trout were to feed solely on d r i f t or the "excess production of . stream invertebrates", their 2 p r e d a t i o n may have l i t t l e e f f e c t on l o t i c i n s e c t p o p u l a t i o n s . The p a u c i t y of experimental data makes i t impossible to d i s t i n g u i s h between these hypotheses. The purpose of t h i s study was to provide experimental i n f o r m a t i o n about the r o l e of predator-prey i n t e r a c t i o n s i n determining the s t r u c t u r e of stream communities. S p e c i f i c a l l y , what e f f e c t does t r o u t p r e d a t i o n have on the abundance and p r o d u c t i o n of stream benthic p o p u l a t i o n s ? In the study, I e x p e r i m e n t a l l y examined the impact of t r o u t p r e d a t i o n on the d e n s i t y , standing crop, d r i f t and p r o d u c t i o n of 9 common i n s e c t s p e c i e s : Nemoura (Malenka) c a l i f o r n i c a ( C l a a s s e n ) , Nemoura (Zapada) c i n c t i p e s (Banks), Tr iznaka d i v e r s a ( F r i s o n ) , P a r a l e p t o p h l e b i a sp_.A, P a r a l e p t o p h l e b i a d e b i l i s (Walker), Cinyqmula S J D . A, Cinygmula S J D . B , Glossosoma peniturn (Banks) and H e t e r l i m n i u s k o e b e l e i (Martin) by manipulating t r o u t d e n s i t i e s i n the headwaters of a small c o a s t a l B r i t i s h Columbia stream. Studi e s on the ecology of l o t i c i n s e c t s have been hindered by a l a c k of taxonomic and l i f e h i s t o r y knowledge (Wiggins 1966; Hynes 1970). Thus a second o b j e c t i v e of the study was to e l u c i d a t e the l i f e h i s t o r y of the i n s e c t s p e c i e s examined in the study. The l i f e h i s t o r i e s of l o t i c i n s e c t s p e c i e s i n the U n i v e r s i t y of B r i t i s h Columbia Research F o r e s t have not been d e s c r i b e d h e r e t o f o r e . 3 DESCRIPTION OF STUDY AREA The study was conducted in the headwaters of Spring Creek in the University of B r i t i s h Columbia Research Forest, located approximately 60 km east of Vancouver, B.C. (Figure 1). Spring Creek is a soft-water (conductivity = 40 uohms), s l i g h t l y alkaline (pH = 7.5), nutrient-poor (nitrate < 0.1 mg/1; phophorus < 0.01 mg/1) spring-fed stream. The drainage basin, which is largely g r a n i t i c , covers about 5.0 km2. The climate i s mild and wet, with an average annual p r e c i p i t a t i o n of 240 cm/yr. The study area consisted of a stream section 37 m long and approximately 3 m wide. The s i t e was bounded by a small shallow pond (area = 960 m2; maximum depth = 1m ) upstream and a culvert downstream. The stream bottom was composed of pebble and gravel (few stones exceeded 10 cm maximum diameter), 5 to 15 cm thick, laying on a g r a n i t i c bedrock base. During the study from May to December 1979, stream discharge varied from 0.004 m3/sec in August to 0.1 m3/sec in October. Water temperature ranged from a maximum of 26.5° C in August to a minimum of 2° C in December (Table 1). Vegetation along the stream.bank was composed of shrubs, Gaultheria shallon and Rubus s p e c t a b i l i s ; deciduous trees, Alnus spp, Salix spp, Acer  macrophyllum and Acer circinatum; and coniferous trees, Tsuga  heterophylla. A resident population of cutthroat trout inhabited the lower portion of Spring Creek. A series of waterfalls located just downstream of the study area prevented the upstream 4 Figure 1. Location of study area. STUDY SITE LOCATION m e t r e s Table 1. Monthly maximum and minimum water temperatures at the study s i t e from June to December, 1979. Temperature (°C) Month Minimum Maximum June 7.5 23.5 J u l y 10.0 26.0 August 12.5 26.5 September 9.0 24.5 October 7.0 16.0 November 3.0 10.0 December 2.0 5.0 d i s p e r s a l of t r o u t i n t o the study a r e a . 7 MATERIALS AND METHODS Experimental Design The stream was divided l o n g i t u d i n a l l y into a control and an experimental study section. Each section was 13 m long, approximately 1 1/2 m wide, and consisted of an upstream r i f f l e (4 1/2 m 2), a shallow pool (7 1/2 m2) and a downstream r i f f l e (7 1/2 m 2). The stream sections were located 12m downstream of the pond and 12 m upstream of the culvert. In mid-July of 1979 the control and experimental sections were separated by an aluminum p a r t i t i o n . A trench was dug between the two sections and aluminum sheets (120 x 30 cm) were placed down the length of the trench. Gaps between the aluminum sheets and the bedrock were f i l l e d with p l a s t i c i n e , and the trench was r e f i l l e d with substrate. Iron rods were driven into the stream bed and fastened to the aluminum p a r t i t i o n to provide support. Each end of the experimental section was enclosed with metal screening (1.2.5 x 1.25 cm mesh). The mesh size of the screening was chosen to be small enough to retain the trout in the enclosure while large enough not to interfere with the movements of invertebrates into or out from each end of the experimental stream section. The aluminum p a r t i t i o n prevented the l a t e r a l movements of trout, d r i f t and benthos between the control and experimental stream sections. Thus a pair of streams, flowing side by side, with similar physical c h a r a c t e r i s t i c s were created within the study area. 8 C u t t h r o a t t r o u t (8.7 to 14.3 cm fork l e n g t h ) , obtained from a downstream s e c t i o n of Spring Creek, were p l a c e d i n t o the enclosure i n mid-August of 1979. They were maintained i n the experimental e n c l o s u r e at approximately 3-4 times the estimated downstream t r o u t d e n s i t y u n t i l the end of November. Fry (4.0 to 6.0 cm fork length) were not used i n the experiment as the screen mesh was not small enough to r e t a i n them i n the e n c l o s u r e . Sampling Methods Benthos Benthic i n s e c t s were sampled using a Hess-type sampler ( M e r r i t t and Cummins 1978) which enclosed a 0.05 m2 a r e a . The sampler was turned s e c u r e l y i n t o the stream bottom and the s u b s t r a t a were thoroughly s t i r r e d . Stones, g r e a t e r than 3 cm, were scrubbed with a brush, then removed from the sample s i t e . I n v e r t e b r a t e s and d e b r i s were swept by the c u r r e n t i n t o the c o l l e c t i n g net attached to the downstream sid e of the sampler. The sampler had a 0.471 mm mesh c o l l e c t i n g net which ended i n a removable p l a s t i c b o t t l e whose bottom was r e p l a c e d with 0.153 mm n i t e x . On each sampling date, e i g h t bottom samples were taken from each of the stream s e c t i o n s , f o r a t o t a l of s i x t e e n samples. A h a b i t a t - s t r a t i f i e d (upstream r i f f l e , p o o l , downstream r i f f l e ) random sampling design was used to l o c a t e 9 the sampling s i t e s w i t h i n each stream s e c t i o n . P a i r s of numbers were drawn from a random number t a b l e to l o c a t e 2 sampling s i t e s i n the upstream r i f f l e and 3 sampling s i t e i n the pool and i n the downstream r i f f l e . The number of samples taken from a h a b i t a t was approximately p r o p o r t i o n a l to the r a t i o of the area of the h a b i t a t to the t o t a l area of the stream s e c t i o n . A sample s i t e was not u t i l i z e d more than once every four months. A l l samples were preserved i n 80% e t h a n o l . D r i f t D r i f t i n g i n v e r t e b r a t e s were sampled using a Surber-type sampler with a 30 x 30 cm aperture (McKone 1975) and a 0.471 mm mesh c o l l e c t i n g net which ended i n a removable p l a s t i c b o t t l e with i t s bottom r e p l a c e d with 0.153 mm n i t e x . Four samplers were p o s i t i o n e d a c r o s s the width of the stream at the downstream end of the stream s e c t i o n s . One p a i r of samplers r e c e i v e d the e n t i r e flow of water from the c o n t r o l s e c t i o n while the second p a i r of samplers r e c e i v e d the e n t i r e flow of water from the experimental s e c t i o n . The samplers were he l d i n pl a c e by i r o n rods d r i v e n i n t o the stream bed. Each sampler f i l t e r e d the e n t i r e water column. On each sampling date, hou r l y d r i f t samples were c o l l e c t e d from each stream s e c t i o n f o r 5 hours, beginning 3 hours before s u n r i s e . A l l samples were preserved i n 80% e t h a n o l . The depth and mid-depth water v e l o c i t y were measured at the mouth of each sampler. 10 Stomach Contents The cutthroat trout were recovered from the enclosure with a hand net immediately after the d r i f t samples had been col l e c t e d (ie 2-3 hours after sunrise). The stomach contents of each trout were removed with a stomach pump (Meeham and M i l l e r 1978). The f i s h were anesthetized during the procedure with t r i c a i n e methyl sulfonate (MS222), held in fresh water u n t i l they had recovered from the e f f e c t s of the anesthetic, and then released back into the enclosure. The stomach contents removed from each trout were preserved in 80% ethanol. This technique had the advantage that the stomach contents of each individual f i s h could be sampled repeatedly during the experiment, thus f i s h adapted to the conditions in the experimental stream section did not have to be s a c r i f i c e d for stomach content analysis. G r i f f i t h (1974) observed cutthroat trout in several Idaho streams and noted that they generally did not feed at night. The stomach contents removed from the trout therefore were assumed to be representative of the feeding habits of the trout during the day. Trout Abundance To determine the minimum number and the s i z e - d i s t r i b u t i o n of trout to be used in the experiment, I censused a cutthroat trout population in a downstream section of' Spring Creek, a week prior to the start of the experiment. Blocking nets were placed above and below a 120 m stretch of stream so that no 11 trout could enter of leave the area. The 120m stretch of stream was electrofished three times, with about a 15 to 20 minute rest period between each e l e c t r o f i s h i n g period. The number of f i s h caught, the fork length and wet weight, measured with a spring balance, of each f i s h and the amount of time spent e l e c t r o f i s h i n g were recorded at the end of each e l e c t r o f i s h i n g period. Since fry could not be weighed accurately in the f i e l d , a small number of them were taken back to the lab and weighed on an triple-beam balance. Scales were removed from a number of trout to determine the age structure of the population. Methods of Analysis Benthos Aquatic insects in each benthic sample were separated from debris, i d e n t i f i e d and enumerated under a dissecting microscope (12 X magnification). Only those benthic insect populations which were abundant (10 or more individuals per sample), were examined further. One benthic sample was randomly selected from the upper r i f f l e , pool and lower r i f f l e from both the control and experimental stream sections from each sampling date and the head capsule width across the eyes of each individual of Nemoura c a l i f o r n i c a , Nemoura c inctipes, Triznaka diversa, Paraleptophlebia sp_.A, Paraleptophlebia d e b i l i s , Cinygmula SJD .A, Cinygmula sp_.B, Glossosoma pen i turn and Heterlimnius  koebelei was measured to the nearest 0.02 mm using an ocular 1 2 micrometer. If the t o t a l number of i n d i v i d u a l s measured of any sp e c i e s was l e s s than 50, than a second sample was randomly s e l e c t e d from the pool and lower r i f f l e from that stream s e c t i o n and the i n d i v i d u a l s of that s p e c i e s measured. Approximately 11,000 of the more than 30,000 i n d i v i d u a l s c o l l e c t e d were measured. L i f e H i s t o r i e s of Insect Spec i e s The l i f e h i s t o r y of each s p e c i e s was e l u c i d a t e d from the a n a l y s i s of the head capsule width-frequency data from the c o n t r o l s e c t i o n of the stream. Head capsule width was used to d i s c r i m i n a t e i n s t a r s of the c a d d i s f l y , Glossosoma peniturn, and the r i f f l e b e e t l e , H e t e r l i m n i u s k o e b e l e i , but no attempt was made to recognize the i n s t a r s of the s t o n e f l i e s or m a y f l i e s . Species i d e n t i f i c a t i o n s were made from a d u l t s specimens c o l l e c t e d i n the d r i f t , benthic or t r o u t stomach samples during the study. A d d i t i o n a l a d u l t specimens from the study area were s u p p l i e d to me by K. Frankenhuyzen (Bi o l o g y Department, Simon Fr a s e r U n i v e r s i t y ) from emergence t r a p samples c o l l e c t e d from September, 1980 u n t i l J u l y , 1981. Specimens of each s p e c i e s were p l a c e d i n the UBC Entomological Museum. 1 3 Insect Density The abundance of individuals with a maximum head or body width less than 0.471 mm was underrepresented in the benthic samples due to the mesh size of the c o l l e c t i n g net. A set of samples therefore was coll e c t e d in March, 1980 with a 0.116 mm mesh c o l l e c t i n g net placed over the o r i g i n a l 0.471 mm mesh c o l l e c t i n g net (double bag sampler). From these samples, the actual proportion of individuals of each species in 0.04 mm head capsule width size classes (from 0.16 to 0.52 mm) col l e c t e d with the o r i g i n a l 0.471 mm mesh c o l l e c t i n g net was estimated. Correction factors were calculated for each 0.04 mm head capsule width size class for each species and the density estimates of each species were adjusted appropriately (Appendix 1). Each benthic insect population had a contagious s p a t i a l dispersion ( i e . variance > mean) ( E l l i o t t 1977; Resh 1979). As a result, the data were transformed with the function ln(x + 1), where In is the natural logarithm and x is the number of individuals of a species in a sample. A three-way f a c t o r i a l analysis of variance, with time (sample date), habitat (upper r i f f l e , pool, lower r i f f l e ) and stream section (control, experimental) as treatment l e v e l s , was used to determine whether the mean population densities of a species d i f f e r e d between stream sections before and after the trout introduction. 1 4 Insect Standing Crop In September and October unpreserved benthic samples were col l e c t e d from the stream. Live individuals of each species were measured, k i l l e d in hot water and placed in small preweighed aluminum weighing pans. These samples were dried at 60° C for 24 hr, cooled to room temperature in a dessicator and then weighed to the nearest 0.001 mg. For small individuals, groups of up to four individuals were weighed together and the mean weight recorded. Live organisms were used in order to avoid possible changes in dry weight due to preservatives (Howmiller 1972; Standford 1972). The log-linear analogue of a power curve, log(W) = log(a) + b log(HW) where, a and b are constants, was used to estimate dry weight (W) from head capsule width (HW) (see Appendix 2). This function has been found to provide the best f i t for weight-length data of insects (Smock 1980). This relationship was used to convert the corrected head capsule width-frequency data (see section above) of each species in each benthic sample into a standing crop estimate. For those samples in which the individuals of a species were not measured, the head capsule, width d i s t r i b u t i o n of the species was assumed to be the same as that found in the analyzed sample in the same habitat on the same date. A three-way f a c t o r i a l analysis of variance, with time 1 5 (sample dates), habitat (upper r i f f l e , pool, lower r i f f l e ) and stream section (control, experimental) as treatment l e v e l s , was used to determine whether the mean standing crops of a species d i f f e r e d between stream sections before and after the trout introduction. The standing crop data of each species were transformed to uncouple the variance from the mean. Taylor's power law ( E l l i o t t 1977) indicated that the square root transformation was appropriate for the data. Insect Product ion The instantaneous growth rate method (Ricker 1946; Allen 1949) was used to estimate the production of each species in the upper r i f f l e , pool and lower r i f f l e habitats in each stream section during the experiment. Since the standing crop estimates of a species were determined i n d i r e c t l y from the head capsule width d i s t r i b u t i o n data, only a single estimate of production for each species in each habitat could be calculated. This method calculates production as: P = G x B where, P i s the production for a given period of t ime, G i s the instantaneous growth rate for the time period and B i s the mean standing crop during the time period. 16 The instantaneous growth rates were calculated for periods between sampling as the natural logarithm of the r a t i o of the mean individual weight at the end of the time period to the mean individual weight at the beginning. If the eggs of a species were hatching during the experiment, then the mean individual weight on each date, was calculated as the mean weight of the 10 largest individuals in the sample. The mean standing crop of a species between sampling periods was calculated as the arithmetic mean of adjacent standing crop estimates (Chapman 1968). Waters and Crawford (1973) and Resh (1977) have shown that t h i s method gives similar results as other methods for ca l c u l a t i n g production for benthic invertebrates. A two-way analysis of variance (randomized block design), with habitat (upper r i f f l e , pool, lower r i f f l e ) and stream section (control, experimental) as factors, was used to determine whether the production of a species d i f f e r e d between stream sections after the trout introduction. D r i f t Aquatic insects in each d r i f t sample were separated from debris, i d e n t i f i e d and enumerated under a dissecting microscope (12 X magnification). Because of the low hourly d r i f t rates of each species, the five hourly samples from each stream section were pooled. A Wilcoxon paired-sample test was used to determine whether the number of individuals of a species d r i f t i n g from each stream section d i f f e r e d after the trout 17 introduction. Trout Abundance Delury's method (Ricker 1975) was used to estimate the density of the trout in the downstream section of Spring Creek. Since fry could not be used in the experiment and as e l e c t o f i s h i n g i s less e f f e c t i v e with small f i s h (McFadden 1961), the density of the fry was estimated separately from the rest of the population. The log-linear analogue of the power curve was used to estimate wet weight from fork length. Since a l l trout were measured when collected, i t was possible to convert the fork length-density data into a standing crop estimate using t h i s r elationship. Trout Stomach Contents Prey organisms removed from the trout stomachs were separated from debris, i d e n t i f i e d and enumerated under a dissecting microscope (12 X magnification). The head capsule width across the eyes of each individual of Nemoura  c a l i f o r n i c a , Nemoura ci n c t i p e s , Triznaka diversa, Paraleptophlebia sp.A, Paraleptophlebia d e b i l i s , Cinygmula sjo.A, Cinygmula S J D . B , Glossosoma pen i turn and Heterlimnius  koebelei was measured to the nearest 0.02 mm using an ocular micrometer. 18 Food Consumption by Trout Following E l l i o t t and Persson (1978), I estimated the dai l y mean number of individuals consumed per trout (C) as: C= (24) (S) (R) where, S is the dai l y mean number of individuals in the stomach and R is the hourly rate of gastric evacuation. Since the stomach contents of the trout were sampled only in the morning (2-3 hours after sunrise), S was assumed to be equal to the mean number of individuals in the trout stomachs at t h i s time. Data from Allan (1981) on the temporal pattern of feeding a c t i v i t y in brook trout (Salvelinus fontonalis M i t c h i l l ) indicated that the mean number of individuals per stomach from samples co l l e c t e d 2 to 4 hours after sunrise approximated the da i l y mean number of prey per stomach. G r i f f i t h (1974) observed that brook and cutthroat trout had similar feeding habits, and found that the mean number of prey per stomach of brook and cutthroat trout in streams inhabited sympatrically was s i m i l a r . Thus my assumption seems reasonable. I was unable to find any published data on the rate of gastric evacuation for cutthroat trout. Therefore I used the data presented by E l l i o t t (1972) on the rate of gastric evacuation for brown trout. E l l i o t t (1972) found that water temperature and the species of prey had a s i g n i f i c a n t effect on 19 the rate of gastric evacuation while trout size, prey size and feeding rate had l i t t l e e f f e c t . Thus R was assumed to be equal to the time for 95% gastric.evacuation of a meal of Baetis  rhodani from the stomach of brown trout at the mean water temperature that occurred on each sampling date ( E l l i o t t 1972, Figure 7). The number of individuals (N) consumed over a time in t e r v a l around each sampling date was estimated as: N = (C) (D) (I) where, D i s the density of trout in the enclosure and I i s the duration of the time interval in days. Since each time interval began and stopped half-way between consecutive sampling dates, the t o t a l number of individuals consumed by the trout during the experiment i s equal to the sum of the number of individuals consumed in each time i n t e r v a l . An estimate of the standing crop (mg dry-wt./m2) consumed by the trout during the experiment was calculated by the same proceedure as outlined above. The dry weight of an individual in the stomach contents was estimated from a regression of dry weight on head capsule width (see Appendix 2). 20 Samplinq V a r i a b i l i t y A number of procedures were undertaken during t h i s study to increase the precision of the insect population density estimates: F i r s t , the loss of individuals in each sample was minimized by: a) using fine mesh (0.471 mm) c o l l e c t i n g nets in the f i e l d ; b) estimating the loss of individuals through the fine mesh c o l l e c t i n g nets; and c) sorting and enumerating individuals from the samples under a stereoscope at 12 times magnification. Second, the hyporheic component of each population was included in the sample. Third, a s t r a t i f i e d random sampling design was used to locate sampling s i t e s and fourth, the t r a n s i t i o n areas between the r i f f l e s and the pool were not sampled; insect population densities were assumed to vary monotonically from one habitat to the other. 21 LIFE HISTORIES OF INSECT SPECIES Stoneflies Three populations of sto n e f l i e s were examined during the study: Nemoura (Malenka) c a l i fornica (Claassen), Nemoura (Zapada) cinctipes (Banks) and Triznaka diversa (Frison). Nemoura c a l i f o r n i c a and Nemoura cinctipes had one generation per year (Figure 2) while Triznaka diversa had a 2 year l i f e cycle (Figure 3). The eggs of Nemoura c a l i f o r n i c a began to hatch in early autumn and continued probably throughout the winter months. Young nymphs lacking c e r v i c a l g i l l s were present from October through March. The head width (HW) of these nymphs measured between 0.14 and 0.16 mm and were probably f i r s t instars (Harper 1973). The nymphs grew rapidly during the spring and summer months. They developed through an unknown number of instars (other species of Nemoura reportedly go through 12-16 instars, Harper 1973; B r i t t a i n 1973), to reach maturity in late summer. Wing pads appeared on nymphs with a HW > 0.70 mm; nymphs with a HW > 0.96 mm had well developed wing pads and were considered ready to emerge. Emergence of adults occurred in late summer. Adults were present on the study s i t e from September through November. Kerst and Anderson (1974;1975) reported a similar l i f e history pattern for Nemoura c a l i f o r n i c a in Oregon. The eggs of Nemoura cinctipes hatched throughout the spring months. Young nymphs lacking c e r v i c a l g i l l s were present 22 Figure 2. Size-frequency histograms of Nemoura c a l i f o r n i c a and Nemoura cinctipes nymphs from May, 1979 to March 1980 . Shaded histograms represent individuals of Nemoura c i n c t i p e s . M indicates mature nymphs (wing-pads well-developed) present. A1 indicates adults of Nemoura c a l i f o r n i c a present. A2 indicates adults of Nemoura c inct ipes present, (next 3 pages) ffitr 6 5? N3- INDIVIDUALS PER O-CS SO- M-K ! K S 2 9 ! l t l l t i 6 N3- INDIVIDUALS PER 0-05 SO- M-B K 8 5! ffl W W ! a " in ! 96,6 "* I 75.4 ID rO- INDIVIDUALS PER 0-06 SO- M-IS- . in S B S 1237 I rO- INDIVIDUALS PER 0-CS SO- M-S tn S 2 ffl co rO- IrCIVICLIALS PER O-CS SO- M* NO- INDIVIDUALS PER O-CS SO- M-M> I^IVIOUALS PER 0-05 SO- M- ND- INDIVIOJALS PER 0-C6 SO- M-f ^ K S l ' f f l W W f e o * m IS R 8 . . . . . . . . o * • ro Ul 26 F i g u r e 3. S i z e - f r e q u e n c y h i s t o g r a m s of T r i z n a k a d i v e r s a nymphs from May, 1979 t o March, 1980 . M i n d i c a t e s mature nymphs (wing-pads w e l l - d e v e l o p e d ) p r e s e n t . A i n d i c a t e s t h a t a d u l t s a r e p r e s e n t , (next 3 pages) 27 MAY H3 1379 JULY 12 1379 2. 2 Him n n •IB 0.24 0-40 0.56 0.72 0-BB 1.04 1-20 1-3E HEAD CAPSULE WIOTH CIvW) 2 o- to- Q-rfl li-n 0-15 0-24 0-40 O-SS 0-72 0-68 1.04 1.20 1.36 HEAD CAPSLLE WIDTH ((*() JUNE 14 1379 AUGUST ,9 1979 2 11 0.12-0*24 0*40 O-SS 0-72 O-SB 1-04 1-20 1-3G K > 0 CAPSULE WIDTH (KM) J-12 0*24 0*40 0-56 0-72 O-fcti 1-04 1-20 1-3G HH>0 CVaLE WIDTH (MJJ SEPTEMBER S 1379 OCTOBER 4 1379 r r i l l l l h n • 12 0 - 2 4 . 0 . 4 0 O - S S 0 - 7 2 0 - 8 8 1-04 1 . 2 0 1 . 3 6 l£AO CAPSULE WIDTH tkW) o- m i n i ri 0 - 1 2 0 - 2 4 0 - 4 0 0 - 5 6 0 - 7 2 O-BB 1 .04 1-20 1-36 rOO CAPSULE WIDTH (M*) SEPTEMBER B3 1979 N3VEM3ER 1 1379 - n . ^ / r T l r T T T h H •IE 0 - S 4 Q.4Q Q.So 0 - 6 8 1-04 1 - 5 0 1-36 r-e/a opsixe WIDTH 0*0 h • • r r i > :TiTK>n 0 - 1 2 0 . 2 4 0 . 4 0 O . S S 0 -73 O . B U 1-04 1-20 1 . 3 S HEAD CAPSULE WIDTH Cr*t) NJVEM3ER 29 1979 n . . r r i m n n n 0-12 0.54 0.40 0-SE 0-72 0.88 1-04 1-20 1-3S HEAD CAPSULE WIDTH CIA(1 MARCH SB 1SS0 e t a xtm. 0*12 0*24 0-40 0-36 0-72 0-( r n , l~h 1.04 i.pO 1-3G HEAD r/fcji r WIDTH (MJJ 30 from A p r i l through July. Nymphs with a HW < 0.30 mm were morphologically indistinguishable from similar sized nymphs of Nemoura c a l i f o r n i c a . Nymphs of both species with a HW between 0.14 and 0.16 mm had no c e r v i c a l g i l l s ; between 0.16 and 0.20 mm had one pair of unbranched c e r v i c a l g i l l s ; and between 0.20 and 0.28 mm had two pair of unbranched c e r v i c a l g i l l s . Nemoura  c a l i f o r n i c a nymphs with a HW > 0.30 mm had branched c e r v i c a l g i l l s and thus could be distinguished from Nemoura cinctipes nymphs which had unbranched c e r v i c a l g i l l s . Growth of the Nemoura cin c t i p e s nymphs was slow during the summer, suggesting the p o s s i b i l i t y that the nymphs undergo a diapause (Harper and Hynes 1970; Harper 1973). The nymphs grew rapidly through the autumn and reached maturity in November. Wing pads appeared on nymphs with a HW > 0.70 mm; nymphs with a HW > 1.0 mm in late November had well developed wing pads and were considered ready to emerge. Emergence of adults probably occurred from late autumn through early spring. Adults were found on the study s i t e in March and A p r i l . A similar l i f e history pattern for Nemoura cinctipes was described by Kerst and Anderson (1974;1975) in Oregon and by Radford and Hartland-Rowe (1971) in southwestern Alberta. The eggs of Triznaka diversa began to hatch in early summer and may have continued throughout the rest of the year. Young, colourless nymphs with a HW between 0.14 and 0.16 mm were present from July through March. The nymphs grew slowly during the summer and autumn and required two years to mature. Nymphs of age 1+ were found with Paraleptophlebia sjo.A nymphs, 31 chironomid and ceratopogonid larvae and nematodes partly ingested, suggesting that at least the older nymphs of T. diversa are carnivorous. The nymphs matured in the spring. Wing pads were found on nymphs with a HW > 1.0 mm. Emergence of adults occurred in the late spring. Mayf1ies Four populations of mayflies were examined during the study: Paraleptophlebia sjo.A, Paraleptophlebia d e b i l i s (Walker), Cinygmula sp_.A and Cinygmula sjo.B. A l l four species had a univoltine l i f e history (Figure 4 and 5). The eggs of Paraleptophlebia sjo.A hatched throughout the summer. Young colourless nymphs with a head width (HW) between 0.14 and 0.16 mm were present from July to September. These nymphs were not f i r s t instars since nymphs with a HW of 0.10 mm were occassionally c o l l e c t e d . Growth of the nymphs was slow during the autumn but a period of rapid growth occurred in the spring. Wing pads were present on nymphs with a HW > 0.70 mm; nymphs with a HW > 1.0 mm had well developed wing pads and were considered ready to emerge. Adult emergence probably occurred from late spring through mid-summer. The l i f e history pattern of this species i s very similar to Paraleptophlebia qreqalis in Oregon (Lehmkuhl and Anderson 1971). The mature nymphs of Paraleptophlebia sjo.A key out as Paraleptophlebia gregalis using the key provided by Lehmkuhl and Anderson. The eggs of Paraleptophlebia d e b i l i s began to hatch during the winter and probably continued u n t i l late spring since young 32 Figure 4. Size-frequency histograms of Paraleptophlebia sp,A and Paraleptophlebia d e b i l i s nymphs from May, 1979 to March, 1980 . Shaded histograms represent individuals of Paraleptophlebia d e b i l i s . M indicates mature nymphs (wing-pads well-developed) present. A indicates adults of Paraleptophlebia d e b i l i s present, (next 3 pages) 33 MAY 23 1S79 JULY 12 1379 2 BB-SS 8 24. M n rfl , • nTh-n m 0-12 0.24 0-40 0-56 0-72 O-SS 1-04 1.20 1-32 2 2B-Si 0.12 0.24 0-40 O-SS 0-72 O-BB 1-04 1-20 1.3 HEAD CAPSULE WIDTH CMM1 HEAD CAPSULE WIDTH CKWI JUNE 14 1979 2 28. s 8 24. • IM 2 2B. 0-12 0.24 0.40 0-S6 0-72 0-88 1-04 1.2J 1-32 ALGLST 9 1379 S 0 - 1 3 0 - 2 4 0-4O 0 - 5 6 0-75 O t U 1-04 l-PO l»3E HEAD CAPSULE WIDTH Ch.M) HEAD CAPS l.E WIDTH CKM) 34 SEFTENfiER S 1379 OCTOBER A 1379 2 aa. 1 0-12 0-24 0.40 O-SG 0-72 0-B8 1-04 1-20 1-32 HEAD CAPSULE WIOTH (KM) S 28-8 24. 4=3-0-12 0-24 0.40 0-56 0«72 0-86 1-04 1-20 1-32 HEAD CAPSULE WIDTH CKM) SEPTEMBER 50 1379 NDVEM3ER 1 1379 2 28-a 8 24. 0*12 0*24 0-40 0-S6 0*72 0-E3B 1*04 1.20 1-32 HtAO CAPSULE WIDTH (KM) A pi M &3-0-12 0-24 0.40 0-56 0-72 O-ftJ 1-04 1-20 lv HEAD CAPSULE WIDTH (KWI N3VEM3ER 23 1379 M . ~ l T h P h m 0-12 0-24 0-40 0-SG 0-72 0-88 1-04 1-20 1.32 HEAD CAPSULE WIDTH (MM) MARCH 28 1SS0 irrfnrn M m-i 0.12 0-24 0-40 0.56 0.72 0-66 1>04 1>2U 1-32 HEAD CAPSULE WIDTH (MM) 36 Figure 5. Size-frequency histograms of Cinygmula S J D . A and Cinygmula sp_.B nymphs from May, 1979 to March, 1980 . Shaded histograms represent i n i d i v i d u a l s of Cinygmula sjo .A . M indicates mature nymphs (wing-pads well-developed.) present, (next 3 pages) 37 MAY S3 1379 JULY 12 1379 0 - 1 2 C 3 5 t u > 2 0 . 1 2 0 - 3 6 HEAD CAPSULE WIDTH CM*) HEAD CAPSULE WIDTH (MMJ JUNC 14 1979 AUGUST 9 1379 5 8 6 -0 . 1 2 0 - 3 5 HEAD CAPSULE WIDTH (KM) 0 - 1 2 C 3 S HEAD CAPSLLE WIDTH CMJ) 38 SEPTEM3ER S 1379 OCTOBER A 1979 2 s I—I—1 IL 6 > 2 0 - 1 2 0 . 3 6 0 - G B 1 . 0 0 J J tL 0 . 1 2 0 - 3 S 0 . 6 6 1 . 0 0 1-32 X X HEAD CAPSULE WIDTH (MM) HEAD CAPSULE WIDTH tKWJ SEPTEMBER EC 1379 rOVE^SER 1 1979 6 > 2 111 0 - 1 2 0 . 3 S 0 - 6 6 1 . 0 0 1 .32 n J=L 0 - 1 2 0 - 3 S 0 . 6 B l .OO 1-32 1 -64 HEAD CAPSULE WIDTH Cf*l) HEAD CAPSULE WIDTH ().*» N3VEK6ER 59 1379 0-12 0.3S 0-6B 1-00 1-32 1-64 HEAD CAPSULE WIDTH CMM) MARCH 2S 1SB0 -H r~ 0-12 0-36 O.SB l-OO 1-32 1-64 HEAD CAPSULE WIDTH 0*4) 40 nymphs with a head width between 0.14 and 0.16 mm were present from March through May. The nymphs grew steadily throughout the year and were mature by mid-autumn. Wing pads were present on nymphs with a HW > 0.70 mm; nymphs with a HW > 0.94 mm had well developed wing pads and were considered ready to emerge. The emergence of adults probably occurred from mid-October through November. A similar l i f e history pattern for Paraleptophlebia  d e b i l i s i s described by Lehmkuhl and Anderson (1971) in Oregon. The eggs of Cinygmula sjo.A, after resting over the winter months, hatched in the spring. Young nymphs with a HW between 0.24 and 0.32 mm were present in May and June. No nymphs were colle c t e d with a HW < 0.22 mm. The nymphs grew rapidly through the summer and matured in early autumn. Wing pads f i r s t appeared on nymphs with a HW of 0.88 mm; nymphs with a HW > 1.4 mm had well developed wing pads and were considered ready to emerge. Adult emergence probably occurred from late September through October. The eggs of Cinygmula sjo.B hatched in the late summer and early autumn. Young nymphs (HW between 0.24 and 0.32 mm) were present from September to November. The nymphs grew steadily through the autumn and winter and matured in the early spring. Wing pads were present on nymphs with a HW > 0.88 mm; nymphs with a HW > 1.36 mm in March and 1.2 mm in May or June had well developed wing pads and were considered ready to emerge. The emergence of adults probably occurred in the late spring. The l i f e history of Cinygmula sjo.B i s similar to that described by Lehmkuhl and Anderson (1970) for Cinygmula r e t i c u l a t a in 41 Oregon. Ca d d i s f l i e s A single caddisfly population, Glossosoma penitum (Banks), was examined during the study. Five instars were i d e n t i f i e d from the head capsule width data (Table 2). These data agree Table 2. Head-capsule widths of l a r v a l instars of Glossosoma penitum from Spring Creek, Instar Head-capsule width (mm) 1 0.16 2 0. 18 - 0.20 3 0.26 - 0.30 4 0.36 - 0.42 5 0.50 - 0.58 well with the instar-head width relationship reported for Glossosoma penitum in Oregon (Anderson and Bourne 1974). Glossosoma penitum had a b i v o l t i n e l i f e - c y c l e (Figure 6). The eggs of the summer generation of Glossosoma penitum began to hatch in June and probably continued though July. The larvae grew very rapidly and by July larvae were present in each stadium. Emergence of adults probably began in July and continued through August. The eggs l a i d by the summer generation probably began to 42 Figure 6. Size-frequency histograms of Glossosoma peniturn larva from May, 1979 to March, 1980 . A indicates adults of Glossosoma peniturn present, (next 3 pages) NO- IhCrviCUALS PER 0-05 SQ- M* NO- INDIVIDUALS PER 0-05 SQ- M-NO- IhOIVIDUALS PER 0-CS SQ- M-NO- INDIVIDUALS PER 0-05 SQ- M-44 SEPTEMBER G 1379 CCTCBER 4 1979 SEPTEMBER S3 1379 NOVEMBER 1 1979 INSTAR INSTAR NO- INDIVIDUALS PER 0-05 SO- M- NO- INDIVIDUALS PER 0-05 SO- M-Ol 46 hatch in August and continued into October as f i r s t instars were present from August u n t i l October. Growth of the larvae proceeded steadily through the autumn. The larvae entered the winter predominently as t h i r d and fourth instars. Pupa were present from the end of March through June. The adults emerged in late spring. Anderson and Bourne (1974) reported a similar b i v o l t i n e l i f e history for Glossosoma penitum in Oregon. R i f f l e Beetles A single r i f f l e beetle population, Heterlimnius koebelei (Martin), was examined during the study. Four instars were i d e n t i f i e d from the head width data (Table 3). Individuals with a head width (HW) between 0.16 and 0.18 mm were assumed to be f i r s t instars because individuals with a HW < 0.16 mm were never co l l e c t e d . Heterlimnius koebelei appeared to require at least 2 years to complete i t s l i f e history (Figure 7). Individual cohorts though could not be recognized from the data since f i r s t , second and t h i r d instar larva were present at a l l times during the year. F i r s t instar larvae were most abundant in the late summer and autumn; t h i r d instar larvae were most abundant in the spring and . early summer ; and fourth instar larvae were most abundant in the early autumn. This pattern suggests that the larvae require one year to develop from an egg to the t h i r d instar and a further year to develop from the t h i r d instar to an adult. Adults began to emerge in the spring and were present in the stream u n t i l November. Pupae were not co l l e c t e d as they 47 Figure 7. Size-frequency histograms of Heterlimnius koebelei larva from May, 1979 to March, 1980 . NO- INDIVIDUALS PER 0-05 S3- M-NO- INDIVIDUALS PER 0-C6 SO- M-NO- INDIVIDUALS PER 0-OS SO- M-NO- INDIVIDUALS PER 0 - 0 5 SO- M-49 SEPTEMBER S 1379 OCTOBER 4 1373 SEPTEMBER 20 1379 NOVEMBER 1 1379 INSTAR INSTAR NO- INDIVIDUALS PER 0-CS SO- KI- NO- INDIVIDUALS PER O-CS SO- M-o r u * in to o 9 I1 £ 7 1 ? 9 51 Table 3. Head-capsule widths of l a r v a l instars of Heterlimnius koebelei from Spring Creek. Instar Head-capsule width (mm) 1 0.16-0.18 2 0.20 - 0.26 3 0.30 - 0.38 4 0.40 - 0.46 are t e r r e s t r i a l (Brown 1972). Qualitative s e r i a l data from previous studies have provided l i t t l e information on the l i f e history of Elmidae beetle species (Hynes 1970). 52 RESULTS Trout Abundance The August density of cutthroat trout in the downstream section of Spring Creek was 0.18 fish/m 2 or 1.17 g/m2 (Table 4). The trout population consisted of fry (age 0+), ranging from 4.0 to 6.0 cm in length and 0.5 to 1.5 g in weight; 1+ year old parr, ranging from 8.2 to 9.2 cm in length and 4.0 to 6.0 g in weight; and 2+ and 3+ year old adults, ranging from 9.5 to 14.3 cm in length and 8.0 to 28.0 g in weight (Figure 8). The re l a t i o n between wet weight (W gm) and fork length (L cm) was determined to be: In W = -5.08 + 3.15 In L based on 18 trout from 5.0 to 14.3 cm fork length (r 2= 0.97). In mid-August therefore, I placed six trout (8.7 - 13.2 cm fork length) into the experimental enclosure (density = 0.3 fish/m 2 or 4.1 g/m2). But trout "disappeared" from the enclosure at a rate of one to two per week. E l e c t r o - f i s h i n g the area upstream and downstream of the enclosure f a i l e d to locate the missing f i s h . Although i t i s possible the trout were escaping from the enclosure and dispersing far downstream, I suspect that predators (eg. raccoons, Procyon lotor) also were removing trout from the enclosure. Beginning in mid-September therefore I stocked the enclosure with eight trout. Trout were added approximately 53 F i g u r e 8. Frequency histogram of fork lengths of c u t t h r o a t t r o u t from S p r i n g Creek at Road F-30, August 1979. Trout were c o l l e c t e d by e l e c t r o f i s h i n g a 360 m2 s t r e t c h of stream. 54 CUTThRDAT TROUT SPRING CJ^EK > IS- „ 10. i 2 - 1 B- 1 J 6 - 1 4- 1 I I i E - 5 4 . 0 6 - 0 8 - 0 1 0 . 0 1 2 - 0 1 4 . 0 1 6 - 0 +H—I FGRK LEMJTH CCM) 55 Table 4. Density and standing crop of c u t t h r o a t t r o u t i n S pring Creek at Road F-30, August 1979. Density Standing Crop (no. / sq. m) (gm wet-wt/ sq. m) Fry (0+) Parr and A d u l t s (1+ - 3+) 0.10 0.08 0.07 1.10 T o t a l P o p u l a t i o n 0.18 1.17 every two weeks to r e p l a c e any that had disappeared. Trout used in the experiment ranged in fork l e n g t h from 8.7 to 14.7 cm and from 6.0 to 28.5 gm i n weight. The estimated mean abundance of t r o u t i n the e n c l o s u r e d u r i n g the experiment was 0.28 f i s h / m 2 or 4.4 g/m2. T h i s abundance was 3 to 4 times that estimated i n the downstream s e c t i o n of Spring Creek. The t r o u t l o s t weight dur i n g the experiment (Table 5). Only two t r o u t gained weight dur i n g a time i n t e r v a l , both in the October 11 to 30„ time p e r i o d . T h i s suggests that the a v a i l a b l e food supply was not s u f f i c i e n t to meet the minimum maintenance requirements of the t r o u t d u r i n g the experiment. 56 Table 5. Mean weight of the trout in the experimental stream section that survived u n t i l the end of each time period. Mean weight of trout (gm wet-wt) Time Period n Beginning End Aug 22 - Sept 1 1 Sept 11 - Sept 25 Sept 25 - Oct 11 Oct 11 - Oct 30 Oct 30 - Nov 20 2 5 7 4 5 12.8 16.0 15.7 13.8 12.5 12.3 14.6 14.6 14.0 12.1 Food Habits of Trout Analysis of the stomach contents suggested that Nemoura  c inctipes, Paraleptophlebia d e b i l i s and Cinygmula sjo.A were important prey species of the trout during the experiment. Paraleptophlebia d e b i l i s was present in the diet of the trout throughout the experiment; Cinygmula sjo.A was present u n t i l they emerged from the stream in mid-October and Nemoura  c inct ipes was present from late September u n t i l the end of the experiment (Table 6). At least one of these three species occurred in 88% of the trout stomachs. Together these three species accounted for 42% of the t o t a l number of organisms present in the trout stomachs. Since the next most abundant taxa were chironomids (20%) and ceratoponogids (18%), Nemoura  c inct ipes, Paraleptophlebia d e b i l i s and Cinygmula sjo.A probably accounted for at least a similar proportion of the biomass consumed by the trout. 57 Table 6. Occurrence of benthic insects in the trout stomach contents. Sampling Date Sept 11 Sept 25 Oct 11 Oct 30 Nov 20 n= 4 5 7 4 5 Nemoura cinctipes - P P P P c a l i f o r n i c a P 0 Triznaka diversa 0+ cohort - - - -1+ cohort P Paraleptophlebia sp. A P P d e b i l i s P P P P P Cinygmula sp. A P P P 0 0 sp. B P Glossosoma penitum - - - P . -Heterlimnius koebelei -n= number of trout sampled P= presence of species in stomach contents 0= absence of species from the benthos and stomach contents -= absence of species in stomach contents The remaining insect populations constituted only a small numerical proportion of the d i e t . Heter1imnius koebelei, the 0+ cohort of Triznaka diversa and the autumn-hatching cohort of Nemoura c a l i f o r n i c a apparently were not u t i l i z e d by the trout while Glossosoma penitum, the 1+ cohort of Triznaka diversa, Cinygmula sjo.B and Paraleptophlebia S JD .A were u t i l i z e d only occasionally during the experiment (Table 6). Together these 58 p o p u l a t i o n s accounted f o r l e s s than 7% of the t o t a l number of organisms in the stomach c o n t e n t s . Immature aqu a t i c i n v e r t e b r a t e s accounted f o r 95% of the organisms present i n the stomach contents of the t r o u t . Winged i n s e c t s , c h i e f l y a d u l t d i p t e r a n s and hemipterans but a l s o i n c l u d i n g subimagos of P a r a l e p t o p h l e b i a debi1 i s and Cinygmula sp_.A composed the r e s t . The t r o u t consumed only a f r a c t i o n of the t o t a l s i z e range of i n d i v i d u a l s of each s p e c i e s present i n the benthos (Table 7) suggesting that v u l n e r a b i l i t y to t r o u t p r e d a t i o n i s a f u n c t i o n of s i z e . Food Consumption by Trout Since Nemoura c a l i f o r n i c a , T r i z n a k a d i v e r s a , P a r a l e p t o p h l e b i a sp_.A, Cinygmula sp_.B, Glossosoma penitum and H e t e r l i m n i u s koebelei accounted f o r l e s s than 7% of the organisms i n the stomach contents of the t r o u t , only the d e n s i t y and standing crop of Nemoura c i n c t i p e s , P a r a l e p t o p h l e b i a debi 1 i s and Cinygmula sp_.A consumed by the t r o u t were estimated. The mean number and dry weight (mg) of Nemoura c i n c t i p e s , P a r a l e p t o p h l e b i a d e b i l i s and Cinygmula sp_.A per t r o u t stomach on each sampling' date are shown i n Table 8. The v a r i a t i o n i n the number of i n d i v i d u a l s per stomach may be p a r t i a l l y e x p l a i n e d by the d r i f t r a t e . A p o s i t i v e r e l a t i o n s h i p was found between the mean number of prey per stomach and the d r i f t r a t e of the prey (r=0 . 9 8 ; P<0.01), which suggests a dependence of 59 Table 7. The range of head-capsule widths of benthic insects in the benthos and trout stomachs between September 6 and November 29. Taxa Head-capsule width (mm) trout stomach benthos minimum maximum minimum maximum Nemoura 0. .48 1 , .02 0. 1 6 1 . .20 Triznaka . 0. ,60 0, .96 0. 1 6 1 , . 1 6 Paraleptophlebia 0. .50 1 , .00 0. 1 6 1 , . 1 4 Cinygmula 0. .88 1 , .60 0. 28 1 , .90 Glossosoma 0. .42 0, .42 0. 1 4 0, .56 Heterlimnius -- -- 0. 18 0, .44 the feeding rate on the abundance of prey in the d r i f t (Figure 9) . The t o t a l density of Nemoura cinctipes , Paraleptophlebia  d e b i l i s and Cinygmula sjo.A consumed by the trout during the experiment was estimated to be 198 individuals per m2 and the t o t a l standing crop to be 42 mg dry-weight per m2. Insect Density and Standing Crop Figures 10 to 12 display the changes in the mean population densities and standing crops in the control and experimental stream sections of the nine benthic species examined. Prior to the introduction of trout into the experimental stream section in mid-August, the mean population densities and standing crops of a l l species except Nemoura 60 Figure 9. Relationship between d r i f t rate of prey and the mean number of prey per trout stomach. Prey species consisted of N. c i n c t i p e s , P. d e b i l i s and C. sp.A. D r i f t was sampled for 5 hours beginning 3 hours before sunrise. Trout stomach contents were sampled 2-3 hours after sunrise. r = .98 , P<.01. EO-IS-1 4 . . . G- 1 61 + + + •4-50 - GO- 100 . 140- 1B0- 500 -ISO'. IfsOIVILXJALS IN CRIFT 62 Figure 10. Mean densities and standing crops of N. c a l i f o r n i c a , T. diversa, P. sp.A and G. penitum in the control C x - x T and experimental (*-•*) stream sections. V e r t i c a l l i n e s represent 1 SE. Cutthroat trout placed into the experimental stream section in mid-August. (next 5 pages) DENSITY OF INEM3JRA DVLIFORWICA 5 0 0 -1979 OATE 64 DENSITY OF TRIZNAKA • I VERSA CO rjD-TJRT) E D O -1979 OATE STAfSOIIVE CROP O F TRI2NAKA D I V E R S A (0+ CQHGRT) 4-0 _ 3*5 .. ' a 3 - ° -6 2-5..-7 2-0.. 1979 •ATE 100-D E N S I T Y D F T R I Z N A K A D I V E R S A (1+ C O H O R T ) 65 B 6 10.1 > Q J U A U EE 1979 D A T E DC NO DE S T A N D I N G C R O P D F T R I Z N A K A D I V E R S A C1+ C O H D R T ) 4.0 T 3.5 1 S5 3 ' ° T 8 ^ 5-5 .. 7 2-0 .. 1-5.. 13 Q i -tn 1-0 1 0.5 0-0 AU S E 1979 • A T E CC NO DE 66 •EJNGITY C F PARALEPTOR - LS IA S P - A GOO-1979 • A T E STANDING CROP C F PARALEP1 G P K . E B I A S P - A 10. T 9. 1 1979 •ATE: DENSITY DF GLDSSOSDMA PENITUM 100 • JM JU AU SE DC NO DE 1979 •ATE 68 Figure 11. Mean densities and'standing crops of N. cin c t i p e s , C. sp_.A and P. d e b i l i s in the control (x-x) and experimental T ± - * ) stream sections. V e r t i c a l lines represent 1 SE. Cutthroat trout placed into the experimental stream section in mid-August, (next 3 pages) 1379 •ATE DENSITY OF CINYLMJ_A SP-A 70 100-2 1379 DATE 5TANDING CROP CF CINYGMJLA SP-A 4-0 3-5 . . 2 JN JU All SE DC NO OE 1979 DATE STANDING L-T?DP CF PARALEPTOPHLEBIA DEBILIS 10. T 3-1 1 1 1 1 1 JU AU SE oc NO OE •ATE 72 Figure 12. Mean densities and standing crops of H. koebelei and C. S J D . B in the control (x-x) and experimental (>-A ) stream sections. V e r t i c a l l i n e s represent 1 SE. Cutthroat trout placed into the experimental stream section in mid-August, (next 2 pages) DENSITY DF CINYGMULA SP-B 74 100' DATE STANDING CROP CF CINYQAJLA SP-B 1-0 DATE 75 Table 8. Mean number and dry weight of Nemoura ci n c t i p e s , Paraleptophlebia d e b i l i s and Cinygmula sp A per trout stomach. Stomach contents were removed with a stomach pump 2-3 hr after sunrise on each date. Date mean number per stomach mean dry weight (mg) per stomach September 1 1 September 25 October 11 October 30 November 20 3 0 1 1 2 2 0 8 0 ,3 ,4 0.73 0.26 0.50 1 .83 0.53 ci n c t i p e s , were not s i g n i f i c a n t l y d i f f e r e n t (P>0.05) between stream sections. While the mean standing crops of Nemoura  cinctipes were not s i g n i f i c a n t l y d i f f e r e n t (P>0.05) between stream sections, the mean densities of Nemoura cinctipes were s i g n i f i c a n t l y higher (P<0.05) in the experimental stream section (Figure 11). The introduction of trout into the experimental stream section had l i t t l e e f fect on the abundance of Nemoura  c a l i f o r n i c a , Paraleptophlebia sp_.A, Glossosoma penitum or the 0+ or 1+ cohorts of Triznaka diversa (Figure 10). The mean population densities and standing crops of these species were not s i g n i f i c a n t l y d i f f e r e n t (P>0.05) between stream sections. The trout introduction however had a negative impact on the population dynamics of Nemoura cinc t i p e s , Paraleptophlebia  d e b i l i s and Cinygmula S J D . A (Figure 11). The mean population densities and standing crops of these species were 76 s i g n i f i c a n t l y lower (P<0.01) in the experimental stream section compared to the control section. The difference in mean density and standing crop between stream sections for each species i s shown in Table 9. These values were calculated by integrating the difference in the mean population densities and standing crops of each population between control and experimental stream sections from mid-August to the end of November. Together, the density of the three species in the experimental stream section decreased 270 individuals per m2 and the standing crop decreased 53 mg dry weight per m2 more than in the control section during, the experiment. Table 9. Difference in mean density and standing crop of N. cinctipes , P. d e b i l i s and C. sp.A between stream sections during the experiment Density Standing Crop (no./sq. m) (mg dry-wt/sq. m) Nemoura cinctipes 142 12.8 Paraleptophlebia d e b i l i s 98 24.2 Cinygmula sp.A 30 16.0 F i n a l l y , the trout introduction had a posit i v e effect on the population dynamics of Heterlimnius koebelei and Cinygmula 77 sjo.B (Figure 12). The mean population densities and standing crops of Heterlimnius koebelei were s i g n i f i c a n t l y higher (P<0.001) in the experimental stream section; the mean densities of Cinygmula sjo.B were also s i g n i f i c a n t l y higher (P<0.05) in the experimental stream section but the mean standing crops were not s i g n i f i c a n t l y d i f f e r e n t (0.06>P>0.05) between stream sections. Insect Product ion The trout introduction had no measurable ef f e c t on the production of any insect population. Production estimates for each l o t i c species were not s i g n i f i c a n t l y d i f f e r e n t (P>0.05) between stream sections (Table 10). Production estimates for Heterlimnius koebelei could not be calculated as the cohorts could not be distinguished. D r i f t More than 80% of the individuals of each species were colle c t e d in the f i r s t 2 hours of d r i f t sampling (3-1 hr before sunrise), indicating that these species were nocturnal d r i f t e r s . Small individuals (head capsule width less than 0.48 mm) of each species were present in the d r i f t during the experiment. As these individuals were underrepresented in the d r i f t samples and were generally not preyed on by the trout (Table 7), they were removed from the data before the d r i f t rates of each species were compared between stream sections. 78 Table 10. Estimates of production (mg dry-wt. / sq. m) of l o t i c species from August to December, 1979 in the control and experimental stream sections of Spring Creek. UR = upstream r i f f l e ; P = pool; DR = downstream r i f f l e . Control Experimental UR P DR UR P DR Nemoura c inctipes c a l i f o r n i c a 1 70 5 80 4 204 <2 1 60 3 38 1 0 146 6 Triznaka diversa 0+ cohort <2 <2 <2 <2 <2 <2 1+ cohort 32 32 24 28 38 36 Paraleptophlebia sp. A 370 266 316 268 250 348 d e b i l i s 208 1 58 1 96 192 1 24 1 20 Cinygmula sp. A 54 68 90 54 48 54 sp. B Glossosoma 1 3 4 8 1 4 5 10 penitum 10 1 2 1 2 1 2 1 6 12 Variation in the d r i f t rates observed for each species during the experiment was p a r t i a l l y due to the variation in the discharge rate of the stream (Figure 13). Unfortunately, only the d r i f t rates of Nemoura cinctipes could be compared between the stream sections because at least on one sampling date in the other species the difference in the d r i f t rate between the stream sections was zero which reduced the sample size below f i v e , the minimum sample size necessary for the test . The d r i f t rate of Nemoura c inct ipes was not s i g n i f i c a n t l y d i f f e r e n t (P>0.05) between stream sections. The trout introduction thus 79 had no observable effect on the d r i f t rate of large individuals (head capsule width greater than 0.48 mm) of any species (Figure 13). The r e l a t i v e l y low d r i f t rate of these species though, made the detection of a difference d i f f i c u l t . 80 Figure 13. Total number of individuals d r i f t i n g from the control and experimental stream sections. Shaded histograms represent d r i f t from the control stream section. D r i f t was sampled for 5 hours beginning 3 hours before sunrise on each date. Stream discharge on each date was 4.2, 51.5, 11.6, 9.8, 117.7, and 55.4 1/sec respectively, (next 3 pages) ^£M•JRA CALIFCRSIICA 81 25. B z n If] 33. 15- 1 i i i i i i i i i i i i i i i i i i NEM3JRA CINCTIFE5 t B a o . | z Lfl 15-H io- J, > ^—H-T R I Z N A K A D I V E R S A 25. 20- 1 1 15- I 10. I I I I I I I I I I I | [ | 1 | i FT1 i AUG SEPT SEPT OCT OCT NOV 11 25 11 30 20 •ATE NO- INDIVIDUALS IN DRIFT NO- INOIVIUDUALS IN DRIFT NO- INDIVIDUALS IN DRIFT o ^ S K S Q i o ' « 8 K 8 111 o oi ' B K S W * • • • « . « • • • « . • • • » . . 03 ro ND« INDIVIDUALS IN DRIFT NO- INDIVIDUALS IN DRIFT o w 6 K S (H o u, S E 8 N 4 1 1 1 1 1 • 4 1 1 1 1 1 03 CO 84 DISCUSSION Effect of Trout Predation on the Abundance of Lotic Populations Selective predation by f i s h has been shown to be an important component of the population dynamics of l e n t i c invertebrates (Northcote et a l . 1978; Hall et a l . 1970; Macan 1966). While studies have indicated stream-dwelling trout to be selective predators ( E l l i o t t 1967; G r i f f i t h 1974; Allen 1981), none of these have considered the e f f e c t of t h i s predation on l o t i c populations. Results of this study suggest that trout, through their feeding a c t i v i t y , can af f e c t the abundance of l o t i c insect populations. The density and standing crop of Nemoura  c inct ipes, Paraleptophlebia d e b i l i s and Cinygmula sjo.A in the experimental stream section decreased, compared to the control, after the trout were introduced (Figure 11). At the start of the experiment more than 90% of the individuals of Cinygmula sjo.A and 50% of the individuals of Paraleptophlebia d e b i l i s were large enough to be considered as prey for the trout. I considered the individuals of a species to be vulnerable to predation i f they were larger than the smallest individual of that species found in the trout stomach contents (Table 8). While individuals of Nemoura cinct1pes were not vulnerable to predation u n t i l September, by the end of the experiment a l l individuals of these three populations had been vulnerable to predation. The d r i f t data indicated that large individuals (head 85 capsule width greater than 0.48 mm) of Nemoura cinc t i p e s , Paraleptophlebia d e b i l i s and Cinygmula sp_.A were present in the d r i f t . In fact, Paraleptophlebia d e b i l i s and Nemoura cinctipes had the highest d r i f t rates (number coll e c t e d per 5 hours) of the species examined. As cutthroat trout are dri f t - f e e d e r s ( G r i f f i t h 1974; Brocksen et a l . 1968), only species present in the d r i f t would be available to the them. Although the d r i f t rate of each species was not measurably d i f f e r e n t between stream sections, a s i g n i f i c a n t reduction in the combined d r i f t rate of these three species in the experimental stream section was observed (Wilcoxon paired sample test, w=1; P<0.05), which suggests that the trout were removing large individuals from the d r i f t without d i f f e r e n t i a t i n g species. Recruitment of young into each of these populations had stopped before the start of the experiment (Figures 2,4,5). Thus the loss of individuals to predation could not be compensated by the recruitment of new individuals. Stomach analysis suggested that these three species were important in the diet of the trout. Trout were estimated to have consumed approximately 74% of the observed difference in mean density and 77% of the observed difference in mean standing crop of these three species between the stream sections during the experiment. Thus trout predation probably was a major cause of the decrease in the density and standing crop of each of these prey populations in the experimental stream section. The trout introduction had l i t t l e a f f e c t on the density 8 6 and standing crop of the autumn-hatching cohort of Nemoura  c a l i f o r n i c a , the 0 + and 1+ cohorts of Triznaka diversa, the winter cohort of Glossosoma penitum and Paraleptophlebia sp.A in the experimental stream section (Figure 1 0 ) . Individuals of Nemoura c a l i f o r n i c a and the 0 + cohort of Triznaka diversa were r e l a t i v e l y small throughout the experiment and not vulnerable to predation. Stomach analysis confirmed that these species were not part of the diet of the trout. Individuals of Glossosoma penitum were not vulnerable to predation u n t i l October and at the end of the experiment less than 5 0 % of the population was large enough to be considered as prey, whereas approximately 9 0 % of the individuals of the 1+ cohort of Triznaka diversa were vulnerable to predation throughout the experiment. Individuals of these populations though were not available to the trout as large individuals were generally absent from the d r i f t (Figure 13). Only when stream discharge was high, were individuals of Triznaka diversa found in the d r i f t and individuals of both species found in the trout stomach contents. Individuals of Paraleptophlebia S J D . A f i r s t became vulnerable to predation in late September and by November 5 0 % of the population was vulnerable. Large individuals were available in the d r i f t from October u n t i l the end of the experiment (Figure 13) and were present in the diet of the trout during this time (Table 6 ) . Trout predation had no s i g n i f i c a n t effect on th i s population u n t i l near the end of the experiment. The marked decrease in standing crop in the 87 experimental stream section in late November suggests that the trout, by feeding on the larger instars from th i s population, may have begun to reduce the standing crop of th i s species at the close of the experiment (Figure 10). Trout predation also had l i t t l e d i r e c t impact on the density and standing crop of Heterlimnius koebelei and Cinygmula sjo.B in the experimental stream section. Heterlimnius  koebelei was not present in the diet of the trout, probably because of the small size of the species. Cinygmula sjo.B was present in the stomach contents of the trout only in late November, when individuals of the population had just become vulnerable (Figure 5) and available (Figure 13). The density and standing crop of Heterlimnius koebelei and Cinygmula sjo.B in the experimental stream section increased, compared to the control section, after the trout introduction. Recruitment of new individuals into each population began approximately at the same time as the start of the experiment (Figures 5 and 7). The increase in density and standing crop of each population, due either tp increased survivorship of young or to reduced emigration, may have been a result of the removal of competitors by the trout ( i e . competitive release). Heter1imnius koebelei, Cinygmula spp., Nemoura cinctipes and Paraleptophlebia d e b i l i s are a l l d e t r i t i v o r e s ( B r i t t a i n 1973; Chapman and Demory 1963; Merritt and Cummins 1978). Thus Heterlimnius koebelei and Cinygmula sjo.B may have benefited from the reduced abundance of their potential competitors: Nemoura c inct ipes, Paraleptophlebia d e b i l i s and Cinygmula sp.A. 88 Alt e r n a t i v e l y , the increase in density and standing crop of Heterlimnius koebelei and Cinygmula sp_.A may have been due to greater egg-deposition in the experimental stream section. Unfortunately, I have no data on egg densities of any species. The eggs of both species were probably deposited before the enclosure was constructed. Thus the enclosure should not have affected the deposition of eggs by the adults of either species. The d r i f t data indicated that the abundance of adults of Heterlimnius koebelei was similar in both stream sections (Figure 13). This suggests that deposition of eggs for this species may have been similar in each stream section. Trout predation therefore can affect the abundance of stream insect populations. Selective predation upon the larger late instars of l o t i c species decreased the density and standing crop of prey populations through increased mortality and may have i n d i r e c t l y increased the density and standing crop of non-prey populations through competitive release. Size-selective predation of stream-dwelling trout has been demonstrated in the laboratory (Ringler 1979) and shown to occur in nature (Allen 1981). If trout select prey from the d r i f t primarily on the basis of size then they would passively switch (Murdock 1969) from one prey species to another as the r e l a t i v e abundance of large prey species in the d r i f t changed during the year. This could account for the seasonal change in the diet of trout•observed here and reported by others (Narver 1972; Allen 1981). Thus trout could p o t e n t i a l l y affect the abundance of numerous l o t i c populations throughout the year. 89 Hall et a l . (1970) found that ponds with f i s h " produced fewer emergent insects than ponds without f i s h . Whether trout predation actually decreased the number of emerging adults of Cinygmula sjo.A, which emerged from the stream in mid-October or Paraleptophlebia d e b i l i s , which emerged between late October and the end of the experiment, i s unknown as emergence data from each stream section was not c o l l e c t e d . However the decreased density of nymphs in the experimental stream section and the presence of emerging subimagos of both species in the stomach contents of the trout suggests that trout predation may have decreased the number of emerging adults of both species. This suggests that trout predation could reduce the number of eggs deposited by a species, p o t e n t i a l l y reducing the density of the following generation. Trout Predation and Insect Production Allen (1951) and Horton (1961) reported that stream-dwelling trout consumed 9 to 150 times the mean standing crop of their prey. As the annual turnover r a t i o (production/biomass) of most benthic invertebrates l i e s between 3 and 7 (Waters 1977), these estimates of prey consumption to prey abundance are c l e a r l y wrong. My results show that trout consumed only 0.4 times the mean standing crop or 9 - 10% of the combined production of Nemoura c inct ipes, Paraleptophlebia d e b i l i s and Cinygmula sp.A during the experiment. The trout therefore consumed only a small fraction of the t o t a l production of their prey in the 90 experimental stream section. The loss of weight by the trout during the experiment suggests that the available food supply was i n s u f f i c i e n t to meet the minimum maintenance requirements of the trout. Although food was apparently abundant, i t appears that the food was not available to the trout. As noted previously, only d r i f t i n g invertebrates were available to the cutthroat trout since the trout are d r i f t -feeders. Thus any factor that e f f e c t s the d r i f t rate of prey w i l l a f f e c t the a v a i l a b i l i t y of prey to the trout. An important factor found to effect the d r i f t rate of invertebrates i s stream discharge (Pearson and Kramer 1972; Chapman and Bjourn 1969; E l l i o t t 1967). The d r i f t rate of prey species in thi s study were observed to be p o s i t i v e l y correlated with stream discharge (Figure 13). The low stream discharge during most of the study therefore, may have reduced the a v a i l a b i l i t y of prey to the trout as the feeding rate of the trout seemed to be dependent upon the d r i f t rate of prey (Figure 9). Only during the last half of October when the stream discharge and d r i f t rate were high, were the trout able to feed s u f f i c i e n t l y well to meet their minimum maintenance requirements. D r i f t rates have been reported to be both s u f f i c i e n t (Mundie 1974; Bishop and Hynes 1969; Warren et a l . 1964) and i n s u f f i c i e n t (Jenkins et a l . 1970; Allen 1981) to support the trout populations found in streams. Possibly, the discharge rate at the time the d r i f t measurements were made, effected the conclusions of these studies. 91 The behavior of the prey i s another factor that may have reduced their a v a i l a b i l i t y to the trout. Large instars of the prey species reported here d r i f t e d primarily at night. A tendency for stream invertebrates to d r i f t nocturnally as they increase in size has been reported in a number of studies (Allen 1978; Steine 1972; Fjellheim 1980). Allen (1978) suggested that this tendency was a predator avoidance adaptation since the risk of predation increases with si z e . Thus, by d r i f t i n g at night, large instars may reduce their a v a i l a b i l i t y to visually-feeding predators. Waters and Hokenstrom (1980) and Bishop and Hynes (1969) have estimated that only 10-20% of the production of l o t i c invertebrates i s available in the d r i f t . If t h i s i s generally true and applies to the present study then the trout were heavily grazing the available food supply since they consumed 9-10% of the production of their main prey species. Mean production of each of the 3 main prey species, Nemoura  c inct ipes, Paraleptophlebia d e b i l i s and Cinygmula sjo.A, was lower in the experimental stream section compared to the control section (Table 10), but these differences were 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 (P>0.05). 92 Predation and Community Structure Connell (1975) suggested that predation is of primary importance in determining community structure. Studies of i n t e r t i d a l and l e n t i c ecosystems, u t i l i z i n g manipulation experiments and comparisons of natural communities with and without the presence of certain species, support t h i s view (see Connell 1975 for examples). While studies comparing stream insect communities with and without trout have suggested that trout predation may be important in structuring l o t i c insect communities (Allan 1975; Straskraba 1965), stream manipulation experiments have not been conducted to v e r i f y these observations. Results of this study suggest that trout predation is important in determining the structure of l o t i c insect communities. Trout appear to be "strong interactors" (sensu Paine 1980) within stream communities in much the same way as s t a r f i s h in ro c k y - i n t e r t i d a l communities (Paine 1966, 1974) and f i s h in l e n t i c communities (Hall et a l . 1970; Macan 1966). Trout, by feeding on the larger individuals available in the d r i f t , reduced the abundance of large individuals in the benthos which may have caused the complementary response of the smaller species (Heterlimnius koebelei, Cinygmula sjo.B) . Thus trout predation may have suppressed the expression of otherwise strong competitive interactions between the species. Further investigation i s c l e a r l y needed to establish the "strength" (sensu Paine 1980) of competitive linkages between l o t i c spec ies. 93 APPENDIX 1 Insect Density Correction Factors Unfortunately, correction factors for each head-capsule width size class could only be calculated for three of the six genera since small instars of Cinygmula and Glossosoma penitum were not present in the benthos when the samples were co l l e c t e d and the number of individuals of Heterlimnius koebelei was i n s u f f i c i e n t to calculate r e l i a b l e values (Table A1.1). Table A1.1 Number of individuals in each instar of Heterlimnius koebelei and Glossosoma penitum coll e c t e d in each c o l l e c t i n g net of a double bag sampler from Spring Creek, March, 1980. Heterlimnius koebelei: Instar 1 2 3 4 Collec t i n g Net 0.471 mm 5 2 2 1 0.116 mm 3 0 0 0 t o t a l 8 2 2 1 Glossosoma penitum: Instar 1 2 3 . 4 5 Collec t i n g Net 0.471 mm - - 5 27 1 1 0.116 mm - - 1 0 0 t o t a l - - 6 27 1 1 A linear relationship was assumed for each genus between head capsule width and the proportion of individuals c o l l e c t e d 94 in the 0.471 mm c o l l e c t i n g net (Figure A1.1). A l l individuals of Nemoura with a head capsule width equal to or greater than 0.50 mm and a l l individuals of T. diversa and Paraleptophlebia with a head capsule width equal to or greater than 0.54 mm were retained in the 0.471 mm c o l l e c t i n g net. The abundance of individuals of Nemoura with a head capsule width less than 0.16 mm and the abundance of individuals of T. diversa and Paraleptophlebia with a head capsule width less than 0.20 mm Table A1.2. Density correction factors for each head-capsule width size class of Nemoura, Triznaka and Paraleptophlebia. Taxa Nemoura Triznaka Paraleptophlebia Size class (mm) 0.16 - 0.20 0.20 - 0.24 0.24 - 0.28 0.28 - 0.32 0.32 - 0.36 0.36 - 0.40 0.40 - 0.44 0.44 - 0.48 0.48 - 0.52 0.52 -16.7 4.9 2.8 2.0 1 .5 1 .3 1 . 1 1 .0 1 .0 1 .0 30.0 6.3 3.4 2.4 1 .8 1 .5 1 .2 . 1 .0 20.0 5.9 3.4 2.4 1 .9 1 .6 1 .3 1 . 1 1 .0 were not corrected. The density correction factors for each head capsule width size class for each genus are given in Table A1 .2. 95 Figure A1.1. Proportion of individuals of Nemoura, Triznaka and Paraleptophlebia retained in the 0.471 mm c o l l e c t i n g net of the double bag sampler. A l l individuals of Nemoura with a head-capsule width > 0.48 mm and a l l individuals of Triznaka and Paraleptophlebia with a head-capsule width > 0.52 mm were retained in the 0.471 mm c o l l e c t i n g net. 96 NEKOLRA 5 ,0. B ° - S -0-1 . C O . HEAD CAPSLLE WIDTH lO-CWIvM SIZE CLASSES) T R I Z N A K A 1 1 .0 S 0 -S W 0-7 I 2 3 °'5 j i 0 , 3 s *— 0-1 jij 0 - 0 HEAD CAPSULE WIDTH CO-CMMM SIZE CLASSES) PARALEPTOPHLEBIA •S i-o 0-9 3 Z B °- S i 5 h- 0 . 1 ^ 0-0 HEAD CAPSULE WIDTH CO-CWKW SIZE CLASSES) 97 APPENDIX 2 Relationship between Head Width and Dry Weight of Aquatic Insects With the exception of Heterlimnius koebelei, the slopes of the functional regressions were not s i g n i f i c a n t l y d i f f e r e n t (P>0.05) between taxa (Table A2.1). Individuals of Heterlimnius  koebelei weighed s i g n i f i c a n t l y more then individuals of other taxa at a l l siz e s . The increased weight of Heterlimnius  koebelei larvae was probably a result of mineral accumulations on the exoskeleton of the larva (Brown 1970). Table A2.1. Values of the regression constants log a and b, with 95% confidence in t e r v a l s , from functional regressions of dry weight (mg) on head-capsule width (mm). n i s the number of individuals; r is the correlation c o e f f i c i e n t . A l l c o r r e l a t i o n c o e f f i c i e n t s were s i g n i f i c a n t at 0.1% l e v e l . n log a b 95% CI I Nemoura 23 -0. 35 2. .76 0. 19 0. .98 Tr iznaka 19 -0. 31 2, .75 0. 1 5 0. .99 Paraleptophlebia 20 -0. 35 2. .84 0. 1 7 0. .99 Cinygmula 16 -0. 72 2, .69 0. 19 0. .99 Glossosoma 16 0. 21 2, .75 0. 26 0. .97 Heterlimnius 1 3 1 . 09 3, .92 0. 37 0. .97 APPENDIX 3 Insect Taxa L i s t for Spring Creek Order Family Spec ies Stoneflies Nemouridae Chloroper1idae Perlodidae Leuctridae Peltoperlidae Perlidae Capni idae Pteronarcyidae Nemoura c a l i f o r n i c a N. cinctipes N. oregonensis N. f r i g i d a N. producta N. cataractae Sweltsa coloradensis Triznaka diversa Skwala p a r a l l e l Isoperla Despaxia augusta Moselia infuscata Yoraperla brevis Calineuria Claassenia Capnia Pteronarcys Mayflies Leptophlebiidae Paraleptophlebia debil Paraleptophlebia sp A Heptageniidae Cinygmula sp A Cinygmula sp B Rhithrogena Epeorus Cinygma Baetidae B a e t i s (2 s p e c i e s ) S i p h l o n u r i d a e Ameletus Ephemerellidae Ephemerella (2 spe c i e s ) C a d d i s f l i e s Glossosomatidae Hydropsychidae Lepidostomatidae H y d r o p t i l i d a e Rhyacophilidae Polycentropodidae B r a c h y c e n t r i d a e Philopotamidae Limnephi1idae Glossosoma penitum Hydropsyche Lepidostoma H y d r o p t i l a Oxyethr i a Rhyacophila Polycentropus Microsema Wormaldia Neophylax Apatania C r y p t o c h i a Onocosmoecus E c c l i s o m y i a B e e t l e s Elmidae H e t e r l i m n i u s k o e b e l e i Narpus Lara 100 101 LITERATURE CITED Allan, J.D. 1975. The d i s t r i b u t i o n a l ecology and d i v e r s i t y of benthic insects in Cement Creek, Colorado. Ecology 56:1040-1053. Allan, J.D. 1978. Trout predation and the size composition of stream d r i f t . Limnology and Oceanography 23:1231-1237. All a n , J.D. 1981. Determinants of diet of brook trout in a mountain stream. Can. J. Fish. Aquat. S c i . 38:184-192. Allen, K.R. 1951. The Horokiwi stream: a study of a trout population. New Zealand Marine Dept. Fisheries B u l l . 10. 238p. Anderson, N.H. and J.R. Bourne. 1974. Bionomics of three species of glossosomatid caddis f l i e s in Oregon. Can. J. Zool. 52:405-411. Bishop, J.E. and H.B.N. Hynes. 1969. Downstream d r i f t of the invertebrate fauna in a stream ecosystem. Arch. Hydrobiol. 66:56-90. B r i t t a i n , J.E. 1973. The biology and l i f e cycle of Nemoura  a v i c u l a r i s . Freshwater Biology 3:199-210. Brocksen, R.W.,G.E. Davis and C E . Warren. 1968. Competition, food comsumption, and production of sculpins and trout in laboratory stream communities. J. of Wildl. Mgmt. 32:51-75. Brown, H.P. 1970. Aquatic dryopid beetles of the United States. Biota of freshwater ecosystems i d e n t i f i c a t i o n manual no. 6. Wat. P o l l . Conf. Res. Ser. E.P.A., Washington, D.C. 82p. Chapman, D.W. and R.L. Demory. 1963. Seasonal changes in the food ingested by aquatic insect larvae and nymphs in two Oregon streams. Ecol. 44:140-146. 1 02 Chapman, D.W. 1968. Production. In: Methods for assessment of f i s h production in fresh waters. W.E. Ricker ed. pp. 182— 196. Blackwell S c i . Publ., Oxford. Chapman, D.W. and Bjourn. 1969. Dis t r i b u t i o n of salmonids in streams, with special reference to food and feeding. In: Symposium on salmon and trout in streams. Pages 153-176. T.G. Northcote, ed. H.R. MacMillan Lectures in Fisheries, Vancouver: Univ. B r i t i s h Columbia. Connell, J.H. 1970. A predator-prey system in the marine i n t e r t i d a l region. I. Balanus glandula and several predatory species of Thais. Ecological Monographs 40:49-78. Connell, J.H. 1975. Some mechanisms producing structure in natural communities: a model and evidence from f i e l d experiments. Pages 460-490. In: M.H. Cody and J.M. Diamond, eds. Ecology and evolution of communities. Harvard University Press, Cambridge, Massachusetts, USA. Corkum, L.D., P.J. Pointing and J.J.H. Ciborowski. 1977. The influence of current v e l o c i t y and substrate on the d i s t r i b u t i o n and d r i f t of two species of mayflies. Can. J. Zool. 55:1970-1977. E l l i o t t , J.M. 1967. Invertebrate d r i f t in a Dartmoor stream. Arch. Hydrobiol. 63:202-237. E l l i o t t , J.M. 1967. The food of trout in a Dartmoor stream. J. Appl. Ecol. 4:59-71. E l l i o t t , J.M. 1972. Rates of gastric egestion in brown trout. Freshwater Biology 2:1-18. E l l i o t t , J.M. 1977. Some methods for the s t a t i s t i c a l analysis of samples of benthic invertebrates. S c i . Publ. Freshwat. B i o l . Assoc. 25. 160p. E l l i o t t , J.M. and L. Persson. 1978. The estimation of dai l y rates of food consumption for f i s h . J. Anim. Ecol. 47:977-991. 1 03 Fjellheim, A. 1980. Differences in d r i f t i n g of l a r v a l stages of Rhyacophila nubila. Holarctic Ecol. 3:99-103. Fox, L.R. 1977. Species richness in streams: an alternate mechanism. Amer. Nat. 111:1017-1021. G r i f f i t h , J.S. J r . 1974. U t i l i z a t i o n of invertebrate d r i f t by brook trout and cutthroat trout in small streams in Idaho. Trans. Amer. Fish. Soc. 103:440-447. H a l l , D.J., W.E. Cooper and E.E. Werner. 1970. An experimental approach to the production dynamics and structure of freshwater animal communities. Limnology and Oceanography 15:839-928. Harper, P.P. 1973. L i f e h i s t o r i e s of Nemouridae and Leuctridae in Southern Ontario. Hydrobiol. 41:309-356. Harper, P.P. and H.B.N. Hynes. 1970. Diapause in the nymphs of Canadian winter s t o n f l i e s . Ecology 51:925-927. Horton, P.A. 1961. The bionomics of brown trout in a Dartmoor stream. J. Animal Ecol. 30:311-338. Howmiller, R.P. 1972. Effects of preservative on weights of some common macrobenthic invertebrates. Trans. Amer. Fish. Soc. 101:743-746. Hynes, H.B.N. 1970. The ecology of running waters. Univ. Toronto Press, Toronto. 555p. Jenkins, T.M. J r . , C.R. Feldmeth and G.V. E l l i o t t . 1970. Feeding of rainbow trout in re l a t i o n to abundance of d r i f t i n g invertebrates in a mountain stream. J. Fish . Res. Bd. Can. 27:2356-2361. Kerst, CD. and N.H. Anderson. 1974. Emergence patterns of Plectoptera in a stream in Oregon, U.S.A. Freshwater Biology 4:205-212. 1 04 Kerst, CD. and N.H. Anderson. 1975. The plecoptera community " of a small stream in Oregon, U.S.A. Freshwater Biology 5:189-203. Lehmkuhl, D.M. and N.H. Anderson. 1970. Observations on the biology of Cinygmula r e t i c u l a t a in Oregon. Pan-Pac. Entomol. 46:268-274. Lehmkuhl, D.M. and N.H. Anderson. 1971. Contributions to the biology and taxonomy of the Paraleptophlebia of Oregon. Pan-Pac. Entomol. 47: 85-93. Macan, T.T. 1966. The influence of predation on the fauna of a moorland f i s h pond. Arch. Hydrobiol. 61:432-452. McFadden, J.T. 1961. A population study of the brook trout. Wildl. Monogr. 7. 73p. McKone, W.D. 1975. Quantitative studies of stream d r i f t with pa r t i c u l a r reference to the McLay model. PhD Thesis, Univer. B r i t i s h Columbia. Meeham, W.R. and R.A. M i l l a r . 1978. Stomach flushing: effectiveness and influence on survival and condition of juvenile salmonids. J. Fish. Res. Bd. Can. 35:1359-1363. Menge, B.A. 1976. Organization of the New England rocky i n t e r t i d a l community: role of predation, competition and environmental hetergeneity. Ecological Monographs 46:355-393. Me r r i t t , R.W. and K.W. Cummins. 1978. An introduction to the aquatic insects of North America. Kendall/Hunt Publ. Co., Dubuque, Iowa. 441 p. Mundie, J.H. 1974. Optimmization of the salmonid nursery stream. J. Fish. Res. Bd. Can. 31:1827-1837. Murdock, W.W. 1969. Switching in general predators: Experiments on predator s p e c i f i c i t y and s t a b i l i t y of prey populations. Ecological Monographs 39: 335-354. 105 Narver, D.W. 1972. A survey of some possible e f f e c t s of logging on two Eastern Vancouver Island Streams. Fis h . Research Bd. Can. Tech. Report No. 323. N e i l l , W.E. and A. Peacock. 1980. Breaking the bottleneck: interactions of invertebrate predators and nutrients in oligotrophic lakes. Amer. Soc. Limno. and Oceanogr. Spec. Symp. 3. Northcote, T.G., C.J. Walters and J.M.B. Hume. 1978. I n i t i a l impacts of experimental f i s h introductions on the macrozooplankton of small oligotrophic lakes. Verh. Internat. Verein. Limnol. 20:2003-2012. Paine, R.T. 1966. Food web complexity and species d i v e r s i t y . Amer. Nat. 100:65-75. Paine, R.T. 1974. I n t e r t i d a l community structure: experimental studies on the relationship between a dominant competitor and i t s p r i n c i p a l predator. Oecologia 15:93-120. Paine, R.T. 1980. Food webs: Linkage, interaction strength and community infrastructure. J. of Anim. Ecol. 49:667-685. Pearson, W.D. and R.H. Kramer. 1972. D r i f t and production of two aquatic insects in a mountain stream. Ecological Monographs 42:365-385. Rabeni, C.F., and G.W. Minshall. 1977. Factors a f f e c t i n g microdistribution of stream benthic insects. Oikos 29:33-43. Radford, D.S. and R. Hartland-Rowe. 1971. The l i f e cycle of some stream insects (Ephemeroptera, Plecoptera) in Alberta. Can. Ent. 103:609-617. Resh, V.H. 1979. Sampling v a r i a b i l i t y and l i f e history features: Basic considerations in the design of aquatic insect studies. J . Fish. Res. Bd. Can. 36:290-310. 1 06 Resh, V.H. 1977. Habitat and substrate influences on population and production dynamics of a stream caddisfly, Ceraclea  ancylus. Freshwater Biology 7:261-277. Ricker, W.E. 1975. Computations and interpretations of b i o l o g i c a l s t a t i s t i c s of f i s h populations. Dept. of the Environment, Fisheries and Marine Service, Canada. B u l l . 191. 382p. Ringler, N.H. 1979. Prey selection by d r i f t feeding brown trout. J . Fish. Res. Bd. Can. 36:392-403. Smock, L.A. 1980. Relationships between body size and biomass of aquatic insects. Freshwater Biology 10:375-383. Standford, J.A. 1972. A centrifuge method for determining l i v e weights of aquatic insect larvae, with a note on weight loss in preservatives. Ecology 54:449-451. Steine, I. 1972. The number and size of d r i f t i n g nymphs of Ephemeroptera, Chironomidae and Simuliidae by day and night in the River Stranda, Western Norway. Norw. J. of Entom. 19:127-131. Straskraba M. 1965. The effect of f i s h on the number of invertebrates in ponds and streams. Verh. Internat. Verein. Limnol. 13:106-127. Warren, C.E., J.H. Wales, G.E. Davis and P. Doudoroff. 1964. Trout production in an experimental stream enriched with sucrose. J. of Wildl. Mgmt. 28:617-660. Waters, T.F. 1972. The d r i f t of stream insects. Annu. Rev. Entomol. 17:253-272. Waters, T.F. and G.W. Crawford. 1973. Annual production of a stream mayfly population: A comparison of methods. Limnology and Oceanography 18:286-296. Waters, T.F. and J.C. Hokenstrom. 1980. Annual production and d r i f t of the stream aphipod, Gammarus pseudolimnaeus in Valley Creek, Minnesota. Limnology and Oceanography 25:700-710. 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            data-media="{[{embed.selectedMedia}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.831.1-0095228/manifest

Comment

Related Items