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The effect of age and environmental factors on the vertical migration and distribution of Chaoborus flavicans… Teraguchi, Mitsuo 1964

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THE EFFECT OF AGE AND ENVIRONMENTAL FACTORS ON THE VERTICAL MIGRATION AND DISTRIBUTION OF CHAOBORUS FLAVICANS (MEIGEN) LARVAE by Mitsuo Teraguchi B i S c , University of B r i t i s h Columbia, 1962 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of Zoology We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October, 1964 In 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 of the requirements f o r an advanced degree at the U n i v e r s i t y of • B r i t i s h Columbia, I agree that the 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 reference and study* I f u r t h e r agree that per-m i s s i o n f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i -c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission*. Mitsuo Teraguchi Department of Znnl ngy , The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8f Canada Date August 28, 1964 ABSTRACT The effect of age and some environmental factors, especially l i g h t , on the v e r t i c a l migration and d i s t r i b u t i o n of Chaoborus flavicans larvae were studied both i n the f i e l d and i n the laboratory at Corbett Lake, B r i t i s h Columbia during the summer of 1963. Dist r i b u t i o n and migration of Chaoborus larvae were studied largely by frequent horizontal Clarke-Bumpus plankton tows made at 1 metre inte r v a l s from the surface almost to the maximum depth of the lake. Marked differences were noted i n daytime v e r t i c a l d i s t r i b u t i o n and d i e l migration of 5 size (or age) classes of larvae. These size classes probably corresponded approximately to l a r v a l i n s t a r s . Class 0 and 1 larvae inhabited the epilimnion i n the daytime throughout the summer, while class 4 larvae were largely confined to the hypolimnion during the day. Class 2 and 3 larvae occupied the epl-, meta-, and hypolimnion i n the daytime during June and July, but were found c h i e f l y i n the hypolimnion during August and September. Only the older larvae (class 2, 3 and 4) underwent marked d i e l v e r t i c a l migration which consisted of 4 phases: l ) daydepth, 2) ascent from day-depth to the surface, 3) gradual descent from surface, 4) rapid descent during dawn. The ascent occurred when subsurface l i g h t was rapidly diminishing at dusk, while the descent took place during darkness and was most marked when l i g h t started to penetrate the subsurface layers during dawn. Seasonal changes i n timing of ascent and descent appeared to be correlated to seasonal changes i n time of disappearance of subsurface l i g h t i n t e n s i t y during dusk. The rates of ascent and descent calculated from the analysis of echo traces were 13.6 and 1.1 m/hr respectively. Further analysis of the echo traces revealed that the Chaoborus scattering layer was i n contact with the lake basin during day-time and descent, but not during ascent. Results from observations of l a r v a l migration i n experimental tubes housed i n a dark room corroborated those of the f i e l d . Class 2 larvae having similar daytime v e r t i c a l d i s t r i b u t i o n (surface and 5 m) as class 0 and 1 larvae underwent v i r t u a l l y no d i e l v e r t i c a l migration i n the tubes, while class 2 and 3 larvae taken from the deeper layers (10-14 m) of the lake did. The d i e l migration consisted of the same 4 phases observed i n the f i e l d , as well as a "dawn r i s e " phase which was p a r t i c u l a r l y evident for class 3 larvae. Complete migration cycles were induced by a r t i f i c i a l l y changing the natural l i g h t i n t e n s i t y over an experimental tube during the period of r e l a t i v e l y constant l i g h t (0900-1900 hours); the larvae responded most markedly to changes i n l i g h t i n t e n s i t y at the 0-1000 lux range. Experiments indicated that the d i e l v e r t i c a l migration of Chaoborus larvae i s an exogenous rhythm controlled by l i g h t . ACKNOWLEDGEMENTS This study has been supported by the B r i t i s h Columbia Fis h and Game Branch and the Institute of Fisheries, Univer-s i t y of B r i t i s h Columbia. The advice and stimulating c r i t i c i s m given by Dr. T. G. Northcote during the study and i n preparation of the manuscript have been greatly appreciated. The invaluable assistance of T. G. Halsey, R. D. Humphreys and E. R. Zyblut deserves special thanks. The occasional assistance of P. Gallagher, G. Hazelwood and D. L. McKay i n sorting plankton samples has been appreciated. The use of property and accommoda-ti o n of the late Mr. Gibson Shaler and Mrs. Eve Shaler has been most h e l p f u l . The c r i t i c a l reading of the thesis by Dr. I. E. Efford, Dr. K. Graham and Dr. C. C. Lindsey i s g r a t e f u l l y acknowledged. i i i TABLE OP CONTENTS Page Abstract i Table of Contents i i i L i s t of Figures v L i s t of Tables • . . . . . . • • i x L i s t of Appendices x Acknowledgements . . . . . . . . . . x i i Introduction . . . . . . . . . . . . . . . . . . 1 Description of the Study Area 3 Physical and Chemical Features . . . . . . . . . . . 3 Bi o l o g i c a l Features . . . . . . . . . . . . 3 Materials and Methods . . 4 Part 1. F i e l d Studies 4 Part 2. Laboratory Studies 9 Results . . . . . . . 12 Part 1. F i e l d Studies 12 Larval Size (or Age) Classes 12 The V a l i d i t y of the Larval Size (or Age) Classes as Instars 13 Abundance of Larval Groups and th e i r Daytime V e r t i c a l D i s t r i b u t i o n . . . . . . . . . . . . . 16 Seasonal Variation i n Horizontal D i s t r i b u t i o n . 17 D i f f e r e n t i a l Migratory Behavior of the Larval Classes . . . . . . . . 19 Migration Pattern of the Older Larvae . . . . . 24 Other Aspects of the Diel V e r t i c a l Migration . . 29 i v Page Part 2. Laboratory Studies 32 Die l V e r t i c a l Migration Under Experimental Conditions . . . . . . . . . . . . . . . . . . 32 Role of Light on Diel V e r t i c a l Migration . . . 39 Discussion . . . . . . . • • • . . • • • . • • • . . . . 43 Seasonal Variation i n Horizontal Distribution... . . 43 The Effect of Age and Environmental Factors on V e r t i c a l D i s t r i b u t i o n . . . . . . . . . . . . . . . 44 The Effect of Age and Environmental Factors on Diel V e r t i c a l Migration 46 Theoretical Interpretation of the Diel V e r t i c a l Migration . . . . . . . . . . . . . . . . 52 Bio l o g i c a l Significance of the Diel V e r t i c a l Migration . . . . . . . . . . . . . . . 53 Literature Cited 55 Appendices . . . . . . . . . . 59 V LIST OP FIGURES Figure Page 1. Map of Corbett Lake showing contour lin e s i n metres, location of sampling Station 1, Cross Section A, i n l e t and outlet streams 5 2* A* The darkroom used to study experimentally the d i e l v e r t i c a l migration of Chaoborus flavicans larvae. B. Cross sectional view of the darkroom showing the arrangement of the p l a s t i c tubes used to hold the Chaoborus flavicans larvae 11 3. The length frequency d i s t r i b u t i o n of the Chaoborus flavicans larvae collected during the 1100 hour sampling periods of the 24 hour f i e l d series of June 16-17, July 20-21, August 17-18 and September 21-22, 1963. The v e r t i c a l arrows indicate means, while the v e r t i c a l bars mark the points of overlap of the calculated theoretical normal curves . . . . . . . . . 14 4. The daytime v e r t i c a l d i s t r i b u t i o n and r e l a t i v e abundance of the l a r v a l classes of C. flavicans i n Corbett Lake based on samples collected during the 1100 hour sampling periods of the 24 hour f i e l d series of June 16-17, July 20-21, August 17-18 and September 21-22, 1963; oxygen and temperature conditions are also shown for each period 18 VI Figure Page 5. The v e r t i c a l d i s t r i b u t i o n of class 0, 1, 2, 3 and 4 larvae of C. flavicans i n Corbett Lake during the 1100, 2100 and 0500 hour periods of the June 16-17 and July 20-21 series, 1963 . . . . . . . . . . . . . * . . • » 22 6. The v e r t i c a l d i s t r i b u t i o n of class 0, 1, 2, 3 and 4 larvae of C. flavicans i n Corbett Lake during the 1100, 2100 and 0500 hour periods of, the August 17-18 and September 21-22 series, 1963 ,23 7. The d i e l v e r t i c a l migration of class 2, 3 and 4 larvae of C. flavicans during September 12-22 series of 1963. The broken l i n e s represent the 14.8°, 12° and 6° C isothe rms; the s o l i d l i n e s represent the 1.0 lux isolume . . . . . . 26 8. The d i e l v e r t i c a l migration of the C. flavicans larvae i n Corbett Lake during June 16-17, July 20-21, August 17-18 and September 21-22 series, 1963. Surface temperature and 10°, 5°, 4° and 3.5° C temperature depths are shown. The 0.1, 1, 10 and 100 lux isolumes are indicated with s o l i d l i n e s 28 9. Echo sounding traces across Corbett Lake at Cross Section A, October 1-2, 1962 (differences vxx Figure Page i n length of trace caused by varia t i o n i n length of run or boat speed; background noise may be ignored). The arrows indicate the " r e s i -due" layer. Time expressed i n hundred hours (P a c i f i c Standard Time) . . 31 10. D i e l v e r t i c a l migration of class 2 larvae (collected from the surface and 5 m depth of Corbett Lake) i n adjacent experimental tubes during August 13-14, 1963. Light i n t e n s i t i e s at the surface of both tubes were i d e n t i c a l , as were temperatures at the surface, 100 cm depth and bottom 34 11. Diel v e r t i c a l migration of class 3 larvae i n the experimental tube during July 16-17, 1963 at Corbett Lake. Surface l i g h t i n t e n s i t i e s (measured with the submarine photometer) and temperatures s t surface, 100 cm depth and bottom of the tube are shown. Probable surface l i g h t i n t e n s i t i e s for .1900, 2000 and 2100 hours ( i f Photovolt Photometer was used) are indicated with s o l i d dots joined with a s o l i d l i n e . . . . 35 12. Diel v e r t i c a l migration of class 2 and 3 larvae i n adjacent experimental tubes during August 16-17, 1963* Light i n t e n s i t i e s at the surface of both tubes were identical,as were temperatures at the surface, 100 cm depth and bottom 36 v i i i Figure Page 13. Diel v e r t i c a l migration of class 2 and 3 larvae i n adjacent experimental tubes during . September 13-14, 1963. Light i n t e n s i t i e s at the surface of both tubes were identical,as were temperatures at the surface, 100 cm depth, and bottom 37 14. The v e r t i c a l movements of class 3 larvae i n adjacent control (exposed to naturally changing l i g h t conditions) and non^control (exposed to a r t i f i c i a l l y changing l i g h t conditions) experimental tubes on August 9, 1963. Identical temperature conditions i n both tubes; i d e n t i c a l l i g h t i n t e n s i t y over both tubes at the beginning, and end of experiment. 41 15. The v e r t i c a l movements of class 3 larvae i n adjacent control (exposed to naturally changing l i g h t conditions) and non-control (exposed to a r t i f i c i a l l y changing l i g h t conditions) experi-mental tubes on September 28, 1963. Identical temperature conditions i n both tubes; i d e n t i c a l l i g h t i ntensity over both tubes at the beginning and end of experiment 42 LIST OP TABLES Table P§,ge 1. Comparison of the monthly t o t a l v e r t i c a l hauls taken at randomly selected stations with No. 10 Wisconsin net, at Corbett Lake i n 1963 20 X LIST OP APPENDICES Appendix Page I Determination of the c a l i b r a t i o n value of the Clarke-Bumpus sampler towed at several depths for a 61 m distance at boat speed of 2.5 knots 59 I I Volume of water passed through and the number of Chaoborus larvae caught by the Clarke-Bumpus sampler (with No. 10 net attached) towed at several depths for a 61 m distance at a boat speed of 2.5 knots 60 I I I Comparison of the volume of water i n the compartments of the subsampler using different t o t a l volumes 61 IV Test of r e l i a b i l i t y of the subsampler, using di f f e r e n t volume of water and diffe r e n t number of larvae. A 0.05 significance l e v e l was used. 62 V Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l larvae i n the l a r v a l classes for samples taken during the 24 hour f i e l d series of June 17-18, .1963. . 65 VI Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l larvae i n the l a r v a l classes for samples taken during the 24 hour f i e l d series of July 20-21, 1963. . . . 67 XI Appendix Page VII Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l larvae i n the l a r v a l classes for samples taken during the 24 hour f i e l d series of August 18-19, 1963 . . . . . . . 69 VIII Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l larvae i n the l a r v a l classes for samples taken during the 24 hour f i e l d series of September 21-22, 1963 71 IX Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l counts of larvae i n samples taken during the 24 hour f i e l d . s e r i e s of June 16-17, 1963 . . . . 76 X Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l counts of larvae i n samples taken during the 24 hour f i e l d series-of July 20-21, 1963 . . . . 78 XI Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l counts of larvae i n samples taken during the 24 hour f i e l d series of August 18-19, 1963 . . . 80 INTRODUCTION The larvae of the dipteran genus Chaoborus (or Corethra) are macroplankters frequently inhabiting ponds ( M i a l l , 1895; Krogh, 1911) and lakes (Muttkowski, 1918; Juday, 1921; Rawson, 1930; Eggleton, 1932; Berg, 1937; M i l l e r , 1941; Deonier, 1943; Lindquist and Deonier, 1943; Davis, 1955; Dendy, 1956; food, 1956; Woodmanse and Grantham, 1961). They are ea s i l y recognized by t h e i r transparent bodies and black paired a i r sacs. Although there are several species i n the genus, they have similar l i f e h i s t o r i e s . The l a r v a l stage l a s t s usually for about 6-7 weeks (sometimes as long as a year), during which time the animals undergo 4 or possibly 5 instars (Muttkowski, 1918; Deonier, 1943; MacDonald, 1956). In general the larvae feed on organisms ranging from phytoplankton to aquatic insects. Deonier (1943) has shown however that the food habits of Chaoborus  astictopus d i f f e r with i n s t a r s , the l a s t 2 - 3 stadia preferring cladocerans and copepods. The most s t r i k i n g aspect of the ecology of Chaoborus i s a marked d i e l v e r t i c a l migration, a characteristic of many planktonic organisms. Typically the migration cycle involves an ascent of daytime benthic larvae into the limnetic zone (to or near the lake surface) about sunset, and a descent which begins i n the following early morning hours and i s completed about dawn (Juday, 1921; Berg, 1937; Davis, 1955; Wood, 1956; Hamilton, 1961; Woodmanse and Grantham, 1961). A t y p i c a l l y i t consists of an ascent of larvae, inhabiting the deep layers of the lake (but not adjacent to the bottom) during the daytime, to the upper strata during the night and a subsequent descent to the lower strata during the early morning hours (Dendy, 1956). In both cases the migrating larvae must encounter changes i n pressure, dissolved gases (especially oxygen), as well as steep temperature gradients i n the summer i f the lake i s eutrophic. Few studies have been made on the effects (both d i e l and seasonal) of age and environmental factors on the basic migration pattern of Ghaoborus larvae. Several workers have shown that younger larvae remain i n the limnetic zone, while the older larvae only temporarily inhabit i t at night (Eggleton, 1932; Berg, 1937; MacDonald, 1956; Yfood, 1956; Woodmanse and Grantham, 1961). The age and size at which t h i s marked change in' migratory behaviour occurs, or the factors responsible for i t s timing, have not been determined i n any d e t a i l . The purpose of t h i s study was to examine the effect of age and several environmental factors on the v e r t i c a l migration and d i s t r i b u t i o n of Chaoborus larvae. Such a study may contribute additional knowledge to the phenomenon of v e r t i c a l migration by planktonic organisms, a subject which has been extensively reviewed (Cushing, 1955; Hardy, 1956; Bainbridge, 1961; Raymont, 1963). 3 DESCRIPTION OP THE STUDT AREA Physical and Chemical Features Corbett Lake i s located on the southern i n t e r i o r piateau of B r i t i s h Columbia at an elevation of 1068 m and about 15 km southeast of M e r r i t t . The lake has a surface area bf 24.2 hectares, a mean depth of 6i8 m and a maximum depth of 19 m. I t becomes thermally s t r a t i f i e d early i n the season with usually no measurable amount of oxygen i n the hypolimnion (below 8m). The lower layers of the lake contain H^S. Vernal and autumnal c i r c u l a t i o n do occur, but are not always complete because the lake i s protected from wind action. The lake has a dissolved s o l i d content of 336 parts per m i l l i o n . The outlet stream at the south-west corner of the lake and an i n l e t entering the north-east end (Fig. l ) flow only during early spring. The bottom of the l i t t o r a l zone extends to a depth of about 4.5 m and i s covered with "marl" and dense shoals of chara. The benthal of the limnetic region i s covered with "marl" and soft black mud. Bi o l o g i c a l Features The lake contains only stocked populations of rainbow trout Salmo gairdneri and brook trout Salvelinus f o n t i n a l i s which are occasionally subject to w i n t e r k i l l . The dominant organisms inhabiting the l i t t o r a l zone are the amphipod Hyallela azteca, chironomid larvae and gastropod 4 Gyraulus sp. (Humphreys, 1964). Numerous plankters inhabit the limnetic zone. Daphnia pulex and Daphnia rosea are dominant cladocerans, while Diaptomus leptopus and Diaptomus nudus are the common copepods. Chaoborus flavicans, C. americanus and C. nyblaei were present i n Corbett Lake with C. flavicans being by far the most abundant (about 96$ of the individuals sampled). MATERIALS AND METHODS Part 1. F i e l d Studies The a i r and lake temperatures were measured using a Cole-Palmer (Model 8425) thermistor with a rapid responding probe. Temperature series were taken only at station 1, the deepest part of the lake. The surface and subsurface l i g h t i n t e n s i t i e s were measured at station 1 with a submarine photometer (Model 15-M-02/1-G.M. Manufacturing Co.) equipped with Weston Photronic Photoelectric "deck" and "sea" c e l l s . Light i n t e n s i t i e s recorded i n micro-ampere units were converted to foot-candles using a Photovblt (Model 200) photometer calibrated d i r e c t l y i n foot-candles. Cloud cover conditions, wind directions and wind v e l o c i t i e s were recorded at each sampling period. Water samples taken monthly with a Kemmerer bottle before or after each 24 hour sampling.series were analyzed for oxygen using an unmodified Winkler Method. A Furuno (Model F-701) 200-kc/sec Sounder was used to make echo traces. In 1962 echo traces were made at station 1 FIGURE 1. Map of Corbett Lake showing contour lines i n metres, location of sampling Station 1, Cross Section A, i n l and outlet streams* and Cross-section A (Fig. l ) using a gain of about 5.75 while the boat was moving at about 1.5 knots (2.9 km/hr). In 1963 traces were made only at station 1 using a gain of 6 and a boat speed of about 0.5 knots (2.8 km/hr). Traces were taken usually before and after plankton sampling at station 1 and about every 15 minutes during dawn and dusk at.Cross Section A. The scattering layers on the traces have been shown by Northcote (1964) to be largely Chaoborus larvae. A Clarke-Bumpus sampler f i t t e d with a No. 10 (0.13 mm) nylon netting was attached to a 4 mm diameter wire towing cable. A 13.6 kgm torpedo-shaped lead weight was t i e d to the end of the cable. Station 1 was the s i t e of plankton sampling i n both 1962 and 1963. The sampler was towed at each sampling depth (surfac and every metre almost to the bottom) for 0.5 or 1 minute. Appropriate corrections were made for wire angle. After each tow the net was washed by splashing water on i t , while the sampler bucket was cleaned by water squirted from a rubber syringe. Samples were preserved i n 10fo formalin solution. Twenty-four hour sampling series were carried out i n August and September, 1962 and usually once a month during the summer (June-September) i n 1963. Six sampling periods (every 4 hours) were carried out during the 24 hour series i n 1962, while 8 were usually maintained during each series i n 1963. In the l a t t e r year sampling was carried out every 4 hours during the daytime (0800-1900 hours - P a c i f i c Standard Time) and every 2 hours at dawn and dusk periods (0400-0600 " hours and 1900-2300 hours). For both years echo traces, l i g h t , temperature and oxygen measurements were made during each sampling period. F i e l d c a l i b r a t i o n of the sampler was attempted i n 1963 i n a similar manner to that described by Clarke and Bumpus (1950). The sampler was towed for a 61 m distance at precisely 2.5 knots. T r i p l i c a t e tows were made at each desired depth (surface 2, 4, 6, 8, 10 and 12 m - uncorrected depths); t h i s was done with and without a No. 10 net attached to the sampler. As the c a l i b r a t i o n values for the sampler with and without the net were unusually high (Appendix 1), a 4.1 l i t e r s / r e v o l u t i o n value extrapolated from the study of Xentsch and Duxsbury (1956) was used. Although a comparable volume of water passed through the sampler (with No. 10 net attached) during t r i p l i c a t e tows, the t r i p l i c a t e samples obtained did not i n a l l cases contain com-parable numbers of Chaoborus larvae (Appendix I I ) . The results suggested that the v a r i a b i l i t y was not due to inconsistency i n operation of the sampler, but rather to the larvae being clumped i n nature. Total counts were made for a l l samples collected i n 1962 and for most of those obtained i n 1963. Estimated counts using a subsampler modified from Elgmork (1959) were made for 1963 samples containing 2000-4000 larvae. The r e l i a b i l i t y of the subsampler was examined using a chi-square test for randomness (Lund et a l , 1958). The subsampler gave random subsamples (Appendices I I I and IV). One-sixth of a sample was usually 8 taken for estimating t o t a l counts, while l / 2 , l/8 or 1/36 portions were taken for determining length of larvae. Total and estimated counts were made for some samples to check accuracy.of estimates. The distance ( i n mm) between the posterior end of the thoracic a i r bladders and the anterior t i p of the abdominal bladders was measured i n accordance with recommendations of Dr. G. G. E. Scudder. The larvae were l a i d on the slides on th e i r r i g h t side so that the body between the anterior and posterior sacs was extended f u l l y , but not stretched. Thirty to s i x t y larvae were placed on a standard microscope slide and covered fi r m l y with a cover s l i p almost equal i n dimensions to the slide before adding water. The larvae were i d e n t i f i e d to species using the key of Cook (1956). Separation of the three species present were based on the presence and location of an antenna! spine, shape of the prelabral appendage, and position of the basal mandibular tooth. A Wisconsin plankton net having an upper ring diameter of 23 cm and f i t t e d with No. 10 nylon net was used i n 1963 to study the horizontal d i s t r i b u t i o n of the Chaoborus larvae i n the limnetic zone. Ten t o t a l v e r t i c a l hauls were made each month (June-September) at ten randomly selected stations where the depth was about 14 m. Samples were preserved i n 10$ formalin solution. A l l larvae were counted i n each sample* A.test of randomness (Kutkuhn, 1958) was used to determine the type of horizontal d i s t r i b u t i o n characteristic of the 9 larvae during the summer. Part 2. Laboratory Studies. Experiments were carried out at Cprbett Lake i n a darkroom (3 x 3 x 3 m) having a wooden frame covered with black poly-ethylene sheets (Pig. 2A).. Two p l a s t i c tubes (2 m length, 14.5 cm inside diameter and 3.2 mm thick wall) marked off i n 20 cm depth intervals were placed inside on a table so that the i r tops (open ends) just protruded out through holes made i n the roof (Fig. 2B). This arrangement made i t possible to study the migration under natural and controlled l i g h t conditions. Two shutters made from furnace vents were used to a l t e r the l i g h t i n t e n s i t y over the tubes. A sheet of wax paper was placed inside each shutter to diffuse the incoming l i g h t . Usually one l i g h t shutter was f i t t e d over the top of the tube, while the other was placed over the search unit of the photometer. Two l i g h t recorders were used usually i n each experiment. The "deck" c e l l of the submarine photometer (Model 15-M-02/1-G.M. Manufacturing Co.) was placed on the roof of the dark room, while the "sea" c e l l was used inside to record the l i g h t gradient along the p l a s t i c tube. A Photovolt (Model 514 M) photometer f i t t e d with phototube C and neutral density f i l t e r s was placed on the ground about 15 m from the dark room to obtain l i g h t readings. The l i g h t intensity recorded by the photometer at ground l e v e l was assumed to represent that at the surface of the tubes. When the surface l i g h t over the experimental tubes was changed with the shutter apparatus, 10 i t was measured at ground l e v e l with the Photovolt photometer whose search unit was covered with the other shutter having the same degree of closure. The thermistor described previously was used to record temperature at every 10 cm depth i n t e r v a l i n the tube. Unfortuna t e l y i t was not possible to control water temperature i n the experimental tubes as the dark room was not insulated and the tops of the tubes were exposed to the outside. The unmodified Winkler Method was used to measure the experimental oxygen conditions. Usually water samples from the surface, 100 cm depth and the bottom were analyzed. Lake water f i l t e r e d with a No. 10 net was used i n a l l the experiments. A dim f l a s h l i g h t was used to count l a r v a l d i s t r i b u t i o n i n the tubes at night. One-hundred fresh larvae collected from desired depths of the lake with the Clarke-Bumpus sampler were used i n every experiment. Diel v e r t i c a l migration experiments were attempted i n July, August and September of 1963. Each experimental 24 hour series consisted of 16 observation periods; every 2 hours during daytime and every hour during dusk and dawn. Each observation consisted of counting l a r v a l d i s t r i b u t i o n for every 20 cm depth i n t e r v a l , measuring l i g h t i n t e n s i t y ( i f necessary^, and recording water temperature. Water samples were taken at the end of each experiment. Larvae used were preserved i n 10$ formalin solution. FIGURE 2A. The darkroom used to study experimentally the d i e l v e r t i c a l migration of Chaoborus flavicans larvae. FIGURE 2B. Cross sectional view of the darkroom showing the arrangement of the p l a s t i c tubes used to hold the Chaoburus flavicans larvae. 12 The importance of l i g h t i n con t r o l l i n g v e r t i c a l migration of the larvae was investigated i n August and, September of 1963. One tube used as a control was exposed to naturally changing l i g h t conditions, while the other was exposed to a r t i f i c i a l l y changing l i g h t i n t e n s i t i e s . The experiments were carried out during the period of r e l a t i v e l y constant l i g h t conditions (0900-1800 hours). Observations were made every 15-30 minutes. After recording the l a r v a l d i s t r i b u t i o n at a given l i g h t l e v e l , the int e n s i t y was changed and measured. Temperatures were measured for both tubes before and after each experiment, while water samples were taken only after f i n i s h i n g the experiment. Larvae used i n the experiments were preserved. RESULTS Part 1. F i e l d Studies Larval Size (or Age) Classes Large numbers (about 2878) of larvae were i d e n t i f i e d and measured to the nearest one-tenth mm bladder-bladder length i n order to determine the number and length ranges of l a r v a l size classes. The animals used were taken from samples collected i n 1963 during the 1100 hour sampling period of the June, July, August and September f i e l d series. A l l larvae of small samples ( 60 larvae) were measured and i d e n t i f i e d , but only fractions of the large samples were examined. Chaoborus flavicans was by far the p r i n c i p a l species (96$ of the larvae sampled), so other species (C. americanus and C. 13 nyblaei) were not considered i n the study* The length frequency data of each month were subjected to probit analysis (Cassie, 1954) to determine objectively the means representing size classes. These means were then used to calculate the corresponding theoretical normal curves using the method described by Snedecor (1957). The points of overlap between adjacent curves were used to determine the length ranges. I t was assumed therefore that each class would contribute to and gain from the adjacent class (or classes) similar numbers of larvae. The smallest and largest lengths of each class were determined for each month and were averaged to obtain length ranges of the classes. During the summer of 1963 there were 5 size (or age) classes as indicated by v e r t i c a l arrows representing means (Pig. 3). A l l 5 occurred on June 17, whereas only 4 were present during the remainder of the summer. The f i r s t 4 classes were present on July 20 and August 18, while the l a s t 4 occurred on September 21. The average bladder to bladder length ranges of class 0, 1, 2, 3, and 4 were respectively 0.87-.157, 1.58-2.77, 2.78-4.39, 4.40-6.11 and 6.12-6.51 mm. The V a l i d i t y of the Larval Size (or Age) Classes as Instars Since an unconventional measuring method (bladder-bladder length) has been used, the question arises as to whether these size classes represent actual in s t a r s . The number and length ranges of l a r v a l instars have not been determined for C. flavicans. MacDonald (1956), using the conventional head capsule measurement, has established the existence of four FIGURE 3. The length frequency d i s t r i b u t i o n of the Chaoborus flavicans larvae collected during the 1100 hour sampling periods of the 24 hour f i e l d series of June 16-17, July 20-21, August 17-18 and September 21-22, 1963. The v e r t i c a l arrows indicate means, while the v e r t i c a l bars mark the points of overlap of the calculated theoretical normal curves. 14 15 instars for two t r o p i c a l species, C. anomalus and C. species B. Deonier (1943), using another recognized method (modifica-t i o n of the mouth parts and anal f i n ) , has established four instars for C. astictopus of Clear Lake, C a l i f o r n i a . He also mentions that the overwintering larvae of the Clear Lake gnat are s i g n i f i c a n t l y larger than those of the l a s t i n s t a r ; an observation similar to that made by Muttkowski (1918)for C. punctipennis of Lake Mendota, Wisconsin. I t appears therefore that temperate Chaoborus larvae can develop through f i v e instars which may be represented by f i v e l a r v a l age (or size) groups based on the body length measurement. As the average t o t a l length measurement of class 0 larvae (determined by bladder-bladder length measurement) i s similar to that (about 1.75 mm average length) of newly hatched C. flavicans larvae measured by Berg (1937), l a r v a l class 0 must represent the f i r s t i n s t a r . S i m i l a r l y the average t o t a l lengths of class 3 and 4 larvae were similar to those (10.8 and 11.2 mm mean length) of two oldest l a r v a l groups ever encountered by Berg i n Esrom Lake, Denmark. As Berg states that larvae having 11.2 mm mean length overwintered, they might represent a f i f t h i n s t a r , thus making the fourth l a r v a l class of Corbett Lake equivalent to the f i f t h i n s t a r . By similar reasoning the t h i r d l a r v a l class present i n Corbett Lake may be considered the fourth i n s t a r . Therefore the seasonal changes i n abundance of the l a r v a l age groups may be attributed to animals undergoing instar changes. 16 Abundance of Larval Groups and th e i r Daytime V e r t i c a l D i s t r i b u t i o n The data used to determine the numbers and average ranges of the size (or age) classes were analyzed i n greater d e t a i l to show the abundance and daytime v e r t i c a l d i s t r i b u t i o n of the l a r v a l groups on a d i e l and seasonal basis. Larval fragments having heads were t o t a l l e d , and appropriate numbers of them were a l l o t t e d to each class. A l l larvae encountered were assumed to be C. flavicans as the inclusion of very few of the other species would not affect the results for the major species. The abundance of larvae at each sampling depth were expressed i n numbers per 100 1; the actual or estimated t o t a l counts were divided by the volume of water supposedly passed through the sampler during each tow. Confidence l i m i t s were calculated for a l l t o t a l estimated counts (Appendices V, VI, VII and V I I I ) . The r e l a t i v e abundance of larvae i n each class changed with the progression of summer i n 1963 (Fig. 4). Class 0 and 1 larvae, having their greatest abundance i n June, decreased markedly i n numbers u n t i l v i r t u a l l y none were present by September. Class 2 animals decreased s l i g h t l y seasonally, while the class 3 larvae increased i n abundance. The few class 4 larvae became s l i g h t l y more abundant by September. There were marked differences i n the v e r t i c a l d i s t r i b u t i o n of larvae with respect to size during the daytime (Fig. 4). The majority of the class 0 and 1 larvae inhabited the surface to 8 m zone (the warmer, oxygenated epi- and metalimnion) during the entire summer, while the few class 4 larvae occupied 17 the 8 to 13 m region. Conversely the class 2 and 3 larvae underwent marked seasonal changes i n daytime v e r t i c a l d i s t r i b u t i o n . The class 2 animals occuppied the surface to 12 m zone on June 17 and July 20 with maximum density occurring at 3 m i n June and somewhat i n July (Fig. 4). In contrast most of them inhabited the 7 to 12 m region on August 18 and September 21 with greatest abundance at about 10 m on both occasions. The class 3 larvae showed a trend similar to that of class 2 animals (Fig. 4). They occuppied the surface to 12 m zone on June 17 with maximum density occurring at about 11 m. In contrast most of the class 3 larvae inhabited the 7 to 13 m (or perhaps deeper) region on July 20, August 18, and September 21 with greatest abundance being at about 10.5 m. The seasonal s h i f t i n daytime v e r t i c a l d i s t r i b u t i o n of class 2 and 3 larvae was not correlated to changes i n l i g h t penetration. The depth at which l i g h t could no longer be measured with the submarine photometer remained at about 15— 16 m during the 4 months. Seasonal Variation i n Horizontal D i s t r i b u t i o n Because the analysis of t r i p l i c a t e samples taken at each of several depths i n 1962 (August 12) and 1963 (June 16) revealed that the Chaoborus larvae might have a clumped horizontal d i s t r i b u t i o n , a sampling design of Ricker (1938) was used during the summer of 1963 to study t h i s aspect more extensively. Ten t o t a l v e r t i c a l hauls were taken during the FIGURE 4. The daytime v e r t i c a l d i s t r i b u t i o n and r e l a t i v e abundance of the l a r v a l classes of C. flavicans i n Corbett Lake based on samples coTlected during the 1100 hour sampling periods of the 24 hour f i e l d series of June 16-17, July 20-21, August 17-18 and September 21-22, 1963; oxygen and temperature conditions are also shown for each period. N O I Q O 2 0 0 P E R I O O L O I O O Si 4 L C L A S S O 1 0 0 2 ? ° 3 , ' 7 3 ? 7 « P P M ° -T E M P A N D O X Y G E N 10 2 0 o N O PER I O O L O I O O •I C L A S S I O I O O °c o 1 P P M . 2 ' j /" / / / .' ' / ' T E M P J U N E 10 2 0 / ' ' T E M P C O S P P M JULY 10 2 0 / / T E M P / AUGUST 1 0 0 2 0 0 O I O O \ 1 0 0 2 0 0 I O O 2 0 0 O I O O C L A S S 2 C L A S S 3 O I O O 2 7 6 3 0 O O I O O 2 0 0 C L A S S 4 O I O O \ O I O O c 4 P P M 10 2 0 I O O 2 0 0 O I O O 2 0 0 S E P T E M B E R 19 late afternoon or early evening of each month (1942-1636 hours on June 16, 1835-1830 hours on July 20, 1903-1955 hours on August 17 and 1738-1836 hours on September 21). The variance over mean r a t i o value of 1,88 calculated with the method of Kutkuhn (1958) was used to determine whether the larvae showed a clumped (negative-binomial) or random (Poisson) d i s t r i b u t i o n . The variance over the mean r a t i o calculated from the sampling data revealed that the larvae had a clumped horizontal d i s t r i b u t i o n on each sampling!;.day, but the degree of clumping increased during the summer (Table 1). The results may indicate a seasonal increase i n the aggregation behavior of the larvae. D i f f e r e n t i a l Migratory Behavior of the Larval Classes Samples collected during the 1100, 2100 and 0500 hour sampling periods of the monthly 24 hour f i e l d series (June 17-18, July 20-21, August 18-19 and September 21-22) of 1963 were used to study behavior of the different l a r v a l classes. As 1100, 2000 and 0500 hour periods were chosen to represent times of daytime d i s t r i b u t i o n , maximum ascent (except i n September) and maximum descent respectively, any substantial migration undergone by a l a r v a l class would be detected i n samples taken during these periods; The confidence l i m i t s of estimated t o t a l counts were calculated (Appendices V, VI, VII and V I I I ) . The younger larvae (class 0 and l ) underwent v i r t u a l l y 20 TABLE I. Comparison of the monthly t o t a l v e r t i c a l hauls taken during late afternoon or early evening at randomly selected stations with No. 10 Wisconsin net at Corbett Lake i n 1963. Haul No. June 16 July 20 August 17 September 1 418 251 199 167 2 422 235 205 145 3 436 271 210 214 4 480 167 108 232 5 433 220 139 124 6 447 298 234 137 7 442 297 292 95 8 356 298 156 85 9 459 294 165 111 10 447 249 155 121 Total 4340 2580 1863 1431 Mean (x) 434 258 186.3 143.1 Sum of Squares Variance (S ) 9632 16570 25240 21115 1070 1841 2804 2347 2.41 7.14 15.08 16.4] 22.19 64.22 135.48 147.5! Pr o b a b i l i t y (p.) 0.01-0.005 0.005 0.005 0.005 21 no d i e l migration, while the older ones (class 2, 3 and 4) showed marked d i e l movements (Pigs. 5 and 6). The few class 0 and 1 larvae present on August 18-19 showed p r a c t i c a l l y no migration; these larvae were v i r t u a l l y absent i n September. Although only few class 4 larvae were present, they neverthe-less showed a marked d i e l v e r t i c a l migration each month. There appeared also to be a seasonal trend of progressively fewer older larvae (class 2, 3 and 4) undergoing ascent (Figs. 5 and 6). The trend was p a r t i c u l a r i t y evident for class 3 and 4 animals. V i r t u a l l y a l l the class 3 larvae inhabiting the 8-13 m zone during the daytime ascended on the 2100 hour period of June 17 and July 20. About 80$ of the class 3 animals moved up on the 2100 hour period of August 18, while only about 60$ of the larvae ascended on t h i s period of September 21. S i m i l a r i l y p r a c t i c a l l y a l l the class 4 larvae occupying the 11 to 13 m depths during the daytime ascended during the 2100 hour period of June 17 and July 20, while fewer of these larvae moved up on the same time period of August 18 and September 19. The discrepancy i n the abundance of class 2 and 3 larvae present during the 1100 and 2100 hour periods i s probably due to ascent of larvae which inhabited depths below 13 m during the day time. The trend may be correlated to seasonal increase i n rate of l i g h t i n t e n s i t y change at dusk as indicated by the isolumes plotted i n Figure 9. I t can be seen that the isolumes (.1, 1.0, 10 and 100 luxes) disappear on June 17 and July 20 (1920-2110 hours) at much slower rate than do those on August 18 (1920-2110 hours) FIGURE 5. The v e r t i c a l d i s t r i b u t i o n of class 0, 1, 2, 3 and 4 larvae of C. flavicans i n Corbett Lake during the 1100, 2100 and 0500 hour periods of the June 16-17 and July 20-21 series, 1963. I IOO JUNE 17-18 1963 2 I O O O 5 0 0 Or 4 : 8 • 12 : I IOO NO. / lOO L. O IOO 2 I O O I IOO JULY 20-21 1963 2 I O O 0 5 0 0 C L A S S O C L A S S C L A S S 2 C L A S S 3 C L A S S 4 0 5 0 0 I IOO PACIFIC S T A N D A R D T IME 1 2 I O O -• : 1 12 O 4 8 12 0 5 0 0 FIGURE 6. The v e r t i c a l d i s t r i b u t i o n of class 0, lf 2, 3 and 4 larvae of C. flavicans i n Corbett Lake during the 1100, 2100 and 0500 hour periods of the August 17-18 and September 21-22 series, 1963. D E P T H IN M E T R E S io co O to oo O to oo - f c - O ru oo A O io oo -b. O D E P T H IN M E T R E S tz 24 and September 21 (1520-1920 hours). Furthermore the class 2 and 3 larvae (especially class 2) appeared to show a seasonal trend for completion of descent to occur progressively e a r l i e r (Figs. 5 and 6). About 30-40$ of the class 2 larvae occupying the upper 4 metres during the 2100 hour period of June 17 and July 20 descended to lower depths by 0500 period of the following days (June 18 and July 21). In contrast about 60-70$ of the class 2 animals inhabiting the upper 4 metres during the 2100 hour period of August 18 and September 21 moved down to lower layers by 0500 period of the following days. This trend may be due to darkness occurring progressively e a r l i e r . Migration Pattern of the Older Larvae Samples collected during the 0800, 1100, 1530, 1930, 2130, 2330, 0330 and 0510 hour sampling periods of the September 21-22 series of 1963 were used to compare the migra-tory pattern of the o l f e r larvae (Class 2, 3 and 4) on a d i e l basis. Samples taken during the other series (June 17-18, July 20-21 and August 18-19) were not used, as i t was v i r t u a l l y impossible to measure and count a l l the larvae i n the subsamples or i n samples collected during the 4 series. Confidence l i m i t s were calculated for the estimated counts (Appendix V I I I ) . Comparison of the d i s t r i b u t i o n patterns for 1930, 2130 and 2330 hours (Fig. 7) indicated that the three l a r v a l classes had approximately similar timing of ascent and descent. In 25 general the migratory pattern of the three l a r v a l classes appeared s u f f i c i e n t l y similar to consider them as a single migrating group. Furthermore the migration may be s p l i t into four phases: 1) day-depth, 2) ascent from the day-depth to the surface, 3) descent from the surface, 4) a more rapid descent during dawn (when sunlight starts to penetrate the water) (Fig, 7), In September the f i r s t phase lasted from about 0800 to s l i g h t l y before 1930 hours. The second phase began about (or before) 1930 hours (dusk) and lasted u n t i l about 2130 hours. The t h i r d phase commenced at about 2130 hours and terminated at sometime before 0510 hours when sunlight started to penetrate the water; t h i s phase occurred therefore during darkness. The fourth phase started at about 0510 hours (dawn); lack of l i g h t and d i s t r i b u t i o n data after t h i s time made i t impossible to determine the termination of fourth phase. The d i e l v e r t i c a l migration seemed therefore to be correlated with d i e l changes i n subsurface l i g h t i n t e n s i t y . Since the 3 l a r v a l classes probably behaved as a single migrating group (Fig. 7) during the entire summer, counts of larvae i n samples collected during the monthly series were made i n order to study seasonal changes i n d i e l v e r t i c a l migration of the class 2, 3 and 4 larvae. Changes i n d i s t r i b u t i o n patterns for the sampling periods during the monthly 24-hour series were attributed therefore to the movements of the older larvae (class 2, 3 and 4), as younger (class 0 and l ) larvae underwent v i r t u a l l y no v e r t i c a l migration. Confidence l i m i t s FIGURE 7. The d i e l v e r t i c a l migration of class 2, 3 and 4 larvae of C. flavicans during September 12-22 series of 1963. The broken l i n e s represent the 14.8°, 12° and 6° C isotherms; the s o l i d l i n e s represent the 1.0 lux isolume. 27 were calculated for the estimated t o t a l counts (Appendices IX, X and XI). The d i e l v e r t i c a l migration of the 3 l a r v a l classes showed a seasonal change apparently correlated with seasonal changes i n timing of subsurface l i g h t extinction (at dusk) and of subsurface l i g h t penetration (at dawn) (Fig. 8). The comparison of the d i s t r i b u t i o n patterns of the 1520 and 1920 hour sampling periods showed that the termination of day-depth phase occurred progressively e a r l i e r (Fig. 8). The phase terminated very shortly before 1920 hours during the June 16-17 and July 20-21 series. This was inferred from the fact that the 1920 hour d i s t r i b u t i o n pattern indicated very s l i g h t ascent over the previous d i s t r i b u t i o n pattern. The phase ended shortly before 1920 hours during the August 18-19 series, as the 1920 hour d i s t r i b u t i o n pattern showed substantial ascent over the 1520 hour pattern. The day-depth phase ended well before 1920 hours during the September 21-22 series as the 1920 hour d i s t r i b u t i o n pattern may represent maximum ascent. Comparison of the d i s t r i b u t i o n patterns for 1920, 2110 and 2310 hours showed similar seasonal changes for the second phase (ascent from the day depth to the surface) of d i e l v e r t i c a l migration (Fig. 8). The phase commenced at about 1920 hours and ended at about 2110 hours during the June, July and August series. I t could not have terminated at about 2310 hours during these series as the comparison indicated the descent to be well under way by 2310 hours. In contrast FIGURE 8. The d i e ! v e r t i c a l migration of the C. flavicans larvae i n Corbett Lake curing June T6-17, July 20-21, August 17-18 and September 21-22 series, 1963. Surface temperature and 10°, 5°, 4° and 3.5° C temperature depths are shown. The 0.1, 1, 10 and 100 lux isolumes are indicated with s o l i d l i n e s * NO PER IOOL i 1 1 O 2 0 0 4 0 0 O8OO I I 2 0 I S 2 0 1 9 2 0 21 IO 2 3 I O 0 3 2 0 0 5 1 0 PAC IF IC S T A N D A R D T I M E 29 the phase began well before 1920 hours and ended shortly after 1920 hours during the -September series. The comparison of the 1920 and 2110 hour d i s t r i b u t i o n patterns indicated the descent to be well under way by 2110 hours. Comparison of the d i s t r i b u t i o n patterns for 1920, 2110, 2310, 0320 and 0510 hours indicated that the t h i r d phase (descent from surface) became progressively longer and appeared correlated with changes i n the duration of t o t a l darkness (Pig. 8). The phase commenced sometime at about 2110 hours and ended at about 0320 during the June, July and August series. I t started shortly after 1920 hours (for reasons described previously) and ended at about 0510 hours during the September series. The fourth phase (a rapid descent during dawn) commenced at about 0510 hours during June, July and August series and sometime after 0510 hours during September series (Pig. 8). The l i g h t conditions (time when l i g h t f i r s t penetrated the water and the depths of the 0.1, 1.0, 10, 100 lux isolumes) during the June, July and August series were s i m i l a r . The time at which the l i g h t f i r s t penetrated the water during the September series was s i g n i f i c a n t l y l a t e r than those of the previous series. Other Aspects of the Diel V e r t i c a l Migration In 1962 echo traces were taken about every 15 minutes during the dusk and dawn periods of October 1-2 at Cross-section A (Pig. l ) using constant gain (volume control of 30 the echo sounder) and boat speed. The traces revealed some interesting aspects on the d i e l v e r t i c a l migration of the l a r v a l population i n r e l a t i o n to the lake basin (Fig. 9). In the daytime (1630 and 1745 hour echo traces) the periphery of the Chaoborus scattering layer appeared to be i n contact with the lake basin and was thinnest at these regions. During the ascent (1800-1915 hour traces) the layer (largely Chaoborus larvae) expanded s l i g h t l y shoreward, but appeared to have l i t t l e contact with the shore. The clear spots over the shore region can be seen on the 1805, 1830, 1845, 1900 and 1915 traces, and p a r t i c u l a r l y well on the 1815 hour trace when background noise was minimal. In contrast the layer became more diffuse and appeared i n contact with the shores during descent (0445-0645 hour traces). The echo traces were also used to calculate the rates of ascent and descent of the larvae (Fig. 9). As the larvae occupying the bottom portion of the scattering layer appeared to ascend f i r s t (compare 1800, 1805 and 1815 hour echo traces), i t was assumed that these larvae moved up to the surface and descended to the o r i g i n a l daytime depth of 12.5 m (Fig. 10). As the ascent commenced between 1745 and 1800 hours, i t was decided a r b i t r a r i l y to have occurred precisely at 1750 hours. As further analysis of these and other traces suggested that the termination of ascent occurred when the "residue" layer (indicated by arrows) was thinnest (compare 1800-1915 hour echo traces), the end of ascent was decided to have occurred at 1845 hours. Since the thickening of the "residue" layer FIGURE 9» Echo sounding traces across Corbett Lake at Cross Section A, October 1-2, 1962 (differences i n length of trace caused by-variat i o n i n length of run or boat speed; background noise may be ignored). The arrows indicate the "residue" layer. Time expressed i n hundred hours (P a c i f i c Standard Time). DEPTH IN METRES 32 indicated descent, the onset of descent probably occurred at about 1900 hours (compare 1900-0645 echo traces). The descent terminated at 0745, the time at which the thickness of the scattering layer f i r s t became constant. Consequently the ascent period lasted 55 minutes (1750-1845), while the descent phase involved 705 minutes (1900-0645). Therefore the rates of ascent and descent were 13.6 and 1.1 m/hour respectively. Part 2. Laboratory Studies Di e l V e r t i c a l Migration under Experimental Conditions Several 24 hour laboratory experiments were carried out during the summer of 1963 to study the d i f f e r e n t i a l migratory behavior of the older l a r v a l classes (only class 2, 3 and 4 larvae migrated). A series was done on July 16-17 with 100 larvae (4.80 mm mean length) collected from about the 13 m depth of the lake. Two series were carried out simultaneously on August 13-14 with 100 larvae batches (3.30 mm mean length for both) taken from the surface and about the 5 m depth of the lake. On August 16-17 two 100 larvae batches (3.80 and 5.20 mm mean length) collected from 10 and 14 m depth respectively were used. F i n a l l y two series were carried out on September 13-14 with larvae batches (4.32 and 5.32 mm mean length) collected from about 10 and 14 m depths i n order to replicate the results of August 16-17. ( I t was not possible to replicate the results of August 13-14 Series i n September as comparable sized larvae were not present i n the lake). 33 The experiments were not started u n t i l the larvae appeared quiescent (lack of darting movements). The majority of the larvae used on July 16-17, August 13-14, August 16-17 and September 13-14 belonged respectively to classes 3, 2, 2 and 3, and 2 and 3. No experiments were conducted with the class 0 and 1 larvae, as they were too small to be observed readily i n the tubes. However i t was f e l t that the class 2 larvae taken from the surface and 5 m depth of the lake would give results since they both had a similar daytime v e r t i c a l d i s t r i b u t i o n i n the lake. The class 2 larvae from the surface layer of the lake (0 and 5 m) showed v i r t u a l l y no v e r t i c a l migration i n the experimental tubes (Pig. 10). The class 0 and 1 larvae underwent p r a c t i c a l l y no migration i n the f i e l d (Figs. 5 and 6). On the other hand class 2 larvae taken from 10 m i n the lake during the daytime showed a d i s t i n c t v e r t i c a l migration i n the tubes (Figs. 12 and 13); the class 2 animals underwent d i s t i n c t v e r t i c a l migration i n the f i e l d (Figs. 5, 6 and 7). Si m i l a r l y the class 3 larvae taken from about the 13 and 14 m i n the lake on July 16, August 16 and September underwent d i e l v e r t i c a l migration (Figs. 11, 12 and 13); the class 3 larvae also showed d i s t i n c t d i e l v e r t i c a l migration i n the f i e l d (Figs. 5, 6 and 7). The migratory pattern of the class 2 larvae i n the tubes was i d e n t i c a l to the f i e l d d i e l movements of the class 2, 3 and 4 larvae (Figs. 12 and 13). I t consisted of the similar FIGURE 10. D i e l v e r t i c a l migration of class 2 larvae (collected from the surface and 5 m depth of Corbett Lake) i n adjacent experimental tubes during August 13-14, 1963. Light i n t e n s i t i e s at the surface of both tubes were i d e n t i c a l , as were temperatures at the surface, 100 cm depth and bottom. AUGUST 13-14 1963 I 2 0 0 1 3 0 0 1 5 0 0 1 7 0 0 1 9 0 0 2 0 0 0 2 I O O 2 2 0 0 2 3 0 0 2 4 0 0 0 2 0 0 0 4 3 0 0 5 3 0 0 7 3 0 0 9 0 0 I I O O I 2 0 0 1 3 0 0 1 5 0 0 1 7 0 0 1 9 0 0 2 0 0 0 2 1 0 0 2 2 0 0 2 3 0 0 2 4 0 0 0 2 0 0 0 4 3 0 0 5 3 0 0 7 3 0 0 9 0 0 I IOO PAC IF IC S T A N D A R D T IME FIGURE 11• Diel v e r t i c a l migration of class 3 larvae i n the experimental tube during July 16-17, 1963 at Corbett Lake. Surface l i g h t i n t e n s i t i e s (measured with the submarine photometer) and temperatures at surface, 100 cm depth and bottom of the tube are shown. Probable surface l i g h t i n t e n s i t i e s for 1900, 2000 and 2100 hours ( i f Photovolt Photometer was used) are indicated with s o l i d dots joined with a s o l i d l i n e . FIGURE 12, D i e l v e r t i c a l migration of class 2 and 3 larvae i n adjacent experimental tubes during August 16—17, 1963. Light i n t e n s i t i e s at the surface of both tubes were identical.as were temperatures at the surface, 100 cm depth and bottom. FIGURE 13. Di e l v e r t i c a l migration of class 2 and 3 larvae i n adjacent experimental tubes during September 13-14, 1963. Light i n t e n s i t i e s at the surface of both tubes were identical,as were temperatures at the surface, 100 cm depth and bottom. S E P T E M B E R 13-14 1963 I I O O 1 3 0 0 1 5 0 0 1 7 0 0 1 9 0 0 2 0 0 0 2 I O O 2 2 0 0 2 3 0 0 2 4 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 7 0 0 0 9 0 0 P A C I F I C S T A N D A R D T I M E -3 38 4 phases: 1) day-depth, 2) ascent from the day-depth to the surface, 3) descent from the surface after ascent, 4) a rapid descent during dawn period (Pigs. 12 and 13). There may have been a morning r i s e at 0500 hours on September 17 (Pig. 13). The migratory pattern of the class 3 larvae i n the tubes was not i d e n t i c a l with the f i e l d d i e l movements of the class 2, 3 and 4 larvae (Pigs. 12 and 13). I t consisted of 5 phases: 1) day-depth, 2) ascent from the day-depth to the surface, 3) descent from the surface, 4) early morning r i s e to the surface ("dawn r i s e " ) , 5) sharp descent during the dawn period (Figs. 11, 12 and 13). The early morning r i s e occurred at about 0400 hours on July 17 (Fig. 11), about 0400-0530 hours on August 17 (Fig. 12) and perhaps at about 0500 hours on September (Fig. 13). The f a i l u r e to depict the morning r i s e i n the f i e l d may. be due to the long time i n t e r v a l between sampling periods (every 2 or 4 hours). In addition the migration cycles of the class 2 and 3 larvae i n the experimental tubes showed a seasonal change (Figs. 11, 12 and 13), p a r t i c u l a r l y i n the time of maximum ascent. For the class 2 larvae maximum ascent was reached at about 2000 hours i n the August 16-17 series and at about 1900 hours on September 13-14. The maximum ascent of class 3 larvae occurred at about 2100 hours i n the July series (Fig. 11), 2000 hours i n the August series (Fig. 12) and 1900 hours i n the September series (Fig. 3). The surface l i g h t intensity was measured during the July 39 series with an instrument less sensitive than that used during the August and September series. As a result the surface l i g h t i n t e n s i t i e s recorded during the 1900, 2000 and 2100 hours of the July series (Fig. 11) were corrected to i n t e n s i t i e s which probably would have been recorded with the sensitive instrument (Fig. 11). Consequently i t was possible to compare the times of l i g h t extinction for the 3 series. The seasonal variation i n time of maximum ascent appeared to be correlated with changes i n time of surface l i g h t extinction (Figs. 11, 12 and 13). During the July series the surface l i g h t became almost immeasurable at about 2100 hours, corresponding with the time of maximum ascent of class 3 larvae (Fig. 11). During the August series the surface l i g h t became immeasurable at about 2000 hours, corresponding with the time of maximum ascent for class 2 and 3 larvae (Fig. 12). During the September series the l i g h t i n t e n s i t y was v i r t u a l l y zero at about 1900, corresponding with the time of maximum r i s e for the class 2 and 3 animals (Fig. 13). A similar seasonal trend was observed i n the f i e l d . The maximum ascent was estimated to have occurred at about 2110 hours during the June, July and August f i e l d series and about 1920 hours during the September f i e l d series (Fig. 8). Role of Light on Diel V e r t i c a l Migration Experiments to test for exogenous rhythm i n the d i e l v e r t i c a l migration of Chaoborus larvae were carried out on August 9 and September 28, 1963 with class 3 larvae collected 40 f r om the 14 m depth of the lake. During each experiment two p l a s t i c tubes were used to hold the animals; the top of one tube was exposed to natural l i g h t conditions (control tube), while that of the other was subjected to l i g h t i n t e n s i t i e s regulated by a shutter system previously described. The tubes were placed adjacent to each other i n the dark room with a black p l a s t i c sheet separating them. At the beginning and end of each experiment, the control and non-control (or experimental) animals were exposed to i d e n t i c a l l i g h t conditions i n order to evaluate whether the two batches of animals behaved s i m i l a r l y (Figs. 14 and 15). The responses were almost i d e n t i c a l at the s t a r t , but somewhat different at the end of the experiments. Discrepancies between two sets of natural l i g h t readings taken near the conclusion of the experiments were probably due to time lapse between readings (Figs. 14 and 15). The temperature regimes i n the control and non-control tubes were v i r t u a l l y i d e n t i c a l during each experiment. I t was possible to induce complete migration cycles, including the morning r i s e phase (Figs. 14 and 15). The cycle was induced at a l i g h t i n t e n s i t y range of 300-1000 luxes on August 9; i t can be seen that non-control (or experimental) larvae responded very l i t t l e to changes i n l i g h t i n t e n s i t y at a range of 1000-15000 luxes (Fig. 14). Extremely s l i g h t changes i n l i g h t i n t e n s i t y made the larvae ascend or descend within 300-J000 lux l i g h t range. The non-control larvae exposed previously to 1450 luxes (at 1452 hours) FIGURE 14. The v e r t i c a l movements of class 3 larvae i n adjacent control (exposed to natur a l l y changing l i g h t conditions) and non-control (exposed to a r t i f i c i a l l y changing l i g h t conditions) experimental tubes on August 9, 1963. Identical temperature conditions i n both tubes; i d e n t i c a l l i g h t i n t e n s i t y over both tubes at the beginning and end of experiment. A U G U S T 9 1963 0 9 3 4 IOIO 1 0 4 5 I I 2 0 1158 1 2 3 0 1306 1342 1416 1452 1 5 2 8 1 6 0 0 1646 1718 1756 1830 1 9 0 5 C O N T R O L HI 4 0 h BOh \20[ I 6 0 r 195 I 2 0 0 h lOO^ 2 0 L I O r ' 1 8 . 8 C O N T R O L ( N A T U R A L L Y C H A N G I N G ) N O N - C O N T R O L ( A P T I F I C A L L Y C H A N G I N G ) " " - - V . 4 0 SO 120 160 H200--IIOO 5 x J 2 0 h- * 1'° 5 3 OL Or 4 0 r 2 U 8 0 h I 2 0 t \60[ I95L N O N C O N T R O L 1 1 _ . . . i l i l l i l i i i 1 H40 8 0 n l 2 0 ^160 0 9 3 4 IO IO 1 0 4 5 I I 2 0 1158 1 2 3 0 1306 1 3 4 2 1416 1452 1528 1 6 0 0 1646 P A C I F I C S T A N D A R D T I M E 1718 1 7 5 6 1830 1905"° 2i.«l|95 FIGURE 15. The v e r t i c a l movements of class 3 larvae i n adjacent control (exposed to naturally changing l i g h t conditions) and non-control (exposed to a r t i f i c i a l l y changing l i g h t conditions) experimental tubes on September 28, 1963. Identical temperature conditions i n both tubes; i d e n t i c a l l i g h t i n t e n s i t y over both tubes at the beginning and end of experiment. S E P T E M B E R 28 1963 PACIFIC STANDARD TIME 43 ascended markedly when exposed subsequently to 620 luxes (at 1528 hours). The experimental larvae descended only s l i g h t l y when the l i g h t was lowered to 410 luxes (at 1606 hours), while they moved down markedly when l i g h t was further reduced to 310 luxes (at 1646 hours). They ascended appreciably upon exposure to 440 luxes (at 1718 hours) and moved up markedly upon subsequent exposure to 350 luxes (at 1756 hours); these results indicated that the ascent response might be greater at 350 luxes than at 440 luxes. A migration cycle was induced at a l i g h t intensity range below 100 luxes on September 28; the non-control larvae responded very l i t t l e to changes i n l i g h t intensity above t h i s range (Fig. 15). The experimental animals exposed previously to 60 luxes (at 1710 hours) ascended markedly when l i g h t i n t e n s i t y was reduced to 5 luxes (at 1740 hours). The larvae descended sharply i n the absence of l i g h t (at 1810 hours), while they ascended upon subsequent exposure to 0.3 lux. DISCUSSION Seasonal Variation i n Horizontal Distr i b u t i o n During the summer of 1963,: the larvae had a clumped horizontal d i s t r i b u t i o n which increased with the progression of summer (Table l ) . The significance of th i s r e s u l t can be speculated upon by the consideration of the seasonal change i n dominant size classes (or i n s t a r s ) . There was a progressive decrease i n the abundance of class 0 and 1 larvae which did not migrate and were largely confined to the epilimnion (Figs. 44 4, 5 and 6). These young larvae are the least s t r u c t u r a l l y d i f f e r e n t i a t e d (for example, the muscles are poorly developed) of larvae present. In fact they may be so undeveloped st r u c t u r a l l y that they are t r u l y planktonic with l i t t l e or no control over movement and are therefore distributed within the eplimnion solely by the s l i g h t water currents i n Corbett Lake. I t may be expected therefore that such animals are randomly distributed. Conversely the class 2, 3 and 4 larvae w i l l be more developed with respect to structure (greater mobility), physiology and possibly behavior. I t i s possible that these older larvae are clumped i n d i s t r i b u t i o n . There-fore the horizontal d i s t r i b u t i o n of the t o t a l l a r v a l population appears least clumped i n June probably owing to the modifying influence of large numbers of randomly distributed small larvae, while i t i s most clumped i n September because there are v i r t u a l l y no class 0 and 1 larvae present. The Effect of Age and Environmental Factors on V e r t i c a l D i s t r i b u t i o n During the daytime (1100 hours) the small larvae occupied the oxygen r i c h epilimnion (above 8 m) of Corbett Lake, while larger and older (class 2, 3 and 4) inhabited the hypolimnion, but not the bottom mud (Fig. 4). A nearly t o t a l l y limnetic summer population of Chaoborus larvae showing this size characteristic has been observed only on rare occasions. Dendy (1956) has observed such a summer population, but did not determine whether the larvae were 45 v e r t i c a l l y s t r a t i f i e d according to size during the daytime. Hunt (1958), investigating a deep Florida lake, has found a t o t a l l y limnetic population whose larvae increased i n size with increase i n depth. Sim i l a r l y Worthington and Ricardo (1936), sampling Lake Edward i n A f r i c a , have found a s t r i c t l y limnetic population whose larvae are v e r t i c a l l y distributed with respect to size. The lakes containing such l a r v a l popula-tions have common chemical characteristics; the lower layers of the lakes are severely depleted of oxygen and contain detectable traces of ^ S . In most lakes containing l a r v a l populations, the larvae are v e r t i c a l l y distributed according to size during the daytime, but with the older larvae being benthic (Muttkowski, 1918; Rawson, 1930; Eggleton, 1932; Berg, 1937; M i l l e r , 1941; MacDonald, 1956; Wood, 1956; Woodmanse and Grantham, 1961). I t i s noteworthy that these lakes containing daytime benthic larvae have no ^ S i n their lower layers. I t seems therefore that highly reduced mud may d i r e c t l y or i n d i r e c t l y prevent the larvae from entering the bottom of productive lakes during the daytime. The fact that larvae enter the bottom mud of Corbett Lake after f a l l overturn supports this speculation. In addition i t may be that l i g h t detectable by the larvae does not penetrate to the bottom i n such supposedly productive lakes, therefore not necessitating the animals to enter the bottom mud. Studies done on both freshwater and marine crustacean plankters indicate similar size (or age) and depth relationships. 46 Gardiner (1933), working with Galanus fimarchius i n the North Sea, has shown that copepbdite stages 3 and 4 l i v e i n the upper layers and stages 5 and 6 i n the deep waters. Langford (1938) have shown that the young stages of a freshwater cladoceran Daphnia longispina remain above the thermocline i n Lake Nipissing during daytime, while the adults remain below i t . The Effect of Age and Environmental Factors on the Die l V e r t i c a l Migration The class 0 and 1 (or f i r s t and second instar) larvae underwent p r a c t i c a l l y no d i e l v e r t i c a l migration i n Corbett Lake while the class 2, 3 and 4 (or respectively t h i r d , fourth and f i f t h instar) larvae did (Figs. 5 and 6). The experimental results seem to corroborate the f i e l d observations; class 2 larvae taken from the upper layers (0 and 5 m) of the lake were assumed to react s i m i l a r l y inasmuch as class 0 and 1 larvae showed v i r t u a l l y no v e r t i c a l movements, while class 2, 3 and 4 animals taken from the lower depths (10-14 m) made obvious migrations (Figs. 10, 11, 12 and 13). Dendy (1956), describing the presence of t o t a l l y limnetic l a r v a l population i n some lakes, do not c i t e the existence of d i f f e r e n t i a l migration a b i l i t y between l a r v a l groups. In contrast, others (Juday, 1921; Berg, 1937; Wood, 1956; Hamilton, 1961; Woodmanse and Grantham, 1961) have observed both benthic and limnetic distributions of larvae, but found invariably that both younger and older animals underwent a d i s t i n c t v e r t i c a l migration. However they f a i l to specify the size 47 ranges of the younger and older larvae. Nevertheless Berg (1937) has shown experimentally that young larvae (1.7 mm mean length) do not migrate v e r t i c a l l y . Nicholls (1934), working with Calanus i n the Clyde area, has noted that the nauplia occur above 30 m and do not undergo d i e l changes i n d i s t r i b u t i o n , while the copepodite stages 1, 2, 3 and 4 undergo v e r t i c a l migration (especially stage 4). In contrast he has found the stage 5 animals to be unresponsive. As a consequence of d i f f e r e n t i a l migratory behavior between l a r v a l groups of Chaoborus, the younger larvae (class 0 and 1) remained i n the warmer oxygenated epilimnion throughout the entire day, whereas the older animals (class 2, 3 and 4) encountered both the epilimnion and colder deoxygenated hypolimnion during the 24 hour period (Pigs. 4, 5 and 6). These observations suggest that there may be several factors which may account for the d i f f e r e n t i a l migration. The smaller larvae may have a much narrower temperature and oxygen range than the older ones. However i t remains to be demonstrated whether differences i n physiological tolerance of these factors exist between the two sets of larvae, and whether or not these may actually affect migration. More l i k e l y the extent of physiological and morphological development of pertinent structures (muscles, sense organs and a i r bladder) i s of greater importance. Deonier (1943), working with Chaoborus  astictopus, has shown that the simple eye i s present i n a l l i n s t a r s , while the compound eye appears i n the t h i r d instar (comparable to class 2 of th i s study) and are developed f u l l y 48 i n the fourth instar (comparable to class 3). Therefore i t may be that the compound eyes (regardless of degree of development) are a v i t a l sense organ for v e r t i c a l migration, providing that l i g h t i s the controlling factor. Berg (1937) has observed the a i r sacs to become f i l l e d with a i r before the C. flavicans larvae hatch, while Akehurst (1922) has shown the a i r to replace f l u i d i n the a i r bladders shortly after hatching. I f entrance of a i r into the a i r sac i s important i n the hydrostatic functioning of the sac, then i t would seem that the i n a b i l i t y of smaller larvae to migrate can not be attributed d i r e c t l y to the structural and physiological development of this organ. Hence one must consider now the p o s s i b i l i t y of v e r t i c a l migration involving s t r i c t l y active body movements which implies that the degree of physiological and structural development of the body muscles i s v i t a l l y important. Although the development of the body muscles i n the larvae has not been examined, i t may well be that the muscles of younger larvae (class 0 and l ) are not as functional as those of older larvae (class 2, 3 and 4). Based on the September samples, there appeared to be no d i f f e r e n t i a l d i e l timing of ascent and descent between migrating l a r v a l groups (Fig. 7). Since no one has attempted to separate the migrating chaoborid larvae into age groups (or i n s t a r s ) , the results can not be compared. I t i s possible that the time intervals (every 2 and 4 hours) between sampling periods are not short enough to demonstrate the existence of d i f f e r e n t i a l d i e l timing. However, since the migrating larvae do have a 49 similar daytime v e r t i c a l d i s t r i b u t i o n (indicating similar response to l i g h t conditions), i t seems l i k e l y that they may have similar d i e l timing of ascent and descent. Fraser (1938) has found that a l l calyptopis stages of Euphausia  superba migrate v e r t i c a l l y and have similar timing of upward . and downward migration. The migration cycles observed i n the f i e l d and laboratory differed only with respect to "dawn r i s e " which occurred under experimental conditions, especially for class 3 larvae (Figs. 7, 8, 10, 11, 12 and 13). Both cycles appeared to be correlated to d i e l l y and seasonally changing l i g h t conditions, with the ascent occurring about dusk and the descent taking place during darkness and dawn. The "dawn r i s e " of the laboratory cycle occurred during the early morning hours. The basic migration pattern observed i n Corbett Lake (Figs. 7 and 8) agrees with those of the same or different species i n other lakes (Juday, 1921; Eggleton, 1937; Berg, 1937; Wood, 1956; Woodmanse and Grantham, 1961). However the time of maximum ascent (consequently time of descent) -of larvae i n Corbett Lake does not correspond to those of larvae i n other lakes. Throughout the summer the maximum ascent occurred between about 1900-2200 hours i n Corbett Lake. Juday (1921) and Eggleton (1931), each working on a different lake, found the maximum r i s e of larvae occurring at about 2200 hours. In contrast Woodmanse and Grantham (1961), Berg (1937) and Wood (1956) studying dif f e r e n t lakes, found the maximum upward movement to take place about 2400 hours. 50 The depiction of "dawn r i s e " i n the experimental tube but not i n the lake i s puzzling. I t may occur i n Corbett Lake, but escaped detection because the time in t e r v a l between sampling periods was not short enough. Failure of other workers to show a "dawn r i s e " i n Chaoborus larvae may be due to the same reason. In addition the p o s s i b i l i t y of i t occurring for a very short time adds to the d i f f i c u l t y of depicting i t i n the f i e l d . The proportion of the older l a r v a l groups undergoing ascent and the time taken for i t to complete the descent decreased as the season progressed (Fig. 8). The seasonal decline i n the percentage of older larvae ascending i s probably due to progressive increase i n the rate of l i g h t intensity change at dusk as i s indicated by the isolumes plotted i n Figure 8. Perhaps the migrating population requires a slow rate of l i g h t change during the transitory dusk period for complete ascent. The migrating larvae completing descent more rapidly with the progression of summer can be attributed perhaps to increasing darkness during the late evening and early morning hours (2200-0500 hours). Experimental results indicate that the rate of descent of the larvae may be a function of rate at which absolute darkness i s approached. The fact that complete migration cycles (including "dawn rise") can be induced i n the laboratory by a r t i f i c i a l l y changing the natural daytime l i g h t conditions (Figs. 14 and 15) indicates that l i g h t controls the timing of d i e l v e r t i c a l migration of Chaoborus flavicans larvae. Furthermore the experiments have shown that the animals respond only to changes of p a r t i c u l a r l y low l i g h t i n t e n s i t i e s (below 1000 l u x ) . Harris and Wolfe (1955) have demonstrated complete laboratory migration cycles of Daphnia magna by c y c l i c a l l y changing the overhead l i g h t i n t e n s i t y using India ink suspension. However, Harris (1963) found the migration cycle of Daphnia magna to pe r s i s t under constant darkness or illumination and therefore concluded the cycle to be a endogenous rhythm. In the present study on Chaoborus larvae, no such experiments have been carried out. However the larvae w i l l remain at one depth i n the experimental tube for several hours voider a constant v i r t u a l l y dark con-d i t i o n ; t h i s further suggests that the larvae have an exogenous rhythm. The d i e l v e r t i c a l migration of the l a r v a l population i n re l a t i o n to the lake basin consisted of very l i t t l e shoreward movement during ascent but some shoreward movement during descent. According to Siebeck (1964), an organism i n the water invariably receives maximum l i g h t i n t e n s i t y almost v e r t i c a l l y from above. Prom th i s i t then follows that the nearly v e r t i c a l ascent of the larvae i s probably a function of the subsurface l i g h t penetration. "Uferflucht" (avoidance of shore) does not appear to be a pertinent factor as larvae are found i n the l i t t o r a l zone during daytime (Humphreys, 1964) and do more inshore during.descent (indicated by echo traces). The shore-ward movement during descent may be a r e f l e c t i o n of the fact that the animals have v i r t u a l l y no v e r t i c a l l i g h t component to orientate to and therefore are moving down i n a random 52 fashion which brings them into the shore region. Theoretical Interpretation of the Diel V e r t i c a l Migration When one attempts a theoretical treatment of v e r t i c a l migration of Chaoborus larvae, the effect of l i g h t on both the active swimming movement and buoyancy must be considered. Many workers have demonstrated that the a i r sacs are buoyancy organs (Krogh, 1911; Bardenfleth and Ege, 1916; Akehurst (1922), Damant, 1924; Holst-Christensen, 1928). On the basis of his study, Berg (1937) speculates that the v e r t i c a l migration i s a s t r i c t phototactic response which he does not describe i n any d e t a i l . On the basis of results from the present study, a simple sign change (positive and negative) of phototaxis i s not s u f f i c i e n t to explain the v e r t i c a l migration. In fact i t may complicate matters by formulating u n r e a l i s t i c l a r v a l behavior. Therefore the migration may involve an interaction of the normal larvae "seeking" an optimum l i g h t zone (low l i g h t intensity range), while having a constant positive response to gravity^ The animals remain i n the optimum zone through passive (buoyancy adjustment) and/or active ( s l i g h t body movements) means. Passive and active movements have been observed i n the laboratory. When the optimum l i g h t zone disappears at dusk, the animals may move down actively or sink as the buoyancy regulation controlled by l i g h t gradually deteriorates. At dawn the larvae may ascend actively or passively to the descending optimum zone i f there i s a stimulating l i g h t 53 gradient created between the animals and the optimum l i g h t zone to invoke responses. I f there i s no such l i g h t gradient, the optimum zone would reach the depth l e v e l of the slowly descending larvae, to invoke the previous interaction. The seasonal v a r i a t i o n i n the migration can be explained also by t h i s hypothesis. Only extremely unfavourable physical and chemical conditions would offset the interactions. B i o l o g i c a l Significance of the Diel "Vertical Migration The b i o l o g i c a l significance can be considered best from the standpoint of the migrating larvae spending at least half of a 24 hour period i n the colder deoxygenated hypolimnion before migrating into the warmer oxygenated epilimnion. Juday (1921), observing that the larvae spend a good portion of the day i n the deoxygenated hypolimnion, has suggested that the significance may involve reduced predation. He argues that most.freshwater f i s h can not tolerate deoxygenated water more than a few hours. Predation would certainly be reduced, especially during the summer. Analysis of trout stomach indicates that the f i s h prey heavily on the larvae during early spring and late autumn i n Corbett Lake. I t i s conceivable also that the f i s h change diets for reasons other than not being able to prey on the larvae. As the larvae encounter da i l y two sets of environment, (aerobic and anaerobic) the adaptive value of d i e l v a t i c a l migration may involve enhancement of optimum population growth. McLaren (1963) has demonstrated empirically that v e r t i c a l 54 migration becomes increasingly important for surface feeding zooplankters as the surface and bottom temperatures become increasingly d i f f e r e n t . The migration under such conditions enables the animals to conserve and divert more energy into growth and fecundity. Migrating larvae feed predominately i n the upper layers of the lake as v i r t u a l l y no food organism (cladocerans and copepods) can withstand anaerobic conditions for any length of time. Temperature difference i s very great between the surface and bottom i n many lakes during the summers Therefore l a r v a l growth and adult fecundity w i l l be enhanced greatly during the summer' i f the larvae migrate. The larvae by spending part of the day i n deoxygenated water which may have also an energy conserving effect w i l l further enhance growth and fecundity (McLaren, 1963). Therefore larvae w i l l be larger than i t would be i f they had not undergone v e r t i c a l migration. Larger larvae became larger pupae which sub-sequently metamorphose into bigger adults with greater fecundity. Ultimately the success of any chaoborid popula-ti o n may depend on these energy conserving effects. 55 LITERATURE CITED Akehurst, S. C. 1922. Larva of Chaoborus c r y s t a l l i n u s (De Grur) (Corethra plumicornis F«). J . Roy. Microscop. S o c , 42: 341-372. Bainbridge, R. 1961. Migrations. Chapter 12 of volume 2 i n "The Physiology of Crustacea", Academic Press, New Xork, 681 pp. Bardenfleth, K. S., and R. Ege. 1916. On the anatomy and physiology of the air-sacs of the larva of Corethra  plumicornis. Videnskabelige Meddelelser f r a Dansk Naturhistorisk Forehing, 67: 25-42. Berg, K. 1937. Contributions to the biology of Corethra Meigen (Chaoborus Lichtenstein). Kgl. Danske Vid. . Selsk., B i o l . Meddel, 13(11): 1-101. Clarke, G-. L., and D. F. Bumpus. 1950. The plankton-sampler -an instrument for quantitative plankton investigation. Amer. Soc. Limnol. Oceanogr., Spec. Publ. No. 5: 1-8 (2nd ed.). Cassie, R. M. 1954. Some uses of probability paper i n the analysis of size frequency d i s t r i b u t i o n s . Aust. J . S c i . , 5(3): 513-522. Cook, E. F. 1956. The nearctic Chaoborinae (Diptera: Culicidae). Univ. Minn. Agr. Expt. Sta., Tech. B u l l . , 218: . 1-102. Cushing, D. H. 1951. The v e r t i c a l migration of planktonic Crustacea. B i o l . Revs., 26: 158-192. Damant, G. C. C. 1925. The adjustment of buoyancy of the larva of Corethra plumicornis. J . Physiol., 54: 345-356. Davis, C. C. 1955. The marine and fresh-water plankton. Michigan State University Press, 562 pp. Dendy, J . S. 1956. Bottom fauna i n ponds with large mouth bass only and with a combination of largemouth bass plus b l u e g i l l . J . Tenn. Acad. S c i . , 31(3): 198-207. Deonier, C. C. 1943. Biology of the immature stages of the Clear Lake gnat. Entomol. Soc. Amer. Ann., 36: 383-388. 56 Eggleton, F. E. 1932. Limnetic d i s t r i b u t i o n and migration of Corethra larvae i n two Michigan lakes. Mich. Acad. S c i . , Arts and Lett. Pap., 15: 361-388. Elgmork, K. 1959. Seasonal occurrence of Cyclops strenus. P o l i a Limnol. Scand., 11: 1-196. Eraser, F. C. 1936. On the development and d i s t r i b u t i o n of the young stages of k r i l l (Euphasis superba). Discovery Rep., 14: 3-192. Gardiner, A. C. 1933. V e r t i c a l d i s t r i b u t i o n i n Calanus finmarehicus. J . Mar. B i o l . Ass. U. K., 18: 575-6T0. ' Hamilton, A. L. MS, 1958. The macroscopic bottom fauna of Kenosee Lake. M.A. Thesis, University of Saskatchewan. Hardy, A. C. 1956. The Open.Sea. 1. The world of plankton. C o l l i n s , London. 335 pp. Harris, J . E. 1963. The role of endogenous rhythms i n v e r t i c a l migration. J . Mar. B i o l . Ass. U.K., 43: 153-166. -Harris, J . E., and U. K. Wolfe. 1955. A laboratory study of v e r t i c a l migration. Proc. Roy. Soc. B, 145: 280-290. Holst-Christensen, P. 1928. Bidrag t i l Kendskabet om Corethra-larvens hydrostatiske Mekanisme. Vedenskabelige Meddelelser f r a Dansk. Naturhistorisk Porening, 67: 25-42. Humphreys, R. D. MS, 1964. Spatial and temporal d i s t r i b u t i o n of invertebrate organisms inhabiting the Chara zone. M.Sc. Thesis, University of B r i t i s h Columbia. Hunt, B. P. 1958. Limnetic d i s t r i b u t i o n of Chaoborus larvae i n a deep Flor i d a lake (Deptera). Flo r i d a Entomol., 41(2): 111-116. Juday, C. 1921. Observations on the larvae of Corethra  punctipennis Say. B i o l . B u l l . , 40(5): 271-286. Krogh, A. 1911. On the hydrostatic mechanism of the Corethra larva with an account of methods of microscopical gas analysis. Skand. Arch, fur Physiol., 25: 183-203. 57 Kutkuhn, J . H. 1958. Notes on the precision of numerical and volumetric plankton estimates from small-sample concentrates. Limnol. Oceanogr., 3(1): 69-83. Langford, R. R. 1938. Diurnal and seasonal changes i n the d i s t r i b u t i o n of the limnetic Crustacea of Lake Nipissing. Univ. Toronto Stud., B i o l . Ser., 45: 1-142. Lindquist, A. W., and C. C. Deonier. 1943. Seasonal abundance and d i s t r i b u t i o n of the larvae of the Clear Lake gnat. J . Kans. Entomol. S o c , 16: 193-202. Lund, J . W. G., C. K i p l i n g and E. D. La Cren. 1958. The inverted microscope method of estimating a l g a l numbers and the s t a t i s t i c a l basis of estimations by counting. Hydrobiologia, 11(2): 143-170. MacDonald, W. W. 1956. Observations on the biology of chaoborids and chironomids i n Lake V i c t o r i a and on the feeding habits of the "elephant-snout f i s h " (Mormyrus kannumi Torsk.). J . Anim. Ecol., 25: 36-53. McLaren, I. A. 1963. Effects of temperature on growth of zooplankton, and the adaptive value of v e r t i c a l migration. J . Fish. Res. Bd. Canada, 20(3): 685-727. M i a l l , L. C. 1895. The natural history of the aquatic insects. MacMillan and Co., London, 395 pp. M i l l e r , R. B. 1941. Some observations on Chaoborus puncti- pennis Say. (Diptera, Culicidae). Can. Entomol., 73: 79-039. Muttkowski, R. A. 1918. The fauna of Lake Mendota. Trans. Wis. Acad. S c i . , Arts and Lett., 1 9 ( l ) : 374-482. Northcotey T. G. 1964. Use of high-frequency echo sounder to record d i s t r i b u t i o n and migration of Chaoborus larvae. Limnol. Oceanogr., 9(1): 87-91. Rawson, D. S. 1930. The bottom fauna of Lake Simcoe and i t s role i n the ecology of the lake. Publ. Ont. Fish. Res. Lab., 40(34): 1-183. Raymont, J. E. G. 1963. Plankton and productivity i n the oceans. Pergamon Press, London, 660 pp. 58 Ricker, W. E. 1938. On adequate quantitative sampling of the pelagic net plankton of a lake. J . Fish. Res. Bd. Canada, 4 ( l ) : 19-32. Siebeck, 0. 1964. Researches on the behavior of planktonic crustaceans i n the l i t t o r a l zone. Verh. i n t . Ver. Limnol., 15(2): 746-751. Wood, K. G. 1956. Ecology of Chaoborus (Diptera: Culicidae) i n an Ontario lake* Ecology, 37(4): 639-643. Woodmanse, R. A., and B. J . Grantham. 1961. Die l v e r t i c a l migrations of two zooplankters (Mesocyclops and Chaoborus) i n a Mi s s i s s i p p i lake. Ecology, 42(4): 619-628. Worthington, E. B., and C. K. Ricardo. 1936. S c i e n t i f i c results of the Cambridge expedition to the East African Lakes, 1930-31 - No. 17. The v e r t i c a l d i s t r i b u t i o n and movements of the plankton i n Lakes Rudolf, Naivasha, Edward and Bunyoni. J . Limn. Soc. Zool., 40: 33-39. Tentsch, C. S., and A. C. Duxbury. 1956. Some of the factors affecting the c a l i b r a t i o n number of the Clarke-Bumpus quantitative plankton sampler. Limnol. Oceanogr., l ( 4 ) : 268-273. APPENDIX I. Determination of the ca l i b r a t i o n value of the Clarke-Bumpus sampler towed at several depths for a 61 m distance at boat speed of 2.5 knots. Without Net With No. 10 Net Towing Depth (i n metres) Corrected Depth (in metres) Average No. of Revolutions (3 Tows) Calibration Value i n L i t e r s per Revolution Average No. of Revolutions (3 Tows) Calibration Value i n L i t e r s per Revolution Surface — 135.3 5.7 126.0 6.2 2 . 1.97 133.3 5.8 125.3 6.3 4 3.80 136.0 5.6 120.0 6.6 6 5.56 137.3 5.7 124.7 6.3 8 7.42 135.3 5.7 124.7 6.3 10 9.21 136.7 5.6 126.3 6.2 12 10.87 135.7 5.7 129.6 6.1 U l vO 60 APPENDIX I I . Volume of water passed through and the number of Chaoborus larvae caught by the Clarke-Bumpus sampler (with no. 10 net attached) towed at several depths for a 61 m distance at a boat speed of 2.5 knots. Corrected Depth ( i n m) Sample No. Time No. of Larvae No. of Larvae per 100 L Surface 1 1252 466 93.20 2 1256 649 119.08 3 1301 956 189.68 1.97 1 1305 1374 267.83 2 1314 1675 340.44 3 1320 1278 237.98 3.80 1 1325 4235 922.66 2 1330 4187 809.86 3 1333 4350 870.00 5.56 1 1338 1833 360.83 2 1345 2128 408.45 3 1350 2067 410.2 7.42 1 1357 710 142.12 2 1402 543 103.43 3 1409 809 159.25 9.21 1 1415 326 63.55 2 1429 317 60.38 3 1434 401 77.56 10.87 1 1439 189 35.73 2 1444 190 35.65 3 1448 214 40.15 61 APPENDIX I I I . Comparison of the volume of water i n the compartments of the subsampler using different t o t a l volumes. Volume of Water i n Each Volume of Water Used Compartment (in ml.) Compartment (in ml.)  about 1000 A 164 B 164 C 163.8 D 164 E 166 F 165 about 750 A 122.2 B 121 C 122 D 122 E 122 F 122.3 about 500 A 81 B 80 C 80.3 D 81 E 81 F 80.8 about 250 A 40.5 B 41 C 41 D 40 E 41 F 41 APPENDIX IV. Test of the r e l i a b i l i t y of the subsampler, using different volume of water and differ e n t number of larvae. A 0.05 significance l e v e l was used. Approximate Volume of Water Used ( i n ml.) No. of Larvae Used Compartment No. of Larvae X i n Each Compartment Value Type of Dis t r i b u t i o n 250 222 500 203 750 239 250 599 A 29 B 40 C 41 D 41 E 44 F 27 A 33 B 40 C 36 D 33 E 27 F 34 A 35 B 48 C 42 D 35 E 37 F 41 A 104 B 94 C 105 D 93 E 104 F 97 N.B. (1 larva l e f t i n the subsampler ) N.B. (2 larvae l e f t i n the subsampler ) 7.14 Random 2.69 Random 3.21 Random 1.50 Random 0> to APPENDIX IV Continued Approximate Volume of Water Used ( i n ml.) 500 750 250 250 No. of No. of Larvae X 2 Type of Larvae Used Compartment i n Each Compartment Value Dis t r i b u t i o n 667 A 122 B 94 9.09 Random C 91 . _ D 108 N.B. (4 larvae E : 93 l e f t i n the F 125 subsampler ) 573 A . 104 B 98 3.64 Random C 103 D 83 E 88 . P 96 1022 • A 161 *B 154 6.96 Random **c 184 N.B. (2 larvae D 159 169 l e f t i n the F 193 subsampler ) *154 A 20 Prom B B 29 2.57 Random Compartment C 25 N.B. (2 larvae D 24 E • 25 l e f t i n the F 30 subsampler ) APPENDIX IV Continued Approximate Volume of Water Used ( i n ml.) No. of Larvae Used Compartment No. of Larvae X i n Each Compartment Value Type of Dis t r i b u t i o n 500 750 500 750 **184 A 29 Prom C B 29 Compartment C 29 D 29 E 33 P 35 ***169 A 32 Prom E B 26 Compartment C 25 D 24 E 32 F 27 1143 A 191 B 181 C 181 D 201 E 202 P 185 1039 A 186 B 165 C 170 D 176 E 174 P 166 N.B. (2 larvae l e f t i n the subsampler ) N.B. (2 larvae l e f t i n the subsampler ) 1.15 Random 2.21 Random 1.98 Random 1.78 Random 65 APPENDIX V. Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l larvae i n the l a r v a l classes for samples taken during 24 hour f i e l d series of June 17-18, 1963 Time Depth Period in. Metres Class 0 Class 1 Class 2 Class 3 Class 4 1120-1207 hours hours .99 64.3 . 79.4 0.3 143.7 166.4 27.2 1.93 10.8 146.5 71.7 2.3 55.5 231.2 155.7 33.9 2.91 45.7 229.4 101.8 11.8 154.9 427.1 244.9 86.3 3.76 42.0 295.5 48.2 7.8 153.2 598.7 163.3 79.3 4.73 64.8 131.0 53.2 191.2 292.9 171.9 5.64 91.8 135.7 31.9 4.8 25.6 303.3 136.5 70.2 6.66 13.5 109.6 30.5 62.5 210.9 93.0 7.52 6.3 46.0 41.5 30.3 37.3 104.6 97.8 80.8 8.56 0.3 6.1 21.0 21.0 5.2 9.3 23.0 47.7 47.7 22.2 8.83 0.1 ' 3.6 7.9 21.6 7.1 18.3 26.7 48.3 9.89 2.0 17.1 36.4 14.5 41.2 68.9 10.60 0.8 10.5 44.1 11.0 31.0 79.5 11.26 0.8 9.0 17.0 0.1 11.5 29.2 41.8 7.3 11.87 0.1 2.0 46.3 1.3 7.0 14.6 82.4 12.8 Surface 0.8 17.3 72.2 4.7 43.9 102.8 203.9 69.0 0.97 0.7 56.3 61.5 45.3 0.7 41.0 172.0 181.5 153.7 41.0 1.90 0.8 46.5 63.0 74.3 4.5 4.2 157.5 186.0 204.0 66.0 2.85 77.6 164.7 120.7 26.7 0.8 213.3 345.1 279.2 123.9 43.9 3.76 80.3 547.5 103.5 7.5 213.0 840.0 249.0 76.5 4.67 52.6 166.3 12.2 170.1 343.9 89.2 5.48 21.0 57.8 7.5 108.0 176.3 76.5 APPENDIX V Continued 66 Time Period Depth i n Metres Class 0 Class 1 Class 2 Class 3 Class 4 hours 6.24 10.6 34.9 0.3 31.3 67.2 9.1 7.52 5.0 6.4 3.1 15.4 17.8 12.0 Surface 0.8 34.1 31.3 0.8 29.3 100.8 95.5 29.3 0.97 39.2 42.2 6.8 110.9 116.0 49.9 1.88 4.1 34.8 53.8 6.6 42.2 102.6 132.4 48.4 2.80 32.8 205.8 32.8 0.8 140.2 408.2 140.2 45.9 3.68 51.3 153.9 76.1 0.8 173.8 334.3 215.2 46.3 4.53 43.9 138.1 32.5 4.9 160.1 308.8 139.0 71.5 5.53 26.7 72.1 4.7 7.8 123.9 203.9 69.0 80.0 6.34 11.3 34.4 7.9 15.2 48.2 86.8 40.6 55.5 7.31 1.6 4.9 62.6 80.5 58.5 71.5 191.0 221.0 8.15 6.9 14.6 43.5 27.2 40.2 82.2 8.99 3.7 5.9 47.6 37.3 42.8 117.1 9.71 0.3 31.8 38.7 9.8 64.3 73.8 10.28 0.3 .2.2 19.2 10.0 16.3 46.2 0.1 7.8 67 APPENDIX YI. Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l larvae i n the l a r v a l classes for samples taken during the 24 hour f i e l d series of July 20-21, 1963. Time Period Depth. i n Metres Class 0 Class I Class 2 Class 3 Class 4 1128-1223 hours 2123-2211 hours 2.88 0.6 2.4 23.8 1.3 6.2 10.4 36.6 16.0 3.78 0.3 , 1.8 125.9 16.4 25.8 216.6 4.67 0.4 6.4 103.8 22.4 46.7 203.5 ( 5.52 0.6 6.4 96.0 21.1 38.3 176.2 6.29 67.6 3.9 . 130.0 28.5 7.13 80.6 6.4 170.8 46.7 7.95 92.1 2.6 193.2 38.6 8.66 82.7 16.5 154.1 55.9 9.53 1.6 58.0 38.8 23.4 120.0 91.5 10.06 4.8 97.4 91.8 70.2 245.9 236.3 10.90 27.2 92.2 92.0 193.2 11.47 7.9 61.5 0.1 28.8 102.4 8.2 12.14 5.7 44.0 22.4 79.0 12.61 0.7 16.2 0.07 6.8 32.3 3.7 Surface 1.6 31.9 167.6 0.8 57.5 136.5 351.2 44.7 0.99 0.8 49.5 73.4 0.8 44.7 167.6 207.5 44.7 1.92 0.8 8.0 37.5 44.7 81.4 146.8 2.87 0.7 16.1 78.3 41.0 95.9 207.8 3.76 82.0 39.0 242.0 166.8 4.64 0.3 13.5 96.6 7.4 18.9 57.7 184.1 44.2 5.48 4.0 87.0 11.2 40.7 180.0 57.5 APPENDIX VI Continued 68 Time Period Depth i n Metres Class 0 Class 1 Class 2 Class 3. Class 4 0511-0600 hours 6.29 0.3 0.3 43.1 1.6 14.9 14.9 97.9 23.4 7.06 0.7 3.7 73.9 0.4 26.3 37.3 156.6 20.5 7.95 0.1 45.9 0.3 7.5 83.1 9.6 8.66 0.3 47.0 2.9 9.6 84.6 17.4 9.43 97.4 245.9 9.95 51.2 2.0 87.8 14.3 Surface 0.1 0.06 9.9 0.9 4.4 3.4 22.4 7.1 .99 0.2 0.08 29.3 0.08 5.9 4.6 52.7 4.6 1.92 0.3 1.3 18.4 0.3 9.6 13.6 44.2 9.6 2.87 0.06 0.1 16.3 1.7 3.4 4.4 31.8 8.8 3.78 0.3 5.9 73.7 0.3 9.6 34.9 141.8 9.6 4.64 0.8 2.4 117.3 0.4 28.7 35.1 221.9 22.4 5.48 2.4 2.4 87.0 2.4 35.1 35.1 180.0 35.1 6.49 0.1 79.8 5.9 4.4 165.0 42.8 7.19 0.4 107.6 20.5 203.4 8.02 93.8 11.2 189.6 57.5 8.75 68.0 28.2 . 147.8 86.0 9.33 0.4 74.6 53.6 24.6 166.8 135.2 10.07 85.4 67.1 226.7 198.0 10.78 8.3 57.3 0.2 38.5 115.9 13.7 11.33 16.0 93.8 0.4 68.2 189.6 22.4 11.99 2.6 16.3 3.3 15.5 39.2 17.0 69 APPENDIX VII. Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l larvae i n the l a r v a l classes for samples taken during the 24 hour f i e l d series of August 18-19, 1963. Time Period 1115-1204 hours 2108-2200 hours Depth . .n Metres Class 0 Class 1 Class 2 Class 3 Class ' 6.86 24.3 44.4 7.63 30.5 1.3 62.1 13.7 8.39 191.4 14.0 396.0 102.7 9.12 213.9 27.1 416.7 126.1 9.83 64.7 67.8 61.5 0.4 146.9 151.7 142.1 22.4 10.24 0.4 58.3 61.5 22.4 137.3 142.1 10.40 20.6 129.1 80.9 244.1 11.49 2.2 68.0 2.2 32.2 147.8 32.2 12.78 14.4 127.7 52.4 215.5 Surface 1.7 46.5 13.8 13.8 62.0 169.7 100.8 100.8 0.95 0.2 0.4 23.0 52.3 2.2 12.0 15.5 61.1 104.7 21.9 1.91 1.8 19.3 35.1 63.2 11.5 150.1 2.76 7.3 56.3 14.6 74.6 172.0 52.7 3.66 77.7 28.7 219.7 133.5 4.50 1.5 35.5 21.2 54.5 139.2 108.9 5.44 1.7 13.8 40.5 62.0 100.8 158.5 6.40 0.1 0.3 18.3 19.3 0.1 7.0 9.0 43.0 44.5 7.0 7.13 25.6 2.7 2.7 41.5 16.0 16.0 7.95 15.3 12.2 2.1 39.4 34.6 15.6 8.66 6.8 11.2 1.4 26.7 34.1 14.8 9.23 3.0 7.2 3.0 17.6 26.4 17.6 9.95 0.07 0.73 6.7 6.1 4.1 7.5 19.0 18.1 APPENDIX VII Continued 70 Time Depth Period i n Metres Class 0 Class 1 Class 2 Class 3 Class 4 0515-0600 hours 10.78 2.0 14.8 2.5 10.5 31.3 11.6 11.61 0.3 5.1 1.2 3.9 13.1 6.4 0.95 0.09 11.5 2.0 3.3 22.4 7.9 1.87 0.09 13.1 1.0 3.5 25.1 6.0 4.46 0.1 0.06 12.8 0.6 4.4 3.4 26.8 6.2 5.44 0.1 0.8 26.9 0.3 7.5 11.7 56.9 9.6 6.13 0.07 0.1 27.9 0.7 3.7 4.8 47.9 6.8 6.86 0.3 55.4 13.1 9.5 95.0 35.9 7.46 74.5 157.9 14.8 63.1 7.99 0.4 67.8 27.5 22.4 151.7 89.0 8.67 0.8 55.1 134.1 0.8 28.7 132.5 244.7 28.7 9.46 2.4 61.5 13.6 35.1 142.1 63.1 9.96 10.7 71.2 1.0 63.9 167.8 35.1 10.40 120.0 254.9 3.6 52.2 11.15 105.9 221.8 0.5 28.2 11.12 42.4 109.2 14.8 63.1 71 APPENDIX VII I . Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l larvae i n the l a r v a l classes for samples taken during the 24-hour f i e l d series of September 21-22, 1963. Time Period 0758-0838 hours 1125-1205 hours 1530-1617 hours Depth L Metres Class 0 Class 1 Class 2 Class 3 Class 8.19 118.5 75.1 237.6 173.7 9.12 47.0 81.7 124.7 177.4 9.83 41.3 86.9 0.9 161.6 238.8 49.2 10.65 6.4 61.5 11.2 46.7 142.1 57.5 11.18 7.0 85.2 17.6 51.4 182.6 75.1 12.14 4.4 50.5 12.3 44.8 130.0 63.2 12.61 3.4 17.2 5.4 13.5 34.8 17.2 7.37 0.05 16.4 10.7 2.7 29.9 21.5 8.19 0.1 18.4 12.3 7.5 44.2 34.6 9.12 27.2 57.1 2.6 92.2 140.5 38.6 9.83 0.4 38.6 47.0 0.4 24.6 119.4 124.7 24.6 10.65 0.4 36.9 99.7 0.4 24.6 . 108.9 203.3 24.6 11.18 23.7 99.7 0.4 86.5 203.3 24.6 12.14 4.1 54.1 0.3 21.1 96.4 10.5 12.61 6.5 38.2 27.8 77.2 14.72 8.4 19.0 0.4 21.7 37.3 24.6 7.37 10.6 . 9.3 26.0 24.1 8.19 60.6 14.9 145.8 69.4 9.12 20.2 63.1 68.3 135.3 9.83 12.3 106.7 9.7 63.2 213.8 57.5 10.65 7.5 133.7 13.1 54.7 254.8 67.3 11.18 0.9 155.3 9.1 51.2 347.2 93.2 APPENDIX VIII Continued 72 Time Period Depth i n Metres Class 0 Class 1 Class 2 Class 3 Class 4 1930-2007 hours 2132-2216 hours 12.14 0.9 53.6 9.7 31.6 135.2 57.5 12.61 1.8 29.0 18.1 25.8 79.6 61.3 Surface 1.8 66.0 7.3 18.7 118.5 31.3 0.97 3.2 36.7 10.1 19.2 72.9 32.6 1.90 .2.1 32.3 4.5 15.6 64.8 21.0 2.78 6.3 19.4 2.1 24.5 45 i-8 15.6 3.63 11.3 22.5 2.3 34.4 52.1 17.1 4.53 6.9 21.4 0.3 26.9 50.3 10.5 6.12 7.6 15.0 29.9 42.3 6.71 2.5 14.2 1.6 11.6 30.4 9.6 7.37 2.8 13.2 0.2 12.8 29.9 5.9 8.19 0.1 10.1 21.4 0.1 8.2 32.6 50.3 8.2 9.12 2.3 28.4 0.3 17.1 60.9 10.5 9.83 1.4 17.9 0.9 14.9 45.1 12.9 10.65 0.9 26.0 0.3 12.9 57.4 10.5 11.18 6.9 21.4 0.9 26.9 50.3 12.9 12.14 0.1 9.2 6.6 4.8 22.1 18.1 12.61 0.2 13.2 5.4 5.6 29.6 17.4 Surface 0.4 44.5 5.1 13.2 89.1 26.3 0.97 0.3- 24.9 13.5 10.5 55.6 38.0 1.91 4.5 35.7 7.2 21.0 69.4 26.2 2.78 7.2 20.5 1.3 26.2 47.4 13.6 3.66 5.9 30.7 3.2 25.0 64.4 19.2 APPENDIX VIII Continued 73 Time Depth Period i n Metres Class 0 Class 1 Class 2 Class 3 Class 4 2322-0002 hours 4.50 0.1 4.1 14.5 0.3 8.2 21.1 39.8 10.5 5.30 5.0 21.4 3.2 23.1 50.3 19.2 6.06 7.5 20.4 0.3 21.1 40.5 7.2 6.63 4.0 19.0 4.5 14.4 37.3 15.4 7.09 4.4 11.9 0.08 16.0 28.0 4.6 8.48 13.5 15.7 38.0 41.6 9.33 3.2 27.2 0.9 19.2 59.1 12.9 9.95 3.2 21.4 19.2 50.3 10.65 1.5 28.4 5.0 14.9 60.9 23.1 11.19 5.6 20.3 2.9 17.2 39.0 12.5 11.99 0.1 16.6 5.0 5.3 33.9 16.3 12.78 1.2 11.4 2.2 9.1 26.8 11.2 Surface 5.9 16.8 5.0 25.0 43.3 23.1 0.98 4.1 11.3 0.7 14.0 25.3 6.8 1.90 5.0 11.9 1.6 16.3 26.9 9.6 2.80 5.0 16.0 0.7 16.3 33.0 7.5 3.60 2.9 7.2 0.4 12.5 19.9 6.4 4.42 4.8 16.2 0.8 22.3 41.7 12.4 5.30. 3.9 20.1 0.3-19.9 47.4 9.9 6.06 13.5 27.2 4.1 38.0 59.12 21.1 6.63 5.0 15.7 2.3 23.1 41.6 17.1 7.28 18.4 34.6 5.3 59.3 .85.1 19.2 APPENDIX VIII Continued 74 Time Period Depth i n Metres Class 0 Class 1 Class 2 Class 3 Class 4 0332-0415 hours 7.99 39.5 30.7 8.8 108.6 93.8 52.3 9.12 1.5 82.5 21.6 55.5 218.9 111.0 9.59 1.3 29.6 6.9 12.9 61.0 25.3 10.65 0.1 21.6 1.3 7.5 49.0 13.6 11.19 0.5 10.6 0.8 5.0 22.0 5.7 11.82 0.1 7.5 2.1 3.2 16.9 8.2 12.61 0.8 7.1 1.1 5.7 16.8 6.4 Surface 1.3 7.9 .05 7.0 18.0 2.7 .97 2.0 6.7 1.4 8.3 16.2 7.0 1.90 1.4 7.9 1.7 7.0 18.0 7.7 2.78 2.1 9.0 0.3 8.2 19.0 3.9 3.66 1.1 12.9 0.4 7.8 27.7 5.9 4.50 2.9 10.1 1.6 12.5 24.3 9.6 5.35 2.3 7.1 2.3 10.3 18.9 10.5 6.18 : 5.9 19.0 0.3 25.0 46.8 10.5 6.93 3.3 15.4 2.0 13.0 31.8 10.1 7.61 14.2 11.3 1.46 30.4 26.0 14.9 8.19 8.3 140.7 1.7 84.4 31.4 59.6 8.79 0.9 83.2 13.1 33.6 183.3 67.3 9.95 . 1.4 39.4 10.6 14.1 75.2 32.4 10.78 0.1 28.4 14.5 8.2 60.9 39.8 11.19 1.2 24.6 3.4 8.6 44.9 13.5 12.29 1.8 13.8 4.4 10.7 30.9 16.0 75 APPENDIX VIII Continued Time Depth Period i n Metres Class 0 Class 1 Class 2 Class 3 Class 4 0513-0555 hours 12.61 .05 8.7 5.2 2.7 19.1 13.7 3.71 5.4 18.2 .09 13.7 31.0 3.2 4.53 5.4 8.0 .05 13.9 17.8 2.6 4.40 4.9 9.2 14.8 21.7 6.18 13.2 24.7 3.7 36.2 53.7 19.2 7.06 13.2 11.2 0.1 36.2 33.0 7.5 7.79 0.1 5.3 26.9 4.5 7.5 22.7 56.9 21.0 8.29 8.8 128.2 8.8 89.6 302.0 89.6 9.01 49.5 116.5 8.0 167.6 274.6 81.4 9.83 9.7 50.5 20.6 57.5 130.0 80.8 10.78 2.4 55.1 6.4 35.1 132.5 46.7 11.75 0.1 39.2 10.2 8.2 76.4 32.6 11.96 0.3 23.7 9.1 10.5 53.9 30.7 12.61 1.9 11.3 7.1 9.6 25.3 18.9 76 APPENDIX IX. Lower and upper confidence l i m i t s expressed i n members per 100 1 for estimated t o t a l counts of larvae i n samples taken during the 24 hour f i e l d series of June 16-17, 1963. Time Depth Lower Upper Period i n Metres Limit Limit 0805-0922 0.97 101.3 130.4 hours 1.90 598.6 910.3 2.87 298.0 453.1 3.76 263.4 495.0 4.67 298.0 525.5 5.60 217.1 288.6 6.45 98.4 149.0 1120-1207 .99 170.2 286.6 hours 1.93 281.4 429.8! 2.91 494.3 730.3 3.76 528.7 816.4 4.73 313.7 578.2 5.64 339.2 587.5 6.66 185.9 312.5 7.52 160.7 115.2 8.56 60.9 103.7 8.83 43.9 79.4 9.89 65.7 107.9 10.60 63.8 106.3 11.26 35.0 68.2 1522-1612 0.97 132.5 196.0 hours 1.96 557.7 862.0 2.87 443.3 713.1 3.80 406.7 682.6 4.54 285.6 512.7 5.64 191.8 259.0 6.34 139.1 202.6 . 7.42 87.2 140.9 1922-2005 Surface 127.7 187.0 hours 0.97 215.3 288.9 1.92 423.6 523.4 2.88 383.1 638.6 3.80 396.4 655.3 4.76 196.1 323.5 5.68 105.6 186.1 6.49 126.9 184.1 7.06 94.5 143.7 2110-2211 Surface 158.5 335.7 hours .97 227.6 423.7 1.90 264.8 477.0 77 APPENDIX IX Continued Time Depth Lower Upper Period i n Metres Limit Limit 2.85 509.8 807.8 3.76 842.9 195.5 4.67 297.5 511.0 5.48 121.5 276.0 6.24 52.9 92.0 7.52 18.7 35.6 2310-1200 Surface 114.5 166.9 hours 0.97 109.9 163.9 1.91 158.8 220.3 2.85 713.0 841.6 3.86 767.3 1107.5 4.76 428.5 437.4 5.53 163.8 227.3 6.62 58.3 97.2 0105-0153 Surface 99.2 150.0 hours .97 110.8 164.1 1.92 565.2 878.3 • 2.85 807.0 1163.1 3.76 855.3 1216.1 4.64 237.0 280.4 5.48 130.1 188.4 6.29 66.2 110.3 0317-0404 Surface 114.7 166.2 hours .96 120.3 177.4 1.90 215.9 290.4 2.85 587.6 889.7 3.76 391.3 652.2 4.64 308.8 398.7 5.60 152.4 215.8 6.43 108.5 158.2 0512-0600 Surface 88.6 183.3 hours 0.97 114.3 222.7 1.88 132.4 244.1 2.80 340.3 582.2 3.68 355.9 612.4 4.53 286.9 516.8 5.53 158.5 335.7 6.34 97.2 176.1 7.31 204.0 404.7 8.99 71.0 152.2 9.71 84.0 131.4 APPENDIX X. Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l counts of larvae i n samples taken during the 24 hour f i e l d series of July 20-21, 1963. Time Depth Lower Upper Period . i n Metres Limit Limit 0815-0909 7.13 142.5 203.1 hours 7.95 132.8 188.9 8.66 323.1 411.0 9.53 240.8 317.7 10.06 209.8 282.2 10.90 74.4 115.9 1128-1223. 2.88 32.3 53.7 hours 3.78 169.8 234.1 4.67 124.1 231.1 5.52 120.0 204.9 6.29 78.0 143.9 7.13 97.0 194.4 7.95 103.2 208.5 8.66 109.1 186.3 9.53 119.7 200.9 10.06 241.9 453.4 10.90 133.0 249.3 11.47 74.6 122.9 12.14 54.9 92.1 1521-1602 9.33 443.4 540.8 hours 9.95 319.4 406.7 1920-2012 9.80 324.5 412.3 hours 10.18 226.7 295.4 2123-2211 Surface 248.2 462.2 hours 0.99 161.2 341.6 1.92 67.1 198.0 2.87 118.5 269.3 3.76 150.2 347.3 4.64 145.2 243.2 5.48 1241.2 231.1 6.29 53.7 113.9 7.06 95.1 186.6 7.95 49.2 87.7 8.66 57.2 95.8 9.43 97.4 245.9 79 APPENDIX X Continued Time Depth Lower Upper Period i n Metres Limit Limit 2307-2354 3.82 224.8 299.1 hours 4.70 279.8 358.8 0320-0400 3.82 191.7 246.8 hours 4.67 , 182.7 250.8 0511-0600 10.07 23.1.6. 284.3 hours 80 APPENDIX XI. Lower and upper confidence l i m i t s expressed i n numbers per 100 1 for estimated t o t a l counts of larvae i n samples taken during the 24 hour f i e l d series of August 18-19, 1963. Time Depth Lower Upper Period i n Metres Limit Limit 0515-0600 6.13 31.3 53.2 hours 6.86 79.1 125.3 7.46 102.2 196.6 7.99 114.1 217.5 8.67 227.5 363.2 9.46 93.8 189.6 9.96 102.4 214.6 10.40 129.5 267.9 11.12 68.6 149.0 

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