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The ocular structure, retinomotor and photobehavioral responses of juvenile Pacific Salmon Ali, Mohamed Ather 1958

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tE^s pttfaergti]} of ^riifelj (EdhmtMa Faculty of Graduate Studies PROGRAMME OF T H E FINAL ORAL EXAMINATION F O R T H E D E G R E E O F : DOCTOR OF PHILOSOPHY "of M O H A I M E D A T H E R A L I B. Sc. Presidency-College, University of Madras, 1952v: M . Sc. Zoology Laboratory, University of Madras, 1954 IN R O O M 187A, BIOLOGICAL SCIENCES BUILDING T U E S D A Y , A P R I L 29, 1958 at 2:30 p. m. C O M M I T T E E I N C H A R G E DEAN G. M . SHRUM, Chairman W. A. C L E M E N S P. CONSTANTINIDES I. McT. COWAN P. F O R D W. S, H O A R C. C. L I N D S E Y , P I N G - T I HO External Examiner: E . Home Craigie University of Toronto A B S T R A C T A histological study of the eyes of juvenile sockeye, coho, pink and chum salmon in fresh water shows that the cones, external nuclear and plexiform layers of the retinae of embryos and alevins are poorly differentiated and do not attain normal: histological or phsyiological proportions until the emergence of fry from the gravel. From a histo-physiological study it is evident that only the emerged fry and older stages are capable of '. retinomotor responses and that these become more marked with age. Differ-ences in rates of adaption are found among the species and stages. Generally, the pigment layer shows a latent period before con-traction in dark. Sensitivity to light is independent of the complete light-adaption of the retinal pigment or visual cells, while full acuity of vision is dependent upon the complete light-adaption of cones. Threshhold value of cones and rods are indicated by the feeding and schooling responses. At light intensities between the cone and rod thresholds the thicknesses of pigment and cone layers obey the Weber-Fechner Law. There is no diurnal rhythm in the positions of retinal pigment and cones of juvenile Oncorhyncbus either under constant light or dark. Results are discussed in relation to the migratory, schooling and feeding behaviour. The rapid down-stream migration of juvenile salmon during a relative short period in the night may be related to a semi-dark adapted state of the eye. P U B L I C A T I O N Al i , M . A . Additions to the Sponge Fauna of Madras. J. Madras Univ. B., 26(2): 1956. A l i , M . A . Development of the Monaxonid Sponge, Lissodendoryx similis Thiele. J. Madras Univ. B;, 26(3): 1957. A l i , M . A . and J. R. Brett. The Structure and Photomechanical Responses of the Pacific Salmon Retina. J. Fish.Res. Bd. Canada 15 (in press) 1958. G R A D U A T E S T U D I E S Field of Study: Zoology History and General Principles of Biology .... W . A . Clemens Biology of Fishes C. C. Lindsey Environmental Physiology W . S. Hoar Fisheries Technology H . L. A . Tarr and W . S. Hoar Biological Methods and Procedures Zoology Staff Comparative Physiology W . S. Hoar Fisheries Biology W . A . Clemens Other Studies: Biometry V . C. Brink Synoptic Oceanography G . L. Pickard Oceanography W . A . Clemens. Special Advanced Oceanography R. F. Scagel -Chemical Oceanography : M . Kirsch Biological Oceanography t R. F. Scagel THE OCULAR STRUCTURE, RETINOMOTOR AND PHOTO-BEHAVIOURAL RESPONSES OP JUVENILE PACIFIC SALMON by MOHAMED ATHER ALI B.Sc, Presidency College, University of Madras, 1952 M.Sc, Zoology Laboratory, University of M&dras, 1954 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the department of Zoology We accept this thesis as conforming to the required standard from candidates for the degree of DOCTOR OF PHILOSOPHY THE UNIVERSITY OF BRITISH COLUMBIA April, 1958 ABSTRACT A histological study of the eyes of juvenile sockeye, coho, pink and chum salmon i n fresh water shows that the cones, external nuclear and plexiform layers of the embryos and alevins are poorly differentiated and do not attain normal histological or physiological proportions u n t i l the emergence of fry from the gravel. From a histo-physiologieal study i t i s evident that only the emerged fry and older stages are capable of retinomotor responses and that these become more marked with age. Differences i n rates of adaptation are found among the species and stages. Generally, the pigment layer shows a latent period before contraction i n dark. Sensitivity to light i s independent of the complete light-adaptation of the retinal pigment or visual ce l l s , while f u l l acuity of vision i s depend-ent upon the complete light-adaptation of cones. Threshold values of cones and rods are indicated by the feeding and school-ing responses. At light intensities between the cone and rod thresholds the thicknesses of pigment and cone layers obey the Weber-Pechner Law. There i s no diurnal rhythm i n the positions of retinal pigment and cones of juvenile Oncorhynchus either under constant light or dark. Results are discussed in relation to the migratory, schooling and feeding behaviour. The rapid downstream migration of juvenile salmon during a relatively short period in the night may be related to a semi-dark-adapted state of the eye. In presenting 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 permission 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 . 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. Department of Zoology  The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada. 0 3 t e A p r i l 16 r 1958  ACKNOWLEDGEMENTS Coming to Canada and doing post-graduate work at the University of British Columbia would have been a Herculean task had i t not been for the kind offices of my friend and well wisher, Dr. E.A. Forsey, Director of Research, Canadian Labour Congress, and I wish to take this opportunity to express my deep gratitude to him for helping me to come to Canada, the interest shown and help given on several occasions. My thanks are due the World University Service of Canada for the award of a bursary during 1954-55. To my teachers and philosophers, Dr. W.A. Clemens, F.R.S.C., and Dr. W.S. Hoar, F.R.S.G., I owe my most humble thanks for the interest shown i n me and my work since the day of my arrival i n Vancouver and for the unfailing guidance, advice and encouragement given so ungrudgingly throughout the last four years. I was f i r s t introduced to the study of vision by Dr. J.R. Brett, Senior Scientist, Fisheries Research Board of Canada, Nanaimo, B.C., while employed there during the summers of 1955 and 1956. I have found this study a rich and rewarding experience and am thankful to him for introducing me to i t . To Dr. C.C. Lindsey, I am thankful for many stimul-ating discussions and for the suggestions and criticisms that improved the experiments and the thesis. The advice given by Dr. P. Constantinides and Dr. P. Ford, F.Z.S., during the course of this work i s acknowledged. This research was carried out in the Department of Zoology and the Institute of Fisheries and I wish to acknow-ledge the encouragement given and f a c i l i t i e s provided by Dr. I. MoT. Cowan, F.R.S.C., Head of the Department of Zoology and Dr. P. Larkin, Director of the Institute of Fisheries. The assistance given by my colleagues i s also gratefully acknowledged. , Mr. R.J.H. Beverton of the Fisheries Laboratory, Lowestoft, while here on an H.R. MacMillan Visiting Lectureship, provided valuable advice on s t a t i s t i c a l methods and analyses for which I am thankful. Prof. Dr. G. von Studnitz, Director of the Natural History Museum, Lubeck, W. Germany, wrote and advised me on several aspects of my problem and also brought my attention to the publications of some German workers and I wish to acknowledge this thankfully. The keen personal interest taken by Thomas Killam in the execution of the attractive diagrams that adorn this thesis i s gratefully acknowledged. To Mr. and Mrs. Hermann Schlagintweit and Miss Sibylle Schlagintweit, I owe my thanks for the assistance given and particularly for improving the nodding acquaintance I had with the German language to a more familiar one. Without this, deciphering and understanding the vast literature i n German, so essential i n an investigation concerning visual physiology, would have been a formidable task. This research was supported by a grant from the Fraser River Hydro and Fisheries Advisory Committee of the University of British Columbia. RESPECTFULLY DEDICATED TO MY PARENTS - i -TABLE OP CONTENTS I INTRODUCTION 1 II MATERIAL AND METHODS 3 A. Material 3 B. Experimental methods 3 1. General........ 3 2. Rates of adaptation 5 a. As seen by photomechanical responses of the retina 5 b. Schooling times 5 e. Feeding rates 7 3. Retinal and behavioural responses to light intensities. 8 a. Retinal photomechanical responses to light intensities...... 8 b. Feeding rates under different light intensities............ 8 c. Observations on intensities at which schools dispersed 9 4. Photomechanical responses under constant light or dark .... 9 5. Histology 10 a. Technique for routine study 10 b. Modified Golgi technique 10 i . Fixation 10 i i . Silver impregnation 11 i i i . Dehydration.. 11 iv. Celloidin imbedding 11 v. Paraffin imbedding 11 v i . Sectioning 12 v i i . Mounting 12 6. Measurements of retinal pigment and cone layers 12 III RESULTS 14 A. Structure of Oncorhynchus eye 14 - i i -1. General shape 14 2. Vertical l i d s 14 3. Cornea 14 4. Sclera 16 5. Annular ligament 16 6. Ir i s 16 7. Lens 17 8. Intra-ocular fluids 18 9. Chorioid 18 10. Falciform process 19 11. Retina 21 B. Differences among stages 27 C. State of a typical light-adapted retina... 29 D. State of a typical dark-adapted retina 29 IV EXPERIMENTAL RESULTS". 30 A. Light-adaptation 30 1. The process of retinal light-adaptation.... 30 2. Embryos 31 3. Alevins (Hatching stage) 31 4. Emerged fry 36 a. Retinal response 36 b. Schooling rates 40 c. Feeding rates 41 5. Late fry 41 a. Retinal response 41 b. Schooling rates 47 c. Feeding rates... • 49 - i i i -6. Smolts . 49 a. Retinal response 49 b. Schooling rates 49 c. Feeding rates 52 B. Dark-adaptation 52 1. The process of dark-adaptation 52 2. Embryos .... 56 3. Alevins (Hatching stage) 56 4. Emerged fry (Post-yolk sac stage) • 59 5. Late fry 59 6. Smolts 60 0. Retinomotor and behavioural responses to different light intensities 64 1. Alevins 69 2. Late fry 72 3. Smolts 75 D. State of the retinal pigment and cone layers under constant light or dark 78 V DISCUSSION 85 A. Structure of the Oncorhynchus eye... 85 1. Ir i s 85 2. Supplementary nutritive device 85 3. Chorioid gland 86 4. Twin cones 86 B. Correlation between the arythmic mode of l i f e of Oncorhynchus and the structure of i t s retina 86 - iv -C. Retinomotor responses 87 1. Phylogenetic occurrence*-.-'*--*- 87 2. Ontogeny of retinomotor response in Oncorhynchus 88 3. Interspecific comparison of retinomotor responses of the species and stages of Oncorhynchus 89 D. Significance of retinomotor response in the l i f e of juvenile Oncorhynchus 89 1. Precedence of light sensitivity to retinomotor responses.. 89 2. Migratory behaviour 90 3. Schooling 93 4. Feeding 93 VI CONCLUSIONS 95 VII LITERATURE CITED 97 - V -TABLES I. Details regarding the fish used in the present investigation 4 II. Details of the rates of light-adaptation in alevins 34 III. Particulars concerning the rates of light-adaptation of emerged fry 37 IV. Details of rates of light-adaptation of late fry.... 44 V. Particulars regarding the rates of light-adaptation of the smolts 51 VI. Summarising results of previous investigations concerning rates of dark-adaptation of various species of f i s h 54 VII. Details regarding the rates of dark-adaptation of the alevins 58 VIII. Details regarding the rates of dark-adaptation of the emerged fry 60 IX. Particulars concerning the rates of dark-adaptation of the late fry 62 X. Particulars regarding the rates of dark-adaptation of the smolts 65 XI. Summarising the rates of dark-adaptation of various f i s h used in previous investigations 67 XII. Details of the state of the eye of the alevins under different light intensities 71 XIII. The state of the eye, feeding and schooling under different light intensities in the case of late fry. 74 XIV. The state of the eye and feeding of smolts under different light intensities 76 XV. Summary of results of previous investigations concerning the state of the eye and acuity of fish of various species 80 - v i -ILLUSTRATIONS 1. Diagram showing the tanks used in the experiments : 6 2. Photomicrograph of a vertical section of the eye of Oncorhynchus 15 3. Photomicrograph showing the falciform process in emerged fry... 20 4. Photomicrograph of retina and diagram of the neurological arrangement therein... 22 5. A. Diagram of retinal pigment, cones and rods in the light and dark-adapted states. 24 B. Diagram showing the arrangement of cones in the regions of the fundus and ora serrata 24 6. Graph showing the rate of expansion of the pigment and contraction of cones on exposure to light in the various stages of sockeye studied 32 7. Graph showing the rate of expansion of pigment and contraction of cones on exposure to light in the various stages of coho.... 33 8. Graph showing the rate of expansion of pigment and contractions of cones on exposure to light in the various stages of pink 35 9. Graph showing the rate of expansion of pigment and contraction of cones on exposure to light in the various stages of chum 38 10. Graph showing the schooling rate of dark-adapted emerged and late fry and smolts on exposure to light.. 39 11. Graph showing the feeding rates of conditioned dark-adapted emerged and late fry and smolts on exposure to light 42 12. Photomicrographs of "light" and "dark"-adapted retinae of late embryos.. 43 13. Photomicrographs of light and dark-adapted retinae of alevins 45 14. Photomicrographs of light and dark-adapted retinae of emerged fry 46 15. Photomicrographs of light and dark-adapted retinae of late fry 48 - v i i -16. Photomicrographs of light and dark-adapted retinae of smolts 50 17. Histogram showing the time for light-adaptation of pigment and cones of emerged fry, late fry and smolts 53 18. Rate of contraction of the pigment and expansion of the cones of the various stages of sockeye 55 19. Rate of contraction''of pigment and expansion of cones in dark of the various stages of coho 57 20. Rate of contraction of the pigment and expansion of the cones of the various stages of pink 61 21. Rate of contraction of the pigment and expansion of the cones in dark of the various stages of chum 63 22. Histogram showing the times taken by pigment and cones of emerged fry, late fry and smolts to dark-adapt 66 23. Graph showing the thicknesses of the pigment and cone layers of sockeye and chum alevins under various light intensities 70 24. Graph showing the thicknesses of pigment and cone layers and feeding rates of late fry belonging to the different species 73 25. Graph showing the pigment and cone thicknesses and feeding rates of smolts belonging to different species under various light intensities 77 26. Histogram showing the cone thresholds of alevins, late fry and smolts of the various species studied... 79 27. Diagram summarising some results obtained in the present investigation together with some other responses of Pacific salmon to various natural conditions are also given 81 28. Graph showing the thicknesses of pigment and cone layers of late chum fry under constant light and dark at intervals of 3 hours for 96 hours 83 I. ,INTRODUCTION Light plays an important role i n the l i f e of animals. Phenomena such as feeding, sexual maturity and migration are governed wholly or partly by light. The chief photoreceptors of most vertebrates are their eyes. For obvious reasons, the human eye was the f i r s t to be studied and seems to have kindled the interest of workers as early as 1,000 B.C. (Sasruta, c. 1,000 B.C.). However, detailed investigations yielding results similar to those available at present were not made un t i l the ninth century A.D. (Hunain ibn Ishak, 806-877; A l i ibn Isa, 940-1010). Since then, much work on the vertebrate as well as the invertebrate eye has been carried out, especially during the last one hundred years, and excellent books dealing with the structure and physiology of the eye with extensive bibliographies are available (Polyak, 1941, 1957; Walls, 1942; Rochon-Duvigneaud, 1943, 1958; Detwiler, 1943; G-ranit, 1947, 1955; von Studnitz, 1952; von Buddenbrock, 1952). The mechanism of accommodation to distant or near vision varies from one vertebrate group to another and often within the same class (Walls, 1942, pp. 272-3). The various classes of vertebrates also show differences i n the way lit hey adapt their eyes for photopie or scotopic vision. This i s accomplished either by photomechanical changes involving the dilatation and contraction of the i r i s or movement of the retinal pigment and visual cells (Walls, 1942; Rochon-Duvigneaud, 1943, 1958; von Studnitz, 1952. Teleosts, with the exception of the eel (von Studnitz, 1933), stargazers and f l a t f i s h (Young, 1931, 1933), light and dark-adapt solely by the migration of the retinal pigment and visual c e l l s . The eel has a contractile i r i s i n addition. The stargazers and f l a t f i s h light and dark-adapt by the contraction and dilatation respectively of their i r i s and i t i s not known whether their retinae are capable of undergoing photomechanical responses. The photomechanical changes in the teleost eye, in response to light or dark are pronounced, equalled only by those seen in the eyes of birds - 2 -(Detwiler, 1943). Several investigations have been carried out to demonstrate the positional changes undergone by the retinal pigment and visual cells of fishes i n light and dark (Garten, 1907; Arey, 1915, 1928; Parker, 1932; Detwiler, 1943; von Studnitz, 1952). In a preliminary study of the Oncorhynchus eye no differences were found in the retinae of the various species (Al i , 1956; A l i and Brett, 1958). This i s perhaps surprising since differences have been observed among the species of Oncorhynchus i n their reactions to light (Hoar, 1951, 1953, 1956, 1958; Hoar et a l , 1957). It seemed possible that a more detailed study of the eye and the ontogeny of photomechanical and behavioural responses of the different species might reveal differences not detected in the i n i t i a l study. If differences are present these might at least partly explain the somewhat different responses of the various species to light during their downstream migration. This study was based on the assumption that such a detailed comparative histophysiological examination of the eye would contribute further to an understanding of the mechanisms of downstream migration of salmon. II. MATERIAL AND METHODS A. Material A l l the experiments were carried out i n 1957. Table I shows the species and stages of salmon used and gives some particulars regarding them. A l l the f i s h , except sockeye and coho smolts were from the hatchery, the Department of Zoology, University of British Columbia. Maximum light intensities at the surface of the water in the fish troughs were 1.5 to 2.5 f t - c , in January and July respectively. Diurnal light period varied with the season of the year. The water temperature ranged from 7°C to 13°C. The f i s h were fed twice daily on a mixture of Clark's dry food*, canned salmon, Pablum**, cod l i v e r o i l and yeast extract. The cod l i v e r o i l supplement was adequate to satisfy Vitamin A requirements essential for the proper functioning of the visual mechanism (Detwiler, 1943; Kampa, 1953). B. Experimental methods 1. General. Experiments were carried out in a light proof room. Two photometers were used for measuring light intensities. They were a Photovolt Model 200 Photometer and a Photovolt Model 520-M Electronic Photometer. The former was used to measure light intensities above 10""*" f t - c . and the latter to measure the lower intensities. The model 520-M was used with f i l t e r s (East-man Kodak wratten f i l t e r s No. 81-EP;CC-10-M; 81-C) to produce a curve with equal spectral response to light of wavelengths 3,000 A 0 to 7,000 A 0. The desired light intensities were created with bulbs of similar spectral ranges. Foot-candle (ft-c.) i s the unit of light measurement employed throughout. One f t - c . , i s equal to 10.764 Lux (Metre-candles). * Obtained through the courtesy of Mr. J.R. Clark of Salt Lake City, Utah, U.S.A. ** Meade Johnson. Prepared mixed cereal. TABLE I - Particulars of the material used in this investigation. Experiments in which f i s h were used Species and Stage Rate of Adaptation Adaptation to L. Intensity Diurnal Rhythm Expt. juengrn in cm. Histo-logical Schooling Feeding Histo-logical School Dispersion Feeding Sockeye Embryo 0.50* Alevin 2.00 + + + Emerged fry 2.75 + + Late fry 3.50 + + . + + + + + i Smolt 6.80 + + + + + Coho Embryo 0.80* +••• Alevin 2.30 + Emerged fry 3.70 + + + Late fry 3.90 + + + + + + Smolt 7.10 + + + Pink Embryo 0.80* Alevin 2.40 + Emerged fry 3.30 + + + Late fry 3.40 + + + + + + Chum Embryo .80* Alevin 2.40 + + + Emerged fry 3.30 + + + Late fry 3.90 + + + + + + + The measurements given for embryos are the diameters of eggs. 2. Rates of Adaptation a. As seen by photo-mechanical responses of the retina In these experiments animals were sampled at intervals following sudden exposure to bright light after a period in darkness or reverse situation. The experiments were carried out using a rectangular galvanised iron trough 137 cm. long, 38 cm. wide and 30 em. high, painted black on the inside. Water circulation was maintained during the experiment. The trough was illuminated (400 ft-e.) by two Sylvania Reflector flood lamps, fixed on opposite ends of the tank. Diagonal wire frames were placed at the ends of the tank to make sure a l l the f i s h were in the same intensity of light, that i s , that none was able to retreat to the less bright corners. The arrangement i s shown in Figure .1. A l l the experiments were carried out during the forenoon.;; Possible variation caused by diurnal rhythm or similar factors was thus minimised. In the light adaptation experiments f i s h were l e f t i n total darkness in the experimental tank over-night. At the commencement of the experiment, the f i r s t samples were taken in total darkness (zero minutes) and then the lights were turned on and the samples taken after the following times (in minutes): 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60 and 70. In the dark adaptation experiments the f i s h were l e f t i n the illuminated tank (400ft-c.) overnight, the f i r s t sample was taken from the illuminated tank (zero minutes); the lights were turned off and samples taken in total darkness at the same intervals as in the case of light adaptation. The frequency of samples was based on earlier work by A l i and Brett (1958) where dark adaptation of sockeye was found to be complete in 50 minutes. b. Schooling times The same tank was used. Only emerged and late fry were studied. A group of 50 f i s h were l e f t i n the tank in darkness overnight. The lights were turned on the next morning — 6 <s* Pig. 1. The tanks used in the various experiments (see text). - 7 -and the number of schooled f i s h recorded at five minute intervals. If more than two fish were swimming in the same direction, at the same speed and in regular formation, this was considered a school. Occasionally two schools were seen in the same tank. In these cases the total number of fish in both schools was taken as the number of f i s h schooled. Observations were continued u n t i l most of the f i s h schooled or t i l l the strength of the school reached a maximum and the number stayed the same for over five minutes. Some of the recently emerged fry did not form schools within a day or two after emerging. In this case 50 fish were f i r s t transferred to another tank under brighter light (400 f t - c ) , where they schooled readily. They were then used in the experiment after they had had schooling experience. c. Feeding rates Fish (six from each group) were f i r s t conditioned for at least a week to feed on Daphnia. The food was presented each morning in association with a vibratory stimulus given by gently tapping the side of the aquarium with a rod. A l l learned quickly and after five or seven days of training were able to feed on about 18 to 20 Daphnia per minute. Six aquaria (30 cm. long, 24 cm. wide and 24 cm. high) were arranged i n a large trough of running water with one conditioned fish in each. Eaeh tank had an opening on the side through which a glass tube, inserted through a rubber stopper, was passed. This glass tube reached almost the bottom of the aquarium. The outer end of the glass tube was attached to a rubber tube with a funnel (Fig. l ) . This arrangement was resorted to so that the fis h could be fed without toQ(imuch disturbance. The next morning the lights were turned on (400 ft-c.) and 100 Daphnia were poured into the aquarium through the funnel after giving the vibratory stimulus. After five minutes the f i s h was quickly removed from the tank and another 100 Daphnia poured into the next aquarium in the same way as i n the case of the f i r s t one after giving the stimulus; after five minutes this was also removed and the same procedure repeated i n a l l the remaining aquaria. After the sixth tank the water from each aquarium was poured out through a net - 8 -and the remaining number of Daphnia in each tank counted. Control experiments without f i s h i n the aquarium showed that the experimental procedure was accurate and no Daphnia were lost during the process of pouring through the funnel, capturing and counting. 3. Retina] and Behavioural responses to light intensities In these experiments the desired light intensities were created by reflecting the light from the white ceiling. The lower light intensities were obtained with weaker bulbs and i n the case of the lowest light intensity used (10~^ f t - c ) , a black wooden box with an aperture diaphragm and with a G—E 7.5 watt bulb was used. The following were the light intensities created; 1G2, 10 1, 10°, 10"1, 10~2, 10"5, 10~"4, 10~5 f t - c , and almost total darkness. Light intensities were read at different areas of the large trough and glass aquaria (30 cm. long, 24 cm. wide and 24 cm. high) set up i n the areas where the light readings were similar. a. Retinal photomechanical responses to light intensities The f i s h were l e f t i n the light intensity at which they were to be fixed for an hour and a half before they were ki l l e d . This time was more than sufficient for the fish to adapt to that particular intensity. With sockeye and chum alevins one group each was kept under constant light and another batch of each i n constant dark for three days before exposing them to different light intensities. This was done to determine whether previous experience had any effect on their reactions t o different light intensities. Since no difference was observed this procedure was discontinued i n subsequent experiments and f i s h from the hatchery were directly transferred to the intensity under which they were to be fixed. b. Feeding rates under different light intensities Three fis h from each group were conditioned to feed on Daphnia as described above. A l l six types of salmon were studied at the same time. The desired intensity of light was set up i n the morning and one conditioned f i s h from each group put in a separate aquarium (30 cm. long, 24 cm. wide and 24 cm. high) and l e f t under the light intensity for an hour and a half after which the Daphnia were fed in the usual manner. After five min-utes the f i s h was taken out and the remainder of Daphnia counted. The three fish were thus studied i n succession. By adopting this procedure the whole series was completed i n nine days. The remainder of the 100 Daphnia were offered to the f i s h after the experiment to ensure that a l l fl.sh got the same amount of food every day. c. Observations on intensities at which schools  dispersed Only the late fry were used in this experiment. The experimental arrangement was the same as above. About ten f i s h were put in the aquarium and l e f t in i t for an hour and a half under the light intensity at which they were to be observed to see whether the school was intact or dispersed. The writer was able to observe this without any d i f f i c u l t y after adapting himself (10-15 minutes) to the intensity i n question. Under 10 f t - c , observations were made by suddenly switching on the light and noting whether the school had broken up. Even under the intens-i t i e s where the animals had formed a school, sudden illumination broke up the school, but i t was possible, immediately after turn-ing on the light (one to two seconds) to notice the intact school and then i t s dispersal. In the event where no school was existing, i t was possible immediately on turning on the light to see the f i s h scattered a l l over the aquarium, some even settled on the bottom. 4. Photomechanical responses under constant l i g h t or dark These experiments to test for diurnal rhythm in the positions of the retinal pigment and visual cells in constant light or dark were carried out using the galvanised iron tank described above (Pig. l ) . A l l the experiments were commenced at noon and ended 96 hours later. Animals to be sampled were l e f t in the tank in light (400 ft-c.) or in dark and sampled every three hours. - 10 -5. Histology a. Technique for routine study Bouin's fixative was used throughout. The whole animal was k i l l e d by dropping directly into separate oars of Bouin's fixative. This method waw resorted to after comparing the retinae of f i s h fixed by three different methods viz., (i) dropping the animal directly in Bouin's ( i i ) beheading and fixing only the head and ( i i i ) anaesthetising f i r s t with chlore-tone and then enucleating the eyes and fixing them. It was found that there was no difference among the retinae fixed by these methods. Direct fixation was less time consuming and also the length of the f i s h and diameter of 1he eye could be measured later. After a day in the fixative, the f i s h was measured and the eyes (in the smolts) or the whole head (embryos, alevins and fry) excised and l e f t i n the fixative for another day. The lenses of the smolts and late fry were removed. The eyes and the heads were kept in 70$ alcohol for one or two days, after which they were dehydrated in 90$ absolute alcohol, cleared i n xylene and embedded in paraffin (Fisher's tissuemat; M.P. 52°C). Sections were cut at eight microns, stained with Harris' haematoxylin, counterstained with eosin and mounted i n Canada balsam. b. Modified Golgi technique To study the neurological arrangement of the retina, Golgi's silver impregnation technique, modified as follows by the author, was used. i . Fixation The fixative was injected into the eye of the anaesthetised f i s h . Subsequently, the enucleated eye was punctured at the sclero-comeal junction and dropped into the fixative. The fixative consisted of the following: 4 parts of 3$ potassium dichromate solution and 1 part of a Ifo solution of osmium tetraxide (osmie acid). Soon after dropping the eye in the fixative more fixative was injected and this process was repeated u n t i l the eye was hard - 11 -enough for the cornea to be removed. When the eye was hard enough, the cornea and the lens were removed. The rest of the eye was l e f t undisturbed for seven days at 4G°G. i i . Silver impregnation After the period of fixation, the eye was taken out of the f ixative and the fixative removed from i t using absorb-ent paper. Then i t was placed in a 1$ silver nitrate solution. This solution was ehanged every seven minutes, twice, after which the eye was l e f t in i t for two days. After this period, the eye was rinsed in fresh silver nitrate solution, so as to remove the precipitation particles. Later, the eye was l e f t i n running tap water for a day to wash the osmium and then transferred to di s t i l l e d water which was changed twice, i i i . Dehydration Subsequently, the eye was dehydrated in 70$, 95$ and 100$ alcohols for half an hour in each strength. After dehydration i t was l e f t i n a mixture of one part of acetone and three parts of absolute alcohol for one hour at 60°0. Cupric sulphate was added to the alcohols a day or two i n advance of use. After this treatment, the eye was washed in several changes of absolute alcohol to remove traces of acetone, iv. Celloidin imbedding One day each in the following: 1:1 ether and alcohol mixture 2, 4, 8 and 16$ celloidin After a day in 16$ celloidin the eye was blocked and l e f t for one hour in chloroform followed by another hour i n 70$ alcohol. v. Paraffin imbedding Two changes of one and one half hours i n xylene were given. When the eye was in xylene, the container was transferred to a cold oven set at 52°C and the heat turned on. In this way the specimen was gradually warmed. When the temp-erature i n the oven reached 52°C the eye was transferred to paraffin (M.P. 52°C) and given two changes of 45 minutes each. After this the specimen was blocked, without using cold water to harden the paraffin. - 12 v i . Sectioning Sections were cut at 60 microns. Celloidin sections were placed on slides, blotted arid covered with. 1$ celloidin and dehydrated i n 75 and 95$ alcohol and cleared in , 1:3 mixture of carbol and xylene and then l e f t i n pure xylene. Afterwards the sections were covered with thick Canada balsam, v i i . Mounting Paraffin sections were also cut at 60 microns, placed on slides and the usual technique for mounting followed. In the case of both celloidin as well as paraffin sections, no cover slips were placed u n t i l the balsam on the slides hardened in an oven set at 50°C for two days. After the balsam hardened, coverslips were placed with weights on top of them and l e f t for three days before examining them under the microscope. 6. Measurements of retinal pigment and cone layers In a l l the eyes, thicknesses of the retina, retinal pigment and cone layers were measured, with.a calibrated ocular micrometer, i n the dorsal region between the ora serrata and the fundus. The pigment layer was measured as the distance between the inner border of the ehorioid and the tips of the pigment projections which varied only four microns. The cone layer was measured from the external limiting membrane to the tips of the cone outer segments, since only the myoid which i s outside the external limiting membrane undergoes elongation and contraction. In this paper thicknesses are compared directly. This was done because i t was observed that the cone and pigment layers do not vary in thickness proportionately to the thickness of the retina. After examining nearly 9,000 eyes the author concludes that in a group of f i s h of the same age and size, the thicknesses of the pigment and cone layers vary at random. Unless a large number of animals of the same age and size group are k i l l e d under exactly identical conditions and their retinae and retinal.layers measured and some sort of relationship among them established s t a t i s t i c a l l y , i t i s very difficult to say with any amount of certainty whether the retinal layers vary in - 13 -proportion to the retinal thickness, size or weight of the animal or the diameter of i t s head or eye b a l l . True, the retina and consequently the various retinal layers are thicker in an older, larger animal but the argument given above applies only to the animals in the same age and size group. In the present invest-igation, in each experiment only animals of same age were used. III. RESULTS A. Structure of the Oncorhynchus eye (Pigs. 2. 3, 5) 1. General shape The eye of the juvenile Pacific salmon i s large and well developed. The eye b a l l i s flattened anteriorly and held in position in the eye socket by six oculomotor muscles (external rectus, internal rectus, superior rectus, inferior rectus, super-ior oblique and inferior oblique). The eyes are capable of a small amount of movement, which i s brought about and coordinated by these oculomotor muscles. 2. Vertical l i d s The Pacific salmon posseses vertical l i d s which cover the circumocular sulcus and eliminate distortive eddies in the slipstream alongside the eyes. The vertical l i d s , which are muscular, are attached to the orbital bones by special ligaments. The l i d complex of the Pacific salmon i s composed of a crescentic and narrow posterior l i d overlapping approximately two thirds of the eye circumference with a triangular and broad anterior fold. The anterior fold i s depressed below the head surface and does not arise from the extreme margin of the circumocular sulcus but seems to arise as a separate conjuctival fold arising from beneath the margin on the anterior side of the membranous orbit. Since the bony orbit i s not complete anteriorly, the drawing of anterior sulcal margin forward w i l l allow a wider range of forward and binocular vision. The anterior l i d fold i s the "false n i c t i -tating membrane". 3. Cornea The anterior, exposed, flattened and transparent corneal surface i s smooth. This may be attributed to the fact that the Pacific salmon i s a fast swimmer. The corneal tissue i s avascular and might be nourished by the intra-ocular f l u i d . Part of the required oxygen might be absorbed from the surround-ing water. The outline of the cornea i s circular and i t s centre i s shifted towards the nasal side as i n most fast swimming f i s h . The outermost surface of the cornea i s made of transparent - 15 -S c l e r a l C a r t i l a g e E p i c h o r i o i d a l Lymph Space Ora Ser r a t a Annular Ligament Suspensory Ligament utockfeiiijpus Layer c l e r a l Layer rmal Layer Lens .Sclera Retina C h o r i o i d Gland JPalciform Process I r i s Optic Nerve C h o r i o i d C o n j u c t i v a Figure 2. Photomicrograph of a v e r t i c a l s e c t i o n of <wi Oncorl^nchue «y«. X90. - 16 dermal layer which, i s a continuation of the head skin. This layer i s two cells thick and i s the thickest layer of the corneal complex. The next two layers, the scleral and auto-ekfefa^pus, are much thinner and are composed of numbers of fibres. The scleral layer remains thin throughout and joins the scleral cartilage at the region where the eye socket commences. The autocanthous layer, on the other hand, becomes broader at the points away from the anterior-most surface of the eye and assumes a triangular shape. A l i t t l e posteriorly, i t becomes narrow again and joins the chorioid at the corneal-scleral junction. In this loc a l i t y an epichorioidal lymph space i s present between the sclera and the chorioid. 4. Sclera * The sclera forms two thirds of the outer covering of the eye and i s cup-shaped. It i s tenacious and opaque and i s enclosed in the eye socket. This tough envelope i s essential to maintain the shape of the eye and to cope with changes i n pres-sure, both intra-ocular and external. The scleral cartilage i n the Pacific salmon i s in the form of a broad ring encircling the eyeball at the region slightly posterior to the corneal-scleral junction. This cartilage i s thin, one c e l l deep. At the post-erior one-thirds of the eye where the sclera!, has no cartilage, i t consists of a tough tenacious, fibrous layer. The sclera i s slightly silvery due to the presence of guanine crystals. 5. Annular ligament A very prominent annular ligament i s present in the Pacific salmon eye and may rightly be called the connexion between the i r i s and the cornea. It merges with the chorioid layer at the corneal-scleral junction. The annular ligament covers most of the i r i s except the central most edge. It does have a secretory appearance, and as Walls (1942) suggested, i s probably the source of the aqueous humour. 6. I r i s The i r i s i s well developed and prominent. The pigment of the i r i s i s a continuation of the epithelial pigment layer of the retina. The i r i s pigment layer i s different from - 17 -the retinal pigment layer in that the former i s found always i n the form of a thin strip and neither expands on illumination nor contracts in dark. It i s made up of small spherical or ellipsoid granules and contains no needle-shaped granules which the retinal pigment does, in addition to the spherical and e l l i p -soid granules near the nucleus. The i r i s pigment joins the retinal pigment a l i t t l e posterior to the corneal-scleral junction and the ora serrata. Another difference between the i r i s and the retinal pigment cells i s that the f ormer do not have finger like processes distally, which the latter do. The i r i s , as such, i s an extension of the retina beyond the ora serrata. The region where the retina becomes the i r i s i s interesting. The visual c e l l layers do not extend beyond the corneal-scleral junction but the external nuclear layer and the other r e t i n a l layers extend a l i t t l e farther anteriorly. However, the i r i s tissue has i t s beginnings from near the corneal-scleral junction and gets broader, almost spindle-shaped anteriorly, tapering into a long columnar epithelium like tissue at the central region which i s i n contact with the lens. Only at the very tip of the i r i s i s pigment present on both the anterior as well as the posterior surfaces. The i r i s of the Pacific salmon i s not capable of photomechanical changes. When sockeye and chum fry were l e f t i n dark individually, i n small, narrow tanks or in irrigated glass tubes and then suddenly exposed to bright light, no contraction of the pupil was observed. Measurements of the i r i s of intact as well as enucleated eyes, exposed to diffuse light of low intensity as well as bright light (400 ft-e.) did not show any changes in diameter (2 mm.). 7. Lens The spherical lens consists of concentrically formed layers and i s an isolated transparent body of c e l l s . The anter-ior half of the lens has, as i t s outer layer (almost like a covering) an epidermal layer which i s not dissimilar to the dermal layer of the cornea but only one c e l l deep. The lens of the Pacific salmon exhibits two different staining properties. The peripheral regions stain with haematoxylin and the core takes - 18 -only eosin. The structure seems more concentric in the centre than on the periphery and the concentric rings are further sub-divided, especially i n the peripheral regions, into cross striations. The lens i s held in position by a suspensory ligament dorsally and a retractor lentis ventrally. The suspensory l i g a -ment arises directly from theregion of the ora serrata while the retractor lentis originates from the falciform process which runs more or less horizontal to the embryonic central fissure. 8. Intra-ocular fluids The small amount of aqueous humour- i n the anterior chamber i s watery and saline*. The posterior chamber i s f i l l e d with gelatinous vitreous humour"; The vitreous humour, in addition to the suspensory ligament prevents, the1 lens from slipping into the posterior chamber. In addition to the retractor lentis, the differences in the densities and pressures between these two fluids play an important role i n the accommodation of the eye for distant or near vision by moving- the lens f orward or retracting i t . 9. Chorioid The chorioid i s located between the retina and the sclera and i s continuous with the outer part of the i r i s . At the time of the formation of the eye, the chorioid extends into the v i t r e a l chamber through the embryonic ventral fissure. The chorioid i s very thin anteriorly and increases in thickness posteriorly, the maximum thickness being around the optic nerve exit. It i s highly vaseularised and has pigment strips both on the retinal and scleral sides. The pigment in these strips i s in the form of spherical or e l l i p t i c a l granules. The increase in the thickness of the chorioid posteriorly i s due to the presence of masses of capillaries, which are known as the chorioid gland. The chorioid gland receives i t s blood supply from the * When taken out with a syriige and tasted i t resembles tears. - 19 pseudobranch (vestigeal hyoid g i l l , found on the inner side of the operculum), through an efferent artery. This artery breaks up into numerous capillaries in the chorioid gland. Blood from the chorioid gland i s u t i l i s e d i n the general chorioidal circu-lation which nourishes the retina. Walls (1942) suggested that the chorioid gland probably reduces mechanical dlstrubance of the retina caused by changes in blood pressure i n the circulation from the heart, by increasing or decreasing the size of the capillaries which form the gland. No guanine was seen in the chorioid of the Pacific salmon, although, guanine was found i n the i r i s . 10. Falciform process As mentioned before, the falciform process runs horizontal to the embryonic ventral fissure. It runs from the ora serrata to the point where the optic nerve leaves the retina and appears like a ridge. The retractor lentis arises from the anterior most tip of the falciform process. The falciform process i s pigmented and numerous capillaries occur in the pigment. The falciform process and the hyaloid vessels (which appear, when present, on the inner surface of the retina) which are mutually exclusive have been termed "supplementary nutritive devices" by Walls (1942). The falciform process i s supplied by an artery which enters the eye at the region of the optic nerve exit. The presence of blood vessels clearly suggests i t s nutritive function. In the sections of the eyes of embryos, alevins and most fry, i t was seen that the ventral fissure has not closed completely and the origin of the falciform process can be followed. In serial sections i t appears to be quite broad which might be due to the ventral fissure being s t i l l open. Figure 3 shows the falciform process in the sockeye emerged fry which appears to encircle a part of the ventral retina. The falciform process presented such an appearance i n a l l the sections of embryos, alevins and fry. In dissections of the eyes of fry and adult salmon as well as trout the author found that the structure of the falciform process was very simple as described earlier and did not present the appearance of a structure encircling a part of the retina. - 20 -R e t r a c t o r L e n t i s Figure 3. Photomicrograph of the f a l c i f o r m of an emerged f r y . xlOO. >1 - 21 Since the whole head of the younger forms mentioned above was sectioned i t appears that this appearance of the falciform process in the sections might be due to the orientation of the eye in the sections. In the sections of enucleated eyes the falciform process did not appear as i t does in Figure 3. This appearance seems to be due to the axis of sections running a l i t t l e diagonally to the optical axis. 11. Retina A brief description of the Oncorhynchus retina has already been given (Ali and Brett, 1958). In this paper a more detailed description w i l l be given based on a larger number of specimens and involving the use of a modified version of the Golgi silver impregnation technique. The retina of Oncorhynchus consists of the following ten layers (Fig. 4): 1. Epithelial pigment layer Non-nervous 2. Rod and cone (visual cell) layer 3. External limiting membrane 4. External nuclear layer 5. External molecular (plexi-form, reticular) layer 6. Internal nuclear layer 7. Internal molecular (plexi-form, reticular) layer 8. Ganglion c e l l layer 9. Nerve fiber layer 10. Internal limiting membrane Neurones of f i r s t order Neuro-epithelial layer Neurones of second order! ) Neurones of ) third order Cerebral portion No fovea was found in any one of the eyes. The blind spot (scotoma) i s e l l i p t i c a l and i s not situated in the centre of the optic cup but slightly ventral to the centre. Its orientation i s naso-temporal (perpendicular to the long-itudinal axis of the eye). Alt Visual c e l l layer External limiting membrane l ! External nuclear layer External plexiform layer Internal nuclear layer v" Internal plexiform layer Ganglion c e l l layer Nerve fibre layer Internal limiting membrane "Horizontal" bipolar c e l l —"Centrifugal" bipolar c e l l "Diffuse" bipolar c e l l 'Amp crine c e l l get" bipolar c e l l TaEasol" ganglion c e l l Nerve fibre 100 MICRONS DIC Figure U» Photomicrograph of the retina, with a diagram of the nearological arraggement therein. - 23 -Figure 4 shows a photomicrograph of the Pacific salmon retina as well as a diagrammatic presentation of i t s neurological arrangement seen in G-olgi preparations. The neurological arrangement shows greater similarity to that of the primate retina as illustrated by Polyak (1941, 1957) than to that of the teleostean retina depicted by Franz (1913). The "parasol" ganglia have not been described by Franz. Further, he shows a greater number of bipolar cells synapsing with each ganglion c e l l than i s the case in Oncorhynchus (Fig. 4). The rod and cone myoids are also seen more prominently in the present investigation. The appearance of the epithelial layer in i t s light as well as dark-adapted states i s shown in Figure 5A. The pig-ment granules near the nucleus are spherical or slightly ellipsoid, whereas distally they are needle-shaped. In the dark-adapted state (Fig. 6A) they contract proximally and form a dense strip leaving the tips of the epithelial cells transparent and devoid of pigment. In some light-adapted eyes a l l the pigment migrates distally leaving the proximal parts of the epithelial cells, near the chorioid, free of pigment. This happens occasion-ally in a l l the species of Oncorhynchus studied and does not indicate any specific differences. However, in most cases only the needle-shaped granules migrate distally on illumination, leaving the spherical and e l l i s o i d ones near the nucleus. The retina of Oneorhynchus possesses cones as well as rods, hence is duplex. Single, twin and rarely unequal twin* cones occur. In the region of the fundus they are arranged in neat, regular mosaics as shown in Figure 5B. This pattern is the same as that described by Eigenmann and Schafer (1900) for Salmo. In the ora serrata this pattern i s not s t r i c t l y adhered to and the cones are arranged in rows (Fig. 5B) without the formation * These may be called double cones but due to their close simil-arity in structure to the twin cones and the striking difference between them and the double cones of other vertebrates, the author prefers to c a l l them unequal twin cones. These occur very rarely in the Oncorhynchus retina, nevertheless, can be seen in a careful examination of preparations. If the sections are stained in eosin for two minutes instead of one, a l l the cone ellipsoids are more prominent. In his earlier investigation (Ali and Brett, 1958) the author did not observe them. - 24 Figure 5. A. Diagram showing parts of the retinal pigment, cones and rods and their positions in the light and dark-adapted stages. B. The arrangement of cones- in the regions of the fundus and oraserrata. o o o Oo 0 o o o FUNDUS OOOOOO B o o o o o o o o oo oo o o o o o ORA SERRATA CONE ARRANGEMENT - 25 -of any definite mosaics. Eigenmann and Schafer do not mention any such arrangement. The outer sections of the cones are typically cone shaped (Pig. 5A) and their ellipsoids are e l l i p t i c a l , almost oval. The ellipsoids of the twin cones are attached while those of the unequal twin cones are attached only partly. The myoids of the twin cones are attached and just before their entrance through the external limiting membrane, they divide and each has a separate nucleus i n the external nuclear layer. They have separate foot-pieces. The only place in which the single, twin and unequal twin cones differ from one another i s in the e l l i p -soid portion. In the ease of the twin cones the ellipsoids are fused with one another but each member can be clearly demarcated (Pig. 5A). In the unequal twin cones one member has a slightly smaller ellipsoid, but the two are attached to one another partially near their bases, unlike the double cones (lacking in Oncorhynchus) where one member i s much smaller and i s not attached to the larger member. In the double cones, the smaller member usually has an o i l globule and migrates very l i t t l e or not at a l l in the dark. In the unequal twins of the Pacific salmon both cones migrate in the dark following the expansion of their myoids. No differences in staining reactions were observed among the single, twin and unequal twin cones. The foot-piece of the cone extends down as f a r as the external molecular layer and expands with several small project-ions at i t s end. These synapse with the branches of the extension from the "midget" bipolar c e l l and often with branches from the "horizontal" c e l l as well. In some areas such as the ora serrata, especially on the dorsal side of the retina, one cone (often 2 or 3) may synapse with branches from the "diffuse" bipolar eells, whose branches also have connexions with five or six rods. In the internal molecular layer, the branches from the "midget" bipolar cells synapse with branches from ganglion cells and the inner branches of the ganglion cells proceed as nerve fibres which are components of the optic nerve. The "horizontal" cells do not send any branches towards the internal molecular layer (Pig. 4). The diffuse bipolar cells make connexions with the ganglion cells of cones or with the "parasol" ganglion cells, which i n turn give off branches which a re also nerve fibres that form the optic nerve. The cones on the dorsal side of the retina are fewer and larger, while those on the ventral side are more numerous and more slender. This i s pronounced in the emerged and late fry stages. In the retinae of these animals the internal nuclear layer and the ganglion c e l l layer are also much thicker. The pigment and cone layers on the ventral side of the retinae of emerged and late fry do not completely contract and expand i n the dark. An idea of the thicknesses of the cone and pigment layers i n the different stages of the four species i n question may be obtained from Figures 6 to 9, 18 to 21. The rods (Fig. 5A) are slender and their outer sections are long, thin and sticklike. Their ellipsoids are small and oval. The rod nuclei are situated below the cone nuclei in most cases and their foot-pieces end in knobs (Fig. 5A) i n the exter-nal molecular layer, where they synapse with the branches from "diffuse" bipolar cells and also, in most instances, with the branches from the "horizontal" c e l l s . The rods are dispersed irregularly in between the cones. They are more numerous in the periphery and are fewer i n the region of the fundus. Since this investigation i s primarily concerned with the retinal photomechanical and behavioural responses of the juvenile Pacific salmon to different light conditions, no attempt was made to study in detail, the number of visual c e l l s in the different areas of the eye, their relative sizes and differences among the different stages. The external nuclear layer contains the nuclei of the rods and cones. These nuclei are more or less ellipsoid. The slightly larger nuclei of the cones l i e closer to the external limiting membrane. The external molecular layer (Fig. 4) consists of the broad foot-pieces of the cones and the knob-like foot-pieces of - 27 the rods. They synapse with the branches of the "midget", "diffuse", "horizontal" and "centrifugal" bipolar cells, in this layer. The internal nuclear layer (Fig. 4) consists of the bipolar cells mentioned above along with the amacrine cells and Miillerian fibres. The internal molecular layer (Fig.4) i s composed of cynapsing fibres of the different bipolar cells and the branches from the ganglion and "parasol" ganglion c e l l s . The ganglion c e l l layer (Fig. 4) contains the cells mentioned in the previous paragraph and varies in thickness in the different stages. In the animals which have more numerous, slenderer cones on the ventral side of the retina, the ganglion c e l l layer i s thicker (2 cells deep). When there are correspond-ingly larger numbers of cones, the internal nuclear layer i s thicker due to the presence of larger numbers of "midget" bipolar c e l l s . Usually each cone synapses individually with a single "midget" bipolar, which in turn synapses with one ganglion c e l l , increasing the thickness of the ganglion c e l l layer also. The nerve fibre layer (Fig. 4), for obvious reasons, i s thinnest in the region of the ora serrata and thickest at the region where the optic nerve leaves the retina. Differences of eye structure among species of Oncorhynchus: No species differences have been noted i n the many Pacific salmon eyes examined. B. Differences among stages Differences, however were observed among the stages studied. The following features are of main interest: The autocaji^fious layer, the cornea and the annular ligament are poorly developed i n the embryo and recently hatched alevin. They attain maximum development i n the late fry stage. The epithelial pigment of the embryo retina i s in a perpetually dark-adapted state (Fig. 10) and exposure to any intensity of light for any length of time does not cause the pigment to disperse. The finger like processes of the - 28 -epithelial cells, devoid of any pigment, are, however, to be seen. Even i n the late alevin stage the pigment migrates only sightly, on illumination. In the older stages such as emerged and late fry, i t undergoes dispersion i n light and concentration in dark. Cones are seen clearly in the embryo retina, but their myoids are always f u l l y contracted, presenting the appearance of a constant light-adapted condition. Exposure to dark for any period of time does not cause an expansion. This situation changes somewhat in the alevin stage where cone myoids do possess the capacity to expand slightly in dark. In the older stages the cone myoids show marked expansion i n dark and contraction in light. No rods were found in the embryo retina. Some are seen in the alevin eye. In older stages the rods are clearly visible in the dark-adapted retinae and their myoids are capable of undergoing expansion in light and contraction i n dark. The internal nuclear layer i s much thicker (approx-imately 10 times) than the external nuclear layer in the embryo. The situation changes as the animal grows older. The external nuclear layer thickens, with the addition of rods, and in the emerged and late fry the internal nuclear layer i s markedly thicker only on hte ventral side of the retina due to the greater proportion of cones there. This situation persists, to a certain extent, even in the smolt. Another difference between the smolt and the younger stages i s that the ventral side of the retina in the latter shows less marked photomechanical changes than those seen i n the ease of the former. The ganglion c e l l layer of the embryo i s also thicker (3 cells deep) than that of older stages. With age i t decreases in thickness, possibly because the area of the retina also increases. In the late fry i t i s about one c e l l deep dorsally and two cells deep ventrally. In the smolts, the ganglion cells are more crowded on the ventral side than on the dorsal side. This, as mentioned elsewhere, i s in accordance with the greater proportion of cones. - 29 -The molecular layers are very thin in the embryos (Pig. 10), and gradually attain the normal proportions with age. In summary, the embryo possesses a l l the ten retinal layers (Pig. 10) which the older, f u l l y developed stages possess; but i n an entirely different proportion i n thickness. The proportions gradually change as the animal gets older, eventually reaching a physiologically balanced state in the emerged or late fry. These observations find a parallel i n the case of the development of the Guppy, Lebistes reticulatus (Mfiller, 1952). 0. State of a typical light-adapted retina (Pig. 4) The pigment has migrated within the finger-like processes of the epithelial cells and by i t s expansion has completely enveloped the rods which are at this stage f u l l y expanded distally due to the elongation of their myoids. The dispersion of the pigment on exposure to light i s thus a device to mask the rods and to absorb excess light. The rods cannot be seen easily in the light-adapted retina unless the surround-ing pigment i s bleached. The cones i n this stage have migrated dist a l l y due to the contraction of their myoids and are seen i n the narrow space between the external limiting membrane and the finger-like process of the epithelial c e l l s , now f u l l of pigment. D. State of a typical dark-adapted eye (Fig. 14) The pigment and the rods have contracted proximally and the cones have expanded distally. The rods may readily be observed i n this state. The pigment, whieh i s contracted (Fig. 16), i s in the form of a thin, dense strip which sep-arates from the rest of the retina in most preparations. - 30 -EXPERIMENTAL RESULTS A. Light-adaptation 1. The process of retinal light-adaptation When a dark-adapted eye (Pig. 16), as previously described, i s exposed to light, the pigment (which i s in a contracted state) starts to disperse and after a certain period, attains maximum dispersion. The cones (whose myoids are expand-ed thus keeping the ellipsoids near the contracted pigment strip) begin to migrate proximally due to the contraction of their myoids. The rods that are seen close to the external limiting membrane in the dark-adapted retina, start migrating di s t a l l y * due to the expansion of their myoids. This process i s triggered when the light-intensity increases to or above the cone thresh-old. It has also been shown that with higher light-intensity light-adaptation i s quicker than with lower light-intensity (von Studnitz, 1933). von Studnitz (1933a) and Wigger (1937) have also found differences in the rates of light-adaptation, during different seasons of the year. The process of li g h t -adaptation i s continuous in the Pacific salmon unlike that i n the goldfish (von Studnitz, 1933a; Wigger, 1937) and the silver mackerel (Zobayashi, 1957) where i t i s interrupted. The Pacific salmon light-adapt in approximately thirty minutes. The pigment, cones and rods usually take different times for complete li g h t -adaptation but they do not show any latent period before commencing to migrate. In contrast, von Studnitz (1933a) and Kobayashi (1957) state that the visual elements of- goldfish and silver mackerel show a latent period before beginning to light -adapt. In general, the cones take the shortest time of the three to light-adapt. No other changes, on exposure to light, are observed i n the Pacific salmon retina. * The terms proximal and distal are used in this paper to denote movements towards and away from the nucleus. - 31 -In the following pages a comparative account of the process and rates of light-adaptation i s given for the various stages of the different species of Pacific salmon studied. The description w i l l be dealt with, stage by stage and interesting features of the different species belonging to the same stage w i l l be described and illustrated with photomicrographs, graphs, histograms and tables. In addition a table, summarising the results of previous investigations i s provided for comparison (Table VI). 2. Embryos None of the embryos, belonging to any of the four species studied, showed any photomechanical response on exposure to lig h t . The pigment remained contracted and the cones that were contracted in the dark, remained so (Pig. 12). The graphs (Pigs. 6, 7, 8 and 9) show the thicknesses of retinal pigment and cone layers of "dark-adapted" embryos exposed to light and sampled at various times mentioned therein. It i s seen that they show no changes i n thickness at a l l on exposure to lig h t . Rods were not observed in any of the embryos examined. 3. Alevins (Hatching stage) (Pigs. 6. 7. 8 and 9) When the alevins kept in dark for a day are exposed to light and sampled at the intervals mentioned, their retinal pigment and cone layers seem to show a slight response as seen in microscopical examination. Their pigment seems to undergo a slight expansion and their cones some contraction. The means of thicknesses of these layers of animals sampled from 0 to 15 minutes after illumination are greater (in the ease of the pig-ment layer) and lower (in the case of the cones) than those of the animals sampled from 20 to 70 minutes (Table II), except i n the case of the pigment layers of coho and chum alevins, indicating that, on the whole, there has been a slight expansion of the pigment layer and contraction of the cone layer on expos-ure to light. This last observation lends support to the histological findings. In the case of the pink salmon alevins, no pronounced changes were observed even in the histological examinations. The most noticeable positional changes of the - 32 Figure 6. Graph showing the rate of expansion of the pigment and contraction of the cone layers on exposure to light in the various stages of sockeye salmon. # Pigment O Cones T I M E IN MINUTES - 33 -Figure 7. The rate of expansion of pigment and contraction of cones on exposure to light in the various stages of coho salmon. • Pigment O Cones T I M E IN M I N U T E S - 34 -TABLE II - The average thicknesses of pigment and cone layers of alevins fixed at 0-15 minutes and 20-70 minutes after illumination. See Pigs. 6, 7, 8 and 9. Average thickness Average thickness of pigment layers of cone layers (in microns) (in microns) Species 0-15 min. 20-70 min. 0-15 min. 20-70 min. after after after after ilium. ilium. ilium. ilium. Sockeye 20.28 22.92 9.72 8.88 Coho 21.55 21.38 14.72 12.08 Pink 20.00 20.41 12.70 12.70 Chum 26.02 21.47 12.27 9.72 - 35 -Figure 8. The rate of expansion of pigment and contraction cones on exposure to light in the various stages of pink salmon. # Pigment O Cones 70 0 10 20 30 40 50 60 TIME IN MINUTES -36 -retinal pigment and eone layers were observed in the case of the coho alevin. Rods were not observed except in very few prepara-tions. The alevins used were newly hatched (about 24 to 48 hours after hatching), had large yolk sacs and stayed on the bottom of the tank without being able to swim about. 4. Emerged fry (no yolk sac) (Figs. 6. 7. 8 and 9) a. Retinal response The response of the retinae of dark-adapted emerged fry, on exposure to light, i s immediate and in none of the animals did the pigment or eone layers have any measurable latent period before beginning to expand or contract, respect-ively. Table III shows the times taken by the retinal pigment and visual cells to light-adapt, the nature of their response, time taken to school on illumination and the times after illumin-ation when the f i s h were able to catch maximum number of Daphnia. The nature of responses given either as linear or exponential are in some cases only approximate. The expansion of the pigment in a l l the species, with the exception of sockeye, i s exponential, while the rate of contraction of the eone layers in a l l these cases, again with the exception of sockeye, i s linear. In addition, the cones of coho, pink and chum light-adapt later than the pigment does, while the cones of sockeye do so earlier than the pigment. When the times taken by the pigment and cone layers to l i g h t -adapt are compared (Fig. 17) i t i s seen that both the pigment and cone layers of chum take the shortest time to light-adapt while the pigment and cone layers of pinks take the longest time. The pigment of the coho light-adapts sooner than the sockeye1s, whereas the cones of the latter take a shorter time to l i g h t -adapt. Since the response (or rate of expansion or contraction) is not uniform in a l l the cases, comparison of curves whose equations have been determined becomes almost impossible. Some of the rates are exponential or almost exponential and in these cases theoretically f u l l adaptation i s attained only at infi n i t e time, but the time of attainment of f u l l adaptation in the case of visual elements that show a linear response i s mathematically TABLE III - The time for light-adaptation of pigment, cones and rods, nature of response, schooling and feeding rates of dark-adapted emerged fry on exposure to light. See Figs. 6, 7, 8, 9, 10, 11 and 17. Species Time for Light Adaptation (in min.) Nature of Response Time taken to school after ilium, (in min.) Minutes after ilium, when max. feeding occurs Pigment Cones Rods Pigment Cones Sockeye 25 20 30 Linear Expo-nential 15 20-25 Coho 15 25 25-30 Expo-nential Linear 20 25-30 Pink 25 25 25 Expo-nential Linear 15 25-30 Chum 10 20 25-30 Expo-nential Linear 15 20-25 - 38 -Figure 9. The rate of expansion of pigment and contraction of cones on exposure to light in the various stages of chum salmon. • Pigment O Cones TIME IN MINUTES - 39 -Figure 10. The rate of schooling of emerged fry, late fry of the four species, when dark-adapted fish are exposed to light. • SOCKEYE O COHO A PINK A CHUM 0 5 10 15 20 0 5 10 15 20 TIME (IN MINUTES) AFTER ILLUMINATION - 40 -i definite, hence comparison of the times for f u l l adaptation of two curves, one exponential and the other linear, i s not possible. However, in the case of both these responses the time taken for • 99$ adaptation i s mathematically definite and can be compared. The time taken for f u l l adaptation i n the case of visual elements that had an exponential response was determined by checking the histological preparations and also the graphs where after a period, say 20 minutes, the pigment or cone layers reached maxi-mum expansion or contraction and when the average of these values (of f u l l y adapted elements) was taken and a line drawn through them, the point where the values of the thicknesses exceeded or f e l l below, this line, was taken as the time for f u l l adaptation. This time for the present purposes seems f a i r l y accurate, especially since i t i s supported by histological observations. Besides, the 99$ values are only very slightly different (about 0.5 microns or less) from the f u l l adaptation values. In view of this, i n a l l the tables and histograms given in this paper by adaptation 99$ adaptation i s meant. A point of significance i s the difference be-tween the thickness of the f u l l y expanded and f u l l y contracted epithelial pigment (Pig. 9) of the emerged chum fry. This difference i s only 15 microns, as opposed to 25 to 35 microns in the case of the pigment as well as the cones of the other species. Even the cone layer of emerged chum fry shows a difference of 25 microns between the f u l l y expanded and f u l l y contracted cones. b. Schooling rates Figure 10 shows the rates of schooling of the emerged fry of a l l the four species studied, when dark-adapted animals are exposed to light. This shows no correlation with the adaptation times of either the pigment or the cone layers. In other words, schooling i s not dependent upon complete li g h t -adaptation. In general, sockeye, pink and chum seem to form a school consisting of most of the animals (50) used in the exper-iment, in about 15 minutes after illumination while the coho takes five minutes longer. Another observation that seems pertinent i s that unlike the other species, coho school less readily and when - 41 -"they do school, only about 85$ of the animals under observation joined the school. The others swam around individually. In comparison, the emerged fry of the other species observed (Pig. 10) formed schools consisting of 96$ or 98$ of the anim-als used in the experiment. Newly emerged fry of a l l the four species studied, which were in the covered trough in the dimly l i t hatchery, do not form schools for the f i r s t two or three days, unless they are subjected to light of quite a high intensity (400 f t - c ) . If l e f t in the trough under unaltered light conditions, they gradually commence forming schools and after about three days are i n a large school, c. Feeding rates The feeding rates of conditioned, dark-adapted emerged fry after various periods of illumination, are shown (Pig. 11). In every case the maximum rate at which the animals capture their prey occurs only at a time when the cones are also light-adapted as shown by histological examination (compare Pigs. 11 and 6, 7, 8 and 9). The time at which maximum feeding occurs does not show any correlation with the time taken by the pigment to light-adapt f u l l y . The time at which maximum feeding occurred also helped to determine the time for f u l l adaptation of the cones for interspecific comparison, referred to above. Prom these experiments i t would appear that maximum feeding indicates that the animal's visual acuity i s at i t s best, and this time corresponds well with the time taken by the cones to contract (light-adapt) completely, showing the relationship between the cones and visual acuity. 5. Late fry a. Retinal response The response of the pigment and cones of the dark-adapted late fry of a l l four species i s immediate and none of them has a measurable latent period (Pigs. 6, 7, 8, 9 and Table IV). The rate of expansion of the pigment i s exponential except in the case of coho, and in the case of the cones their rate of contraction i s also linear, the chums, i n this case, - 42 Figure 11. The rate of feeding of conditioned dark-adapted emerged fry, late fry and smolts of the four species, on exposure to light. - 43 -A Figure 12. Photomicrographs of l i g h t - a d a p t e d (A) and dark-adapted (33) r e t i n a e of l a t e embryos. x l l 5 . B TABLE IV - The time for light-adaptation of pigment, cones and rods, nature of response, schooling and feeding rates of dark-adapted late fry on exposure to light. See Pigs. 6, 7, 8, 9, 10, 11 and 17. Species Time for Light Adaptation (in min.) Nature of Response Time taken to school after ilium, (in min.) Minutes after ilium, when max, feeding occurs Pigment Cones Rods Pigment Cones Sockeye 10 15 15-20 Expo-nential Expo-nential 10 15-20 Coho 20 10 20 Linear Expo-nential 20 10-15 Pink 20 20 25 Expo-nential Expo-nential 10 20-25 Chum 20 10 20 Expo-nential Linear 10 10-15 - 45 A Figure 13. Photomicrographs of l i g h t - a d a p t e d (A) and dark-adapted (B) r e t i n a e of a l e v i n s . x l l O . B - 46 -A Figure 14. Photomicrographs of l i g h t - a d a p t e d (A) and dark-adapted (B) r e t i n a e of emerged f r y . x l 2 0 . B - 47 -being the exception. The pigment of the sockeye late fry i s light-adapted ten minutes after illumination while that of the other three species i s light-adapted in 20 minutes (Fig. 17). The cones of coho and chum light-adapt earliest, ten minutes, while that of the sockeye light-adapt in 15 minutes followed by pink in 20 minutes. Certain points of interest may be mentioned. In the case of the late sockeye fry the rate of pigment expansion on illumination i s exponential as compared with that of the emerged sockeye fry where i t i s linear. The eones of the late sockeye fry light-adapt five minutes after the pigment, as opposed to the case i n the emerged fry and smolts whose cones light-adapt before the pigment. In the case of the coho, the cones of the late fry light-adapt i n ten minutes in contrast to the cones of the emerged fry which take 25 minutes to contract maximally. In the case of the late chum fry, the difference between the f u l l y contracted and the f u l l y expanded pigment layer i s 50 microns as compared with t he emerged fry where- the difference i s only 15 microns. The emerged sockeye fry showed-differences from the emerged fry of the other species whereas the late sockeye fry do not show any marked differences from the other species. The only aspect in which some difference i s seen between late sockeye fry and late fry of other species i s the time taken by the pigment to light-adapt. The pigment of the late sockeye fry takes the least time to light-adapt (19 minutes), b. Schooling rates (Fig. 10) The dark-adapted late fry of sockeye, pink and chum form schools consisting of 98$ of the fi s h under observation in ten minutes while the coho take 20 minutes to do so. As in the case of the emerged coho fry, the late fry also do not school readily and when they do school, only about 80$ of the experiment-a l animals form the school. The time taken to form a school, i n the case of the sockeye and late coho fry corresponds with the time taken by their pigment layers to expand maximally. In the case of pink no correlation either with the pigment or cones i s seen while in the chum the time taken to school i s the same as the time taken by the cones to light adapt (10 minutes). Figure 15. Photomicrographs of l i g h t - a d a p t e d (A) and dark-adapted (B) r e t i n a e of l a t e f r y . x l 2 5 . B - 49 -c. Feeding rates The coho and chum are able to capture 96$ to 98$ of the Daphnia offered ten minutes after illumination while the sockeye and pink take longer times (Fig. 11 and Table IV). However, in a l l cases the time after illumination at which max-imum feeding occurred i s the same as that taken by the cones to light adapt. In the case of the pinks where the pigment and cones take the same time (20 minutes) to light-adapt this correlation extends to the pigment also. As mentioned in the case of the emerged fry, here also, i t i s seen that f u l l visual acuity i s reached when the cones are light adapted, enabling the capture of maximum number of Daphnia possible. 6. Smolts a. Retinal responses The pigment and the cones start expanding and contracting immediately after the lights are turned on. The pig-ment of coho shows a slower movement for the f i r s t five minutes (Fig. 7). The rate of response of the pigment as well as the cones of the sockeye i s exponential while that of the pigment and cones of the coho i s linear. In both the species the pigment light adapts in 20 minutes and the cones in 15 minutes (Figs. 6 and 17). b. Schooling rates The smolts of sockeye and coho failed to form schools in the experimental tank on exposure to light*. A group of 50 f i s h were observed at five minute intervals for half an hour and at one half hour intervals for the next hour and a half yet they showed no signs of school formation. * The smolts as well as the emegged and late fry of a l l the four species seemed to be alarmed (?) on illumination and showed something that resembled escape behaviour, but this lasted only for a brief period (about a minute, sometimes less) after which they quieteneddown. This alarm reaction seemed more accented in the case of the smolts, probably because they swim faster and are much larger than the fry. Among the fry, both emerged as well as late, the sockeye and coho seemed to show a more pronounced reaction than the pink and chum fry. These observations seem to agree with those of Hoar et a l (1957). At the time the present experiments were conducted, the author was not aware of the findings of Hoar et a l loc. c i t . , or he would have observed these reactions more closely. - 50 -A Figure 16. Photomicrographs of light-adapted (A) and dark-adapted (B) retinae of smolts. x500. TABLE V - The time for light-adaptation of pigment, cones and rods, nature of response and feeding rates of dark-adapted smolts exposed to light. See Pigs. 6, 7 and 11. Species -Time for Light Adaptation (in min.) Nature of Response •Minutes after ilium, when max, feeding occurs Pigment Cones Rods Pigment Cones Sockeye 20 15 20-25 Expo- Expo- 15-20 nential nential H Coho 20 15 20-25 Linear Linear 15-20 -52 -Occasionally, two or three f i s h were observed to swim together but this lasted f o r only a short time after which they separated. Nothing like the school of the emerged and late fry described above was ever observed. c. Feeding rates In both the species maximum feeding occurred 15 minutes after illumination (Fig. 11). Here also, as i n the case of the emerged and late fry, i t i s seen that this time i s the same as that taken by the cones to light-adapt (Figs. 6, 7, 8, 9 and 11). The sockeye smolts take the same time as the late fry to light adapt (15 minutes), but a longer time than the emerged fry (Fig. 17). The coho smolts, on the other hand, take a longer time than the late fry but a shorter time than the emerged fry (Fig. 17). 33. Dark-adaptation 1. The process of dark-adaptation When a light-adapted eye, whose description has been given previously (Fig. 4), i s subjected to darkness, the disper-sed, needle-shaped pigment granules which are in the finger-like processes of the pigment epithelial cells (Fig. 5A) start to concentrate proximally, eventually (approximately 45-5 minutes, depending on the species and/ or stage) forming a thin, very dark strip (Fig. 5A) around the nuclei, leaving the processes of the pigment epithelial cells transparent. The cone ellipsoids which are seen near the external limiting membrane (Fig. 5A) due to the contraction of their myoids, begin to migrate dist a l l y due to the elongation of their myoids, f i n a l l y taking their dark-adapted position close to the dense pigment strip. The rod myoids which are elongated in light (Fig. 5A) commence contracting, bringing the rod ellipsoids closer to the external limiting mem-brane, where they remain in the dark-adapted state (Fig. 5A). Latent periods before the contraction of pigment and expansion of cones in dark have been described both in Trachurus (Kobay-ashi, 1957) and Oncorhynchus nerka (Ali and Brett, 1958). In the present investigation no latent period prior to the expan-sion of cones in dark was observed in any of the species or - 53 -Figure 17. Histogram showing the times taken by the pigment and cones of emerged fry, late fry and smolts of the four species, to ligh t -adapt. SOCKEYE COHO PINK CHUM EMERGED FRY SOCKEYE COHO PINK CHUM LATE FRY CD m z H SMOLT o o z m CO 10 20 0 10 20 0 10 20 TIME ( IN MINUTES) FOR LIGHT ADAPTATION - 54 -TABLE VI - Summarising the latent periods and time for light-adaptation of certain other species of f i s h studied by previous workers. Species Latent Period Time for Light-adaptation Reference Pigment Cones Rods Pigment Cones Rods Leuciscus rutilus L. aula Anguilla Carassius  auratus Anguilla Ameiurus  nebulosus Pundulus  heteroclitus Abramis  crysoleucas Carassius  auratus C. auratus enucleated eye Trachurus  japonicus Oncorhynchus nerka 2 2 30 60 120 30 35-40 60 60 45-20 40 30 30 45 40 30 45 16-18 15-20 40 Pergens, 1896 Chiarini, 1904 von Hess, 1902 von Studnitz, 1933a do. Arey, 1916 do. do. do. Wigger, 1937 Kobayashi, 1957 A l i and Brett, 1958 - 55 -Figure 18. Rate of contraction of pigment and expansion of cones in dark in the various stages of sockeye salmon. • Pigment O Cones T I M E IN M I N U T E S -56 -stages studied but in the case ,of the pigment a latent period generally occurred prior to the commencement of contraction in the dark. On the whole, the process of dark-adaptation i s slower than that of light-adaptation. The reason for this appears to be that light i s an active stimulus whereas darkness i s not. In the following pages the process of dark-adaptat-ion in the different stages of the four species of Pacific salmon in question w i l l be dealt with, as in the case of light-adaptat-ion, stage by stage. Tables summarising the information for each stage are given (Tables VII, VIII, IX and X). In addition, the results of previous workers have been tabulated (Table XI) for comparison. 2. Embryos As seen in Figures 12, 18, 19, 20 and 21, none of the embryos exhibit any movement of either the pigment layer or the cones, on exposure to dark. Histologically also, no changes i n the thicknesses of the pigment and cone layers are observed when the "light-adapted" embryo i s subjected to darkness. Rods were not seen in any of the preparations. 3. Alevins (hatching stage) Graphs (Figs. 18, 19, 20 and 21) show the thicknesses of the pigment and cone layers of light-adapted alevins sampled at various times after darkening the room and demonstrate no pronounced changes in thicknesses of these layers. However, examination of the sections of their eyes reveals a slight contraction of the pigment and expansion of the cones i n the dark. Also, i t i s seen that in the alevins of a l l the four species the means of the thicknesses of the pigment layer and cone layer sampled from 0 to 15 minutes are consistently higher or lower, respectively, than the means of the thicknesses of these layers of animals fixed from 20 to 70 minutes (Table VII). The points on the graphs show marked variation, but on the whole, a tendency of the pigment layer to contract and the cones to expand can be observed (Fig. 13). Very slender rods were seen in some of the preparations. - 57 -Figure 19. Rate of contraction of pigment and expansion of cones in dark in the various stages of coho salmon. • Pigment O Gones - 58 -TABLE VII - Average thicknesses of pigment and cone layers in alevins fixed at 0-15 minutes and 20-70 minutes after light-adapted alevins were exposed to dark. See Pigs. 18, 19, 20, 21 and 22. Species Average thickness of pigment layers (in microns) Average thickness of cone layers (in microns) 0-15 min. after dark 20-70 min, after dark 0-15 min. after dark 20-70 min. after dark Sockeye Coho Pink Chum 19.86 20.29 19.30 25.27 17.36 18.75 16.80 24.33 8.80 10.41 12.08 13.99 10.00 12.22 15.41 16.11 - 59 -4. Emerged fry (post yolk sac) On subjection to dark, the pigment of the emerged fry-retina shows a latent period before beginning to contract. This varies interspecifically. The pink has the shortest (5 minutes) latent period, while the chum has the longest (15 minutes). The sockeye and coho both have a latent period lasting for ten min-utes (Pig. 22 and Table VIII). After this latent period the pigment begins to contract and in a l l species except the sockeye, i t i s linear. The contraction of the sockeye's retinal pigment is exponential. The retinal pigment of the pink takes the longest time to contract maximally (45 minutes). Another point of inter-est in the case of the pink i s that i t s pigment remains in the half contracted state for 25 minutes (from 15 to 40 minutes after dark), between the time i t starts to contract after the i n i t i a l latent period, and the time i t attains maximal contraction. As in the case of light-adaptation, the recently emerged chum fry exhibit only a small difference in the thickness between the f u l l y expanded and f u l l y contracted epithelial pig-ment layer (17 microns). In comparison, the difference between their f u l l y contracted and f u l l y expanded cone layers i s 22 microns. The cones of the species have a latent period before commencing migration distally. In a l l eases, the response i s linear (Pigs. 18, 19, 20, 21 and Table VIII). The chum cones take the shortest time (20 minutes) and the coho cones take the longest (40 minutes). 5. Late fry The epithelial pigment layers of a l l the species with the singular exception of the pink possess a latent period before commencing contraction (Pigs. 18, 19, 20, 21, 22 and Table IX). It may be recalled that in the emerged fry stage the pink had a latent period of five minutes, which was the shortest as compared with the other species. The rate of contraction of the pigment i s exponential in the coho, pink and chum, while in the sockeye i t i s linear. The sockeye pigment i s maximally contracted after 45 minutes, in contrast with the pink whose pigment takes the TABLE VIII - The time for dark-adaptation of pigment, cones and rods, the latent periods and natures of response of emerged fry. See Pigs. 18, 19, 20, 21 and 22. Species Latent period before the commencement of the contraction of pigment (in min.) Time for Dark-adaptation (in min.) Nature of Response Pigment Cones Rods Pigment Cones Sockeye 10 30 35 15 Expo-nential Linear Coho Pink Chum 10 5 15 40 45 30 40 25 35 25 20 15 Linear Linear Linear Linear Linear Linear - 61 -Figure 20. Rate of contraction of pigment and expansion of cones in dark, in the various stages of pink salmon. # Pigment O Cones TABLE IX - Times for dark-adaptation of pigment, cones and rods, latent periods and natures of response of late fry. See Figs. 18, 19, 20, 21 and 22. Species Latent period before the commencement of the contraction of pigment (in min.) Time for Dark-adaptation (in min.) Nature of Response Pigment Cones Rods Pigment Cones Sockeye Coho Pink Chum 15 10 15 45 40 15-20 35-40 35-40 20 30 40 35 15 35 20 Linear Expo-nential Expo-nential Expo-nential Linear Expo-nential almost Expo-nential Linear - 63 -Figure 21. Rate of contraction of pigment and expansion of cones in dark, in the various stages of chum salmon. # Pigment O Cones 80 0 10 20 30 40 50 60 70 TIME IN MINUTES - 64 -shortest time (30 minutes) to contract f u l l y . None of the species shows a latent period before the commencement of the expansion of i t s cone myoids in dark (Pigs. 18, 19, 20, 21, 22 and Table IX). In the case of the cones also, those of the sockeye take the longest time to expand f u l l y , that i s , 40 minutes. The other species take 35 minutes to do so. In addition, the response of the sockeye cones i s linear while those of the others show an exponential rate, although that of the pink's i s not so clear i n that i t almost resembles an exponential and certainly i s not linear. 6. Smolts The pigment layers of both sockeye and coho possess latent periods, but that of sockeye i s somewhat shorter (25 min-utes) than that of the coho's (30 minutes). The rate of contract-ion of the sockeye retinal pigment i s linear while that of the coho i s exponential, but they both take the same time to contract f u l l y Pigs. 18, 19, 20, 21 and Table X). The cones of both the species possess no latent period before beginning to expand, but like the pigment, the sockeye cones show a linear response, as against the exponential response of the coho cones. The cones of sockeye take a longer time to light-adapt (50 minutes). The coho cones do so after 40 minutes of dark. C. Retinomotor and Behavioural Responses  to Different Light Intensities Retinal photomechanical changes elicited by low light intensities are similar to the process of dark-adaptation. When the intensity of light f a l l s below their threshold the retinal elements commence to migrate towards their dark-adaptation states. After the light intensity starts decreasing below a certain level, the pigment begins to contract and seems to take various positions ranging from the f u l l y expanded (light-adapted) state to the maximally contracted (dark-adapted) state, thereby regulating the amount of light absorbed inside the optic cup. This process exposes the rods which have been shielded by the pigment. •TABLE X - Times for dark-adaptation of pigment, cones and rods, the latent periods and natures of response of smolts. See Pigs. 18, 19 and 22. Species Latent period before the commencement of the contraction of pigment (in min.) Time for Dark-adaptation (in min.) Mature of Response P O Pigment Cones Rods Pigment Cones Sockeye 25 40 50 20-25 Linear Linear Coho 30 40 40 20 Expo- Expo-nential nential - 66 -Figure 22. Histogram showing the times taken by the pigment and cones to dark-adapt. Solid positions of the bars represent the latent time before reactions;. S O C K E Y E C O H O PINK C H U M S O C K E Y E C O H O PINK C H U M E M E R G E D F R Y L A T E F R Y I S M O L T JL 0 2 0 4 0 0 2 0 4 0 0 2 0 4 0 T I M E ( IN M I N U T E S ) F O R D A R K A D A P T A T I O N 6 0 - 67 -TABLE XI -Times for dark-adaptation of pigment, cones and rods, the latent periods of certain f i s h studied by previous investigators. Latent Period Time for Species Light-adaptation R ^ f e r e n c e Pigment Cones Rods Pigment Cones Rods Leuciscus 1 rutilus 1. aula 5 Ameiurus  nebulosus Pundulus -heteroclitus Abramis  crysoleucas Carassius -auratus Trachurus 5-10 japonicus Oncorhynchus 12 nerka 5-10 16 20 60 60 45-60 30 30 30 90 56-60 55-60 24 Pergens, 1896 Chiarini, 1904 Arey, 1916 do. do. do. Kobayashi, 1957 A l i and Brett, 1958 - 68 -When the light intensity f a l l s below their threshold, the cones commence migrating distally so as to enable the rods (so i t would appear) to take their positions closer to and directly i n the path of the incoming light quanta. Thus, i t i s seen, that declining light intensity, when i t reaches a certain level or levels triggers these three individual responses. At very low intensities a l l the three retinal elements assume dark-adapted states. When the cones are closest to the external limiting membrane (light-adapted state) due the contraction of their myoids, the visual acuity of the animal i s at i t s best and in the case of the Pacific salmon this i s demonstrated by their feeding rates, which are at their maximum when the animal is f u l l y light-adapted. As the animal's eyes become adapted for scotopic vision, the cones are fu l l y elongated, since the light intensity is far below their threshold. In this state, the scotopic visual elements, the rods, with their low thresholds come into play. As long as the light intensity i s above their threshold, the light sensitive rods can detect movements and large objects and the presence or absence of light. In the case of the Pacific salmon, i t may be said that rod threshold i s indicated by the dispersion of a school at a particular intensity or in the case of the feed-ing response, the Intensity above the one at which feeding altogether stops due to the inability of the animal to disting-uish the silhouette or movement of the prey (Daphnia). The change from photopic or scotopic vision i s also indicated by the change in the fish's mode of capturing i t s prey. In photopic vision the animal swims about in the tank at a l l depths and quickly captures the Daphnia that i t sees, swallowing one and spotting another simultaneously. In this state feeding rate i s at i t s highest (19 to 20 a minute). When the shiftover to sco-topic vision occurs, the animal resorts to an altogether different method. It stays in the bottom third of the tank, i t s body at a small angle with the bottom and "spots" the animal by i t s movement and shadow, then makes a dash upward, captures i t and returns to the bottom of the tank again. The rate at which - 69 -i t captures the prey depends on the light intensity, for as the intensity declines more and more there is greater d i f f i c u l t y in "spotting" any movement or silhouette. But as long as the intensity exceeds the rod threshold, some amount of feeding i s feasible, but once the intensity f a l l s below the rod threshold, feeding or capturing the prey by visual means stops altogether and whatever prey capturing occurs after this i s due to the employment of some other sensory perception. In the case of the Pacific salmon, the rate of capture of prey in a completely dark room was found to be often 0 and rarely one or two in five minutes. The positions of the pigment and cones and the rates of feeding at the intensities between rod and cone thresholds obey the Weber-Pechner Law (Pigs. 23, 24 and 25). In the following pages the positions of retinal pigment and cones as well as the feeding rates under different light intensities (in the case of late fry and smolts) of the Pacific salmon w i l l be described. The intensities at which schools dispersed (in the case of the late fry only) w i l l also be given. The data for each stage have been tabulated (Tables XII, XIII and XIV) and those for a l l the species studied summarised diagrammatically (Pig. 27), together with values of various light intensities in nature. The results of previous investigations with f i s h have also been presented (Table XV) for comparison. 1. Alevins (Two or three weeks after hatching - Pig. 23 and Table XII). Only sockeye and chum alevins were used. The alevins used were not newly hatched as were those in the adaptation experiments but about two weeks older, some almost approaching the emerging stage. These showed notable positional changes of visual c e l l layers under different light conditions. Ho differences were observed in the reactions of the retinal pigment and cones to different light intensities between the alevins that were kept in darkness and those that were kept in light, for four days, prior to the experiment (Pigs. 23, 26 and Table XII. The pigment of the sockeye was f u l l y expanded - 70 -Figure 23. The thicknesses of pigment and cones of sockeye and chum alevins in various light-intensities. (Light) and (Dark) refer to the fish that were kept under light or dark, respectively for three days before exposing them to the various light intensities (see text). • Pigment O Cones - 71 -TABLE XII - Intensities at which pigment and cones start dark-adapting and become f u l l y dark-adapted in the case of sockeye and chum alevins. One batch of each were held in light and another each in dark, for three days before exposure to different intensities. See Pigs. 23 and 26, Intensity (ft-c.) Intensity (ft-c.) at which f u l l y at which f u l l y Species light adapted dark adapted Pigment Cones Pigment Cones Sockeye (held in light) Sockeye (held in dark) Chum (held i n light) Chum (held in dark) 10l 10v 10 10 -1 10J 10J 10 10 -1 10 -2 10 -2 10 -2 10 -2 10 -1 10 -1 10 - 3 10 - 3 - 72 -un t i l the intensity f e l l below 10 f t - c , and was f u l l y contract--2 ed at intensities below 10 f t - c The chum pigment seemed to have a lower threshold for the commencement of contraction (10""^  f t - c ) , and:/was seen to be f u l l y contracted at intensities _2 of 10 f t - c , or lower. The cones of both sockeye and chum were maximally expanded u n t i l the intensity decreased below 10 f t - c . The cones of the former were fu l l y contracted at 10 _ 1 f t - c , or lower, while those of the latter did not do so u n t i l the intens-i t y f e l l below 10 ^ f t - c , or lower. 2. Late fry (Figs. 24, 26 and Table XIII) The pigment layer of a l l species, except sockeye, remains f u l l y expanded u n t i l the intensity of light f a l l s below 10° f t - c . That of sockeye starts contracting when the intensity f a l l s below 1Q"*" f t - c . Maximal contraction of the pigment occurs at various intensities in the different species (Fig. 24 and Table XIII). Here the two extremes seem to be coho (10~^ ft-c.) and chum (10""1" f t - c ) . The cones of a l l species except coho do not start expanding (dark-adapting) unless the light intensity decreases below 10° f t - c . (cone threshold). This intensity i s lower (10" 1 ft-c.) in the case of coho (Fig. 26 and Table XIII). The cones are maximally expanded at 10""^ " f t - c . , i n the case of pink -2 and chum, while in the sockeye this occurs at 10 f t - c , and at 10 f t - c . in the case of the coho. The feeding rates under the different light intensities studied show a correlation with the state of adaptation of the cones (Fig. 24 and Table XIII). Changes in feeding behaviour, when the light intensity f a l l s below the cone threshold, are similar in a l l the species. When vision changes from photopic to scotopie, the animals stay in the bottom third of the tank and capture their prey by "spotting" their movements and s i l -houettes. It appears from these experiments that the cones of coho have the lowest threshold (10""^  f t - c ) , while the others have a higher threshold (10° f t - c ) . - 73 -Figure 24. The thicknesses of pigment and cone layers and the feeding rates of late fry "belonging to the different species under various light intensities. • Pigment O Cones A Feeding Rate LIGHT INTENSITY ( FT-C.) - 74 -TABLE XIII - Intensities at which, pigment and cones are light and dark-adapted, feeding i s at i t s maximum or stops, and schools disperse in the late fry. See Pigs. 24 and 25. Fully Fully Gone Feeding Rod Light-adapted Bark-adapted -Threshold Stops at Threshold at f t - c . at f t - c . f t - c . f t - c . f t - c . Species Pigment Cones Pigment Clones Sockeye 10 1 10° 1(T 2 10" 2 10° 10"5 10~ 4 Coho 10° 10" 1 10""5 10~3 lCT 1 10'5 10" 4 Pink 10° 10° 10"2 10" 1 10° 10~5 1(T 4 Chum 10° 10° 10" 1 10" 1 10° 10~5 10~ 4 - 75 -No feeding occurs at intensities of 10"^ f t - c . or lower, The rods of a l l the species studied have the same threshold (10" 4ft-c.). The observations on the feeding rates and changes in feeding behaviour at different light intensities agree with those made with coho fry in an earlier investigation ( A l i , 1957). 3. Smolts (Figs. 25, 26 and Table XIV) The pigment of the sockeye smolt, like that of the sockeye late fry, does not commence contracting u n t i l the light intensity decreases below 10 1 f t - c , which i s higher than the minimum intensity at which the coho pigment i s f u l l y expanded (10° f t - c ) . As in sockeye, the coho smolts also show similarity with the fry. The retinal pigment layer of sockeye i s maximally contracted at a lower light intensity (10"^ ft-c.) than that at which the coho pigment maximally contracts (10~ f t - c ) . The cones of both sockeye and coho smolts do not commence their migration distally unless the light intensity f a l l s below lO""1 f t - c . (Figs. 25, 26 and Table XIV). However, the maximum intensities of light at which they are f u l l y expand-ed (dark-adapted) are different. They are 10"" f t - c . (sockeye) and 10" 5 f t - c (coho). In the case of the smolts also, feeding rates show agreement with the state of adaptation of the cones (Fig. 25). The change in the mode of capture of the prey when light intensity f a l l s below the cone threshold i s also the same as in the fry, described above. The cone (10""1 ft-c.) and rod (10~ 4 ft-c.) thresholds, the intensity at which feeding by visual means stops (10~^ ft-c.) are the same in both the sockeye and coho smolts (Figs. 26, 28 and Table XIV). In a l l cases, the pigment and cone layers at light intensities lower than their thresholds showed semi-contracted and semi-expanded stages respectively, with the exception of the pigment and cone layers of the late chum fry, the cones of the pink fry and sockeye smolts. This i s perhaps understandable in the case of the pigment layer for i t s function seems to be to - 76 -TABLE XIV- Intensities at which pigment and cones are light-adapted and dark-adapted, feeding i s at i t s maximum or stops, and cone and rod thresholds in the smolts. See Pigs. 26 and 27. Fully _ Fully Gone Feeding Rod Light-adapted Dark-adapted Threshold Stops at Threshold at f t - c . at f t - c . f t - c . f t - c . f t - c . Species figment'Cones Figment Cones Sockeye 10 1 10" 1 10~ 4 10" 2 10*"1 10~5 10" 1 Coho 10° 10"1 10~2 10"3 10 _ 1 10~5 10""4" - 77 -Figure 25. The thicknesses of pigment and cone layers and the feeding rates of sockeye and coho smolts under various light intensities. • Pigment O Cones A Feeding Rate L I G H T I N T E N S I T Y IN F O O T " C A N D L E S - 78 -control, by expansion or contraction, the amount of light absorbed inside the optic cup, but one would assume that the cones would stay light-adapted as long as the light intensity exceeds their thresholds and then when i t decreases below the thresholds, they would dark-adapt. These intermediate stages seen i n the ease of cones perhaps suggest that they are not altogether useless at light intensities below their thresholds. The absence of intermediate states of contraction and expansion of pigment and cones respectively in the exceptions mentioned above may be due to the reason that the intermediate states occur at some light intensity or intensities between the two intensities at which they were f u l l y light-adapted and dark-adapted. On the other hand, this may not be the case, but may be characteristic of these particular stages. Further investigation alone, can answer this. D. State of the Retinal Pigment and Oone Layers  Under Constant Light or Dark If the positions of the pigment and cone layers were to {Trust exhibit a diurnal rhythm, they wouldkpresent the appearance of completely dark-adapted states during the day even i f the f i s h were kept in constant dark. On the other hand i f there were a diurnal rhythm in their positional changes, they would not be f u l l y light-adapted during the night even i f the f i s h were main-tained in constant light. In general, according to previous workers (Welsh and Osborn, 1937; Wigger, 1941; Arey and Mundt, 1941) diurnal rhythm i s seen only in dark or i s more pronounced in dark than under constant light. In instances where diurnal rhythm i s observed i t i s more marked on the f i r s t day and less so on the second and eventually disappearing altogether. The period > of i t s persistance varies. The retinal pigment and cones of animals that do not possess a diurnal rhythm in their positions w i l l always be ligh t -adapted, when kept in light and sampled either during the day or night. Similarly, i f they are kept in dark and sampled i n day-time or night-time they w i l l s t i l l present the appearance of being - 79 -Figure 26. Histogram showing the cone thresholds of the various stages of the different species. ALEVIN LATE FRY SMOLT SOCKEYE COHO PINK CHUM 1 1 1 1 1 1 1 1 1 1 I 1 I I I J L--5 -3 -I | -5 - 3 -I 0 - 4 -2 -10 10 10 10 10 10 10 10 10 10 10 LIGHT INTENSITY IN FOOT" CANDLES TABLE XV - Summary of the results of previous investigations using different f i s h in various light intensities. Animal Lowest Intensity at which in a Light-adapted State Highest Intasity at which in a Dark-adapted ..State Lowest Intensity at which Acuity at Maximum Highest Intensity at which Acuity Pails Reference (ft-c.) (ft-e.) (ft-c.) (ft-c.) Pigment Cones Pigment Cones Rods Phoxinus  Lepomis Ennocanthus Lepomis  Salmo  Phoxinus Lepomis  Lepomis Microcanthus 1.8x10 -4 7.4x10' -4 (School disperses at 2.2x10 i to 3.2x10"" ^ ft-c.) 3.7x10 29.57 100 5.6x10 -4 3x10" 5x10" 4.6x10 6.5xl0""5 -4 12.1 9.3x10 i o - 1 0 4.6x10 -7 -1 Brunner, 1934 Wolf & Zerrahn-Wolf, 1935 Crozier et a l , 1936 Kampa, 1953 Woodhead, 1957 Jones, 1956 Grundfest, 1932 Clarke, 1936 Yamanouchi, 19 5 6 Continued on next page TABLE XV - Summary of the results of previous investigations using different f i s h i n various light intensities. Continued from page 80. Animal Lowest Intensity at which in a Light-adapted State (ft-c.) Highest Intensity at which in a Dark-adapted State (ft-c.) Lowest Intensity at which Acuity at Maximum (ft-c.) Highest Intensity at which Acuity Fails (ft-c.) Refer-ences Pigment Cones Pigment Cones Rods 0. kisutch Trachurus Carassius -.46 Miagurnus 6.5x10 Lateolab-rax Cyprinus -2 9.3xl0"2 2.8xl0~2 7.4xl0" 4 2.1x10 -2 7.4x10 -4 2.1xl0~ 2 1.9xl0"4 1.9xl0" 4 ,-3 7.4x10 -4 3.7x10 (before midnight) 9.3x10"^ " (after midnight) 4.6xlO"5 (before midnight) 5.6xl0"6 (after midnight) 10 10~4 A l i , 1957 ) ) \Kobayashi, ; 1957 ) Tamura, 1957 - 81 -Figure 27. Diagram summarising the results obtained under various light intensities, with some other responses of Oncorhynchus. Light intensities under natural conditions are also indicated for comparison. R O D T H R E S H O L D S C H O O L D I S P E R S E S F E E D I N G MINIMUM C O N E T H R E S H O L D L O W E S T L I G H T A T W H I C H M A X . F E E D I N G O C C U R S N O F E E D I N G R O D ( S C O T O P I C VISION ) A D O W N S T R E A M D I S P L A C E M E N T £ 1 C O N E ( P H O T O P I C VISION ) fl 3 1 0 1 0 . -4 I d 3 C L E A R N E W M O O N N I G H T F U L L M O O N N I G H T 1 0 2 1 0 " ' 1 0 ° 1 0 1 L I G H T I N T E N S T Y ( F T - C > 1 0 ' D A W N 8 D U S K 10" 1 0 B R I G H T S U M M E R DAY C L O U D Y D A Y - 82 -fu l l y dark-adapted irrespective of the time at which they are sampled. In the sockeye alevins, late fry, smolts; coho late fry; chum alevins and late fry which were subjected to constant darkness and light and sampled every three hours for 96 hours, no differences were observed between the positions taken by the retinal pigment of the animals fixed during the day and those fixed furing the night. In a l l the histological preparations the retinae of animals subjected to constant light appeared fu l l y light-adapted whether they were sampled during the day or during the night. Likewise, the retinae of animals kept i n darkness were f u l l y dark-adapted irrespective of the time of the day they were sampled. It does not appear that the pigment or cones have a diurnal rhythm or any rhythm indeed. However, when the thicknesses were plotted against the time the animals were sampled, i t was seen that except in a few cases the points of the graph did not present the appearance of lying in a straight line as i s seen for example in the case of chum fry in dark, whose data are presented (Pig. 28). In most cases, although the pigment layer was f u l l y light-adapted or dark-adapted, depending on whether the animal was held in light or dark, i t s thickness showed marked variation. This was less frequent in the case of the cones. The data for chum fry in light are presented (Pig. 28) as an example of an extreme case of variation. In addition, i t also shows a tendency of the pigment layer to be slightly expanded in the nights of f i r s t two days. The reason for this, perhaps, i s the building up of acid as has been described by von Studnitz (1933a) and Wigger (1937, 1941) in the case of the gold f i s h . On the other hand, i t might be that constant light i s a stress and as such disturbs the physiological balance in the animal's metabolism. Once the physiological balance i s altered or upset, the changes in the secretion of hormones that control phenomena such as pigment dispersion or contraction could bring about changes in the thickness or position of pigment which are not diurnal but random, depending on the effect of endocrines. It has been - 83 -Figure 28. Thicknesses of pigment and cone layers of late chum fry at three hour intervals for 96 hours, under constant light or dark. Pigment # Cones O 90 80 70 60 CO O 50 CC y 2 40 -z. - 30 CO CO Ul § 70 CJ I 60 Ist DAY 2nd DAY 3 r d DAY 4" DAY 5,h DAY o o o o ^ o O ~ () LIGHT o o o o o o o o o o o o o ° 0 0 ° 50 40 _L o o 0 o o DARK • • # • • o o o 12 24 12 24 12 24 HOUR OF DAY 12 24 12 - 84 -found that intermedin regulates the migration of retinal pig-ment in fishes (Vilter, 1942, 1946). In a l l other cases whether in constant light or constant darkness, the conditions varied between these two extremes. In general, the variation i s greater in older animals such as smolts, most probably due to the greater thickness of their retinae. Statistical analyses were carried out and no s i g n i f i -cant differences could be established either between 1he day data and night data or between the midnight data and the midday data or between the thicknesses of these layers of animals sampled furing the f i r s t two days and those sampled during the last two days (3rd and 4th days). It seems safe to conclude that in Oncorhynchus there i s no diurnal rhythm in the positions of retinal pigment and cone layers and that the variations in the thicknesses of the pigment layer may be related to the action of intermedin. - 85 -V. DISCUSSION A. Structure of the Oncorhynchus eye The eye of the Pacific salmon possesses features such as the sclera, cornea, lens, i r i s and an inverted duplex retina with a l l the ten layers, that are characteristic of a typical vertebrate eye, including the human. However, since i t i s a teleost eye, i t shows certain structural and functional features that are peculiar to most teleosts. These can be l i s t e d as the cup shaped, oval eye with i t s flattened anterior surface, pro-portionately larger cornea, spherical lens, retractor lentis, non-contractile i r i s , a 11 supplementary nutritive device" consisting of a falciform process, chorioid gland, accommodation (for distant or near vision) by shifting the lens axis, the presence of twin cones and the capacity of the visual cells and retinal pigment to undergo photomechanical changes. 1. I r i s Some teleosts such as the eel (von Studnitz, 1933), and some flounders and stargazers (Young, 1931, 1933) have a contracile i r i s . A l l the other teleosts that have been studied, have been observed to have a fixed pupil (Walls, 1942; Rochon-Duvigneaud, 1943, 1958). 2. Supplementary nutritive device, The mutually exclusive falciform process and hyaloid vessels form, what Walls (1942) refers to as the "supplementary nutritive device". The falieiform process has i t s precursor in the holostean (Lepisosteus) eye but occurs prominently only among some teleosts. In the teleosts that do not possess i t or possess i t in a reduced form, hyaloid vessels occur between the nerve fibre layer and the internal limiting membrane (Walls, 1942). The same artery that supplies the falciform process supplies the hyaloid vessels in the latter forms. The falciform process' connexion with the chorioid and the highly vascularised nature of i t s pigment layer suggest i t s function as a supplementary nutritive device. - 86 -3. Chorioid gland Only those teleosts that possess a pseudobranch or false g i l l , found on the inner side of the operculum, possess the chorioid gland. An efferent artery supplies aerated blood to the chorioid gland after entering the eye i n the vi c i n i t y of the optic nerve. In the forms that lack the pseudobranch (incidentally, most of these are small eyed forms such as the catfishes), the chorioid gland i s also absent. Since the blood vessels from the chorioid gland branch further and enter the chorioid, i t appears that the function i s mainly nutritive. However, as Walls (1942) suggests i t might also counteract fluctuations in the pressure of blood from the heart, so as to minimise mechanical disturbance of the iretina. 4. Twin cones Twin cones are a teleostean monopoly. Walls (1942) considers that these might have evolved from the holostean double cones by an equalisation of the two individuals forming the double cones. The structure of the unequal twin described in this paper (Pig.5) would appear to support this view. In the closely related genus Salmo. Verrier (1935) and McBwan (1938) have described similar unequal twin cones. Of the three types of cones present, viz., single, twin and unequal twin cones, the last are rarely seen but occur mostly in the fundus and the surrounding regions. B. Correlation between the arythmic mode of l i f e of  Oncorhynchus and the structure of i t s retina The heavily pigmented ephithelial layer, three types of cones and the abundance of rods (Pig. 5) together with the abi l i t y of these visual elements to undergo remarkable photo-chemical changes, suggest that the Pacific salmon i s , what Walls (1942) would c a l l "arythmic". This is certainly borne out by the results obtained in this as well as an earlier ( A l i , 1957) investigation. It has been shown (Pigs. 24, 25, 27 and Tables XIII and XIV) that the juvenile Pacific salmon are capable of carrying on activities - 87 -such as feeding and schooling under widely different light conditions. The presence of three types of cones and the complex neurological arrangement of the retina suggest that the Pacific salmon might be capable of colour vision. C. Retinomotor (retinal photomechanical) responses 1. Phylogenetic occurrence The vertebrate eye adapts i t s e l f for photopic (bright light) or scotopic (dim light -Dammerungssehen) vision by shifting the position of i t s visual cells and retinal pig-ment (most teleosts) as shown in the several figures i n this paper or by the constriction or dilation of i t s i r i s (mammals). The eel among the teleosts (von Studnitz, 1933a), anural amph-ibians, several reptiles and birds (Detwiler, 1943) employ both methods. Cyclostomes show neither retinal nor pupillary responses (Walls, 1928b, 1942). Elasmobranchs show no retin-omotor responses but have a contractile i r i s (von Studnitz, 1933b). Diurnal ganoids (Amia) show retinal photomechanical responses but nocturnal forms (Acipenser) do not. Some dipnoans and cladistians possess a contractile i r i s but show no retin-omotor response (Walls, 1942). Teleosts as a group show extensiveretinal photo-mechanical responses (Garten, 1907; Arey, 1915, 1928; Parker, 1932; Walls, 1942; Detwiler, 1943; von Studnitz, 1952). In some teleosts such as flounders and stargazers (Young, 1931, 1933) pupillary responses have been observed but information on whether or not their retinae are capable of retinomotor movements i s not available. Amphibians with the exception of caecillians show photomechanical changes in their retinae, particularly of the pigment and cone layers. Although, not much i s known about the distribution of retinomotor responses among the reptiles, enough i s known (Detwiler, 1943) to indicate that some of them do show a slight migration of visual c e l l and retinal pigment layers. Birds are capable of extensive retinomotor responses. - 88 -In some eases their pigment migration i s so extensive that in the light-adapted state i t extends down to the external limiting membrane. Mammals light or dark-adapt by a dilation or contract-ion respectively, of their i r i s and show no retinal photomechan-i c a l changes (Walls, 1928a). Retinomotor responses are more primitive as compared with pupillary responses (Walls, 1942), but are more efficient, their only disadvantage being the slowness with which they occur. In fast moving forms with high visual acuity, such as the birds, this method of adaptation has been retained and the whole mechanism of adaptation perfected by the evolution of a contractile i r i s as well. 2. Ontogeny of retinomotor responses in Onchorhynchus As has been shown in this paper (Pigs. 6 to 9, 12, 18 to 21), retinal photomechanical changes occur neither in,the embryos prior to hatching nor in the newly hatched alevins of any Oncorhynchus species. The situation changes as the alevins become older. Two or three weeks after hatching, the alevin with a comparatively small yolk sac shows retinomotor responses to different light conditions (Pig. 23 and Table XII). In the emerged fry, the ab i l i t y of the retina to undergo extensive photomechanical changes (Pigs. 6 to 9, 14, 18 to 21) and emergence from the gravel, with the consequent exposure to different light conditions, coincide. This a b i l i t y i s more marked in the late fry and i s perfected in the smolts (Pigs. 15 and 16). Since no study of the ontogeny of photomechanical responses in other teleosts or in any other vertebrate group i s available, i t i s not possible to abate with certainty whether this situation i s common to a l l teleosts or to other verte-brates whose retinae are capable of undergoing photomechanical changes. The development of the rat (Detwiler, 1932) and the guppy (Muller, 1952) retinae suggest that i s i s l i k e l y . - 89 -3. Inter-specific comparison of retinomotor responses  of the species and stages of Oncorhynchus Clemens (1953) wrote, "On the "basis of morphological, physiological, lifei-history and behaviour studies to date, i t appears that spring and coho salmon are related on the one hand, pink and chum on the other, and that sockeye occupy a position more less intermediate between the two pairs". Based on several years of study of the behaviour of juvenile Oncorhynchus. Hoar (1958) has come to the conclusion that the coho are the closest to the parental, trout-like type, while the pink and chum are the most specialised. Specific differences have been found among the juvenile Oncorhynchus in their reactions to light (Hoar et a l , 1957). However, no differences in their ocular structure were found i n this as well as an earlier investigation (Ali and Brett, 1958). Hence, i t was of interest to note whether they showed any inter-specific differences in their retinomotor responses. As has been shown in the results, the various species and stages studied did show differences in their rates of light or dark-adaptation and retinal responses to different light intensities. However, these differences are not consistent (Pigs. 17, 22, and 26) and do not indicate inter-specific relationships. D. Significance of retinomotor responses  in the l i f e of juvenile Oncorhynchus 1. Precedence of light sensitivity to retinomotor responses It has been shown that the eyes of the newly hatched alevins are not capable of undergoing-photomechanical responses. However, they are photosensitive as seen by their negative response to light. Hoar (1958) considers the tendency to hide under stones (as a result of photonegative response) to be their complete behaviour at this stage. The Oncorhynchus alevins become less photonegative and increasingly photopositive with age. This coincides with the greater development of the aetinal elements resulting in their increased a b i l i t y to respond to - 90 -light (Pigs. 13 and 23), culminating in the photopositive emerged fry that i s also capable of undergoing marked photomechanical changes (Pigs. 6, 7, 8, 9, 14, 18, 19, 20, 21 and 22), and also possesses f u l l visual acuity as shown by the feeding experiments (Pigs. 11, 24). The newly hatched demersal alevins of fresh-water f i s h are known to be photonegative (Buckland, 1863; Hein, 1906; White, 1915; Smith, 1916; Gray, 1928; Stuart, 1953; Woodhead, 1957). It is not known whether the retinae of these alevins are capable of undergoing photomechanical changes or not. In contrast to the demersal alevins of the fresh-water species, the pelagic larvae of marine fish that have been studied so far are a l l reported to be photopositive (Dannevig, 1932; Franz, 1909; Soleim, 1942; Tavolga, 1950; Buckmann et a l , 1953). Again, i t i s not known whether their eyes show retinomotor responses. 2. Migratory behaviour As Clemens (1951) remarked, "The migration of Pacific salmon is another excellent il l u s t r a t i o n of the delicate inter-relations between organisms and the environment; in other words, of the interplay between a physico-chemical organism and a physico-chemical environment". The downstream migration, which takes place at dusk i s a combination of the fish's response to light and i t s individual behaviour pattern ((Hoar, 1953). It has been shown that the eyes of sockeye and coho smolts as well as pink and chum fry are in the process of dark-adapting at the time of the commencement of downstream migration (Ali and Brett, 1958). Hoar (1953, 1958), Neave (1955) and McDonald (1956) have shown that as the light intensity decreases at dusk, the fry of the migrating species (sockeye, pink and chum) rise to the surface and either swim with the current or are displaced. The mechanism of downstream migration is similar in fry and smolts. The coho fry do not show the same marked increase in activity as the other species at dusk. Due to this, they ordinarily do not rise to the surface and become displaced at dusk. They are however, subject to some displacement, - 91 -particularly in times of high, water (Hoar, 1958). The fact that the cone threshold of coho fry (10 _ 1 ft-c.) i s lower than that of the other species (Pig. 26) while their rate of dark-adaptat-ion i s very similar (Pig. 22), might be partly or wholly responsible for this difference in their behaviour. They w i l l , in short, be able to see at lower light intensities. Evidence has been presented in the present investig-ation to show that when the light intensity f a l l s below the cone thresholds (ICf 1 to 10° ft-c.) the eyes of fry and smolts commence to dark-adapt (Pig. 24 and 25). It has also been shown that the process of dark-adaptation takes ordinarily (with the exception of emerged chum fry) 35 to 40 minutes in the case of fry and 40 to 50 minutes in the ease of smolts (Pig. 22). In the face of this evidence, i t i s suggested that these fish commence migration as the light intensity begins to decrease rapidly and f a l l s below the cone threshold. This may result in a state of partial night-blindness. At this stage the rate of decrease of light intensity in nature i s very rapid and decreases from 1.0 f t - c . to 0.002 f t - c . in 30 minutes (Ali and Brett, 1958, Pigs. 9, 10 and 11). Its rate is greater than the rate of dark-adaptation as found in this investigation. This leaves the animal in a semi-dark-adapted state which results in i t s losing i t s a b i l i t y to maintain position with relation to some reference point and i t swims with the current, or i s displaced downstream. Since the process of dark-adaptation takes 35 to 40 minutes for completion in the fry, those that have :risen to the surface at the time of dusk are in a semi-dark-adapted state for 35 to 40 minutes and consequently, swim with the current or get displaced during this entire period. When the process of dark-adaptation i s completed the light intensity at the surface of the water i s well above the rod threshold (10~ 4 f t - c ; Tables XIII and XIV) and the fry are able to see large objects such as rocks and use them as reference points and migration ceases or slows down considerably. This suggests that the states of adaptation of the eyes are responsible for the marked peak in the downstream migration of the juveniles at dusk. This peak - 92 -in migration lasts for a longer time in the case of sockeye smolts (Dr. W.A. Clemens, personal communication). This appears to be due to the process of dark-adaptation taking a longer time (50 minutes) in the case of sockeye smolts (Pig. 22). This peak in the downstream migration of the Oncorhynchus juveniles may have survival value, especially in the case of smaller fry. As Hoar (1958) remarks, "When many small f i s h must face a fixed number of predators, the shorter and more precise the period of contact, the better w i l l be their chances of survival". The slow rate of dark-adaptation coupled with the rapid decrease in light intensity, triggers their mass migration lasting for a brief period, with i t s obvious advant-ages. Another point that warrants mention here i s the fact that these migrations at dusk are related, not to the time of the day, but to the light intensity (McDonald, 1956). In other words, as the summer day gets longer and longer, the commence-ment of migration shifts to later times in the evening correspond-ing to the intensity of light. The absence of diurnal rhythm i n the positions of pigment and cones, as shown in this investigation (Pig. 28), makes possible this response to light intensity, for, i f there were a sharp diurnal rhythm in the positions of the visual cells and pigment layer, the animal would possess an eye in a particular state of adaptation irrespective of the intens-i t y of light available. This would result in i t s coming up to the surface and swimming' or being displaced with the current at the same time every day. It would appear that in guiding juvenile downstream migrants around barriers such as dams, using visual stimuli (e.g., illuminated screens), the fact that the eye of the migrating fis h i s i n a semi or f u l l y dark-adapted state (Ali and Brett, 1958) and that i t takes 10-20 minutes for i t to light-adapt f u l l y (Pig. 17) may be important. Unless the eye is in a light-adapted state, visual acuity, so necessary f o r the animal to be able to follow the moving screen, i s not at i t s best as shown by the feeding experiments conducted in the - 93 -present investigation (Pig. 11, also compare Pig. 17). 3. Schooling That sight is the primary requisite in the formation and maintenance of fish schools has been established (Keenley-side, 1955; Hoar, 1958), and this investigation presents more evidence to support this. It has been shown in the results that schools of .juvenile Oncorhynchus disperse in total darkness and in light intensities lower than the rod threshold (10~ 4 f t - c . Table XIII). Pink, chum and sockeye fry, which are known to school readily (Hoar, 1958) form schools ten to fifteen minutes after dark-adapted fish are exposed to light (Pig. 10). The coho, on the other hand, school less readily as shown by the longer time they take to school and the fewer animals that participate in this process (Pig. 10). The time taken by the dark-adapted animals on exposure to light, to school i s less than the time taken for f u l l light-adaptation (compare Pigs. 10 and 17). Prom this i t would appear that f u l l acuity of vision i s not necessary to recognise and join another fish of the same species to form a school. This is-supported by the fact that schools persist in dim light when the eyes are adapted for scotopic vision. Once formed, a school stays intact (other conditions remaining the same) u n t i l the light intensity f a l l s below the threshold for rods, when even the shapes of large objects are not recognisable. 4. Feeding As Hoar (1958) observed, "The Pacific salmon i s basic-a l l y a surface feeding fish, depending on i t s eyes for the location and capture of its- food"". ~ This has been- well exempli-fied by the results obtained in this as well as an earlier investigation (Ali, 1957). Active feeding stops in the dark, save for the occasional, chance capture of prey. In feeding experiments, in which liv e prey are used, the behaviour of the prey under different light intensities w i l l affect the results significantly. However, in the case of Daphnia, i t has been shown (Harris and Wolf, 1955) and also noticed during the course of the present investigation that on exposure to total - 94 -darkness, the Daphnia w i l l sink to the bottom only after ten to fifteen minutes, whereas experiments in this investigation did not last over five minutes. It has been saown that feeding does not occur when the light intensity i s lower than the rod threshold (Pigs. 24 and 25). It occurs at higher intensities but between the rod and eone thresholds (scotopic vision), i s proportional to the c logarithm of the light intensity (Weber-Fechner Law). When light intensity increases to cone threshold or higher, i t reaches i t s maximum. Whether very high intensities reduce feeding rates i s a matter of conjecture, but intensities as 2 high as 63 x 10 f t - c , did not affect maximum feeding in coho fry ( A l i , 1957). The change in the mode of capture of the prey as the intensity f a l l s below the cone threshold i s interesting and indicates the inability of the rods to resolve as the cones do. That the visual acuity i s lost in the scotopic visual f i e l d of the animal i s shown by the change to the "silhouette method" of feeding whereby the animal makes use of the sensitivity of rods and "spots" the shadow of the prey against the brighter background. With decreasing light intensity the difference between the shadow and background diminished, making the location of prey more and more d i f f i c u l t . This accounts for the reduction in feeding rates, in proportion to the logarithm of the light intensity (Pigs. 24 and 25). When the light intensity f a l l s below the rod threshold, the shadow of the prey cannot be distinguished from the background, by the animal, resulting in i t s inability to spot the prey, and feeding stops. - 95 -VI. CONCLUSIONS 1. The structure of the Oncorhynchus eye i s typical of vertebrate and teleost eyes. It i s very similar to the Salmo eye described in the literature. 2. The i r i s of the Oncorhynchus i s immobile. 3. The Oncorhynchus eye has a chorioid gland and a falciform process. 4. The neurological arrangement of the Oncorhynchus retina i s similar to that of the primate retina. 5. The eyes of the late embryos are not ful l y developed histologically and physiologically and are not capable of retinomotor responses. 6. The eyes of the alevins are also not f u l l y developed, but show some response to light. Their a b i l i t y to undergo photomechanical changes increases with age. 7. As the animals become older, they show a general trend in shortening of the time required for light-adaptation. 8. Dark-adaptation takes a' longer time than does light-adaptation. 9. The time taken for dark-adaptation shows a tendency to increase with age. 10. The pigment layers of a l l the f i s h studied, except that of the late pink fry, have a latent period before the.commence-ment of contraction in the dark. 11. The latent period i s longer in older sockeye and coho. It does not change with age in the chum salmon. In the case of the pink i t i s very short (5 minutes) in the emerged fry and is absent in the late fry. 12. The four species of Pacific salmon studied show differences in their retinal and behavioural responses to light. 13. In a l l cases, the state of the cones and the a b i l i t y to capture maximum number of Daphnia are correlated. - 96 -14. The sockeye show a lowering of the cones thresholds with age; in the coho there are no differences among the early l i f e history stages, while the chum fry have a higher cone threshold than do the alevins. Only the fry of pink were studied. 15. The rod threshold (10"*4 ft-c.) i s the same in a l l species and stages. 16. The feeding rates and thicknesses of the pigment and cone layers obey the Weber-Fechner law in the intensities between the rod and cone thresholds. 17. Under constant light or constant dar, there i s no diurnal rhythm in the positions of the pigment and cone layers of the Pacific salmon. 18. Based on this research i t i s suggested that the downstream migration of juvenile Pacific salmon occurs as a result of their eyes being in a semi-dark-adapted state for a short period at dusk, due to the rapid decrease in the incident light intensity and the relatively slower rate of dark-adaptation. This results in the fi s h losing their reference points and swimming with the current or being displaced downstream. VII. LITERATURE CITED A l i ibn Isa 940-1010. Tadhkirat al-Kahhalin (Oculist's memorand-um book). Latin Ed. in P. Pansier, "Collectio opthal-mological veterum auctorum", Vol. 3, J.B. Bailliere & F i l s , Paris, 1903. (Prom Polyak, 1941). A l i , M.A. 1956. Studies on the retina of the Pacific salmon. Proe. British Columbia Academy of Sciences. Tenth Sci. Conference, p. 6 (Abstract). A l i , M.A. 1957. Rate of feeding in coho (Oncorhynchus kisutch) under different light intensities. Proc. British Columbia Academy of Sciences. Eleventh Sci. Conference, p. 14 (Abstract;. A l i , M.A. and Bretl} J.R. 1958. The structure and photomechanical responses of the Pacific salmon retina. J. Pish. Res. Bd. Canada, 15: (in press). Arey, L.B. 1915. The occurrence and the significance of photo-mechanical changes in the vertebrate retina - an historical survey. J. comp. Neurol., 25: 535-554. Arey," L.B. 1916. The movements in the visual cells and retinal pigment of the lower vertebrates. J. comp. Neurol., 26: 121-201. Arey, L.B. 1919. A retinal mechanism of efficient vision. J. comp. Neurol., 30: 343-354. Arey, L.B. 1928. Visual cells and retinal pigment. "Special Cytology", Vol. 2, Section 25, 887-926. E.V. Cowdry, Editor. Paul B. Hoeber Inc., New .York. Arey, L.B. and Mundt, G.H. 1941. A presistent diurnal rhythm in visual cones. Anat. Rec, 79 (Supp):5. Boll, P. 1877. Zur anatomie und physiologie der retina. Arch. Anat. Physiol., 4.: 783-787. Brown, P.A. 1936. Light intensity and melanophore response in the minnow, Ericymba buccata Cope. Biol. Bull. Woods Hole, 70: 8-15. Brunner, Gertrud, 1934. Tiber die Sehscharfe der Elritze (Phoxinus laevis) bei verschiedenen Helligkeiten. Z. vergl. Physiol., 21: 296-316. Buckland, P.T. 1863. Pish hatching. London. Buckmann, A., Harder, W.S. and Hempel,G. 1953. Unsere Beobach-tungen am Hering (Clupea harengus L). Kurze Mitt. a.d. Pischereibiol. abt. d. Max-Planckflnstituts, Wilhelmshaven, 3: 22-42. - 98 -Buddenbrock, ¥. von, 1952. Vergleichende Physiologie Bd.l. Sinnesphysiologie. Verlag Birkhausen, Basel. Chiarini, P. 1904. Changements morphologiques que l'on observe dans l a retine des vertebres par l 1 a c t i o n de l a lumiere et de l 1obscurity. Premiere Partie. La retine des poissons et des amphibes. Arch. i t a l . Biol., 42: 303-322. Clemens, W.A. 1951. On the migration of Pacific salmon (Oncorhynchus). Trans. Roy. Soc. Can., 3rd Ser. 45 (Sec. V): 9-17. Clemens, W.A. 1953. On some fundamental problems in the biology of Pacific salmon. Trans. Roy. Soc. Can. 3rd Ser. 47 (Sec. V): 1-13. Crozier, W.J., Wolf, E. and Zerrahn-Wolf, G. 1936. On c r i t i c a l frequency and c r i t i c a l illumination for reaction to flickering light. J. gen. Physiol., 20: 211-228. Dannevig, A. 1932. Fl/devigens Utklekningsansalt 1882-1932. IV Torsk, Utklekning og unders^kelser. Arsberetn, ved. Norges. Pisk. 4: 17-30. Detwiler, S.R. 1932. Experimental observations upon the developing rat retina. J. comp. Neurol., 37: 481. Detwiler, S.R. 1943. Vertebrate photoreceptors. MacMillan Co., New York. Eigenmann, C.H. and Schafer, CD. 1900. The mosaic of single and twin cones in the retina of fishes. Amer. Nat., 34: 109-118. Franz, V. 1909. Einige- Versuche zur biologie der fischlarven. Int. Rev. Hydrobiol., 2: 557-579. Franz, V. 1913. Sehorgan. Oppel's Lehrbuch der vergleichende mikroskopische Anatomie der Wirbeltiere. G. Fisher, Jena. Garten, S. 1907. Die Veranderungen der Netghaut durch Licht. Von Graefe-Saemisch Handbueh der Augenheilkunde. Parti, Vol. 3, Cap. 12. 2nd Edn. Granit, R. 1947. Sensory mechanisms of the retina. Oxford University Press, London. Granit, R. 1955. Receptors and sensory perception. Yale University Press, New Haven. - 99 -Gray, J. 1928. The growth of fi s h . II. The growth rate of the embryo of Salmo fario. J. exp. Biol., 110-124. Grundfest, H. 1932. The sensibility of the sun-fish, Lepomis. to monochromatic radiation of low intensities. J. gen., Physiol. 15: 307-328. Harris, J.E. and Wolfe, Ursula K. 1955. A laboratory study of vertical migration. Proc. Roy. Soc. London, Ser. B, 144-j- 329-354. Hein, W. 1906. Zur Biologie der Porellenbrut I. Versuche uber das Nahrungsbedilrfnis der Bachforellenbrut im Bruttrog und im kunslichen Brutbett. Allg. Pisch. Ztg., 31: 239-243. Hoar, W.S. 1951. The behaviour of chum, pink and coho salmon in relation to their seaward migration. J. Pish. Res. Bd. Canada, 8: 241-263. Hoar, W.S. 1953. Control and timing of fish migration. Biol. Rev. 28: 437-452. Hoar, W.S. 1954. The behaviour of juvenile Pacific salmon, with particular reference to the sockeye (Oncorhynchus- nerka). J. Pish. Res. Bd. Canada, 11: 69-97. Hoar, W.S. 1956. The behaviour of migrating pink and chum salmcfn fry. J. Pish. Res. Bd. 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