UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Salinity preferences : an orientation mechanism in salmon migration. McInerney, John Edward 1963

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

Item Metadata

Download

Media
831-UBC_1963_A1 M2 S2.pdf [ 3.8MB ]
Metadata
JSON: 831-1.0105606.json
JSON-LD: 831-1.0105606-ld.json
RDF/XML (Pretty): 831-1.0105606-rdf.xml
RDF/JSON: 831-1.0105606-rdf.json
Turtle: 831-1.0105606-turtle.txt
N-Triples: 831-1.0105606-rdf-ntriples.txt
Original Record: 831-1.0105606-source.json
Full Text
831-1.0105606-fulltext.txt
Citation
831-1.0105606.ris

Full Text

SALINITY PREFERENCE - AN ORIENTATION MECHANISM IN SALMON MIGRATION  by  JOHN EDWARD McINERNEY B . S c , St. Patrick*s College, University of Ottawa, 1959 M.Sc,  University of B r i t i s h Columbia, 1961  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the DEPARTMENT OF ZOOLOGY We accept t h i s t h e s i s as conforming t o the standard required from candidates f o r the degree of DOCTOR OF PHILOSOPHY  Members of the Department of Zoology The University of B r i t i s h Columbia A p r i l , 1963  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 l m e n t of the requirements f o r an advanced degree at the University 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 representatives.  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 written permission.  Department of Zoology The University of B r i t i s h Columbia, Vancouver 8, Canada.  The U n i v e r s i t y of B r i t i s h Columbia FACULTY OF GRADUATE STUDIES PROGRAMME OF THE FINAL ORAL EXAMINATION FOR THE DEGREE OF DOCTOR OF PHILOSOPHY of JOHN EDWARD McINERNEY B . S c , U n i v e r s i t y of Ottawa, 1959 -M,Sc, The U n i v e r s i t y of B r i t i s h Columbia, 1961 IN ROOM 3332, BIOLOGICAL SCIENCES BUILDINC TUESDAY,. APRIL 16, 1963, at 9:30 A.M. COMMITTEE IN CHARGE Chairman: F.H. Soward R.N. Band I . McT. Cowan J.F. Eisenberg C.V. Finnegan  W.S. Hoar P.A. L a r k i n G.L. Pickard P. Remnant  E x t e r n a l Examiner: Dr. J.P. T u l l y F i s h e r i e s Research Board of Canada Nanaimo, B:C,  SALINITY PREFERENCE - AN ORIENTATION MECHANISM IN SALMON MIGRATION ABSTRACT The preferred s a l i n i t i e s of f i v e P a c i f i c Salmon species (genus Oncorhynchus) were studied. Each species was shown to undergo a temporal sequence of preference changes. The sequence began with a preference f o r freshwater then changed i n the d i r e c t i o n of increasing seawater concentration, the terminal pattern i n d i c a t i n g a preference f o r water of open ocean concentration. This preference sequence was shown to p a r a l l e l c l o s e l y the h o r i z o n t a l s a l i n i t y gradients t y p i c a l of e s t u a r i e s , through which j u v e n i l e salmon pass on t h e i r seaward migration. On the basis of t h i s evidence the following o r i e n t a t i o n mechanism was proposed: that j u v e n i l e salmon are able to use estuarine s a l i n i t y gradients as one of the d i r e c t i v e cues i n t h e i r seaward migration. Further study of t h i s o r i e n t a t i o n mechanism showed that the i n i t i a l part of the preference sequence develops unaffected by seawater exposure. By contrast the l a t t e r part of the sequence (corresponding to the seaward end of the estuary) was found to depend on a period of exposure to seawaterotherwise a regression to a freshwater preference took place. The sensory s t i m u l i leading to the s a l i n i t y pre- . ference response were shown to depend on a complex i n t e r a c t i o n of n a t u r a l l y occuring sea s a l t s . Experimentally the simplest s a l t mixture which would e l i c i t a normal response consisted of two cations (Na and C a ) and one anion ( C l " ) . I t was shown further that taste or the common chemical sense was the primary sensory modality underlying the response and that j u v e n i l e salmon have an a b i l i t y approaching absolute s a l i n i t y d i s c r i m i n a t i o n . +  ++  Speculation concerning the e v o l u t i o n of the s a l i n i t y preference o r i e n t a t i o n mechanism was presented. Published evidence favors the view that migratory salmonids have evolved from nonmigratory forms with l i m i t e d osmoregulatory a b i l i t i e s . On t h i s basis i t was proposed that o r i g i n a l l y the a b i l i t y to o r i e n t w i t h respect to seawater concent r a t i o n was of d i r e c t s u r v i v a l value to the stenohaline a n c e s t r a l salmonid. Later as diadromous movements expanded along with e u r y h a l i n i t y , s a l i n i t y preference became integrated into a temporal sequence of changes and thereby an o r i e n t a t i o n mechanism u s e f u l f o r migration.  GRADUATE STUDIES F i e l d of Study:  Zoology  . Quantitative Methods i n Zoology Comparative Physiology Comparative Ethology Seminar i n Ethology Marine F i e l d Course Ichthyology Endocrinology F i s h e r i e s Seminar  P.A. L a r k i n W.S. Hoar W.S. Hoar M.D.F. Udyardy J.F. Eisenberg P.A. Dehnel C.C. Lindsey W.N. Holmes Staff  Other Studies: I n t r o d u c t i o n to Synoptic Oceanography I n t r o d u c t i o n to B i o l o g i c a l Oceanography Introduction to Dynamic Oceanography  G.L. R.F. G.L.  Pickard Scagel Pickard  ABSTRACT  The preferred s a l i n i t i e s of f i v e P a c i f i c salmon species were studied. Each species was shown t o undergo a temporal sequence of preference changes. The sequence began with a preference f o r f r e s h water then changed i n the d i r e c t i o n of increasing seawater concentration, the terminal pattern i n d i c a t i n g a preference f o r water of open ocean concentration. was  This preference sequence  shown t o p a r a l l e l c l o s e l y the h o r i z o n t a l s a l i n i t y gradients t y p i c a l of  estuaries through which juvenile salmon pass on t h e i r seaward migration.  On  the b a s i s of t h i s evidence the following o r i e n t a t i o n mechanism was postulated: that juvenile salmon are able t o use estuarine s a l i n i t y gradients as one of the d i r e c t i v e cues i n t h e i r seaward migration. Further study of t h i s o r i e n t a t i o n mechanism showed that the i n i t i a l part  o of the preference sequence develops unaffected by seawater exposure.  By  contrast the l a t t e r part of the sequence (corresponding t o seaward end of the estuary) was found t o depend on a period of exposure t o seawater otherwise a regression t o a premigratory freshwater preference took place. The sensory s t i m u l i leading t o the s a l i n i t y preference response were shown t o depend on a complex interaction of n a t u r a l l y occuring sea s a l t s . Experimentally the simplest s a l t mixture which would e l i c i t a normal response consisted o f two cations (Na*" and C a ) and one anion ( C l ~ ) . + +  It was shown  further that t a s t e or the common chemical sense was the primary sensory moda l i t y underlying the response and that juvenile salmon have an a b i l i t y approaching absolute s a l i n i t y discrimination. Speculation concerning the evolution of the s a l i n i t y preference o r i e n t a t i o n mechanism was presented.  Published evidence favours the view that  migratory  salmonids evolved from nonmigratory forms with l i m i t e d osmoregulatory a b i l i t i e s .  ii  On this basis i t was proposed that originally the a b i l i t y to orient with respect to seawater concentration was of direct survival value to the stenohaline ancestral salmonid.  Later, as diadromous movements expanded along  with euryhalinity, salinity preference became integrated into a temporal sequence of changes and thereby an orientation device useful for migration.  ACICNOWLEDGEMMTS  t I wish t o thank e s p e c i a l l y Dr. W. S. Hoar. d i r e c t i o n and assistance are deeply appreciated.  His i n s p i r a t i o n , Thanks are due also  t o other members of my committee including Drs. P. A. Larkin, C. V. Finnegan, C. C. Lindsey, R. L. Band, J . F. Eisenberg and T. G. Northcote. The salmon used i n t h i s study were supplied through the generous cooperation of a number of agencies including the Canadian Department of F i s h e r i e s , F i s h e r i e s Research Board of Canada, the F i s h and Game Branch of the B r i t i s h Columbia Department of Recreation and Conservation, the Washington State F i s h e r i e s Department and the International P a c i f i c Salmon Commission.  Special thanks are due Dr. H. Harvey of t h e Salmon Commission  f o r h i s generous assistance.  Food f o r the experimental salmon stocks was  generously supplied by Mr. J . Terpenning of the B r i t i s h Columbia Department of Recreation and Conservation and the J . R. Clark Company, Salt Lake C i t y , Utah.  TABLE OF CONTENTS Page I. II.  INTRODUCTION  .  1  MATERIALS AND METHODS  7  Experimental Animals . . . . . . .  .  7  S a l i n i t y Preference Test  7  S a l i n i t y Preference Chemical Methods  ...12  S t a t i s t i c a l Procedures  16  Seawater Acclimation and Resistance III.  . . . . . . . . . . . . .  RESULTS  2L  Temporal Changes i n S a l i n i t y Preference  21  E f f e c t s of Repeated Testing  .30  E f f e c t s of Seawater Exposure  31  E f f e c t s of Size  35  Sensory Basis of S a l i n i t y Preference  ....35  Temporal Changes i n Schooling Behaviour and A c t i v i t y . . . . . Development of S a l i n i t y Tolerance 17. V. VT.  20  . . . . . . . . . . . . . .  40 &  DISCUSSION  45  SUMMARY  50  LITERATURE CITED  60  LIST OF FIGURES Page Figure 1.  Figure 2.  V a r i a t i o n i n salmon l i f e h i s t o r i e s related t o environmental s a l i n i t y . . . . . . . . . . . . . . .  2  University of B r i t i s h Columbia seasonal water temperatures and photoperiods  •  .  8  Figure 3.  S a l i n i t y preference t e s t tank  11  Figure 4.  Details o f s a l i n i t y preference t e s t apparatus . . . . .  13  Figure 5.  S t a b i l i t y of t e s t gradients  15  Figure 6 .  Temporal changes i n s a l i n i t y preference, chum salmon . . . . . . . . . . . . . . . . . . . . . .  Figure 7»  Temporal changes i n s a l i n i t y preference,  Figure 8.  pink salmon Temporal changes i n s a l i n i t y preference, coho salmon . . . . . . . . . . . . . .  Figure 9.  23 26  Temporal changes i n s a l i n i t y preference, spring salmon  Figure 10.  22  27  Temporal changes i n s a l i n i t y preference, sockeye salmon  29  Figure 11.  E f f e c t s of repeated S a l i n i t y Preference Tests . . . . .  32  Figure 12.  E f f e c t s of seawater exposure on s a l i n i t y preference  33  Figure 13*  E f f e c t s of f i s h s i z e on s a l i n i t y preference . . . . . .  36  Figure 1 4 .  Response of pink salmon t o a NaCl solution  38  Figure 15.  E f f e c t s of Ca + and S0^ concentration on the response of coho salmon t o a NaCl solution 39 Figure 16. Temporal changes i n schooling behaviour and a c t i v i t y of chum salmon 42 Figure 17. Comparison of the development o f s a l i n i t y tolerance i n the f i v e species of P a c i f i c salmon . . . . 44 +  V  page Figure IS. Figure 19.  S a l i n i t y gradient structure i n a northern B r i t i s h Columbia r i v e r outflow  46  Underyearling coho s a l i n i t y preference pattern and the estuarine boundary l a y e r . . . . . . . . . .  Figure 20.  52  Diagrammatic representation of the s a l i n i t y preference o r i e n t a t i o n mechanism  •  ..56  LIST OF TABLES  Table I.  Data on experimental salmon stocks  9  INTRODUCTION  Many d i f f e r e n t avenues of thought may l e a d t o an understanding One which I f i n d stimulating i s the idea that l i f e confers on the  of l i f e .  possessor  the a b i l i t y t o t u r n to advantage features of the environment i n i t i a l l y d i s advantageous.  To salmon, f o r example, the range of s a l i n e media encountered  during t h e i r extensive migrations presupposes the development of an almost unique osmoregulatory capacity.  The i n a b i l i t y of most f i s h e s t o t o l e r a t e even  a small part of the salmon's natural s a l i n i t y range evidences the evolutionary scope of t h i s adaption.  Once freed of the osmoregulatory problem however,  salmon have been ablte t o use s a l i n i t y gradients t o advantage as guiding cues during t h e i r migrations. Before considering the t o p i c of o r i e n t a t i o n , the character and scope of the salmon's s a l i n i t y environment w i l l be reviewed b r i e f l y .  Three more or l e s s  d i s t i n c t regions may be recognized namely, lakes and r i v e r s , estuaries and the open ocean.  Although the f r e s h water areas and the open ocean form the  opposite ends of the s a l t concentration spectrum they are s i m i l a r i n that both are extensive areas of r e l a t i v e l y uniform salt concentration.  Estuaries i n  sharp contrast are characterized by t h e i r steep s a l i n i t y gradients ( F i g . l ) . The importance of each of these regions to the salmon's migratory movements i s dependent on two main f a c t o r s .  F i r s t , the migratory behaviour of  p a r t i c u l a r species shows considerable v a r i a t i o n . gorbuscha) f o r example may  Pink salmon (Oncorhvnchus  spawn under e s t u a r i a l conditions and thereby eliminate  the f r e s h water region from t h e i r l i f e h i s t o r y . (Hanavan and Skud, Although conclusive evidence i s l a c k i n g , Huntsman  1954)•  (1938) speculated that 3ome  A t l a n t i c salmon (Salmo salar) remain i n estuaries a f t e r migrating downstream  18  [•  U|6 o 0  14  2,2  or tlOl UJ  u z o u  8  < <  2  UJ  to  FRESHWATER  ATLANTIC  SOCKEYE  ESTUARY  OCEAN  PINK  O. GORBUSCHA^  SALMON  SALMON SALMO SALAR  NERKA  SALMON  FIGURE  I  VARIATION IN S A L M O N LIFE HISTORIES R E L A T E D TO E N V I R O N M E N T A L SALINITY  -3-  and thus eliminate the ocean region.  An i n d i c a t i o n of the range of l i f e  h i s t o r y v a r i a t i o n i s provided i n Figure 1 . Secondly, the geographic features of an area may markedly a l t e r the dimensions of each region.  The volume of r i v e r discharge and presence or  absence of offshore i s l a n d s , f o r example, may region.  a l t e r the s i z e of the estuarine  The v a r i a t i o n s of lakes and r i v e r s are w e l l known and do not require  elaboration here.  The l e a s t variable of the three regions both i n r e l a t i v e  s i z e and physical properties, such as temperature, i s the ocean. This b r i e f review of the salmon's range serves two purposes.  First, i t  provides an i n d i c a t i o n of the o r i e n t a t i o n problem a r i s i n g from the wide v a r i e t y of environments encountered during migration.  Second, i f consideration  i s r e s t r i c t e d t o salt gradients i t indicates that the p o t e n t i a l usefulness of s a l i n i t y as an o r i e n t a t i o n cue i s probably r e s t r i c t e d mainly i f not t o the e s t u a r i a l region.  entirely,  Without pursuing the matter In d e t a i l at t h i s point,  i t seems probable that the salt gradients of t h i s region are of such an order of magnitude i n r e l a t i o n t o the rest of the salmon's environment as t o provide the most promising migration guide.  A d e t a i l e d consideration of estuarine  s a l i n i t y structure w i l l be reserved f o r the Discussion. Most s c i e n t i s t s would probably agree that salmon migrations are r e markable examples of navigation.  However, the o r i e n t a t i o n mechanisms which  they use are as yet poorly understood.  In t h i s regard It should be pointed  out that there i s no reason t o expect that orientation i s r e s t r i c t e d t o a s i n g l e mechanism e i t h e r f o r a part or f o r the whole journey. may be advanced f o r t h i s opinion.  Two  reasons  F i r s t , alternate mechanisms, p a r t i c u l a r l y  those dependent on unrelated environmental cues, would be advantageous where there was the p o s s i b i l i t y of one cue being obscured.  For example, to depend  -4-  solely on visual cues i n turbid estuarine waters would be inadaptive. Second, the evolution of anadromous migrations probably took place i n stages correlated with the subdivisions of the environment described earlier.  A variety of orientation mechanisms, each specially adapted to  the contingencies of a particular environmental subdivision seems reasonable to expect. Many possible orientation mechanisms can be imagined as useful to migrating salmon. However, the problem of providing a credible demonstration that salmon actually use a particular mechanism i s d i f f i c u l t . that the following c r i t e r i a should be satisfied.  I propose  F i r s t , the environmental  cues used for. orientation should be shown to be sufficiently predictable to provide a reliable migration guide.  A l l that i s necessary i s a predictable  series of changes along the migration route.  Second, the animal should be  shown to possess both the sensory and central nervous organization necessary for coordinating i t s movements i n relation to the environmental cue.  Third,  the animal'8 motivational response to the range of environmental cues should be shown to undergo an orderly sequence of temporal changes which parallel spatial changes i n the environmental cue along the migration route. The last i s the most subtle and therefore the most d i f f i c u l t criterion to satisfy. The purpose of this thesis w i l l be to present evidence supporting the idea that salmon use salinity gradients as migration guides.  Original  evidence satisfying the second and third c r i t e r i a w i l l be presented along with already published evidence bearing on the physical nature of estuarine s a l i n i t y gradients satisfying the f i r s t criterion. Prior work on the general relations of fishes to salinity i s abundant. Much of i t however deals with the problems of osmoregulation while those of  -5-  orientation receive scant attention.  In the l a t t e r group four works are of  p a r t i c u l a r importance i n the present context. Shepard (194&) allowed juvenile chum and coho salmon t o choose between p a r a l l e l flows of fresh and s a l t water.  He found a sharp contrast i n the  behaviour of two species, chum f r y showing a preference f o r salt water and coho f r y avoiding i t .  He found also that as the season progressed, chum  f r y showed an increasing preference f o r seawater. Later Houston  (1956, 1957) and Baggerman (i960) using steep, standing  gradients demonstrated that four species of P a c i f i c salmon show changes i n s a l i n i t y preference correlated with t h e i r stage of development and with season.  In the case of Baggerman*s studies the response t o a single  concentration of seawater was used as an index of migration d i s p o s i t i o n , without investigating the s i g n i f i c a n c e of the behaviour.  Houston's studies  provided the basic groundwork essential t o understanding the b i o l o g i c a l s i g n i f i c a n c e of the behaviour.  In p a r t i c u l a r he showed that stream resident  underyearling coho avoided concentrated seawater but showed a preference f o r d i l u t e seawater. seawater.  Migrant coho smolts by contrast p r e f e r r e d concentrated  In the present study these observation w i l l be shown t o form part  of a l o g i c a l sequence of s a l i n i t y preference changes, which act as a guiding Influence i n seaward movement of young salmon. In a different type of study Fontaine and Vibert  (1952) investigated  the p o s s i b i l i t y of A t l a n t i c salmon using gradients of dissolved s a l t s as o r i e n t a t i o n cues i n r i v e r s .  When c a r e f u l l y examined, the i n s t a b i l i t y and  i r r e g u l a r i t y of the r i v e r gradients l e d them t o conclude t h a t , "Un t e l gradient ne pent done "etre considers comme l e facteur guidant l e Saumon jusqu'a ses fraye'res".  -6-  In summary, i t w i l l be the purpose of t h i s t h e s i s t o show that young salmon are able t o use estuarine s a l i n i t y gradients as orientation cues during t h e i r seaward movements.  MATERIALS AND METHODS  Experimental Animals The f i v e species of salmon endemic t o Canada's west coast were used i n the study.  These are:  Oncorhynchua keta (chum), 0 . gorbuscha  (pink),  0. tshawytscha (spring), 0. nerka (sockeye) and 0. k i s u t c h (coho). Details of each experimental stock are given i n Table I.  Each group w i l l  be referred t o i n the text by the code numbers l i s t e d (eg. COHO B-62). Unless otherwise s p e c i f i e d each species was maintained i n running fresh water i n the University of B r i t i s h Columbia aquarium.  Seasonal changes i n  temperature and photoperiod approximated those occurring n a t u r a l l y at t h i s latitude.  Representative aquarium data on these variables are given i n  Figure 2. The animals were fed three times d a i l y during the summer months and twice d a i l y i n the winter.  Of three types of food, each experimental stock  received one predominantly and the two others occasionally as indicated i n Table I.  The three types of food included Clark's Trout Food, a dehydrated  product, frozen adult brine shrimps (Artemia salina) and a wet food preparation consisting of beef l i v e r 65& raw or cooked f i s h 21$, Pablum 8%, brewer's yeast 6% and 1 teaspoon of t a b l e salt per 300 grams mixture.  Due  t o the r e l a t i v e cleanness of brine shrimp, f i s h maintained i n r e c i r c u l a t i n g seawater f o r extended periods were fed s o l e l y on t h i s food.  S a l i n i t y Preference Test Most of the evidence f o r t h i s t h e s i s i s derived from the " s a l i n i t y preference, t e s t " , a method developed by Baggerman (1957) and Houston (1957). In essence t h i s teat permits young salmon t o choose between fresh water and some concentration of seawater.  -1.7  FIGURE 2  UNIVERSITY OF BRITISH COLUMBIA - SEASONA L WATER TEMPERATURES AND P H O T O P E R I O D S  DATA ON EXPERIMENTAL SALMON STOCK  Species Chum  Pink  Code A-60  Coho  Spring  «*  A-63  tt  A-6l  yearlings  Food  May 6, i960  Clark's Food  May 2, 1962  tt  tt  Smith Falls Hatchery B.C. Cheakamus River B.C. Cultus Lake, B.C.  Nov. 20, 1962  Skeena River, B.C.  F a l l of I960  tt  n  Spring, 1961  tt  tt  March 3, 1962  tt  tt  B-62  tt  A-63  tt  Robertson Creek, B.C.  Oct. 26, 1962  Jones Creek, B.C.  Summer, 1961  A-61  underyearlings  Date  Smith Falls Hatchery B.C. n it tt  underyearlings  B-62  n  C-62  tt  A-63  tt  B-62 A-63  Sock eye  underyearling  A-62  B-61  Source  Age  A-62 B-62 A-63  underyearlings tt underyearl ings yearlings underyearlings  Cheakamus River April 27, 1962 B.C. Samish Hatchery, May 8, 1962 Washington State, U.S.A. Samish Hatchery, Nov. 27, 1962 Washington State, U.S.A.  Wet Food n  tt  Clark's Food  Samish Hatchery May 8, 1962 Washington State, U.S.A. Samish Hatchery Nov, 27, 1962 Washington State, U.S.A.  Wet Food  Cultus Lake, B.C.  May 8, 1962  Clark's Food  tt  tt  it  May 16, 1962  Wet Food  tt  tt  n  March, 1963  -10-  In the tank pictured i n Figure 3-A,  an incomplete p a r t i t i o n divides  the tank into two compartments each containing an a i r stone and bottom centre i n l e t .  In the actual t e s t procedure the i n l e t s i n each compartment  were f i r s t stoppered with rubber plugs, one s o l i d and the other pierced by a one-quarter inch diameter hole, leaving the l a t t e r compartment continuous with the surrounding water bath ( F i g . 3-B).  Next the t e s t seawater solution  (0-18$oCl) natural seawater or an experimental mixture was poured into the sealed compartment to the l e v e l of the p a r t i t i o n ( F i g . 3-C)»  Then the  running water bath was f i l l e d with fresh water so that the unsealed compartment was flooded t o a l e v e l approximately one-half inch from the top of the p a r t i t i o n ( F i g . 3-D).  At t h i s point eight f i s h were placed i n the  l a t t e r compartment, the a i r stones were turned on i n both compartments and the apparatus was allowed t o stand unchanged f o r three hours (10 A.M. 1 P.M.)(Fig. 3-E).  This was done f o r two reasons.  an opportunity t o adjust t o t h e i r new  to  F i r s t , i t gave the f i s h  environment and second, i t permitted  the water i n the t e s t compartments t o reach a uniform temperature  equal t o  that of the running water bath. After three hours the a i r supply was turned o f f and the l e v e l of the surrounding water bath was again raised very slowly (10-15 min.) so that a freshwater "bridge" was formed across the p a r t i t i o n allowing the f i s h freedom of movement between the two compartments ( F i g . 3-F).  The depth of  the bridge was adjusted t o suit the size of f i s h being t e s t e d . By making the bridge with freshwater, a stable s t r a t i f i c a t i o n was obtained with the l e s s dense water on top.  This i s shown diagrammatically i n Figure 3-G»  It i s important t o remark at t h i s point that every t e s t was  i d e n t i c a l except  f o r the concentration or composition of the salt water i n the sealed com-  ^-WATER BATH  FIGURE  3  SALINITY PREFERENCE TEST TANK  -12-  parbment. A f t e r establishing the bridge, the number of f i s h i n each of the two compartments was recorded over a period of two and one-half hours according t o the following schedule:  once every two minutes f o r 20 minutes, followed  by a 10 minute pause, with the 20-10 minute cycle repeated f i v e times. In the present study the apparatus consisted o f twenty t e s t tanks arranged i n the p a r a l l e l fashion diagrammed i n Figure 4.  Mounted above  each tank was a mirror, two l i g h t s (General E l e c t r i c Lumiline) and two r e f l e c t o r s , the l a t t e r functioning t o prevent an image of the l i g h t being r e f l e c t e d back from the water surface t o the observer.  The entire apparatus  was surrounded with black walls and curtains except f o r an eye-level s l i t through which observations were made from a darkened background.  The i n -  t e n s i t y of i l l u m i n a t i o n i n the tanks was 1.5*0.1 foot candles measured at the water surface d i r e c t l y over the p a r t i t i o n .  By v i r t u e of the symmet-  r i c a l arrangement of l i g h t s , the s l i g h t shadowing produced by the r e f l e c t o r s was uniform i n both compartments.  The temperature  of the running water  bath and the t e s t tanks, was equal (±0.2°C) t o that of the aquarium holding tanks i n which the f i s h were normally kept.  In summary, t h e apparatus was  designed t o provide a constant set of conditions devoid of changing external s t i m u l i which might i n t e r f e r e with the s a l i n i t y preference t e s t . S a l i n i t y Preference Chemical Methods Natural seawater varying seasonally between twelve and f i f t e e n parts per thousand chloride, was used except where noted. pared with dechlorinated fresh water.  Dilutions were pre-  More concentrated seawater was  made by adding appropriate amounts of k i l n half-ground s a l t up t o 1B% C l . 0  FIGURE 4  DETAILS OF SALINTY P R E F E R E N C E TEST APPARATUS  -14-  The s a l t thus added never accounted f o r more than one-third of the t o t a l chlorinity. The l a r g e number of t e s t s performed precluded checking each seawater concentration.  Instead spot checks were made using the modified Mohr  t i t r a t i o n (5-10$ AgN03, 0.1% saturated s l e x t r i n e ) .  2, 4, dichlorofluorescein i n alcohol and  An o v e r a l l d i l u t i o n accuracy of i0.2$o CI was  obtained  including a t i t r a t i o n error of -O.l^o CI using 5 ml* seawater aliquots. Special salt water mixtures were made from reagent grade chemicals and dechlorinated tap water according to the composition t a b l e s of Barnes*  (1954) and Hale (1958). Considerable data on the s t a b i l i t y of the test t r a d i e n t s was by Houston (1957).  obtained  In the present study a comparison of mixing i n tanks  (a) without f i s h (b) with small, r e l a t i v e l y inactive f i s h and (c) with large very active f i s h i s presented i n Figure 5. the center of the sealed compartments.  Water samples were taken from  The data are intended t o give an  i n d i c a t i o n of the range of v a r i a b i l i t y i n gradient breakdown. f o r the l a r g e a c t i v e f i s h represent an extreme condition. F i s h these q u a l i t i e s of s i z e and a c t i v i t y were r a r e l y tested.  The results approaching  A more nearly  t y p i c a l condition i s that i l l u s t r a t e d by the smaller, r e l a t i v e l y inactive fish. Because the gradients break down with time a problem of interpretation arises.  A l l the seawater concentration values given i n the Results r e f e r  to the concentration at the beginning of each t e 3 t .  Since the s a l i n i t y  preference response was measured during the succeeding one hundred and f o r t y minutes, while the gradient was diminishing, the response cannot s t r i c t l y be r e l a t e d t o the i n i t i a l seawater concentration.  However, i t i s important  18 COHO  BC-62  LENGTH X 8.9 C M ACTIVITY 81-1 PARTITION. CROSSINGS PER 5MIN. OBSERVATION  12  A - NO FISH 12  JO  60-  'O  ±  30 SALINITY  FIGURE 5  ± ± 60 90 120 P R E F E R E N C E T E S T " MINUTES STABILITY  OF TEST  GRADIENTS  150  180  -16-  t o r e a l i z e that f o r the purposes of t h i s t h e s i s i t w i l l not be  necessary  to r e l a t e s a l i n i t y preference t o s p e c i f i c seawater concentrations.  Only  the r e l a t i v e responses to an increasing sequence of concentrations w i l l be necessary.  Although more stable gradients have been designed and might have  been used, the great advantage i n the present method i s that the preferences f o r each concentration may be tested separately.  The importance of t h i s  fact w i l l be considered i n the Discussion.  S t a t i s t i c a l Procedures Generally, a s e r i e s of f i v e seawater concentrations (0, 3» 6, 12, lB%o was tested simultaneously.  Cl)  F i f t e e n t o twenty test r e p l i c a t i o n s f o r each  concentration were found to give consistent r e s u l t s .  Four days were r e -  quired t o run such a s e r i e s , performing twenty t e s t s per day, the maximum number p o s s i b l e , with a t o t a l of sixteen r e p l i c a t i o n s f o r each concentration. The d i s t r i b u t i o n of t e s t s i n the twenty tank apparatus was arranged that the v a r i a b i l i t y a r i s i n g both from the apparatus and f i s h was  dis-  t r i b u t e d as evenly as possible between the various test concentrations. any one concentration t h i s was  so  For  accomplished by (a) d i v i d i n g the twenty tank  apparatus i n t o four groups of f i v e tanks each ( i n the case of 16 r e p l i c a t i o n s ) and using one different tank i n each group of f i v e on four successive days, (b) using an equal number of front and back compartments f o r the  experimental  seawater concentrations ( F i g . 4) and (c) arranging the d i f f e r e n t seawater concentrations i n a s t a t i s t i c a l l y random sequence i n the apparatus. t o avoid any danger of unconsciously adding a personal bias t o the  Further, obser-  vations, the d i s t r i b u t i o n s of f i s h were recorded without knowing which compartments held which concentration of seawater.  -17-  Eight f i s h were used i n each t e s t .  The normal behaviour of juvenile  salmon always includes some form of s o c i a l aggregation, used rather than single f i s h .  hence groups were  Eight proved to be a convenient number, both  f o r the size of tank i n r e l a t i o n t o the s i z e range of animals and f o r rapid d i s t r i b u t i o n enumeration. A t o t a l of 640O d i s t r i b u t i o n observations was concentration  obtained f o r each seawater  (8 f i s h per observation x 50 observations per f i s h per  r e p l i c a t i o n x 16 r e p l i c a t i o n s ) .  Past workers have used a v a r i e t y of  s t a t i s t i c a l t e s t s of s i g n i f i c a n c e i n evaluating these d i s t r i b u t i o n r e s u l t s . The use of these t e s t s i s of doubtful v a l i d i t y f o r the following reasons?^  The p r o b a b i l i t y statement derived from any t e s t of s i g n i f i c a n c e  i s i n part a function of the sample s i z e .  Thus when a small percentage  difference between two variables e x i s t s , a l a r g e sample w i l l produce a s t a t i s t i c a l l y s i g n i f i c a n t difference and a small sample a non-significant difference. Consideration of the s a l i n i t y preference r e s u l t s i n Figure 6 f o r example, w i l l show that differences of greater than 30% are rare and that the average differences i n response to d i f f e r e n t concentrations are generally much l e s s than 30%%  of seawater  The sample s i z e w i l l then s e r i o u s l y  affect the outcome of any t e s t of significance applied t o t h i s type of data. S t a t i s t i c a l l y the sample s i z e i s equal to the t o t a l number of i n dependent observations.  It was  stated e a r l i e r that 64OO d i s t r i b u t i o n  observations were obtained f o r each seawater concentration.  The r e a l sample  s i z e however i s smaller since these cannot be considered to be independent observations  f o r the following reasons.  statistically  F i r s t , the eight f i s h  used i n each test did not always behave as independent units but  frequently  showed a strong schooling behaviour during the s a l i n i t y preference t e s t .  -18-  To complicate matters f u r t h e r , the degree of group response (schooling) showed very marked seasonal v a r i a t i o n ( F i g . 16A). dependence i s temporal  in origin.  The second source of  Because the d i s t r i b u t i o n observations  are taken i n f a i r l y rapid sequence any one observation i s , t o a greater or l e s s e r extent, dependent on the proceeding one.  The degree of dependence  here i s d i f f i c u l t t o estimate and once again probably quite v a r i a b l e depending on the changing seasonal a c t i v i t y of the animals ( F i g . 16B). The net effect of these considerations i s t o make the " e f f e c t i v e " sample s i z e , so uncertain as t o be meaningless.  One might p l a u s i b l y  argue i n the l i g h t of the above considerations that instead of 64OO observations per t e s t concentration that there were r e a l l y closer t o 16 (the  number of r e p l i c a t i o n s ) s t a t i s t i c a l l y independent observations. E a r l i e r work consisted mainly, i n comparing the response of two  water concentrations (0%o C l and 12-15  %» C l ) .  sea-  Under these conditions  s t a t i s t i c a l t e s t s were intended t o supply a basis of comparison f o r successive t e s t s and, more important, t o provide a frame of reference f o r interpreting the b i o l o g i c a l meaningfullness of any differences i n response which were found.  In the present work these aims have been achieved with-  out the use of these kinds of s t a t i s t i c a l t e s t s .  By simultaneously comparing  the response t o a range of seawater concentrations., approaching the salmons natural s a l i n i t y range, evaluations of b i o l o g i c a l significance were based on the pattern of response t o the various concentrations.  In other words,  the preference response t o any single concentration was  evaluated by com-  paring i t t o o v e r a l l pattern of response t o the entire range of seawater concentration tested. In the Results a l l the d i s t r i b u t i o n observations have been converted t o percent differences i n d i s t r i b u t i o n i n the seawater (sealed) compartments.  -19-  The response t o 0%  o  C l (fresh water) i s used as the base l i n e .  A negative  value indicates a negative preference (avoidance) i n comparison t o f r e s h water and a p o s i t i v e value a p o s i t i v e preference i n comparison t o freshwater. I f desired, any percent f i g u r e may be reconverted t o absolute values according t o the following method.  For example, i n Figure 6-A a  of -12% i s shown t o 9$° C l seawater.  response  In absolute terms a t o t a l of (-12%  x  8 f i s h per observation x 50 observations per f i s h per r e p l i c a t i o n x 20 r e p l i c a t i o n s =) 960 fewer f i s h were observed i n 9%a C l seawater than i n the 0%o C l seawater (freshwater). t o 3%<> C l seawater.  In Figure 6-C a response of -20$ was shown  Back c a l c u l a t i n g a t o t a l of (-20% x 8 f i s h per obser-  vation x 50 observations per f i s h per r e p l i c a t i o n x 16 r e p l i c a t i o n s =)  1280  more f i s h were observed i n 3%° seawater than i n 0%o seawater (freshwater). The terms 8 and 50 are the same i n a l l the data i n t h i s t h e s i s .  The percent  difference i n d i s t r i b u t i o n and number of r e p l i c a t i o n s are presented i n the appropriate section of Results. The present experimental design l e d t o r e s u l t s showing rather small percentage differences i n d i s t r i b u t i o n and considerable response i n individual experiments.  variability  When active species of f i s h are confined to  r e l a t i v e l y small tanks and t h e i r positions are recorded at r i g i d two minute i n t e r v a l s just such r e s u l t s would be anticipated.  By so confining the f i s h  however, one i s assured that the animals frequently "choose" between the t e s t compartments and i n addition a l a r g e number of t e s t s can be done simultaneously.  It should be emphasized also that the interpretations of  t h i s t h e s i s are based mainly on the patt erns of response t o seawater concentration so that v a r i a b i l i t y  i n the response pattern i s of greater im-  portance, i n the present context, than the v a r i a b i l i t y  i n i n d i v i d u a l preference  -20-  tests.  The former w i l l be considered i n the Results.  Seawater Acclimation and Resistance The effects of p r i o r exposure t o seawater on s a l i n i t y preference constitutes a major l i n e of supporting evidence.  For short periods of  exposure, (3 hours) the f i s h were simply put into the seawater compartment of the s a l i n i t y preference tank instead of the usual freshwater partment.  com--  For longer periods an eighty l i t e r r e c i r c u l a t i n g seawater system  was used, i n which the water was changed weekly.  The animals were fed  s o l e l y on brine chrimp and the temperature was maintained equal t o i0.2°G that of the running dechlorinated water i n the aquarium, by a thermos t a t i c a l l y controlled r e f r i g e r a t i o n u n i t . S a l i n i t y resistance t e s t s were made, i n the case of small animals, i n glass museum j a r s holding eight l i t e r s of solution. placed i n a running water bath, and each was  The j a r s were  supplied with compressed a i r .  Duplicate t e s t s using f i v e f i s h per test were usually done. Tests on large f i s h were c a r r i e d out i n a f o r t y l i t e r r e f r i g e r a t o r bath.  In a l l t e s t s the temperature was equal ±0.2°C t o the temperature of  the normal holding tanks of the animals.  The seawater d i l u t i o n s were pre-  pared i n the same manner as described f o r s a l i n i t y preference except that c h l o r i n i t i e s , more concentrated than the a v a i l a b l e natural seawater, were obtained by f r e e z i n g out freshwater, rather than by adding s a l t . Counts of surviving f i s h were taken three or four times d a i l y f o r a period of no l e s s than one hundred hours.  A f i s h was considered t o be dead  when no operculor movements could be detected.  In the present context the  r e s u l t s have been confined t o showing the maximum seawater concentration at which 100$  s u r v i v a l was  obtained.  RESULTS  Temporal Changes i n S a l i n i t y Preference The temporal changes i n s a l i n i t y preference, which chum salmon f r y undergo are i l l u s t r a t e d i n Figure 6 (A-E).  The single most important  feature of these graphs i s the orderly sequence of s a l i n i t y preference changes s t a r t i n g with a modal preference f o r freshwater (0$  o  Cl) i n May  through 3%° C l i n June, 6 $ e C l i n J u l y , approximately 9 $ o C l i n August and f i n a l l y 10%<> C l i n October.  This feature i s emphasized i n Figure 6-F.  Several other points require comment.  F i r s t , i t should be remembered  that these f i s h were retained i n freshwater throughout the months of testing.  B a s i c a l l y then the temporal sequence of changes seems t o be  independent of any prolonged exposure t o seawater such as the f i s h would normally experience during t h e i r seaward movement. Second, Figure 6 shows the general consistency of the response patterns, a feature of basic importance t o t h i s t h e s i s .  Figure 6-A e s p e c i a l l y shows  the high degree of response r e g u l a r i t y t o successive seawater concentrations even though each was t e s t e d independently.  Third, not only does the pre-  f e r r e d concentration change i n an o r d e r l y sequence but the i n t e n s i t y of response t o the modal concentration does also.  From a 14.3$  response to  3%° C l the response i n t e n s i t y increases t o 25-30$ f o r the subsequent modal c one ent rat ions• Fourth, the shape of the curves indicates a rather wide l a t i t u d e i n preference.  In other words the response t o the modal concentration does  not d i f f e r r a d i c a l l y from the response to the adjacent concentrations.  The  s i g n i f i c a n c e of t h i s feature w i l l be considered i n the Discussion. Comparable data f o r pink salmon are given i n Figure 7 ( A - l ) .  Graphs  + IOr  6 9 12 SEAWATER CONCENTRATION % o  FIGURE 6.  TEMPORAL CHANGES IN SALINITY PREFERENCE - CHUM S A L M O N  FIG.  6 , CHUM A - 6 2  12 r UJ  I—  <  TEMPORAL SEQUENCE OF MODAL PREFERENCES  I UJ  U  J  o  O  MAY  1 JUNE  | JULY  |  AUG  I  SEPT  I  OCf  I  +20r  PINK FRY B - 6 2 APRIL 4 - 1 6 / 62 2 0 REPLICATES  + 10  o  UJ  u z  UJ  CH UJ  u_ UJ  *-IO  JUNE 21- 23/62 15 REPLICATES '  B  O ll  o  # O  ULl  u z  _  O  UJ  cr  UJ  u_  UJ  CL-IO  >  JULY 13-17/62 16 REPLICATES  <  CO  -f-IOh  JL  -10  6 9 12 SEAWATER CONCENTRATION °/ooCL  FIGURE 7  TEMPORAL CHANGES IN SALINITY P R E F E R E N C E — PINK S A L M O N  18  FIG.7 , PINK  A-62  SEAWATER CONCENTRATION °/oo CL  FIG.7  PINK  PINK A-61 MARCH 8-12/62 16 REPLICATES  PINK B-62 AUG 17-27/62 16 REPLICATES  H  H E L D IN I 4 ° / O O C L SEAWATER F R O M JUNE 21/62  PINK  _L B-62  N O V 28-DEC 16  5/62  REPLICATES  H E L D IN I 4 % O C L SEAWATER FROM  JUNE 21 / 62  6  ±  9  12  SEAWATER CONCENTRATION % o C L  15  18  7-A t o 7-G  are f o r f i s h maintained i n freshwater and cover almost a f u l l  calendar year.  The A p r i l response ( F i g . 7-A)  suggests that the f i s h do  not orient to salt concentration at t h i s early stage of development.  From  June through September the responses (7 B-E) are similar to those f o r chum, showing an orderly change i n the modal preference and i n response i n t e n s i t y . The November response (7-F) shows a complete reversal t o the June condition when freshwater was preferred. Pinks tested as yearlings i n March demonstrate the c y c l i c a l nature of changes i n s a l i n i t y preference, by redeveloping a preference f o r concentrated seawater (ll%  0  Cl).  Graphs H and I show the responses of pink f r y maintained i n 14$ o C l seawater from June 21, 1962,  instead of freshwater.  In contrast t o the  freshwater f i s h which p r e f e r 3$° C l i n August ( F i g . 7rP)> f i s h at t h i s time p r e f e r seawater.  l8$o  t  n  e  seawater  C l , a concentration approaching open ocean  Thus, while both chum and pink undergo a p a r t i a l sequence of  s a l i n i t y preference changes when held i n freshwater, the data obtained from pinks held i n s a l t water shows that seawater exposure has the double effect f i r s t , of accelerating the rate of change of the preference sequence and second, of extending the sequence t o include concentrations approaching those common i n oceanic waters. Contrasting the s a l i n i t y preference of the freshwater f i s h i n l a t e November and early December ( F i g . 7-F) with the 3eawater f i s h f o r the same period ( F i g . 7-l) another difference may be seen. regress t o a freshwater preference.  The seawater f i s h do not  Instead they maintain t h e i r 18$ p C l  preference and even increase t h e i r i n t e n s i t y of response from 23.7$ i n August t o 35«1$ i n November.  The meaning of the minor preference peaks  occurring at 3$° C l i n both seawater f i s h graphs i s not evident.  -25-  A more d e t a i l e d series of data on the early stages of the s a l i n i t y preference sequence are given i n Figure 8 (A-G).  These observations are  f o r coho, a species which commonly migrates t o sea as a smolt a f t e r spending a year i n freshwater.  Graphs A t o C, showing a change i n pre-  ference from freshwater to 3%o C l , provides evidence that the sequence i s a smooth, continuous series of preference changes rather than a number of discontinuous steps.  A f i n a l proof of t h i s point would require a much more  detailed series of observations than was possible i n t h i s study. Figure 8-D  shows a series of observations made during July and August  of 1961, whereas a l l the other data of Figure 8 were obtained during 1962. The s i m i l a r i t y of s a l i n i t y preference response patterns f o r the two years demonstrates the r e p l i c a b i l i t y of the response f o r a single species obtained from different sources, and f o r successive year classes.  Figure 8  (E-G)  indicates that unlike chum and pink the coho s a l i n i t y preference response pattern does not change beyond a preference f o r 3%° C l during t h e i r f i r s t year, a result consistent with t h e i r non-migratory behaviour at t h i s age, and the fact that they are often found i n the inner estuarine region (Hoar, 1958).  Nor does i t regress to a freshwater preference as with pinks kept  i n freshwater. Figure 9 i l l u s t r a t e s the sequence of s a l i n i t y preference changes which spring salmon f r y undergo when maintained i n freshwater.  Though the data  are l i m i t e d the sequence most c l o s e l y resembles those of pink and chum. A gradual change i n preference from freshwater t o 3%° C l seawater ( F i g . 9, A-C)  i s followed by a regression t o a freshwater preference (9-D).  The  pattern i s again consistent with the species normal migratory pattern since the w i l d stock from which these animals were obtained (Samish Hatchery,  + IOr  0  C O H O FRY BC / 62 MAY 2 4 - 2 9 /62 2 0 REPLICATES  B  O  JUNE 2 5 - 2 7 / 6 2 15 REPLICATES  11  <~>+IOf0  -9.  o  O UJ <J  Ob  2  UJ  a:  UJ  u. UJ  cr Q.  - 10  JULY 3 0 - A U G 2/62 16 REPLICATES  < , + 10  12  FIGURE 8  SEAWATER CONCENTRATION °/oo T E M P O R A L CHANGES IN SALINITY P R E F E R E N C E - COHO S A L M O N  FIG 8  COHO  B C - 6 2 EXCEPT D  FIG LU  o  8 , C O H O BC-62  6 9 12 SEAWATER CONCENTRATION °/oo CL  FIGURE 9  TEMPORAL CHANGES IN SALINITY PREFERENCE - SPRING S A L M O N  FIG 9  SPRING FRY B-62  -23-  Washington State Department of Fisheries) normally migrates seaward as underyearlings.  The results are also consistent with the earlier suggestion  that a period of exposure to seawater i s necessary for the complete development of the preference sequence, otherwise a regression to a freshwater preference takes place. For this species, at least, these alternate routes of development may also be associated with their relatively v&rialba migratory behaviour.  In some instances spring f r y remain i n freshwater f o r  a year or more before migrating t o the ocean (Clemens and Wilby, 1961 Carl et a l , 1959). For sockeye underyearl ings (fry) and yearlings (smolts) the observed sequence of salinity preference changes i s puzaling i n some features. Sockeye f r y show two distinct preference peaks daring Kay and June (Fig. 10-A). The peak occuring at approximately- 3%o C l i s similar t o that shown by coho f r y at about the same time. l/$o C l i s unique to sockeye*  The second peak at approximately  Measurements of salinity tolerance indicate  that the second peak coincides with the upper limit of survival of the species at this time (Fig. 17).  The reality of the two peak preference i s  supported by the July data (Fig. 1©-B) which shows substantially the same pattern. From September t o December (Fig. 10, C-E) the modal preference i s approximately 3%o C l though some traces of the anomalous second peak are always i n evidence.  The January pattern (Fig. 10-F) clearly shows a single  preference peak (3%° Cl) and this i s followed by an orderly sequence of changes i n the modal.pref erence u n t i l a terminal pattern i s reached i n August (Fig. 10, G-l) where 18&> C l i s preferred. When tested late i n October (Fig. 10-J) no pattern i s evident suggesting a regression similar  +20r-  S O C K E Y E FRY A-62 M A Y 14 J U N E 1 4 / 6 2 2 0 R E P L I C A T E S  + I 0 h  LLI U  z  UJ  cr  UJ  u_ UJ  cc  JUNE 1 8 - 2 4 / 6 2 16 REPLICATES  CL  £  SEPT 3 - 6 / 6 2 16 REPLICATES  r-  <  co  6  9  12  SEAWATER CONCENTRATION °/oo CL  FIGURE 10  15  T E M P O R A L CHANGES IN SALINITY P R E F E R E N C E - S O C K E Y E SALMON  18  FIG + IOr  IO  S O C K E Y E FRY A - 6 2 OCT 12-23/62 6 REPLICATES  DEC 7 - 1 1 / 6 2 16 R E P L I C A T E S  O  n  _i U+IO| o  O UJ  U  z  UJ  O  cc LLl  U. uJ  a: CL  > -IO'  JAN 17-23/63 16 REPLICATES  _J <  o420h  •4-IO  O  6 9 12 SEAWATER CONCENTRATION °/ooCL  15  18  FIG +20  IO  SOCKEYE SMOLTS  r  6 9 12 SEAWATER CONCENTRATION % o  CL  B-62  FIG  IO  SOCKEYE SMOLTS  +20r  6 9 12 SEAWATER CONCENTRATION °/oo CL  B-62  -30-  to those of pink and spring kept i n freshwater (Figs* 7 and 9). Several points are important.  F i r s t , i n nature this species would be  expected to migrate as yearlings (smelts). The results are therefore broadly consistent with their normal migratory pattern though the s i g n i f i cance of the 14-15$o Cl preference peaks Is not evident. According to Clemens and Wilby (1961) a few sockeye may migrate to sea as f r y (underyearlings).  These anomalous peaks may i n some way, as yet not clear, be  connected with this fact.  Second, sockeye unlike chum, pink and spring,  seem to undergo a complete sequence of salinity preference changes when retained exclusively (except for seawater contact during testing), i n freshwater.  Third, Figure 10-1, although superficially similar to the  double preference peak patterns obtained earlier f o r fry, must be evaluated quite differently since i t i s the terminal pattern i n an orderly sequence of changes. The f i s h would, according t o their preference patterns be entering oceanic waters at this time and the double peak probably results from a decreasing preference for waters through which the migrants have most recently passed. Effects of Eepeated Testing The demonstration of the foregoing temporal sequences of salinity preference i s based on a number of assumptions.  First i t i s assumed that  the laboratory measurements approximate the changes occuring i n nature. The good agreement, already mentioned a number of times, between the observed results and the published information on the migratory habits of the various species makes this a broadly plausible assumption. Second, that the sequence of preference changes i s not merely the  -31-  result of repeated testing i s demonstrated by the results presented i n Figure 11.  Curve B represents the response of eoho f r y which had already  been tested on seven previous occasions. Curve A i s f o r animals which had never before been tested, but which were otherwise kept under the same laboratory conditions.  Despite the differences i n response intensity the  clearly similar modal preferences (3&> Cl) demonstrates that the development of the preference sequence i s certainly not a laboratory artefact nor is prior contact with seawater necessary for triggering the preference changes in the early part of the sequence. The Effects of Seawater Exposure The effects of seawater exposure on the salinity preference pattern were examined further by exposing underyearling coho to several seawater concentrations for varying  periods prior to testing.  In the f i r s t experi-  ment the f i s h were simply put into the salt water compartment of the salinity preference tanks for the i n i t i a l three hour adjustment period, instead of the usual freshwater compartment. Sixteen replications for each of the five seawater concentrations (0, 3, 6, 12, 18%o Cl) were pertformed. The results are diagrammed in Figure 12-A.  Comparing the seawater  f i s h with the controls (Fig. 8-F) the same modal preference (3%° Cl) i s evident i n both cases.  In the second and third experiments coho f r y were  kept i n 1 2 # o Cl seawater for periods of ten days (Fig. 12-B) and six weeks (Fig. 12-C) respectively.  In both instances the modal preference remained  unchanged at 3%° C l . In summary i t can be concluded that the i n i t i a l part of the preference sequence develops unaffected by seawater exposure.  6 9 12 ^ SEAWATER CONCENTRATION % o CL  FIGURE  II E F F E C T S OF R E P E A T E D PREFERENCE TESTING  SALINITY  MOr  COHO B C / 6 2 DEC 29 "JAN 3 / 6 3 6 REPLICATES  uJ  cr  3 HRS SEAWATER EXPOSURE  UJ  a_  UJ  cr  -2C4 JAN 16  UJ LLl  DC UJ U_ UJ CC  2 WKS SEAWATER EXPOSURE  ^-20  FEB 1 2 - 2 0 / 6 3 16 REPLICATES  h-  <  CO  £  0  -10 6 WKS SEAWATER EXPOSURE -2C*  FIGURE  SEAWATER CONCENTRATION" */oo C L  12  E F F E C T S OF SEAWATER EXPOSURE ON SALINITY P R E F E R E N C E  18  -34-  Combinlng these results with those presented earlier for pink f r y where i t was shown that the latter part of the preference sequence only developed i n those f i s h which had been exposed to seawater f o r three months (Fig.  7-H), the following generalizations emerge* The i n i t i a l part of the preference sequence (O-approx* 6-9%o Cl)  normally develops independently of any exposure to seawater and i t i s not altered by continuous seawater exposure up to s i x weeks*  Development of  the l a t t e r part of the sequence i s dependent on some period of exposure to seawater greater than that provided i n salinity preference testing and less than 3 months (except i n the case of sockeye where the f u l l sequence develops with only salinity preference test exposure).  The minimum seawater con-  centration necessary for the development of the f u l l preference pattern cannot be specified precisely except as being less than 12-15& C l . However i t i s probably safe to speculate that concentrations of the order of 3%o to 6%o C l would be adequate since these are the concentrations which the seaward migrants would encounter normally during the early part of the preference sequence* Not only i s the f u l l development of the preference sequence dependent on a period of seawater exposure but the timing of the sequence or the rate of ehange of preference i s also affected by exposure to seawater* best illustrated by the pink salmon data*  This i s  The groups kept i n freshwater  showed a preference for 3%° Cl in late August (Fig* 7-D) followed by 6%o Cl i n late September (Fig* 7-E).  Contrasted with this, the f i s h kept i n  seawater from June, already preferred 18%e C l i n late August (Fig. 7-H). The general applicability of these conclusions w i l l require future confirmation i n the case of spring and chum*  -35-  It i s apparent from the foregoing considerations that a completely satisfactory description of the entire preference sequence and i t s normal timing would require f a c i l i t i e s where the f i s h could be held in their most preferred seawater concentration, instead of being confined t o freshwater or t o some single seawater concentration* The Effect of Size* Baggerman (I960) i n her studies demonstrated that the length of the daily photoperiod i s at least partly responsible for the seasonal timing of salinity preference changes*  In the present study a further variable  size, as a rough index of the development, was investigated. It would be anticipated that i f larger f i s h reflected a more advanced state of development they might in turn show a more advanced stage in the sequence of salinity preference changes* to be true*  Examination of Figure 13 shows the reverse  Thus, while the modal preference i s d e a r l y 3% C l for the D  three size groups of coho f r y , the intensity of response i s the strongest for the small f i s h and weakest for the large fish*  Why this should be so  i s not clear* Sensory Basis of Salinity Preference What property of seawater do salmon detect i n showing the salinity preference response?  Data presented so far indicate that the response i s  f i r s t of a l l dependent on seawater concentration*  Among the seawater  properties which salmon might be expected t o detect are density, osmotic concentration or salty taste. A summary of earlier data follows (Mclnerney, 1961).  The response  of chum f r y to an a r t i f i c i a l seawater (made from nine reagent grade salts,  COHO B C - 6 2 16 REPLICATES S M A L L — X 8-9 C M MEDIUM— X 9-5 C M LARGE — X IO-3CM  NOV 23-27/62 NOV 5-13/62 NOV 5-13/62  < to o  O  F I G U R E 13  4  6  8  SEAWATER CONCENTRATION °/oo C L E F F E C T S OF FISH SIZE ON PREFERENCE SALIN ITY  10  -37-  Hale, 1958) when compared with the response to natural seawater showed no appreciable differences*  Altering the pH of seawater from 7*9 to 6*3  did not change the response* When a l l the anions i n the a r t i f i c i a l seawater were replaced with Cl~, and i n a separate test a l l the cations with Na , the response of the chum fry was not affected* +  On the basis of the  l a t t e r two results especially, a normal response to a pure sodium chloride solution would hare been predicted. When this experiment was performed on three different occasions using two species, pink (Fig. 14) and coho (Fig. 15, A-B), the response consistently Indicated an avoidance of the pure NaCl solution.  The conclusion must then  be drawn that the response depends on a mixed salt solution* and that either a cation mixture or an anion mixture i s adequate and more generally that the salinity preference response i s not dependent on a unique mixture of salts. Because of the probability that, for any one concentration, a large number of salt mixtures might be made which would e l i c i t approximately the same salinity preference response, this l i n e of investigation was abandoned. Instead, two salts (CaCl2  NagSO^) were chosen, to study their  individual effects on modifying the salinity preference response to a pure NaCl solution.  This was done by testing the response of eoho f r y to salt  solutions containing the normal amount of NaCl i n 3%° C l a r t i f i c i a l seawater plus one of six amounts of GeClo ranging from zero to the normal amount i n 3%o Cl a r t i f i c i a l seawater (Barnes, 1958).  These observations were then  compared with the response shown to 3&> C l , natural seawater. The results Illustrated i n Figure 15-A show an optimum C a concentration 44  with a decreasing preference f o r Ca "* concentrations above and below this 4  optimum*  Compared to natural seawater three types of response may be  PINK S A L M O N A - 61 SEPT 21-24/61 16 R E P L I C A T I O N S  4°/ooCL NATURAL SEAWATER  NA C L S O L U T I O N I S O S M O T I C WITH 4 ° / o o C L SEAWATER 60 90 120 SALINITY P R E F E R E N C E TEST MINUTES  F I G U R E 14  R E S P O N S E O F PINK S A L M O N TO A N NA C L SOLUTION  +10,  C O H O B C - 6 2 16 R E P L I C A T E S DEC 13-18/62  0"<LU  +5  coZco  S o Z  UJ  cr  UJ  u_  UJ  cr a.  J_  5  Q99 GRAMS  O  ±  2.97 3.96 1.97 GM. NA CL714 L. CA C L +53.03  4.95  2  II  d+io  B  o o  COHO B C - 6 2 JAN 5-12/63 16 REPLICATES  O UJ  Z+5 UJ  cr  UJ UJ CL >  Z _l < CO  O  -5 U  -10  -15  oo  1  O  FIGURE  "83h 7IO 1.77 3.55 5.32 GRAMS + 53.03 NA CL / 14 E F F ENA C T SSOOF CA & GM. SO;~CONCENTRATION ON 15 THE RESPONSE OF C O H O SALMON TO AN NA C L SOLUTION fc  +  t+  -40-  recognized.  F i r s t . a region of supernormal response, second, two con-  centrations of calcium e l i c i t i n g a response the same as natural seawater (points X, I) and third, a subnormal response to the extremes of calcium cone entration. The same general relationship was obtained when the SO^—  ion was  similarly tested except that the entire curve was transposed to the negative preference (avoidance) region (Fig. 15-B).  The differences i n response  intensity between A and B reflect a temporal variability i n response i n tensity. These results permit the following conclusions.  F i r s t , though stimuli  arising from osmotic properties and density cannot be entirely eliminated as providing accessory or alternative information, i t i s clear that the salinity preference response i s primarily elicited by a non-specific mixture of sea salts.  Thus, though the osmotic and density properties of seawater  may be duplicated i n a variety of ways only certain salt combinations w i l l e l i c i t a normal salinity preference response.  Second, though not tested  experimentally, i t seems reasonable to infer that taste or the common chemical sense i s the sensory modality mediating the salinity preference response.  Third, extrapolating from the Ca "* and SO4— results i t may be 4  concluded that i n nature, the salinity preference response depends on a complex interaction of salts found in natural seawater, and on both anion and cation interaction. Temporal Changes i n Schooling Behaviour and Activity Salinity preference data already presented for pink f r y showed that for those animals kept i n seawater from the previous June 21, the terminal (18%> Cl)  salinity preference pattern was reached by mid-August.  In other words  they preferred water equal i n concentration to the open ocean*  Support  for the contention that this timing approximates that occuring under natural conditions i s provided by the findings of Hanzer and Shepard  (1962)* Their  work showed that by mid-August i n Chatham Sound* pink f r y were virtually absent from the mainland shore and scarce along offshore islands* indicating by comparison to earlier sampling a progressive movement into oceanic waters* The changes i n schooling intensity and activity paralleling those of salinity preference from Hay to September are presented i n Figure 16 for chum fry.  Schooling intensity was measured by noting the size of each group  which crossed the salinity preference test tank partition during continuous five minute periods of observation. Schools were taken to consist of two or more f i s h showing a close and constant spatial relationship while proceeding i n the same direction across the tank partition*  When two or more  f i s h crossed the partition simultaneously* but without showing an obvious "association", these were not recorded as schools*  A measure of general  activity was obtained by totalling a l l the animals crossing the partition during the same observation period* As a result, activity i s at least partly a function of schooling intensity. In the present context one point i s important.  A very marked increase  in schooling and activity occurs during the same time that salinity preference i s changing i n i t s orderly sequence (refer to Fig. 6, A-E). Whether or not these behaviour changes are affected by the animals being confined to freshwater was not established.  CHUM SALMON FRY 960 96 962  CHUM S A L M O N FRY  50  V - I960 X - 1961 - 19 62  9  40  30  H  <  20  MAY  F GURE  JUNE  6  JULY TEMPORAL BEHAVIOUR SALMON  AUG  SEPT  OCT  C H A N G E S IN SCHOOLING AND ACTIVITY OF C H U M  -43-  Development of Salinity Tolerance The development of salinity tolerance was studied i n each of the five species as underyearlings*  The results of these tests are presented to  Figure 17* Pink and chum show a very early development of tolerance for concentrated seawater. Spring and sockeye f r y occupy an Intermediate position and coho shows the most restricted tolerance pattern of a l l . These results are i n agreement with Black's (1951) for chum and coho and are consistent with the known migratory behaviour of each species. Thus pink and chum migrate seaward shortly after emergence. Spring and sockeye have both been recorded as migrating in their f i r s t year though i t would appear from Figure 17 that either they remain i n dilute seawater during the summer or their a b i l i t y to tolerate concentrated seawater depends on a gradual process of acclimation. Coho f r y unlike the other species have never been recorded as migrating to the ocean though they have been reported i n the upper estuarine region. Both facts are consistent with their salinity preference pattern described earlier and their restricted a b i l i t y to osmoregulate as shown in Figure 17* Of the five species studied coho appear to be the most stenohaline.  POINTS P L O T T E D A R E MAXIMUM SEAWATER CONCENTRATIONS AT W H I C H IOO°/o SURVIVAL WAS OBTAINED  FEB  MARCH  FIGURE 17  APRIL  MAY  JUNE  JULY  A COMPARISON OF THE D E V E L O P M E N T T O L E R A N C E IN FIVE SPECIES OF PACI F IC  AUG OF SALINITY SALMON  DISCUSSION  It was proposed i s the Introduction that three c r i t e r i a were necessary to credibly demonstrate any orientation mechanism useful for migration* The f i r s t criterion stated that the environmental cue used for orientation must be sufficiently predictable to provide a reliable migration guide* Estuarine salinity gradients w i l l now be considered i n this regard. The work of the Pacific Oceanographic Group of the Fisheries Research Board of Canada on estuaries i s especially useful.  First i t constitutes  the most comprehensive information available on estuarine dynamics and structure* and second, i t deals with estuaries typically frequented by the five species of salmon used in the present study. In Figure 18-A the surface salinities of a northern river outflow are shown diagraamatically. A regular seaward progression i f increasing salinities is plainly evident.  Examination of the elevation diagram (Fig.  shows vertical as well as horizontal gradients.  18-B)  Representative vertical  sections are shown in greater detail i n Figure 18-C. Some understanding of the stability of stratification and the dynamics of the system may be gained when i t i s realized that fresh water flowing into the ocean overruns the adjacent coastal seawater forming a surface layer of brackish water. This *  surface water remains on the surface  and when removed from the influence of coastal streams the salinity of the sea surface increases in the direction of flow because of progressive dilution with underlying seawater and removal of fresh water by evaporation." t h i s process "  3n  deep water i s transferred upwards but no freshwater i s  ELEVATION B p Z5 i  30 i  15 i  C  UPPER  20 i  IS i  B  20 i  10 1  1  "20 1  1  30S%« 1  V_^^N^  1r""\  MCTlRS  s  INTER MEDIATE I  MODIFIED  o  IO  IS  GRADIENTS C A F T E R  TULLY, 1952  F I G U R E 18 SALINITY GRADIENT STRUCTURE IN THE VICINITY OF A NORTHERN RIVER OUTFLOW  -47-  transferred downwards." "Freshwater influence i s f i n a l l y lost by infinite dilution with seawater when the upper zone becomes indistinguishable from the lower zone" (Tully, 1952). A number of factors may influence estuarine salinity structure including the geography of the surrounding land, t i d a l fluctuation and currents, wind and changes i n river discharge.  When, for example, the seaward surface  flow i s opposed by strong winds the surface layer increases in depth which in turn has the effect of displacing the series of isohalines shown i n Figure 18-A toward the river mouth. In contrast a prolonged increase i n river discharge shifts the isohaline pattern (Fig. 18-A) in a seaward direction (Tully, 1952, Tabata and Pickard, 1957). Additional examples of estuarine salinity structure are given by Tully (1949# 1952) for Alberni Inlet and Chatham Sound, by Tabata and Pickard (1957) for Bute Inlet, by Tully and Dodimead (1957) and Waldichuck (1952, 1957) for Georgia Strait and by Picard (1961) in a review of British Columbia mainland inlets. In summary i t may be said that estuarine salinity gradients satisfy the requirements of the f i r s t criterion as orderly, persistent and predictable features of river outflows.  The horizontal gradients are of  special importance since salmon migrations are in the main horizontal movements. The possibility of more subtle vertical movements, in relation to vertical salinity gradients, i s not discounted though they w i l l not be considered here.  Likewise the possibility that salmon could use the more subtle  coastal salinity gradients (Tully, 1952, Lane, 1962) i s a problem open to future investigation.  Measurements approaching the order of magnitude of  these offshore gradients were not attempted in the present study.  -48-  Attention w i l l now be turned to the second criterion.  This states that  the animal must be shown to possess both the sensory and central nervous organization for detecting and, coordinating i t s movements in relation t o the environmental cue. One of the unique values of the salinity preference test i s that both aspects of the second criterion can be satisfied i n the one experiment.  First  the entire body of the salinity preference data presented i n the Results clearly demonstrates that juvenile salmon are capable of positioning themselves within salinity gradients, (the single exception t o this was pink f r y i n April,  At this date Pinks, under natural conditions, would just be  emerging from the redd (Carl et a l , 1959) so that their apparent inability to orient their movements i n the salinity gradient i s probably a reflection of their early stage of development).  Second, i t may logically be inferred  from the a b i l i t y t o position themselves within the test gradients, that the animals possess the necessary sensory capacity for detecting salinity, thus satisfying both aspects of the second criterion. An especially important piece of evidence bearing on the salmon's a b i l i t y to direct i t s movement with respect to salinity gradients i s derived from the peculiarities of the test gradient used in the present study. By testing the response t o each seawater concentration separately i t was demonstrated that juvenile salmon are capable of showing a specific response to particular seawater concentrations even when removed from the natural context of continuous salinity gradients.  This a b i l i t y to show a specific  response t o a particular stimulus value without reference to two or more values of the stimulus (in other words, without a scale of reference) may be termed absolute discrimination as opposed to differential discrimination.  -49-  Juvenile salmon apparently can recognize specific seawater concentrations without the necessity of referring to other points i n the gradient though under natural circumstances this would normally be possible. Evidence concerning the sensory basis of the response may be summarized as follows:  The response i s dependent f i r s t and foremost on seawater con-  centration.  Secondly, for any single seawater concentration the response  depends on a sea salt mixture though not a unique mixture. Thus the response to natural seawater may be imitated by a variety of salt mixtures which may be as simple as two cations (Na*. Ca'H-) and one anion (Gl~).  The response  to natural seawater probably depends on a complex interaction of a l l the salts present. By deduction i t may be concluded that the sense of taste or common chemical sense mediates the response (Hasler, 1957). In summary, i t i s clear that juvenile salmon are able to detect and to position themselves with respect to differences i n salinity thus satisfying the second criterion. The third criterion requires that the animals response to the range of environmental cues be shown to undergo a sequence of temporal changes which parallel spatial changes in the environmental cue along the migration route. Stated i n a different way. the potential of salinity gradients and salinity preference as parts of an orientation mechanism useful for migrations, depends on the correspondence between the temporal sequence of preference changes and spatial sequence of environmental salinity changes. A comparison of these two factors shows that juvenile salmon undergo a series of salinity preference changes which closely matches the horizontal salinity gradients of the estuarial region through which they must pass on  -50-  their seaward migration. A l l three criteria having been satisfied, the following hypothesis i s proposed: that the salinity preference behaviour of juvenile salmon reflects an a b i l i t y to use estuarine salinity gradients as a method of orientation during their seaward migration. The role played by the proposed orientation mechanism i n salmon migration w i l l now be considered in somewhat greater detail.  I have already  cited the work of Fontaine and Vibert (1952) as demonstrating that gradients of dissolved salts within rivers are so irregular as to provide l i t t l e likelihood of salmon using them for orientation.  Moreover, when  i t i s considered that juvenile salmon moving downstream find themselves on a one-way "conveyor belt" to the ocean, there i s really no evident need for such an orientation mechanism during this phase of the journey. Once the upper estuary i s reached, however, the net transport, although s t i l l seaward, i s masked by the effects of complex t i d a l oscillations. Moreover inner estuarine water i s frequently turbid and the overall dimensions of the environment gradually increase so that the depth and horizontal extent become many times those of the river proper.  In an other-  wise d i f f i c u l t environment, horizontal salinity gradients offer a reliable set of orientation cues. Coho salmon, as underyearlings remain i n rivers, never progressing further than the upper estuarine region.  Consistent with this behaviour,  their salinity preference pattern changes from a freshwater preference i n May to a preference for 3%o Cl seawater in July. It then remains unchanged u n t i l at least the following February (Fig. 19-A).  I am indebted to Dr.  -51-  J . P. Tully for suggesting a possible explanation why 3%  0  C l should form  the terminal modal preference* In Figure 19-B i t i s shown that as the tide rises, a "salt wedge* 1  intrudes into the river mouth forming a boundary layer between the freshwater and intruding seawater. Since the seawater and freshwater oppose each other the boundary zone i s formed as a tide rip or convergence which moves back and forth i n the river mouth according to the stage of the t i d a l cycle.  Figure 19-C shows that the boundary layer actually forms a salinity  barrier such that a 3%o C l salinity preference corresponds to the river side of the boundary layer (Tully and Dodimead, 1957). We w i l l return now to a consideration of those species whose preference sequence continues to change in the direction of increasing seawater concentration.  To decide precisely how salmon direct their movements in  relation to estuarine salinity gradients requires that we know something of their normal activities i n this region.  Recent studies (Manzer, 1956,  Manzer and Shepard, 1962) indicate that unlike the abrupt and shortlived downstream migration, passage through estuaries i s a leisurely a f f a i r covering a period of three to five months (April to August approximately) in the case of pink and chum salmon. Judging from the rapid growth during t h i s period, i t i s not unreasonable to think of salmon u t i l i z i n g the estuarine region as a feeding area prior to moving into the open ocean. With this in mind, the rather wide latitude in the most preferred seawater concentraion, which i s a feature of a l l the salinity preference data presented, gains some significance.  Too narrow a preference range,  though perhaps better for a more direct and therefore more rapid migration, could restrict the usefulness of the estuarlal region as a feeding area.  On  COHO FRY  cr UJ  BC-6.2  TEMPORAL SEQUENCE OF MODAL PREFERENCES  < < LLl  CO  U 0 0  O  MAY JUNE JULY AUG SEPT OCT NOV DEC JAN  EST UA RIAL  BOUNDARY  FEB  J  LAYER DIAGRAM  B  •B&C MODIFIED AFTER TULLY & DODIMEAD 1952 O KILOMETERS 5 IO  FRASER RIVER BOUNDARY LAYER DURING RISING T I D E  FIGURE  19  UNDERYEARLING COHO TEMPORAL SALINITY PREFERENCE SEQUENCE AND THE ESTUARINE BOUNDARY LAYER  the other hand a f a i r l y wide latitude i n preference i s consistent with both a nursery function and an oriented migration, though perhaps a less direct and slower movement. When the size of the seaward pink salmon migrants (up to 15 cm., Manzer and Shepard, 1962) i s considered in relation to the steepness of the estuarine gradient (approx. 0.<3000035$o salinity/cm., calculated from Fig. 18-A) a taxis form of orientation seems extremely unlikely.  Even  admitting the possibility that salinity receptors occur over the entire body surface, the steepness of the normal estuarine gradient would not be sufficient to produce two intensities of stimulation at different points on the body. According to Fraehkel and Gunn's (1961) classification of orientation then, the salinity preference mechanism would be classed as a kinesis. Two types of kinesis have been described.  In the f i r s t , orbhokinesis,  locomotion i s random in direction and the speed or frequency varies according to the intensity of stimulation. Where the preferred environment i s restricted to a single area which constitutes only a small percentage of total available environment, such as with salinity preference, orbhokinesis would be an ineffective mechanism. In the second type of kinesis, klinokinesis, the rate or frequency of turning i s proportional to the intensity of stimulation. This coupled with the phenomenon of sensory adaptation enables the animal to remain within a narrowly circumscribed region of preference.  The " t r i a l and error" behaviour of paramecia remaining within a  drop of water more acid than the surrounding region i s a familiar example of klinokinesis (Fraenkel and Gunn, 1961). Sufficient evidence i s not available to be able to definitely characterize  - 5 4 -  the salinity preference orientation mechanism as klinokinetic.  In fact,  the absolute salinity discrimination already discussed might be regarded as incompatible with the sensory adaptation essential to klinokinetic orientation.  A H that can be concluded i s , that,of the two types of  kinesis which,have been described, salinity preference orientation more closely resembles klinokinesis than orthokinesis. However, the salinity preference orientation mechanism can only be referred to the Fraenkel and Gunn classification in a very general way for the following reasons. F i r s t , an additional variable, not considered i n their classification, i s superimposed on the simple kinesis.  This i s the temporal sequence of  salinity preference changes, by virtue of which each single orienting movement i s f i n a l l y woven Into a pattern leading to the orderly seaward displacement of the young salmon. Second at the same time that migration through the estuarine region 9  i s taking place, measurements of schooling Intensity and activity i n the laboratory show these behaviours to be at the peak of their seasonal development. While a school of f i s h cannot be treated as a super-organism, individuals i n the school probably do gain certain advantages. By virtue of the intimate and orderly structure of the school, each Individual enjoys the benefit of an enlarged sensory capacity. It i s even possible, though completely speculative, that a group might be large enough to permit aligning of the long axis of the school with the salinity gradients and thereby t o achieve a taxis type of orientation.  An additional effect of intense  schooling would be to average out any differences In preference of i t s individual members.  -55--  Some evidence of the factors affecting the development of the temporal sequence of salinity preference changes i s available*  Baggerman (I960)  found with coho f r y that by increasing daily photoperiod, changes in preference could be accelerated.  In the present work, study on this topic was  largely restricted to the effects of seawater exposure*  Using coho fry also,  i t was shown that the i n i t i a l part of the preference sequence develops unaffected by seawater exposure (Fig.  20-A). For the latter part of the  sequence two routes of development are possible depending on seawater exposure.  Those animals kept i n freshwater (except for periodic tests)  undergo a partial sequence of preference changes (Fig.  20-B) whereas animals  kept i n seawater undergo a complete series of preference changes* the terminal pattern being a preference f o r seawater of open ocean concentration (Fig* 20-C)*  The single exception to t h i s generalization was sockeye. which  underwent a complete sequence of changes when held i n freshwater. It appears then that the normal seawater exposure experienced by the migrant during the early stages of seaward movement i s necessary for the f u l l unfolding of the preference sequence otherwise the pattern reverses to a freshwater preference.  It i s interesting to speculate that this regression  might be a normal feature in adult f i s h returning prior to spawning, i n other words* that they use the preference sequence i n reverse as an orientation mechanism. Because the timing of the salinity preference sequence i s partlydependent on seawater exposure, the present methods of study permitted only a rough measurement of this feature. With pink fry. kept i n seawater from l a t e June, an entire sequence of preference changes was completed between April and late August (Fig. 20).  This compares favourably with data pre-  sented by Manzer and Shepard (1962) for the entry of pink salmon into the  >  RIVER  MAY  ESTUARY  —= FIGURE  >  >-  OCEAN  AUGUST  20  DIAGRAMMATIC REPRESENTATION OF T H E SALINITY P R E F E R E N C E ORIENTATION MECHANISM--.  -57-  estuarine region and their f i n a l offshore movements. To obtain a more precise measurement would require that the animals be kept i n their most preferred seawater concentration as the season progressed instead of being confined to freshwater or to some single seawater concentration. In conclusion. Figure 20 diagrammatically summarizes the close correspondence between the temporal sequence of salinity preference changes and the horizontal salinity gradients of the estuarial region, the two features most essential to the proposed orientation mechanism. Speculation Concerning the Evolution of the Salinity Preference Orientation Mechanism Both freshwater and the ocean have been plausibly defended as the original habitat of the ancestral salmonid, a non-migratory form (Tchernavin, 1939# Jones, 1959). The marine proponents recognize freshwater spawning as offering a better chance of survival to the young fish whereas the freshwater proponents see feeding advantages i n the ocean habitat for the prematuring animal. Whichever i s the true ancestral habitat, i t i s reasonable to assume that the development of anadromous movements took place in gradual stages through the intermediate estuarine region. The f i r s t problem to be overcome in such an evolutionary expansion would be of osmoregulation.  Assuming that euryhaline osmoregulation  de-  veloped gradually, then an obvious selective pressure would exist for a mechanism of orientation related to salinity.  In other words, the a b i l i t y  to detect and to orient with respect to salinity gradients was originally related to the stenohaline condition of the ancestral salmonid and only as complete eurhalinity developed did i t become part of a general pattern of  -58-  behaviour associated with migratory movements* If this i s true then within the five members of the genus Oncorhynchus used In the pressnt study, the species whose pattern of salinity preference i s s t i l l a matter of survival i s probably most closely related to the ancestral salmonid and the ancestral habitat.  This assumes, of course,  that the inability to osmoregulate under certain conditions i s not of secondary derivation. Data on salinity tolerance presented i n the Results shows that coho f r y are the least tolerant of the five species (Fig. 17).  When their  osmoregulatory limitations are considered i n relation to their salinity preference pattern as f r y (Fig. 8 or 19) i t i s apparent that their modal preference (0-3% Cl) prevents them from wandering into seawater concentrations beyond their osmoregulatory a b i l i t y .  Coho therefore are postu-  lated to most closely resemble the ancestral salmonid.  Further because  they are restricted in their a b i l i t y to osmoregulate i n concentrated seawater, freshwater i s postulated to be the ancestral habitat. This agrees with, and provides additional support for, Tchernavin's conclusion that the family Salmonidae originated i n freshwater (1939). It also provides independent support for Hoar's (1958) evolutionary conclusions that the coho i s closest to the parent type, where the study was based on an extensive examination of the migratory behaviour of four species of Pacific salmon.  SUMMARY  1*  Juveniles of five species of Pacific salmon were shown to undergo a  temporal sequence of salinity preference changes. The sequence began with a preference for freshwater, continued to change in the direction of i n creasing seawater concentration and was terminated by a preference for seawater of open ocean concentration. 2.  This temporal preference sequence was shown to correspond to the  spatial sequence of seawater concentrations found as horizontal gradients in estuaries and through which young salmon pass on their seaward migration. 3.  On the basis of the foregoing evidence an orientation mechanism was  postulated; that by means of salinity preference, juvenile salmon are able to use estuarine salinity gradients as directive cues during their seaward migration. 4*  A period of seawater exposure was found to be necessary for the  development of the complete preference sequence otherwise a regression to a freshwater preference occurred. 5.  The sensory stimuli leading to the salinity preference response were  found to depend on the complex interaction of the natural constituents of seawater. Experimentally the simplest mixture which would e l i c i t a normal response consisted of two cations (Na* and GaH-) and one anion (Cl"). 6.  A variety of evidence pointed to taste or the common chemical sense as  the sensory modality underlying the response.  In addition evidence of an  a b i l i t y approaching absolute salinity discrimination was considered. 7.  It was postulated that originally the a b i l i t y to recognize seawater  concentration was of direct survival value to the stenohaline ancestral  -60-  salmonid.  Later, as migratory movements developed along with euryhalinity.  salinity preference became integrated into a temporal sequence of changes and thereby an orientation device useful for migrations.  LITERATURE CITED  Baggerman, B. An experimental study of the timing of breeding and migration in the three-spined stickleback (Gasterosteus aueuleatus* L.), Auch. Neerland Zool. 12, 105-318 (1957J. Baggerman, B. Salinity preference, thyroid activity, and the seaward migration of four species of Pacific salmon (Oncorhynchus)» J. Fish. Res. Bd. Canada, 17, 296-322, I960. Barnes, H. Some tables for the ionic composition of seawater. J. Exp. Biol. 31, 582-588, 1954Black, W. S. Changes i n body chloride, density and water content of chum (Oncorhynchus keta) and coho (0. Kisutch) salmon fry when transferred from fresh to sea water. J. Fish. Res. Bd. Canada 8, 164-176, 1951. Carl, G. C , Clemens, W. A., Lindsey, C. C. The freshwater fishes of British Columbia. B. C. Provincial Museum, Handbook No. 5, 1959. Clemens, ¥• A., Wilby, G. V. Fishes of the Pacific coast of Canada. Bulletin No. 68, Second Edition, Fish. Res. Bd. Canada, 1961. Fontaine, M., Vlbert, R. Migration f l u v i a l e anadrome du Saumon (Sal mo salar L.) et gradient de salinite. Annales de l a Station Centrale d»Hydrobiologie Appliquee, 4, 339-345, 1952. Fraenkel, G. S., Gunn. D. L. Inc. N. ?., 1961.  The orientation of animals.  Hale, L. J. Biological laboratory data. 94-96, 1958.  Dover Publications  John Wiley and Sons Inc., N. T«,  Hanavan, M. G., Skud, B. E. Intertidal spawning in pink salmon. U. S. Dept. Int., Fish and Wildlife Ser. Fish Bull. 56, 167-185, 1954. Hasler, A. D. Olfactory and gustatory senses of fishes, i n Physiology of Fishes, Edited, M. E. Brown, Academic Press, N. T., Vol. I I , 187-207, 1957. Hoar, W. S. The evolution of migratory behaviour among juvenile salmon of the genus Oncorhynchus. J . Fish. Res. Bd. Canada, 15, 391-428, 1958. Houston, A. H. An experimental study of the responses of young Pacific salmon to sharp sea water gradients. M. A. Thesis, Dept. of Zoology, University of British Columbia, 1956.  ^2-  i-  Houston* A. H. Responses of juvenile chum* pink and coho salmon to sharp seawater gradients. Can. J. Zool. 35, 371-383* 1957. Huntsman, A. G. Sea behaviour i n salmon, Salmon and Trout Magazine, Ho. 90, 1938. Jones, J. W.  The Salmon.  Collins Publishers, London, 1959*  Lane, K. R. A review of the temperature and salinity structures i n the approaches to Vancouver Island, B r i t i s h Columbia. J . Fish. Res* Bd. Canada, 19, 45-91, 1962. Manzer, J. I. Distribution and movement of young Pacific salmon during early ocean residence. Progress Rpts. Pacific B i o l . Sta. Fish. Res. Bd. Canada, No. 106, 24-28, 1956. Manzer, J. I., Shepard, M. P. Marine survival, distribution and migration of pink salmon off the British Columbia coast. Pink Salmon Symposium, Institute of Fisheries, University of B r i t i s h Columbia, 113-122, 1962. Mclnemey, J. E. An experimental study of salinity preference and related migratory behaviour. M. Sc. Thesis, Dept. of Zoology, university of British Columbia, 1961. Pickard, G. L. Oceanographic features of inlets in the B r i t i s h Columbia mainland coast. J. Fish. Res. Bd. Canada, 18, 907-999, 1961. Shepard, M. P. Response of young chum salmon (Oncorhvnchus keta) to changes in seawater content of the environment. M. A. Thesis, Dept. of Zoology, University of B r i t i s h Columbia, 1948. Tabata, S., Pickard, G. L. The physical oceanography of Bute Inlet, British Columbia. J. Fish. Res. Bd. Canada, 14, 487-520, 1957* Tchernavin, A. The origin of salmon. Salmon and Trout Magazine, No. 95* 1-21, June, 1939. Tully, J. P. Oceanography and prediction of pulp mill pollution i n Alberai Inlet. Bulletin No. LXXXIII, Fish. Res. Bd. Canada, 1-169, 1949. Tully, J. P. Notes on the behaviour of fresh water entering the sea. Seventh Pacific Science Congress, 3> 1-22, 1952. Tully, J . P., Dodimead, A. J. Properties of the water in the strait of Georgia, B r i t i s h Columbia, and influencing factors. J. Fish. Res. Bd. Canada, 14, 241-319, 1957.  -63-  Waldichuk, M. Oceanography of the strait of Georgia. 1. Salinity Distribution. Progress Rpts. Pacific Biol. Sta. Fish. Res. Bd. Canada No. 93, 26-29, 1952. Waldiehuk, M. Physical Oceanography of the strait of Georgia. Res. Bd. Canada, 14, 321-486. 1957*  J . Fish.  

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0105606/manifest

Comment

Related Items