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The effects of intraspecific competition for food on reproduction in the guppy (Poecilia reticulata).. Morrell, Michael Rowland 1973

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c-l  THE EFFECTS OF INTRASPECIFIC COMPETITION FOR FOOD ON REPRODUCTION IN THE GUPPY (POECILIA RETICULATA) AND THE IMPLICATIONS FOR THE STOCK AND RECRUITMENT PROBLEM  by  MICHAEL ROWLAND MORRELL B . A., S t a n f o r d U n i v e r s i t y , 1 9 6 8  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS' FOR THE DEGREE OF MASTER OF•SCIENCE i n t h e Department of Zoology  We a c c e p t t h i s t h e s i s as conforming t o t h e required standard.  THE UNIVERSITY OF BRITISH COLUMBIA September, 1973  In presenting  t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r  an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may by h i s representatives.  be granted by the Head of my Department or I t i s understood that copying or p u b l i c a t i o n  of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission.  Department of  ~~&>Q I  y  The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada  ii  ABSTRACT  Laboratory experiments were conducted with female guppies to determine the effects of intraspecific competition f o r food on fecundity, f r y size, and gestation period. Two groups of females which had been raised at different levels of intensity of competition were -compared. Gestation period was s i g n i f i c a n t l y longer i n the high competition group.  The effects on fecundity and f r y size were  dependent on the size of the adult female. The largest females i n the high competition group did not d i f f e r with respect to these  parameters  from individuals of the same size i n the low competition treatment; the smaller females produced fewer but larger f r y i n each brood under high competition.  The t o t a l weight of f r y per brood produced by  females of a given size was not different between treatments.  These results are discussed from the standpoint that the reproductive characteristics of the individuals of a population can be viewed as t a c t i c a l components of a strategy whose objective i s to maximize the present value of the number of surviving progeny produced by each individual i n the course of i t s l i f e t i m e . The optimal d i s t r i b u t i o n over time of energy devoted to reproductive ends and the optimal d i s t r i b u t i o n at one point i n time of a given amount of energy among many or few offspring are expected to vary among populations according to the particular age- and size-specific mortality rates faced by each population.  Both magnitude and v a r i a b i l i t y of mortality are important.  I t i s suggested that by measuring the appropriate reproductive parameters  iii  o f t h e a d u l t members o f a p o p u l a t i o n ,  i t s h o u l d be p o s s i b l e t o make  p r e d i c t i o n s about t h e shape and expected v a r i a b i l i t y o f t h e s t o c k / r e c r u i t relationship.  iv  TABLE OF CONTENTS Page TITLE PAGE  .  .  i  ABSTRACT.  i i  TABIiE O F C O N T E N T S LIST  O F TABXJES©  o e e e s o * « e v e « o e o e * o » » * « * e * < » » * O 0 e o o 0 o » » o * o o * * « e «  • « e a o e « o * * o * > a o e a o 0 * « * « o * « « o e o « o « « * * o o o o o a e o o * o «  LIST OF FIGURES.  v i i  ACKNOWLEDGEMENTS. INTRODUCTION.  ....  ix  '.  1  MATERIALS AND METHODS  '  Experiment 1...... Experiment 2......  •  Experiment 3«»«  Experiment 1 . Experiment 2............  10 12 »  RESULTS o o o o o o . « . . o o . o 0 « . . . . . . o . . o e « . o . . . . o . . . . . o .  9  . . . . . . . .  15 ......  •  17 17 21  Experiment 3............*.....0.**..*.000*0*0000**0.******  ^3  Comparison of Experiments 1 and 3«  31  DISCUSSION. . o . . o . . o © . . . . . . o . . « . « . o . . . . © « . o o . e o e . . . . . © o . o f t . . . . . .  45  Reproductive strategy....  46  F i e l d studies on p o e c i l i i d reproduction................... Other laboratory studies of fecundity and n u t r i t i o n . . . . . . . Studies on exploited marine populations................... Reproductive strategy theory and stock/recruitment........  5^ 65 67 82  CONCLUSIONS. SUMMARY BIBLIOGRAPHY APPENDIX  vi  LIST OF TABLES  Table  I II  Page  Analysis of variance of standard length of f r y . Experiment 3 — high food vs. low food ..... " The calculated fecundity of female plaice 37 cm. i n length and estimates of reproductive l i f e span at different l o c a l i t i e s . « . . o o o e o o e e o . . . . o e o . o « o . . . . . . e o o . o o o  30  71  III  Reproductive parameters of Atlantic herring t r i b e s . o  76  IV  Environment encountered by l a r v a l Atlantic herring  81  vii  LIST OF FIGURES  Figure  Page  1  A family of Ricker s t o c k / r e c r u i t curves  3  2  A f a m i l y o f B e v e r t o n and H o l t  3  3  stock/recruit curves0.  Schematic drawing o f t h e a p p a r a t u s u s e d i n E x p e r i m e n t s 2 and 3 t o i s o l a t e e x p e r i m e n t a l f e m a l e s ,  13  4 " The r e g r e s s i o n o f t h e l o g a r i t h m o f numbers o f f r y p e r b r o o d on m a t e r n a l l e n g t h . Experiment 1 . . . • 5  6  7  8a  8b  9a  9b  10  11  12  13a  19 r  The r e g r e s s i o n o f t h e l o g a r i t h m o f numbers o f embryos o b t a i n e d by d i s s e c t i o n on m a t e r n a l l e n g t h . Experiment 1 .  20  The r e g r e s s i o n o f f r y l e n g t h on m a t e r n a l l e n g t h . Experiment 1 . .  22  The r e g r e s s i o n o f d a i l y consumption o f d r y f o o d on wet weight o f f e m a l e . Experiment 2  24  The r e g r e s s i o n o f numbers o f f r y p e r b r o o d on m a t e r n a l l e n g t h . Experiment 3 — h i g h f o o d t r e a t m e n t  26  The r e g r e s s i o n o f numbers o f f r y p e r b r o o d on m a t e r n a l l e n g t h . Experiment 3 — low f o o d t r e a t m e n t  27  The r e g r e s s i o n o f f r y l e n g t h on m a t e r n a l l e n g t h . Experiment 3 — h i g h f o o d t r e a t m e n t .  28  The r e g r e s s i o n o f f r y l e n g t h on m a t e r n a l l e n g t h . Experiment 3 — low f o o d t r e a t m e n t  29  Comparison o f t h e r e g r e s s i o n l i n e s o f t h e l o g a r i t h m o f numbers o f f r y p e r b r o o d on m a t e r n a l l e n g t h . Experiment 1 v s . Experiment 3». •  33  Comparison o f t h e r e g r e s s i o n l i n e s o f f r y l e n g t h on m a t e r n a l l e n g t h . Experiment 1 v s . Experiment 3««  •••  35  The r e g r e s s i o n o f t h e d r y w e i g h t s o f b r o o d s o f Experiment 3 on ^ ( S L ^ * 8 ) . * .  37  The r e g r e s s i o n o f e s t i m a t e d b r o o d d r y weight on m a t e r n a l l e n g t h . Experiment 1 .  39  viii  Figure  13b  Page  The  r e g r e s s i o n o f a c t u a l b r o o d d r y weight on m a t e r n a l  length. 14a  The  The  16  17  Experiment  19  40  1.....  Experiment  42  3**«««  •  •  43  The r e l a t i o n s h i p between t h e i n s t a n t a n e o u s r a t e o f p o p u l a t i o n g r o w t h , m, and p o p u l a t i o n d e n s i t y f o r r s t r a t e g i s t s and K- s t r a t e g i s t s . . . .  50  Average numbers o f embryos c a r r i e d by female Gambusia manni c o l l e c t e d at two l o c a t i o n s i n t h e Bahamas a t d i f f e r e n t seasons  64  The  r e g r e s s i o n o f f e c u n d i t y o f p l a i c e from d i f f e r e n t  a r e a s on r e p r o d u c t i v e l i f e 18  •  r e g r e s s i o n o f i n d e x o f r e p r o d u c t i v e e f f o r t on m a t e r n a l  length. 15  3•••• •  r e g r e s s i o n o f i n d e x o f r e p r o d u c t i v e e f f o r t on m a t e r n a l  length. 14b  Experiment  span...  73  Average d r y weight o f r i p e eggs o f v a r i o u s s t o c k s o f C l u p e a harengus o f t h e n o r t h e a s t A t l a n t i c p l o t t e d a g a i n s t month o f spavining  77  F e c u n d i t y - e g g s i z e diagram f o r some w i n t e r - s p r i n g and summer-autumn spawning s t o c k s o f C l u p e a h a r e n g u s .  78  ix  ACKNOWLEDGEMENTS  My s u p e r v i s o r , D r . Norman J . W i l i m o v s k y , gave g e n e r o u s l y o f h i s time and t a l e n t throughout t h e s t u d y .  Financial  support came from  N a t i o n a l R e s e a r c h C o u n c i l o f Canada g r a n t s t o D r . W i l i m o v s k y .  D r . Donald M c P h a i l f i r s t  suggested t h a t I c o n s i d e r my problem  from t h e p o i n t o f v i e w o f r - and K- s e l e c t i o n .  My u n d e r s t a n d i n g o f t h e  theory o f reproductive strategy increased considerably as a r e s u l t o f c o n v e r s a t i o n s w i t h D r . E r i c Charnov and Mr. Stephen S t e a r n s .  I n t h e f i n a l phases o f t h e s t u d y , I r e c e i v e d more t h a n a l i t t l e h e l p from my f r i e n d s .  Ms. V a l e r i e Best and Ms. Sharon H e i z e r t y p e d t h e  m a n u s c r i p t , and Ms. S t e p h a n i e Judy p r e p a r e d s e v e r a l  figures.  D r . Robin Harger gave encouragement and sound, a d v i c e a t a c r i t i c a l j u n c t u r e ; w i t h o u t h i s h e l p t h e work would c e r t a i n l y n o t have been completed.  To R o b i n , I d e d i c a t e t h i s  thesis.  1  INTRODUCTION  One  o f t h e c e n t r a l problems o f f i s h e r y b i o l o g y i s t h e n a t u r e o f  t h e r e l a t i o n s h i p between t h e and t h e  s t o c k o f spawning a d u l t s o f a  population  s t r e n g t h o f t h e r e c r u i t y e a r - c l a s s which t h e y p r o d u c e .  problem can be r e s o l v e d i n t o two  The  components, b o t h o f i n t e r e s t t o  the  f i s h e r y manager: 1)  What i s t h e average number o f r e c r u i t s produced by spawning stock o f a g i v e n s i z e ? o v e r a range o f s t o c k  t o determine t h e o p t i m a l 2)  How  This r e l a t i o n s h i p  s i z e s d e f i n e s the  f a m i l i a r s t o c k - r e c r u i t c u r v e and  shape o f  the  a l l o w s t h e manager  spawning stock  much v a r i a t i o n i s t o be  a  size.  expected about t h e  average c u r v e , and what are t h e o f d e p a r t u r e s from t h e average?  specific  causes  Variation i n  s t r e n g t h o f y e a r - c l a s s e s produced by  similar-sized  spawning s t o c k s i s o f t e n v e r y l a r g e i n n a t u r a l populations.  The  causes o f t h e v a r i a t i o n  u s u a l l y v e r y d i f f i c u l t t o i d e n t i f y and f o r r e a s o n s reviewed by G u l l a n d  are  quantify  (1965).  If this  q u e s t i o n c o u l d be answered, i t would be p o s s i b l e t o p r e d i c t the  strength of r e c r u i t i n g  some time b e f o r e the a c t u a l t i m e o f and t h e r e f o r e t o t a l stock  recruitment,  s i z e s c o u l d be  The t y p i c a l approach t o e s t i m a t i o n o f t h e s h i p has been t o c o l l e c t e s t i m a t e s  year-classes  forecast.  stock-recruit relation-  o f spawning s t o c k  s i z e and  resultant  2  recruitment for a series of years and to f i t a curve t o the data. Larkin (1973)  has discussed the different types of curves which may be used.  Curves described by Ricker (1954, 1958) and Beverton and Holt (1957) are the two types most often used i n management; examples of these curves are shown i n Figures 1 and 2 .  In both families of curves, recruitment  increases with stock size at low population levels; the major difference between them i s that at high stock levels the Ricker curve indicates an absolute drop i n recruitment, whereas the Beverton and Holt curve approaches an asymptote. In both families of curves recruitment per unit stock reaches a maximum at intermediate stock sizes and decreases with further increases i n spawning stock; thus both curves imply the existence of density-dependent  processes regulating population size.  The viewpoint adopted i n t h i s study i s that the stock/recruit curve for a population represents the result of a l l the interactions of the reproductive characteristics of the population with particular aspects of the environment. Reproductive characteristics of individual members of the population are assumed to have evolved by natural selection; the combination of reproductive t r a i t s shown by a given individual - age at f i r s t , maturity, fecundity, reproductive lifespan, etc. - can be considered as t a c t i c s i n a reproductive strategy whose objective i s to maximize individual f i t n e s s .  The hope i s that from an  understanding of these reproductive t a c t i c s i n an evolutionary context w i l l come an understanding of t h e i r population consequences - that i s , the stock/recruitment relationship of the population.  The observed year-  to-year variation i n recruitment indepent of stock density i s assumed t o  2.0  a  J 1  SPAWN E R S - W  1  2  Number of eggs x  F i g u r e 1.  A family of Ricker stock-recruit curves. The g e n e r a l e q u a t i o n i s Z = We ( a  1 - W  ).  I n curve  a, a =  i n b, a = 1.0; i n c , a =""2.678. A f t e r R i c k e r (195S)..  0.667;  Figure 2.  i :  1  3  4  10 '°(E) -  A f a m i l y o f B e v e r t o n and H o l t s t o c k / r e c r u i t curves. The g e n e r a l e q u a t i o n  1  i s R = cc+fi/E.  Holt  (1957).  A f t e r B e v e r t o n and  4  r e s u l t from d e n s i t y - i n d e p e n d e n t  fluctuations i n the relevant  environmental  v a r i a b l e s ; i f t h i s v a r i a b i l i t y has been p r e s e n t i n much t h e same form over e v o l u t i o n a r y time, t h e r e p r o d u c t i v e t a c t i c s o f t h e p o p u l a t i o n should r e f l e c t  The  some accommodation t o i t .  experimental  part of the present  f e c u n d i t y and c l o s e l y r e l a t e d v a r i a b l e s .  study d e a l s w i t h  absolute  B a g e n a l ( l 9 7 3 ) has r e v i e w e d  t h e p o s s i b l e r e l e v a n c e o f f e c u n d i t y t o t h e s t o c k and r e c r u i t m e n t He c i t e s s e v e r a l f i e l d  s t u d i e s showing d e c r e a s e s  i n average  absolute (1963b,  f e c u n d i t y w i t h i n c r e a s i n g p o p u l a t i o n d e n s i t y , namely Bagenal  I 9 6 5 ) , Hodder (1963), K i p l i n g and F r o s t (1969), R a i t t (1968).s t u d i e s by Bagenal ( 1 9 6 9 a ) , H e s t e r evidence  observed  provide  The h y p o t h e s i s which r e s u l t s from t h i s i s t h a t  reductions i n fecundity at high population density r e s u l t i n d i v i d u a l f e e d i n g l e v e l mediated by i n t r a s p e c i f i c  t h i s h y p o t h e s i s has been suggested field  Laboratory  t h a t f e c u n d i t y o f i n d i v i d u a l females i s p o s i t i v e l y r e l a t e d t o  nutritional level.  reduced  ( 1 9 6 4 ) , and S c o t t (1962)  problem.  s t u d i e s c i t e d above.  from  competition;  several times i n connection with t h e  B a g e n a l (1973) p o i n t s out t h a t t h e observed  r e l a t i o n s h i p between f e c u n d i t y and p o p u l a t i o n d e n s i t y i s o f t h e form necessary and  t o produce t h e s t o c k - r e c r u i t m e n t  curves o f Beverton  o f R i c k e r , although i t i s evident t h a t other f a c t o r s ,  and H o l t  such as  compensatory m o r t a l i t y o f eggs and l a r v a e , c a n produce t h e same e f f e c t .  5  The objectives of t h i s study, then, are the following: 1)  to describe quantitatively the effects of intraspecific competition f o r food on fecundity and related reproductive parameters, and  2) to relate these findings to the stock and recruitment problem.  Herein competition i s taken to mean the demand by two or more organisms for the same resource i n excess of immediate supply; t h i s d e f i n i t i o n i s essentially that of Milne ( 1 9 6 1 ) .  I t i s of interest to  distinguish two different courses that competition may follow: "contest" and "scramble", i n the terminology of Nicholson (1954)•  In contest  competition, some of the competitors are able to satisfy completely t h e i r requirements for the scarce resources, while others secure l i t t l e or none. In scramble competition, on the other hand, the available resource i s divided more or less randomly among the competitors, and a l l receive less than t h e i r f u l l requirements.  The effect of competition,  regardless of the course i t takes, i s to reduce consumption of the scarce resource (relative to f u l l requirements) for some or a l l of the competitors.  When scarce food i s the object of competition, at least some of the competitors v a i l be forced to exist on reduced rations.  This  reduced energy input to an individual may be allocated among the c o n f l i c t i n g internal demands of maintenance, growth, and reproduction i n different ways. Energy expenditure i n a l l categories may be reduced, or expenditures  6  i n one category may be maintained at a high l e v e l at the expense of the others.  In t h i s study attention i s focussed on reproduction, but i t i s  important to remember that reproduction i s only one of several processes which may be affected by food shortage.  I t would be extremely d i f f i c u l t to pursue the objectives set out-above i n a natural f i s h population. The intensity of competition must depend on population density and the available supply of scarce resources; the i d e n t i f i c a t i o n of c r i t i c a l resources and measurement of available supply would be very involved. Furthermore, i t i s probable that f i s h fecundity at a given time i s significantly influenced by events occurring from one to several reproductive seasons previously (Bowers and Holliday, '1961; Hester,  1964;  Hodder,  1963;  Kipling and Frost,  1969);  therefore, i t i s desirable to know the history of a given individual over several reproductive seasons. Both of these problems are f a r more easily handled i n the laboratory than i n the f i e l d .  The guppy F o e c i l i a r e t i c u l a t a (Peters) was chosen as the experimental animal for several reasons.  Guppies are r e l a t i v e l y easy to maintain i n  the laboratory and reproduce readily throughout the year.  In addition,  t h e i r short gestation period (about one month) makes i t possible to collect a considerable amount of data on reproduction i n a reasonable time.  7  In P o e c i l i a f e r t i l i z a t i o n i s internal, and the embryos develop inside the ovary.  The f r y are released by the female at an advanced  state of development; they can swim actively at b i r t h and normally begin to feed soon after release.  The schedule of stages of oogenesis and embryonic development have been worked out f o r P o e c i l i a reticulata by Stolk (1951)•  The  ovary of a mature female normally contains oocytes and embryos of three discrete stages:  ( l ) many small undifferentiated oocytes of less than  0.1 mm. diameter, (2) a smaller number of medium-sized oocytes up to 0.3 mm, diameter containing some yolk, (3) either a set of large yolky oocytes (or ova) or a similar number of developing embryos. I f we take the day of parturition of one brood as day 0 and assume a constant brood period of 2.8 days, we may follow the development of the oocytes that w i l l eventually constitute one brood.  On day 0 some of the many  small oocytes begin' to differentiate and accumulate yolk. By day 28 these oocytes have become medium-sized oocytes as defined above; they j o i n a pre-existing group of medium-sized oocytes, according to Stolk. Following the next parturition (day 56 i n t h i s model), some of the medium-sized oocytes lay down the bulk of t h e i r yolk supply very rapidly, maturing to large ova, and are f e r t i l i z e d .  The resultant f r y are born on  day 84.  On the basis of Stolk's work, i t seems possible that.events, occurring up to three brood periods before parturition may affect the numbers and sizes of f r y produced.  In an experimental study of the  8  effects of feeding on guppy reproduction, Hester (1964) found that brood size was s i g n i f i c a n t l y affected by feeding regime of the female during the two brood periods immediately preceding parturition.  Hester vras  unable to rule out the p o s s i b i l i t y of three period effects.  I f an individual guppy reduces the amount of energy dedicated to reproduction, one would expect some or a l l of the following effects: 1)  a reduction i n number of f r y per brood,  2)  a reduction i n size of individual f r y ,  3)  an increase i n time between broods.  A l l of these potential effects are examined i n t h i s study. .  9  MATERIALS AND METHODS  The guppies used i n t h i s study were descendants of f i s h collected i n August, 1967j i n Trinidad, West Indies. The i n i t i a l c o l l e c t i o n consisted of something over 100 adults and immatures taken by B. Seghers from the Tacarigua River about four kilometers northeast of i t s confluence with the Caroni.River.  The f i s h were shipped by a i r to Vancouver, where  the stock was maintained i n aquaria i n the laboratory of Dr. N.R. L i l e y at the University of B r i t i s h Columbia.  At the time of the i n i t i a l c o l l e c t i o n the Tacarigua at the collecting site was three meters broad with a rather uniform depth of about ,15m. Current was measured, at about .4m/sec, and the water was clear; however, after a rainstorm, the stream becomes turbid, and the flow may increase by an order of magnitude. Aside from an algal f i l m on the stream bottom, there were no aquatic plants at the collecting s i t e . Water temperature was not recorded i n 19°7» but at a similar site on the nearby Aripo River, temperature varied from 25.5 to 26.5°C i n the summer of I969.  The. c i c h l i d Crenicichla a l t a and the characid Hoplias malabaricus,  both of which prey on adult guppies, were present at the Tacarigua s i t e . Other fishes present included the c i c h l i d Aequidens pulcher, the characids Astyanax bimaculatus and Corynopoma r i s e i , the l o r i c a r i i d Hypostomus r o b i n i i , and a pimelodid of the genus Rhamdia (Seghers, 1973» and pers. comm.).  10  From September, 19°9i several hundred of the descendants of the o r i g i n a l Tacarigua stock were maintained i n the laboratory as a source of experimental animals. aquari  The f i s h were kept i n 64 l i t e r capacity glass  with floating plants, Ceratophyllum sp., f o r cover. Water  temperatures were maintained at 26 + 1° C by the use of thermostatically controlled aquarium heaters. Aquaria were not exposed to natural l i g h t ; the^main illumination for the tanks was provided by 20 watt fluorescent l i g h t s about 35 cm. above the surface of the water.  Lights were  automatically controlled to maintain a constant day length of 14 hours throughout the study.  A l l aquaria were f i l l e d with tap water to which  was added one gram of rock salt per l i t e r ; water lost by evaporation 1  was replaced at frequent intervals with tap water.  On a weekly basis  28 1. of water were removed from each tank and replaced with fresh tap water plus s a l t .  Each tank.was equipped with a glass wool and charcoal  f i l t e r to c i r c u l a t e , aerate and f i l t e r the water.  - The main food during most of the study was l i v e Tubifex sp. (Oligochaeta); f i s h i n stock tanks were fed once daily i n amounts sufficient to maintain an almost continuous supply of l i v e Tubifex on the tank bottom.  This basic diet was supplemented with small quantities  of Tetramin, a commercial dry f i s h food.  Departures from t h i s basic  diet w i l l be discussed below.  Experiment 1: i  Measurement of Reproductive Parameters of Isolated Females Fed on Excess Tubifex _______________  In August, 197O5 24 healthy mature females were selected from stock tanks as experimental animals.  Fish were selected so as to  11  represent, equally a l l lengths of mature females present i n the stock " tanks (14-45 mm. standard length).  Individual females were isolated  i n glass jars of 1.95 !• capacity containing 1.3 1. of water with floating Geratophyllum as cover for f r y .  Jars were kept i n 26° C  water baths.  The f i s h were fed once daily, s i x days a week with excess Tubifex.  Each day before feeding, feces and food remaining from the  previous day were siphoned from the jars, and the water was passed through a charcoal and glass wool f i l t e r and returned to the j a r . At weekly intervals half the water i n each j a r was removed and replaced with tap water plus salt..  , At lease once every 24 hours, jars were carefully examined f o r the possible presence of f r y .  Upon discovery, f r y were removed from the  j a r , anaesthetized with MS-222, and measured, to the nearest 0.1 mm. standard length under a compound microscope, using a technique adapted from Counts ( l 9 o l ) .  After a l l f r y had been released, the female was anaesthetized with MS-222 and measured to the nearest 0.1 mm. standard length with d i a l calipers.  The female was then quickly blotted on a paper towel and  placed i n a pre-weighed beaker of water; the change i n weight of the beaker plus f i s h was recorded to the nearest .01 g.  The female was then  returned to her o r i g i n a l j a r , to which two males had been added.  The males  were removed after f i v e days, during which time they were presumed to have inseminated the female (see Rosenthal, 1952).  12  Experiment 2 ; Measurement of Ration of Dry Food Experiment 1 provided descriptive information on reproduction of f i s h fed i n excess; the next step was to measure the effects of reduced rations.  In order to set up a treatment of reduced rations f o r  f i s h of different sizes, i t was necessary to establish a curve of f u l l rations as a function of body size.  Two separate attempts were made to measure guppies* actual consumption of Tubifex fed i n excess. Neither .experiment produced usable results; the main problems were the d i f f i c u l t y , of accurately weighing the l i v e Tubifex before feeding and of recovering a l l of the uneaten . portion after feeding.  To circumvent the d i f f i c u l t i e s of weighing l i v e  Tubifex, dry food (Tetramin) was used.exclusively i n a l l experiments and i n the stock tanks beginning i n June, 1971•  In t h i s and subsequent experiments'a different system was used to isolate individual females i n such a way as to.minimize predation on fry and to provide improved c i r c u l a t i o n and f i l t r a t i o n of water. Nylon mesh was attached to a plexiglass frame, using Pliobond glue or silicone aquarium sealant, to form 10 separate compartments, each measuring 10.2 x 12.5.x 2 7 . 8 cm. Each framework was placed i n a 64 1..capacity aquarium with a f i l t e r system as described above.  The water l e v e l was  maintained so that each individual compartment enclosed about 3»5 1« of water. Figure 3 i s a schematic drawing of the apparatus. Each compartment which contained an adult female (A.C,D,G,I,J) had fine mesh on three sides; even newborn guppies were too large to penetrate  13  A  j  B  y  C  D  ^/ ////// F  Figure 3«  G  1  J  Schematic drawing of the apparatus used i n Experiments 2 and 3 to isolate experimental females. The outside dimensions are 20.4 x 62.5 x 27.8 cm. a) Lateral view b) Top view. Solid lines indicate small mesh; broken l i n e s indicate large mesh.  14  t h i s mesh. The fourth side consisted of larger mesh through which frybut not adults could pass.  Thus f r y being pursued by t h e i r mothers were  able to escape through the large mesh into the adjacent refuge compartment (B,E,H~). Each refuge compartment was divided diagonally by a fine mesh barrier i n order to separate the f r y of different females.  Fry  were observed to use the refuges i n the intended manner.  Twenty experimental females were selected from stock tanks so as to give equal representation to a l l size classes present i n the stock tanks (14-36 mm.).. The experimental animals were placed i n the individual mesh compartments of the i s o l a t i o n apparatus.  After a four day acclimation  period, during which f i s h were fed and excess food was removed once daily, the experimental procedure was begun.  For four consecutive days a weighed quantity, of dry food was placed in'each compartment, and the f i s h were allowed to feed undisturbed for one hour.  (A p i l o t study showed that v i r t u a l l y a l l feeding occurs  i n the f i r s t 15 minutes after food i s presented, and that after an hour none of the f i s h studied showed any further interest i n the remaining food.)  At the end of the hour the amount of food remaining i n each  enclosure was recorded qualitatively (0, trace, or +), and any excess was removed. On the following day the amount of food offered was increased for f i s h which had consumed a l l or nearly a l l of the previous day's ration, and food was reduced for the others. In t h i s manner, i t was possible to bracket roughly the actual amount of food consumed i n one feeding by an individual f i s h .  15 Experiment 3 * Measurement of -Reproductive Parameters of Isolated Females at F u l l and Half Rations of Dry Food .. In August, 1971> two groups of 1 0 healthy females were selected from the stock tanks for use i n t h i s experiment. Before the f i s h were actually selected,  1 0 values of standard length were picked which  covered the range of sizes available i n the tanks (14-28 mm.).  Fish  from the stock tanks were then measured, and the f i r s t ones f a l l i n g within 0.5  mm.- of the predetermined lengths were assigned to f u l l or half rations  by the toss of a coin.  Each selected f i s h was placed i n the i s o l a t i o n  compartment which had been previously designated at random for that specific length and ration.  Thus f i s h wefe randomly assigned to food  treatment and tank position, with the r e s t r i c t i o n that each treatment group contained the same range and distribution of sizes.  The isolated guppies were daily fed weighed quantities of Tetramin representing f u l l or half rations f o r t h e i r particular weight. Feces and food from the previous day were removed p r i o r to each feeding. Following each parturition, rations were adjusted to the appropriate percentage of post parturn weight of the female ( 3 . 0 % for high, 1,5%> for low food treatment) i n order to allow for weight changes due to growth during the experiment.  Tanks were checked at least once daily for newborn fry, and when births occurred the same procedures were followed as described f o r Experiment 1. In addition, i n t h i s experiment after the standard  16  length of each f r y had been measured, the entire brood was placed i n a drying oven at 60° C, and the dry weight of the brood as a whole was measured to the nearest 0.1 mg. after 48 hours, by which time the weights were constant.  17 RESULTS  In Experiments 1 and 3 mortality of experimental f i s h was high.  The causes of the mortality are not known; possibly the r e s t r i c t i o n  of normal a c t i v i t y imposed by the i s o l a t i o n chambers was somehow involved. Typically f i s h stopped feeding and remained inactive for a few days or a week before dying.  Dead f i s h showed no external growths, lesions, or  parasites, and post-mortem dissections revealed no i n t e r n a l abnormalities.  Because of the losses of experimental animals, both experiments were terminated e a r l i e r than anticipated.  In Experiment 1, only three  f i s h survived to day 50; of these, one died on day 60, and the other two were alive on day 70, when the experiment was stopped.  Experiment 3  was terminated on day 51'with four survivors. As discussed i n the Introduction, i t was anticipated that reproductive effects of n u t r i t i o n would be apparent only after a time lag of up to three months. Thus, most, i f not a l l , of the data on reproduction collected i n the two experiments reflect feeding conditions i n the stock tanks p r i o r to the experiments themselves.  This idea i s supported by the results of  Experiment 3t i n which there are no significant differences between the high and low food treatments with respect to either numbers or sizes of f r y produced.  Experiment 1 I t was anticipated that maternal size would have significant effects on both numbers and sizes of f r y produced. Figure 4 shows  18  the relationship between the natural logarithm of number of f r y i n a brood and the standard length of the mother, the points a l l l i e near a straight l i n e with positive slope s i g n i f i c a n t l y different from zero (p< .001).  Eighteen of the experimental females which died were dissected, and t h e i r ovaries were examined. Fifteen of these contained developing embryos, althouth i n one case decomposition had progressed so f a r that i t was impossible to count the embryos. Of the three females without embryos, one had produced a brood five days before dying, and the next batch of large oocytes had not yet been f e r t i l i z e d ; i n the other two cases there was no development of oocytes beyond the smallest, least differentiated stage.  Figure 5 shows the regression of the natural logarithm of number  of embryos on maternal length.  The positive slope i s s i g n i f i c a n t l y  different from zero (p< .001).  A comparison of the regressions of Figure 4 and Figure 5 may provide a measure of the extent of maternal cannibalism, since the embryos counted by dissection had not, of course, been exposed to predation. The comparison was carried out by means of analysis of covariance. The hypothesis that the two regressions have a common slope was  accepted  at a low l e v e l of probability (p=.08). There was a nonsignificant tendency for the excess of embryos over f r y actually born and recovered to be greater for smaller females than for larger ones; the largest females had about the same numbers of progeny i n both cases (the regression lines intersect at a standard length of  39«7  mm.).  The hypothesis that the  elevation of the two regression lines i s the same was rejected (p=.01);  19  EOCK„  ±00 °±  U .  LL a  m ID  STANDARD LENGTH OF FEMALE F i g u r e 4»  a )  The r e g r e s s i o n o f t h e l o g a r i t h m o f numbers o f f r y p e r b r o o d on m a t e r n a l l e n g t h . Experiment 1.  ±00°..  STANDARD LENGTH OF FEMALE ( Figure 5. The regression of the logarithm of numbers of embryos obtained by dissection on maternal length. Experiment 1.  21  i f the regressions are given the same slope, then for a l l lengths of females there were s i g n i f i c a n t l y more embryos than f r y . This result suggests that the numbers of a l l broods actually born may have been reduced by cannibalism before the f r y were removed and counted. However, the d i f f e r e n t i a l effect of maternal size i s the opposite of what would be expected i f predation were the cause of the difference. Significant pre-natal mortality could also account for t h i s effect. The data collected i n Experiment 3 shed more l i g h t on t h i s problem.  The relationship of length of f r y to length of the mother i s positive (Figure 6), but there i s considerable variation of length of f r y within broods. A straight l i n e f i t to the points has a positive slope s i g n i f i c a n t l y different from zero (p< .001), but accounts for only about one-fourth of the t o t a l sum of squares.  Addition of  higher order terms (up to quartic) to the regression equation does not s i g n i f i c a n t l y reduce the residual.  Two females produced two broods each, and two others produced three and four broods.  Thus four individuals gave seven values of time  between broods; the mean i n t e r v a l was 24.8 days (standard deviation:2.9; range:22-28 days)  e  Experiment 2 After four days of experimental feeding, five f i s h had not eaten a significant amount on any day; these were a l l very inactive and were excluded from the analysis. Of the remaining 15, three produced  22  7»0.. s  10» ,  15*  SO"  S5»  30°  35=  40"  45«  50*  STANDARD LENGTH DF FEMALE ( M M O F i g u r e 6.  The r e g r e s s i o n o f f r y l e n g t h on m a t e r n a l l e n g t h . Experiment 1. The r e g r e s s i o n l i n e was c a l c u l a t e d on t h e b a s i s o f t h e i n d i v i d u a l l e n g t h s o f 343 f r y . F o r c l a r i t y , o n l y t h e mean l e n g t h o f each brood + two s t a n d a r d e r r o r s has been p l o t t e d .  23  broods during the i n i t i a l acclimation period; one of these died the day after giving b i r t h .  One male was added to each of the other two  compartments i n order to maintain a supply of fresh sperm for the reproductive study which was to follow; the data on feeding for these compartments were also excluded from the analysis.  The remaining twelve individuals a l l consumed a l l or nearly a l l of the food offered on at least one day. Where different amounts were eaten on different days, the higher value was used i n the analysis. The s o l i d l i n e of Figure 7 shows the results of the regression analysis of weight of dry food eaten on l i v e weight of female; the linear regression accounts f o r almost 9&f of the t o t a l sum of squares. 0  The Y-intercept of the l i n e i s at 0.62 mg.  A regression l i n e passing  through the origin (broken l i n e i n Figure. 7) f i t s the data almost as well; f o r convenience t h i s second l i n e was chosen as the basis f o r calculations of rations for Experiment 3«  Experiment 3 This experiment was designed to measure the effects of reduced . rations on reproduction. I t was intended that the experiment should l a s t for three brood periods i n order to allow for the p o s s i b i l i t y that brood size i s determined early i n the process of oogenesis.  However,  mortality of experimental f i s h was very high (half the f i s h died i n the f i r s t 28 days), and the experiment was terminated on day 51 with four survivors.  24  0«  200*  400»  600°  WEIGHT OF FEMALE  Figure 7 .  600°  CMGO  Hie r e g r e s s i o n of d a i l y consumption of dry food on wet weight of female. Experiment 2, S o l i d l i n e : best l i n e a r f i t (Y = .6236 + .0287 X). Broken l i n e : best l i n e a r f i t passing through o r i g i n (Y = .02984 X ) .  1000"  25  In the course of the experiment, 11 different females produced 16 broods--eight broods i n each treatment group.  Within each treatment group there i s a significant positive regression of the natural logarithm of number of f r y on maternal length; the hypothesis of zero slope was rejected i n each treatment group (high food:p< . 0 1 ; low food i n Figure 8.  .05>p>.02).  The two regressions are shown  The residual error about the regression l i n e i s  s i g n i f i c a n t l y greater for low food than for high; t h i s violates the assumptions of analysis of covariance, so s t a t i s t i c a l comparison of the two treatments was carried out as recommended by Snedecor ( 1 9 5 6 : 9 7 ) by means of a t-test using 6 rather than 12 degrees of freedom.  Hypotheses  of equal slopes and equal intercepts were both accepted (p>.5)«  In Figure 9 f r y size i s plotted as a function of maternal size for each treatment group.  Two broods from the high food group were  born prematurely; f r y from these broods had large yolk sacs which imposed an appreciable curve on the body, preventing accurate measurement. These two broods were excluded from t h i s part of the analysis.  For  neither treatment i s the slope of the regression l i n e s i g n i f i c a n t l y different from zero. Possible differences i n average size of f r y between treatment were tested by means of a two-level nested analysis of variance (Table I ) .  There were highly significant differences i n  average standard length of f r y among broods within treatments (p<.005), but the difference i n average f r y length between treatments was not significant i n comparison (.25>p>.10).  26  aoo*  T  STANDARD LENGTH DF F E M A L E  Figure 8a.  (MM-)  The regression of the logarithm of numbers of f r y per brood on maternal length. Experiment 3 — high food treatment.  Figure 8b.  The regression of the logarithm of numbers of fry per brood on maternal length. Experiment'3 — low food treatment.  28  —N  n  10«  15°  SO-  E5»  30«  35=  40»  STANDARD LENGTH DF F E M A L E  45»  (MM*)  Figure 9a. The regression of f r y length on maternal length. Experiment 3 — high food treatment. The regression l i n e was calculated on the basis of the individual lengths of 47 f r y . The mean length of each brood + two standard errors has been plotted.  50«  29  7°0.  w  6»a.  >LL  B°SJ-  LL a  5*4..  z  a  < D  B«0.  < CJJ  5»a. 5oa(_ 10«  f~ ±5«  so<  E5«  30 <  45«  40«  STANDARD LENGTH DF FEMALE  (  Figure 9b. The regression of f r y length on maternal length. Experiment 3 — low food treatment. The regression l i n e was calculated on the basis of the individual lengths of 5° f r y . The mean length of each brood + two standard errors have been plotted.  \  50 <  30  TABLE I . Analysis of variance of standard.length of f r y — high food vs. low food  Source of Variation Between treatments  d. f.  SS  MS  1  .879  -879  12  4.912  .409  89  2.890  .0325  102  8.681  F 2.149(1,12) NS  Among broods within treatments Within broods; error Total  12.58(12,89) **  31  Comparison of Experiments 1 and 3 The f a i l u r e of the f i s h of Experiment 3 to demonstrate significant differences i n reproduction under different feeding regimes i s not surprising i n view of the response time-lags predictable on the basis of Turner's (1937) and Stolk's (1951) work on oogenesis and Hester's (1964) experiments on reproduction and n u t r i t i o n . Most of the broods of Experiment 3 were born before the experimental treatment was expected to have taken effect.  The data from t h i s experiment probably result from  conditions of l i f e i n the stock tanks prior to the experiment; the same i s probably true for the f i s h of Experiment 1, although i n t h i s case the experimental feeding regime was th*e same as that of the stock tanks.  The f i s h of Experiment 1 were fed l i v e Tubifex i n excess, both i n the stock tanks and i n the experiment. For two months prior to the beginning of Experiment 3» f i s h i n the stock tanks were fed exclusively on Tetramin given i n a single daily feeding.  I have no information on  the relative n u t r i t i o n a l value of Tubifex compared to Tetramin, but experienced aquarists are generally of the opinion that l i v e food i s better for f i s h than dry preparations.  After introduction to the  aquaria, uneaten Tubifex survived i n d e f i n i t e l y on the bottom, and f i s h fed on them throughout the day.  The dry food, on the other hand,  was taken primarily from the surface or as i t sank through the water column; uneaten flakes lying on the bottom were usually ignored. Thus the Tubifex-fed f i s h had the opportunity to feed at w i l l throughout the day, whereas the dry food group effectively was restricted to less than one hour of feeding each day.  32  Maximum sizes attained by f i s h fed on Tubifex were considerably greater than those of f i s h raised on dry food.  In selecting the experimental  f i s h for Experiment 1, the largest female measured had a standard length of 45«2 mm.;  females larger than 30 mm. were common i n a l l stock tanks.  In setting up Experiment 3» on the other hand, the largest female encountered measured only 27.8 mm.  This i s an indication that growth  and/or survival rates were lower when only dry food was provided.  Densities of f i s h i n the stock tanks were not measured, but they were similar under both feeding regimes. Each 64 1. capacity stock tank contained approximately 100 to 150 adult guppies plus a similar but more variable number of f r y and immatures.  I t appears that the stock tank histories of the f i s h used i n Experiments 1 and 3 are different enough that comparison of the results of the two experiments provides v a l i d information on the central problems of t h i s study.  Unfortunately, densities and feeding regimes  i n stock tanks were not carefully controlled; however, the qualitative history of the f i s h of Experiment 1 was of good nutrition and low intensity of competition, whereas the f i s h of Experiment 3 experienced poorer nutrition and consequently more intense competition i n the months preceding the experiment. On t h i s basis the results of the two  treatment  groups of Experiment 3 have been pooled for comparison with Experiment 1.  In Figure 10 are reproduced the lines of best f i t of the regressions of the natural logarithm of numbers of f r y on maternal  33  > LL LL a  ra z  10-  IS"  so  45"  50'  STANDARD LENGTH OF F E M A L E ( M M O  Figure 10. Comparison of the regression l i n e s of the logarithm of numbers of f r y per.brood on maternal length. Broken l i n e : Experiment 1 (points are shown i n Figure 4) • Solid l i n e : pooled data from Experiment 3 .(poi^s are shown i n Figures 8a and 8b).  34  length f o r the two experimental groups; the points on which the lines are based are shown i n Figures 4 and 8. Of course, only the section of the lines corresponding to females under 28 mm. i s v a l i d f o r comparative purposes, since no females above that size produced broods i n Experiment 3«  The data were compared by analysis of covariance,  and the hypothesis that the two regressions were samples from populations having the same slope was rejected (p=.05).  The smaller females i n  Experiment 3 produced s i g n i f i c a n t l y fewer f r y per brood than did females of the same size i n Experiment 1.  The lines intersect where maternal  length i s 26„9 mm.; i n other words, the largest females of the dry food group produced about the same numbers of f r y per brood as females of corresponding size i n the Tubifex group.  This i s the same sort of  effect that appeared as a non-significant trend i n Experiment 1, when numbers of embryos carried by f i s h of a given size were compared with numbers of f r y actually born and recovered for counting.  Figure 11 shows the regression lines of f r y lengths on maternal lengths for Experiment 1 and the pooled data for Experiment 3 ; the points are shown i n Figures 6 and 9« The two regressions were compared by analysis of covariance, and the difference i n the slopes was found to be highly significant (p=.004).  Again, i n the size range of interest  (maternal length 28 mm.), the smaller females i n the two groups show a substantial difference, while the larger ones perform about the same.  The lines intersect at a maternal length of 2 6 . 6 mm. (almost  i d e n t i c a l to the point of intersection of the lines representing numbers per brood).  35  7«0U-  S  e»a.  >-  (Z LL, LL. a  I ID —™ro  /Cm  LJ  a rz < n  <  I— UJ  6°a10"  "I————+• 15«  SO"  30»  33°  4™ 40"  STANDARD LENGTH DF FEMALE  4tj<  (MMO  Figure 11. Comparison of the regression lines of f r y length on maternal length. Broken l i n e : Experiment 1 (points are shown i n Figure 6). Solid l i n e : pooled data from Experiment 3 (points are shown i n Figures 9a and 9b).  36  and l e n g t h o f f r y ) a r e not s u f f i c i e n t t o d e s c r i b e unambiguously d i f f e r e n c e s i n r e p r o d u c t i v e response o f t h e two It  the  experimental groups.  seems obvious t h a t t h e s m a l l e r females responded d i f f e r e n t l y  under  t h e d i f f e r e n t n u t r i t i o n a l regimes, but i t i s not c l e a r whether t h e t o t a l commitment o f energy t o r e p r o d u c t i o n by t h e s m a l l females i n Experiment 3 a c t u a l l y d e c r e a s e d o r was fewer and l a r g e r f r y p e r b r o o d . it  s i m p l y d i s t r i b u t e d among 3»  By t h e b e g i n n i n g o f Experiment  had become e v i d e n t t h a t t h e t o t a l weight of. each b r o o d s h o u l d be  measured i n a d d i t i o n t o t h e l e n g t h s o f i n d i v i d u a l f r y ; a c c o r d i n g l y , i n Experiment 3 t h e d r y weight o f each b r o o d as a whole was  measured,  as d e s c r i b e d above under M a t e r i a l s and Methods.  I n o r d e r t o o b t a i n an e s t i m a t e o f t h e d r y weights o f broods 1,  produced i n Experiment  t h e f o l l o w i n g t e c h n i q u e was  used.  Assuming  t h a t d r y weight o f an i n d i v i d u a l f r y i s p r o p o r t i o n a l t o i t s l e n g t h r a i s e d t o some exponent, D  Vood  =  a  ^  +  S L X  i t follows that  )  ( 1 )  where —brood (ISL )  1  3  t  h  e  w e  i&ht  i s t h e sum  o f an e n t i r e b r o o d ,  o f t h e s t a n d a r d l e n g t h s o f t h e fry-  making up t h e b r o o d , i n d i v i d u a l l y r a i s e d " t o exponent x,  an  and  a and b are t h e c o n s t a n t s o f a l i n e a r r e g r e s s i o n e q u a t i o n . E q u a t i o n ( l ) was  f i t to  o f x r a n g i n g from 0 . 1 m i n i m a l when x was  t h e d a t a o f Experiment 3 u s i n g t r i a l v a l u e s  to 10.0.  4.8;  The r e s i d u a l sum  o f squares  f o r t h i s v a l u e o f t h e exponent,  was  the equation accounted  f o r almost 97$ o f t h e observed v a r i a t i o n i n d r y weight o f broods ( t h e f i t t h e p o i n t s i s shown i n F i g u r e 1 2 ) .  This equation  to  F i g u r e 12.  The r e g r e s s i o n o f t h e d r y w e i g h t s o f broods o f Experiment 3 on 2 _ ( S L * +  )•  See t e x t f o r e x p l a n a t i o n .  38  was then used to calculate estimate of the dry weight of broods produced i n Experiment 1 from the data on standard length of f r y i n those broods.  In Figure 13 are plotted the natural logarithms of brood dry weights as a function of maternal length f o r the two experimental groups:  the positive slopes are obviously significant ( p < . 0 0 1 i n both  cases). When the two regressions are compared by analysis of covariance, the tendency f o r the smaller females of Experiment 3 to produce.lighter broods than those of Experiment 1 i s found not s t a t i s t i c a l l y significant; the hypothesis that the regressions have a common slope i s accepted (p=12).  Additional variance may have been introduced into the data  of Experiment 1 by the use of estimates rather than direct measurements of dry weight of broods;  i t i s possible that t h i s added error plus the  small sample sizes have resulted i n the erroneous acceptance of a false n u l l hypothesis that the slopes are equal.  In the same  analysis the hypothesis that, given the regressions have a common slope, the elevation of the lines i s not different, i s also accepted  (p=.42).  Gadgil and Bossert ( 1 9 7 0 ) suggest that reproductive e f f o r t , defined as the fraction of an organism's energy resources devoted to reproduction at a given time, be measured as the ratio of weight of sex products to t o t a l body weight. I have calculated the ratio of dry weight of each brood to the wet weight of the mother just after parturition; t h i s index of reproductive effort should be nearly proportional to Gadgil and Bossert's measure.  39  Figure 13a.  i  The regression of estimated brood dry weight on maternal length. Experiment 1.  40  EOCK  100* —N B  STANDARD LENGTH DF FEMALE CMMO  F i g u r e 13b.  The r e g r e s s i o n o f a c t u a l brood d r y weight on • maternal length. Experiment 3 . — p o o l e d d a t a .  41  In both Experiment 1 and Experiment 3 the index of reproductive effort tended to increase as maternal length increased (Figure 14).  Although there i s a good deal of scatter about the  regression lines shown i n Figure 14, the positive slope of the l i n e i s s i g n i f i c a n t l y different from zero for Experiment 1 (p=.03).  In  Experiment 3 the increase of reproductive effort with size i s not significant (p=.l8)„  Comparison of the two sets of data by analysis  of covariance showed that the two regressions were indistinguishable with respect to either slope (p=.99) or elevation (p=.31) of the lines of best f i t .  In Experiment 3 five different females produced two broods each. 2.95;  The mean time between broods was 2 9 . 8 days (standard deviation: range:  25-32 days).  Although sample sizes are small, a  comparison of these results with those of Experiment 1 (mean gestation period= 2 4 . 8 days) leads to the conclusion that the mean time between broods was s i g n i f i c a n t l y smaller i n Experiment 1 than i n Experiment 3 (t= 2 . 8 6 ,  10 d.f., p < . 0 2 ) .  Following i s a b r i e f summary of the conclusions from the comparison of the results of Experiment 1 with those of Experiment 3« Under conditions of r e l a t i v e l y poor nutrition and, presumably, more intense competition, time between broods increases f o r a l l females of the sizes studied.  The smaller females i n an aquarium respond to  deteriorating n u t r i t i o n a l conditions by producing fewer but larger f r y i n each brood, while larger females are essentially unaffected.  As  42  STANDARD LENGTH DF F E M A L E ( M M - )  Figure 1 4 a . The regression of index of reproductive effort on maternal length. Experiment 1,  0»OHJL  Figure 14b.  The regression of index of reproductive effort on maternal length. Experiment 3 — pooled data.  accurately as can be determined from the present study, females of the same size produced about the same t o t a l weight of f r y per brood under both n u t r i t i o n a l regimes, but there i s a suggestion that smaller females produced lighter broods under the poorer feeding regime.  45  DISCUSSION  Svardson (1949) noted that, a l l other things being equal, natural selection should favor genotypes producing higher numbers of eggs, and so average fecundity within a population should increase with every generation. Since natural populations do not show t h i s steady increase i n fecundity over.time, he reasoned that there must exist some selective pressure towards decreasing egg number and that observed fecundities must represent a compromise between opposing forces. Svardson suggested four selective forces which might operate to l i m i t fecundity i n f i s h ; h i s discussion i s summarized i n the following paragraphs.  F i r s t , a f i s h must dedicate some energy to growth and maintenance i n addition to reproduction.  Increased energy devoted to growth and  maintenance processes should increase the probability that an individual w i l l survive u n t i l the next reproductive season. Also, since f i s h fecundity often increases with body size, energy put into growth during one time period may result i n increased fecundity i n subsequent spawnings. Thus i n an evolutionary sense the advantages of increased fecundity at a given time may be counterbalanced by the future reproductive advantages l i k e l y to result from alternative uses.of available energy.  Second, there may be a physiological strain involved i n producing increasing numbers of eggs. This strain may d i r e c t l y reduce an individual's chance of surviving u n t i l the next reproductive season.  46  T h i r d , f o r species which provide some form of p a r e n t a l care f o r t h e i r eggs and young, the l i m i t s on fecundity may be set not by the number of eggs which a female can produce but by the number of eggs or young which can be s u c c e s s f u l l y cared f o r .  The argument here f o l l o w s  that of Lack (1948) f o r b i r d s .  F i n a l l y , i f there i s a premium on large s i z e of i n d i v i d u a l eggs, some compromise must be made between egg s i z e and egg number, s i n c e , f o r a given input of energy t o reproduction, any increase i n energy per egg must reduce the t o t a l number of eggs produced.  Svardson c i t e d  evidence from several sources t o show t h a t l a r g e eggs produce l a r g e l a r v a e , which have an advantage over smaller i n d i v i d u a l s i n i n t r a s p e c i f i c competition.  Svardson*s paper generates the hypothesis t h a t f i s h  populations whose larvae face intense i n t r a s p e c i f i c competition w i l l produce fewer and l a r g e r eggs than populations of the same species f o r which i n t r a s p e c i f i c competition i s a l e s s important source of l a r v a l m o r t a l i t y ; i n these l a t t e r p o p u l a t i o n s predation and unfavorable p h y s i c a l f a c t o r s , which are assumed t o act more or l e s s independently of l a r v a l s i z e , would be the important causes of l a r v a l deaths.  Reproductive Strategy Ideas s i m i l a r t o Svardson's have been developed i n more general and d e t a i l e d form i n the recent l i t e r a t u r e on r - and K- s e l e c t i o n . MacArthur and Wilson  (1967: 145-180) coined  the terms i n d i s c u s s i n g  optimal reproductive s t r a t e g i e s i n uncrowded and crowded environments. They argued that i n an uncrowded environment w i t h abundant resources, those genotypes w i l l be favored which can u t i l i z e the most resources  47  and produce the largest numbers of progeny per unit time,, maximizing r , 1 /  the i n t r i n s i c rate of population increase of the l o g i s t i c equation. —> Such individuals w i l l be expected to predominate, for example, i n a situation where climate i s seasonally severe and a few survivors of the unfavorable season each year recolonize under conditions of abundant supply of resources.  On the other hand i n crowded environments with a  relatively uniform and favorable climate, genotypes which can replace themselves at the lowest density of available resources (maximizing K, carrying capacity i n l o g i s t i c terminology) w i l l be favored; these genotypes are termed K-strategists. Pianka (1972) has summarized the development of the concepts of r- and K~ selection.  He states that r - strategies w i l l evolve i n  populations which are usually maintained at low density relative to carrying capacity by a high rate of density-independent mortality,, In such a situation resources w i l l be abundant relative to the population's demand, and competition w i l l be.light, so each individual w i l l have a high t o t a l income of energy. A high proportion of t h i s energy income w i l l be channeled into reproduction since alternative demands w i l l be low; there i s l i t t l e benefit to be derived from energy devoted to gaining competitive advantage over conspecifics, and the high, often catastrophic, mortality makes i t a better bet f o r an individual to  l/ The l o g i s t i c equation defines a sigmoid curve of population growth over time. I t s d i f f e r e n t i a l form i s dN „ / K - N s where N = dt ~ K population size i n numbers, r = i n t r i n s i c rate of population increase, and K = maximum attainable value of N , or carrying capacity. The equation was f i r s t derived by Verhulst (1838). r  K  ;  48  maximize present reproduction than to invest energy i n growth, for example, i n order to gamble on reproductive benefits i n an uncertain future.  Since the progeny w i l l also face l i g h t competition but  potentially high density-independent mortality, the optimal strategy for the parent i s to minimize the amount of energy input per offspring so as to maximize the number of individuals produced. The net result i s that r- selected populations w i l l show rapid development, early maturity, high fecundity, and a tendency towards semelparity (a single reproductive period followed by death as i n Onehorhynchus); individual offspring w i l l be of "low quality" - small, without well-developed competitive or anitpredator mechanisms.  Pianka theorizes that high population density, with consequent increased intensity of competition and reduced resource a v a i l a b i l i t y , produces K- selection.  Individual energy income i s low, and a smaller  proportion (relative to the r - selected case) i s available for reproduction since more energy must be diverted to competitive ends. I f the offspring w i l l also encounter intense competition, more energy input per offspring i s favored, and numbers per parent must be reduced.  The  reproductive consequences of K- selection should include slower development, greater competitive a b i l i t y , lower resource requirements, delayed maturity, low fecundity, iteroparity (more than one reproductive attempt i n a l i f e t i m e ) , and high quality offspring.  It i s important to emphasize that natural selection always favors those genotypes which maximize m, the b i r t h rate minus the death  49  rate.  The difference between r - s t r a t e g i s t s and K- s t r a t e g i s t s l i e s i n  the response of t h i s parameter to changes i n population density, or more p r e c i s e l y , a v a i l a b i l i t y of resources.  Figure 15,  modified s l i g h t l y from  Gadgil and Bossert (1970: F i g . l ) , i l l u s t r a t e s the point.  When population  density i s low and resources are abundant, the r - s t r a t e g i s t w i l l have a higher m as a r e s u l t of i t s higher b i r t h rate, since mortality under these conditions i s presumably p r i m a r i l y density independent and a f f e c t s r - s t r a t e g i s t s and K- s t r a t e g i s t s approximately  equally.  However, the  high b i r t h rate of _r- s t r a t e g i s t s i s achieved at the cost of " q u a l i t y " of i n d i v i d u a l s ; therefore, as population density r i s e s and  competition  becomes more intense, the death rate of r - s t r a t e g i s t s r i s e s r a p i d l y , and m i s reduced accordingly.  K- s t r a t e g i s t s have low b i r t h rates as a  consequence of dedicating a s i g n i f i c a n t proportion of t h e i r energy budget to non-reproductive  a c t i v i t i e s , which act i n one way  or another to keep  death rates r e l a t i v e l y low even under conditions of high population density and low resource a v a i l a b i l i t y .  Thus m f o r a K- s t r a t e g i s t f a l l s  r e l a t i v e l y slowly with increasing population density. density (marked A i n Figure 15)  Above some  the output of surviving o f f s p r i n g i s  higher f o r the K- s t r a t e g i s t than f o r the r - s t r a t e g i s t .  I f population  density (or resource a v a i l a b i l i t y ) i s c o n s i s t e n t l y on the l e f t side of A, then r - s t r a t e g i s t s w i l l predominate i n the population; Kw i l l be favored to the right of A.  strategists  I f conditions f l u c t u a t e to e i t h e r  side of A, one might predict that the population would be polymorphic f o r reproductive strategy or that some intermediate strategy would evolve. A t h i r d a l t e r n a t i v e would be the evolution of the a b i l i t y to switch strategies according to conditions; t h i s would seem p a r t i c u l a r l y l i k e l y  50  Population Density —> <= Resource Availability :  Figure 1 5 . The relationship between the instantaneous rate of population growth, m, and population density f o r r strategists and K- strategists. See text f o r explanation. From Gadgil and Bossert ( 1 9 7 0 ) .  51  i f the fluctuation showed a regular periodicity.  Pianka (1970) and Gadgil and Solbrig (1972) stress that i d e a l r and K- strategies represent the poles of a continuum.  The l a t t e r workers  state that the position of a population on t h i s continuum relative t o other closely related populations can be quantified according to the proportion of t o t a l energy income dedicated d i r e c t l y to reproduction. They suggest that one measure of t h i s proportion i s the t o t a l weight of offspring produced relative t o weight of the parent.  Gadgil and Solbrig  emphasize that increased reproductive output does not by i t s e l f constitute evidence for r - strategy since increased a v a i l a b i l i t y of resources t o a population, occasioned perhaps by a sudden catastrophic mortality, results i n increased energy income for the survivors, which i n turn can cause increased reproductive output without any change i n the proportion of t o t a l energy used for reproduction; they term t h i s the population dynamic effect.  The l i t e r a t u r e on "r- and K- selection so far discussed represents one approach to the problem of the evolution of l i f e history characteristics. I find i t an i n t u i t i v e l y appealing introduction, but as so f a r stated, i t needs some q u a l i f i c a t i o n .  I t i s oversimple to think that a l l reproductive  strategies can be placed somewhere along a one-dimensional continuum. This concept implies that l i f e history characteristics such as age at f i r s t maturity, growth rate, and fecundity are so highly inter-dependent that they w i l l always vary together i n a predictable way i n response to population density, resource a v a i l a b i l i t y , and mortality schedules.  52  I n f a c t , t h e r e l a t i o n s h i p s a r e so complex t h a t we s h o u l d n o t be s u r p r i s e d t o d i s c o v e r many p o p u l a t i o n s "K-selected" t r a i t s .  d i s p l a y i n g m i x t u r e s o f " r - s e l e c t e d " and  T i n k l e and W i l b u r  (1973)  review s e v e r a l  instances  o f o c c u r r e n c e o f such "mixed s t r a t e g i e s " - e.g. e x t r e m e l y h i g h e f f o r t expended by p o p u l a t i o n s  reproductive  whose a d u l t d e n s i t y i s q u i t e s t a b l e and  presumably n e a r c a r r y i n g c a p a c i t y .  Such d e p a r t u r e s  from what would be expected on t h e b a s i s o f r -  and K- s e l e c t i o n arguments c a n b e s t be u n d e r s t o o d i f we c o n s i d e r t h e basic objective of reproductive  strategy.  By d e f i n i t i o n n a t u r a l s e l e c t i o n  f a v o r s t h o s e genotypes which make t h e g r e a t e s t c o n t r i b u t i o n t o t h e gene p o o l o f succeeding reproductive  generations;  i tfollows that the objective of  s t r a t e g y , as o f any o t h e r e v o l v i n g system, must be t o  maximize t h i s c o n t r i b u t i o n , o r f i t n e s s .  Schaffer b. + p.•  3.  (1972)  has shown t h a t t h e q u a n t i t y t o be maximized i s :  +1  where b ^ i s t h e number o f progeny produced by a female a t age i which s u r v i v e t o r e p r o d u c e , and p_. i s t h e p r o b a b i l i t y t h a t t h e female v  s u r v i v e from age i t o age i +  reproductive device  value  1,  +  1  and — — — —  i s Fisher's  ^0  f o r a female age i + 1.  f o r expressing  (1930)  This l a s t quantity i s a  a l l expected f u t u r e b i r t h s t o an i n d i v i d u a l i n  terms of. t h e i r e q u i v a l e n t v a l u e analogous t o t h a t o f p r e s e n t economics.  i  i n present  discounted  b i r t h s ; t h e concept i s  value  o f f u t u r e income i n  S c h a f f e r ' s formulation i s d e c e p t i v e l y simple;  can be s u b d i v i d e d  will  each term  i n t o s e v e r a l components, many o f which a r e i n t e r r e l a t e d .  53  The term b ^ i s t h e a b s o l u t e i  f e c u n d i t y o f an a d u l t female a t  m u l t i p l i e d by t h e p r o p o r t i o n o f t h o s e  reproducing  adults.  Fecundity  age  eggs which s u r v i v e t o become  i s an i n c r e a s i n g f u n c t i o n o f  reproductive  e f f o r t , t h e p r o p o r t i o n o f t h e organism's t o t a l energy income d e v o t e d to reproduction.  Fecundity  f o r a given reproductive  effort i s also  l i k e l y t o be an i n c r e a s i n g f u n c t i o n o f p a r e n t a l s i z e i n organisms l i k e fish", which c o n t i n u e t o grow a f t e r r e a c h i n g f e c u n d i t y a t age i n previous reproductive  i can be  a decreasing  sexual maturity.  function of reproductive  s u r v i v a l of o f f s p r i n g i s probably  often a f u n c t i o n of density,  i n some c a s e s be  a f u n c t i o n o f egg  of reproductive  level  i s t h e p r o b a b i l i t y o f a d u l t s u r v i v a l from t h e  reproductive  f u n c t i o n of present  season t o t h e n e x t . reproductive  i s r e l a t e d t o s i z e , i t may seasons.  effort.  a l s o be  I t i s l i k e l y t o be a  decreasing  To t h e e x t e n t t h a t a d u l t  a function of reproductive  survival  effort i n  A d d i t i o n a l l y , i t i s l i k e l y t o be a g e - s p e c i f i c .  f i n a l term i s F i s h e r ' s r e p r o d u c t i v e v a l u e .  adult t h i s i s the reproductive  size,  effort.  The term  The  survival  or l a r v a l  which i n t u r n i s n e g a t i v e l y r e l a t e d t o f e c u n d i t y a t any g i v e n  previous  Pre-  a v a i l a b i l i t y , p h y s i c a l e n v i r o n m e n t a l f a c t o r s ) ; however,  o f young stages may  present  effort  seasons t o t h e e x t e n t t h a t growth has been r e d u c e d .  f a c t o r s beyond t h e c o n t r o l o f t h e a d u l t ( e . g . p o p u l a t i o n resource  Thus,  expected p r o d u c t i o n  F o r an  individual  of s u r v i v i n g o f f s p r i n g i n a l l f u t u r e  seasons, reduced by t h e p r o b a b i l i t y o f t h e a d u l t ' s s u r v i v i n g  t o each season.  A l l of t h i s i s f u r t h e r discounted  according to  the  54 p o p u l a t i o n growth r a t e .  I f t h e p o p u l a t i o n i s i n c r e a s i n g , t h e n one  surviving o f f s p r i n g i n the present  represents a higher proportion o f the  p o p u l a t i o n t h a n one produced i n t h e f u t u r e and i s e v o l u t i o n a r i l y worth more; t h i s i s s i m i l a r t o t h e s t a n d a r d  economic model w h e r e i n money which  can b e g i n e a r n i n g i n t e r e s t now i s worth more than, t h e same amount t o be received i n the future.  However, i f t h e p o p u l a t i o n i s d e c l i n i n g , i t i s  as i f t h e r e were a n e g a t i v e i n t e r e s t r a t e , and f u t u r e r e p r o d u c t i o n i s worth more t h a n  present.  I n summary, a r e p r o d u c t i v e s t r a t e g y i s a c o m b i n a t i o n h i s t o r y c h a r a c t e r i s t i c s manifested  of l i f e  by an organism which may be thought o f  as r e p r e s e n t i n g a s e t o f t a c t i c a l c h o i c e s by t h e organism w i t h r e s p e c t t o several interrelated variables.  One group o f t h e s e c h o i c e s has t o do w i t h  t h e magnitude and d i s t r i b u t i o n o f r e p r o d u c t i v e e f f o r t o f t h e i n d i v i d u a l over i t s l i f e t i m e . m a t u r i t y and onset lifetime and  V a r i a b l e s i n c l u d e d i n t h i s group a r e ages o f f i r s t o f senescence, number o f r e p r o d u c t i v e attempts i n t h e  (the choice o f semelparity or i t e r o p a r i t y i s i n c l u d e d here),  energy t r a d e o f f s among r e p r o d u c t i o n and o t h e r energy-consuming  functions.  The magnitude o f t h e energy i n p u t t o each i n d i v i d u a l egg o r  l a r v a i n a p a r t i c u l a r r e p r o d u c t i v e attempt r e p r e s e n t s a separate of  category  c h o i c e ; f o r a g i v e n t o t a l energy i n p u t t h e organism must choose one o f  t h e i n f i n i t e number o f c o m b i n a t i o n s o f a b s o l u t e f e c u n d i t y and energy of  i n d i v i d u a l eggs o r l a r v a e ; i . e . t h e same t o t a l energy c a n be d i s t r i b u t e d  among many s m a l l young o r a few l a r g e ones. choices (that i s , d i f f e r e n t  present  output  D i f f e r e n t combinations o f  s t r a t e g i e s ) w i l l r e s u l t i n d i f f e r e n t numbers o f  young which t h e m s e l v e s s u r v i v e t o b r e e d . of  content  S t r a t e g i e s which o p t i m i z e t h e sum  p l u s t h e p r e s e n t v a l u e o f f u t u r e output  by n a t u r a l s e l e c t i o n i f t h e y a r e h e r i t a b l e .  w i l l be f a v o r e d  55  Species of the Family Poeciliidae have several characteristic l i f e history features which must be considered i n discussing t h e i r reproductive strategies.. The shallow water areas they inhabit are often quite unstable over time, and population levels may fluctuate considerably. Generation time i s t y p i c a l l y short, varying from several weeks to a few months. Each adult female may produce several broods over a r e l a t i v e l y long reproductive season. These features combine to make i t very l i k e l y • that successive generations, or even the same individual at different times, w i l l face quite different sources and levels of mortality. Under such conditions the a b i l i t y to make rapid adjustments of reproductive strategy to immediate and anticipated conditions should be of great selective value.  Aside from the purely descriptive information on guppy reproduction, the most interesting fact which emerges from my data i s that under conditions of poorer nutrition and presumably more intense competition, the smaller females redistributed the same reproductive effort among fewer, larger fry (the same size as f r y of larger females), while the larger females showed no change i n strategy. This result prompts several questions for which I have hypothetical answers, most of which are testable.  What i s the advantage to the smaller females of increased size of fry? When competition among f r y i s intense, larger f r y may have a large enough survival advantage that, i n spite of the reduction i n  56  absolute fecundity, the fitness of the adult i s increased by putting more energy into each individual o f f s p r i n g  0  This i s -essentially the same as  Svardson's (1949) fourth hypothesis (see page 2 3 ) .  Magnuson (1962)  found i n a laboratory study that when competition for food was intense, larger juvenile medaka (Oryzias latipes) successfully dominated smaller ones and showed significantly, faster growth. Bagenal (1969b) i n a f i e l d experiment showed that brown trout (Salmo trutta) f r y hatching from larger eggs had a higher survival rate than those from smaller eggs l i v i n g i n the same enclosures; he suggested that the mechanism producing t h i s effect might have been that the larger f r y were more effective at maintaining t e r r i t o r i e s .  I f the explanation of the f i r s t question i s correct, then why did only the smaller females change strategy?  One hypothesis i s that  reduction i n rations acted as the signal for the change i n strategy and that, as a result of contest-type competition, only the smaller females actually suffered reduced rations i n the experiment.  The d i f f i c u l t y  with t h i s hypothesis i s that i f the change of strategy i s adaptive, and i t s effects are related to d i f f e r e n t i a l survival of f r y of different sizes, then i t i s only the smaller females which are receiving the messag about what conditions w i l l be l i k e for t h e i r f r y . Since f r y of both larg and small females presumably face a similar competitive situation, the fitness of the larger females should be considerably reduced by t h e i r i n a b i l i t y to read the environment and respond with a corresponding increase i n size of f r y . I t i s possible that the difference i n response of large as opposed to small females i s an a r t i f a c t of the  experimental  57  situation, i n which food was localized i n time and space, and where the population was spatially restricted i n an environment offering l i t t l e v i s u a l i s o l a t i o n among competitors: Magnuson  (1962)  found that both of  these conditions increased the intensity of contest competition. Possibly under natural conditions competition i s closer to the scramble type, and a l l sizes of adults are s u f f i c i e n t l y affected by deteriorating conditions to receive the message and make the appropriate switch of strategy.  Why did reproductive effort not decline when the n u t r i t i o n a l regime worsened? A simple hypothesis i s that the l e v e l of reproductive effort i s genetically fixed and cannot vary much except on an evolutionary time scale. Alternatively, i t seems l i k e l y that when adult mortality i s high and/or growth i s slow, as i n Experiment 3» there i s l i t t l e benefit to be gained by postponing reproductive effort u n t i l such time as . probability of survival of f r y might increase.  One of the major deficiencies of the present study i s that I have only scanty information on the ecology of my experimental animals under natural conditions, where the characteristics under investigation evolved and presumably have adaptive value.  I do have two pieces of information  from Ben Seghers (personal communication) which seem relevant.  First,  although the Tacarigua River i s r e l a t i v e l y stable i n comparison with other rivers of the area, i t does undergo regular fluctuations i n flow and related variables as a result of marked seasonal variations i n r a i n f a l l . Second, predation on adult guppies by c i c h l i d s i s probably r e l a t i v e l y  1  58  intense.  I n an attempt t o make up f o r t h e l a c k o f f i e l d d a t a on my own  organisms, I w i l l review problems i n r e l a t e d  i n some d e t a i l f i e l d i n v e s t i g a t i o n s on s i m i l a r  species.  F i e l d S t u d i e s on P e o c i l i i d J.D.  McPhail  Reproduction  (unpublished  data) has made p r e l i m i n a r y f i e l d and  l a b o r a t o r y s t u d i e s o f r e p r o d u c t i o n o f another Neoheterandria  t r i d e n t i g e r , i n Panama; h i s r e s u l t s show some  p a r a l l e l s w i t h mine. small forest season.  small P e o c i l i i d , striking  He worked p r i m a r i l y w i t h p o p u l a t i o n s i n h a b i t i n g  streams which d r i e d up almost c o m p l e t e l y  H i g h wet season p o p u l a t i o n s a r e reduced  streams d r y up; a v e r y  during the dry .  d r a m a t i c a l l y as t h e  small proportion of these populations  survives  t h e d r y p e r i o d i n i s o l a t e d p o o l s i n deeper p a r t s o f t h e stream b e d . When t h e r a i n s come a g a i n , f l a s h f l o o d i n g causes a sudden expansion o f a v a i l a b l e h a b i t a t , which t h e s u r v i v o r s r e c o l o n i z e . t h a t t h e onset  o f t h e wet season a l s o b r i n g s about an i n c r e a s e i n  a v a i l a b l e f o o d , which i n t h e s e  The  streams i s mostly  of t e r r e s t r i a l  changes i n r e p r o d u c t i v e p a t t e r n t h r o u g h t h e y e a r  that Neoheterandria although McPhail's use  I t seems p r o b a b l e  different  populations  suggest  s w i t c h s t r a t e g y a c c o r d i n g t o t h e season,  d a t a do n o t show d i r e c t l y t h a t s i n g l e  s t r a t e g i e s at d i f f e r e n t times.  females i n t h e i s o l a t e d  origin.  individuals  L a t e i n t h e d r y season  streambed p o o l s suspend r e p r o d u c t i o n  entirely.  E a r l y i n the. wet season females from t h e expanding p o p u l a t i o n s produce r e l a t i v e l y l a r g e numbers o f s m a l l f r y a t a p p r o x i m a t e l y  10 day i n t e r v a l s  ( t h i s s p e c i e s shows s u p e r f o e t a t i o n , so t h e g e s t a t i o n p e r i o d f o r an  59  i n d i v i d u a l brood i s about 20 days d u r i n g t h i s s e a s o n ) .  Females t a k e n  from t h e dense p o p u l a t i o n s o f t h e l a t e wet season and e a r l y d r y season produce much s m a l l e r broods o f l a r g e young a t 15 day i n t e r v a l s .  It  l o o k s a s though t h e s e p o p u l a t i o n s a r e u s i n g r - s t r a t e g y when p o p u l a t i o n d e n s i t y i s low and K- s t r a t e g y when t h e a v a i l a b l e h a b i t a t i s r e l a t i v e l y full.  McPhail the  same a r e a .  a l s o examined f e m a l e s from l a r g e r permanent streams i n I n most c o l l e c t i o n s females t e n d e d towards t h e low  f e c u n d i t y / l a r g e f r y end o f t h e spectrum, a s one would expect benign  environment.  i n a stable,  However, i n some l o c a l i t i e s t h e f e m a l e s showed t h e  r - s e l e c t e d p a t t e r n ( h i g h f e c u n d i t y and s m a l l f r y ) ; t h e s e p o p u l a t i o n s a l l l i v e d i n l o c a l i t i e s where f i s h p r e d a t o r s were abundant and a d u l t m o r t a l i t y was p r o b a b l y  high.  Krumholz  (1948)  studied the fecundity of populations of the  P o e c i l i i d Gambusia a f f i n i s i n n o r t h e r n I l l i n o i s and southern where t h e y had been i n t r o d u c e d f o r mosquito c o n t r o l .  Michigan,  These p o p u l a t i o n s ,  at t h e n o r t h e r n l i m i t o f t h e s p e c i e s ' range, s u f f e r e d h i g h w i n t e r m o r t a l i t y f o l l o w e d by r a p i d i n c r e a s e t o h i g h d e n s i t i e s d u r i n g t h e s p r i n g and summer months.  During  one r e p r o d u c t i v e season Krumholz made monthly c o l l e c t i o n s  o f Gambusia from e s t a b l i s h e d p o p u l a t i o n s i n two ponds, one o f which appeared t o have low and t h e o t h e r moderate b i o l o g i c a l p r o d u c t i v i t y . E a r l y i n t h e f o l l o w i n g s p r i n g he i n t r o d u c e d mature v i r g i n f e m a l e s and males t o a v e r y p r o d u c t i v e  stock-watering  pond; he made c o l l e c t i o n s  from t h i s p o p u l a t i o n throughout t h e summer.  From t h e p r e s e r v e d  Krumholz measured l e n g t h f r e q u e n c i e s , r e p r o d u c t i v e  s t a t e o f both  collections sexes,  60  and numbers o f embryos c a r r i e d by g r a v i d f e m a l e s .  U n f o r t u n a t e l y he  made no measurements o f s i z e o f young.  Females o f two d i f f e r e n t l i f e h i s t o r y t y p e s o c c u r r e d i n a l l o f Krumholz' p o p u l a t i o n s .  Females b o r n i n mid t o l a t e summer grew t o  moderate s i z e and o v e r w i n t e r e d a s immatures; t h e s e f e m a l e s began t o reproduce i n e a r l y  s p r i n g and produced t h r e e t o f i v e b r o o d s b e f o r e d y i n g ,  sometimes a f t e r a p e r i o d o f s t e r i l e autumn.  The s p r i n g progeny  s e n i l i t y i n t h e i r second summer o r  o f t h e o v e r - w i n t e r e d f e m a l e s showed r a p i d  growth and matured a t a much s m a l l e r s i z e t h a n d i d t h e i r mothers; produced from one t o t h r e e broods d u r i n g l a t e died.  they  s p r i n g and summer and t h e n  Thus w i t h i n t h e same p o p u l a t i o n , f e m a l e s b o r n i n s p r i n g  distributed  t h e i r r e p r o d u c t i v e e f f o r t a c c o r d i n g t o r - s t r a t e g y , and f e m a l e s b o r n i n summer behaved l i k e K- s t r a t e g i s t s ; c h o i c e o f s t r a t e g y depended n o t on parentage b u t on season o f b i r t h .  In  b o t h l i f e h i s t o r y t y p e s i n a l l ponds f e c u n d i t y a t a g i v e n  l e n g t h d e c r e a s e d w i t h each s u c c e s s i v e brood from a maximum a t t h e f i r s t or  second brood; another way o f s t a t i n g t h e same t h i n g i s t h a t  average  f e c u n d i t y f o r l e n g t h showed a more o r l e s s steady d e c r e a s e w i t h i n a g i v e n pond a s t h e season p r o g r e s s e d .  Comparisons  among t h e t h r e e ponds  showed t h a t f i s h i n t h e newly s t o c k e d , p r o d u c t i v e pond matured a t a s m a l l e r s i z e and produced more broods i n t h e c o u r s e o f t h e season r e l a t i v e t o t h e o t h e r ponds.  F o r comparable  life  h i s t o r y t y p e s and  t i m e s o f y e a r , average f e c u n d i t y a t a g i v e n l e n g t h i n c r e a s e d w i t h • • i n c r e a s i n g pond p r o d u c t i v i t y .  61  Krumholz attributed the decrease i n fecundity over time to physiological changes associated with senescence i n the females. However, his data do not rule out the p o s s i b i l i t y that decreasing fecundity (which may have been accompanied by an increase i n size of individual fry) represented a switch i n reproductive strategy i n response to increasing population density and competition as the season advanced.. I t i s suggestive that the drop i n fecundity with time was most pronounced i n the least f e r t i l e pond and that fecundity i n the newly stocked, productive pond continued at a high l e v e l throughout the summer.  Krumholz (1963) reported the results of a study on Gambusia manni i n two ponds i n the Bahamas. The techniques were similar to those described for the previous study.  In Daniels' Pond there were no aquatic  predators; the pond at Calloo Mangrove was connected to the sea at high tide and contained several predatory marine species, including  juvenile  barracuda. Although he made no direct measurements of population densities, Krumholz f e l t that density was much higher i n Daniel's Pond. The Daniels' Pond population contained a high proportion of large senile individuals, p a r t i c u l a r l y i n late summer and autumn at the end of the reproductive season; the maximum size observed at Calloo Mangrove was much smaller, and there was no evidence of s e n i l i t y .  Krumholz reasoned  that predators eliminated senescent Gambusia at Calloo Mangrove.  Krumholz concluded that growth was faster i n the Daniel's Pond population due to presumed higher b i o l o g i c a l productivity of the pond. As indirect evidence for t h i s hypothesis, he cited his observations that  62  both maximum size and size at f i r s t maturity were higher i n Daniels' Pond. The plots of length frequencies f o r successive collections at Daniels' Pond show a clear advance of modal length frequencies from month to month, from which growth of individual cohorts can be estimated. The modal length frequencies for the Galloo Mangrove population are much less clear within any collection, and they vary e r r a t i c a l l y with time; possibly t h i s i s due to rapid changes i n the age and size composition of the population due to high predation, but very rapid growth could have the same effect.  Since the difference i n predation mortality probably  also.accounts f o r the observed differences i n maximum length between ponds, Krumholz' contention about differences in.growth rates rests on the.fact that the Calloo Mangrove f i s h mature at a smaller size. I t has been shown f o r many species of f i s h that early maturity i s often associated with fast growth; Svardson  (1943)  showed that fast-growing  male guppies matured at an e a r l i e r age and smaller size than slower growers. In sum, on the basis of the limited evidence, i t seems more l i k e l y that growth i s more rapid at Calloo Mangrove.  The two populations showed marked differences i n reproductive strategy.  Calloo Mangrove, females had higher average fecundity throughout  the year i n spite of being considerably smaller on the average than Daniels* Pond f i s h .  In addition, peak reproduction, as measured by average  fecundity and proportion of females which were gravid, started e a r l i e r i n the year and continued longer at Calloo Mangrove.  63  In Figure 16 I have plotted fecundity against size of female for the two populations at three different times, which are representative of collections made during the early, middle, and late parts of the reproductive season. In a l l Calloo Mangrove collections fecundity increased exponentially with maternal length; the regression of fecundity on length for Daniels' Pond has zero slope early and late i n the season, but-at the peak of the reproductive period the slope and height of the regression were quite similar to Calloo Mangrove. My explanation of the data i s that the Calloo Mangrove population follows r - strategy at a l l seasons due to high predation pressure, which keeps population density low, resource a v a i l a b i l i t y high, and competition low.  The explanation  of the Daniels' Pond data i s more complicated and involves some speculation.  I suggest that i n January and July the population was  resource-limited and was following K- strategy; the January population density was probably lower but so was b i o l o g i c a l productivity i n the moderately seasonal Bahamas. In late January and early February a period of extremely low temperature caused heavy mortality i n Daniels* Pond. The reduced population density, which coincided with seasonally increasing productivity of the pond, must have reduced the intensity of competition, and the population appears to have switched towards r_strategy for several months (February through May).  In the absence of  data one can only speculate, that the increased fecundity i n A p r i l was accompanied by a reduction i n average size of f r y and that i n the early and late collections average size of f r y was greater.  64  Daniels' Pond 10  19 J a n .  J  25  Mangrove  23 J a n .  o  20  Gal loo  o »o  I  °9  I  °eo  J  I  May  '  1  I  24 May  « 15 >  •  o  o  E a) 0)  XI  E  I  5 00  J  (  19 J u l y  L  J  I  I  I  I  40  45  18 J u l y  15  o  10  0°,  ° 0. 20  Figure 16.  J  0  25  •°°  0  L  30  ° « 35  40 45 20 25 Total length of female (mm,)  J  30  L  35  Average number of embryos carried by female Gambusia manni collected at two locations i n the Bahamas at different seasons. Each point i s an average of several individuals. Data from Krumholz ( 1 9 6 3 : Tables 15 and 1 7 ) .  65  Other L a b o r a t o r y S t u d i e s o f F e c u n d i t y and  Nutrition  H e s t e r (1964) i n v e s t i g a t e d t h e e f f e c t s o f reduced r a t i o n s f e c u n d i t y o f a domestic, s t r a i n o f P o e c i l i a r e t i c u l a t a , .  on  His r e s u l t s  i n d i c a t e d t h a t low r a t i o n s d u r i n g one brood p e r i o d s i g n i f i c a n t l y t h e numbers o f f r y i n t h a t brood and t h e one f o l l o w i n g .  reduced  He observed  no  t r e n d i n s i z e o f f r y w i t h f e e d i n g l e v e l o r m a t e r n a l s i z e , and t h e r e were no changes i n l e n g t h o f t h e g e s t a t i o n p e r i o d , by H e s t e r was  The  stock of guppies  somewhat l e s s f e c u n d f o r l e n g t h t h a n my  f r y he measured averaged much smaller' t h a n t h o s e i n my  s t o c k , and t h e study.  There are s e v e r a l d i s c r e p a n c i e s between H e s t e r ' s r e s u l t s those of the present study. l a r g e and  s m a l l females  H e s t e r found t h a t reduced  rations  and  affected  s i m i l a r l y ; t h i s i s p r o b a b l y due t o t h e f a c t  h i s e x p e r i m e n t a l f i s h were i s o l a t e d i n i n d i v i d u a l a q u a r i a f o r t h e experiment. whereas i n my  Thus t h e r e was  no c o m p e t i t i o n f o r f o o d i n H e s t e r ' s  s t o c k t a n k s l a r g e r i n d i v i d u a l s may  used  that  entire  experiments,  have been a b l e t o g a r n e r  a d i s p r o p o r t i o n a t e share o f f o o d at t h e expense o f t h e s m a l l e r f i s h . the l i g h t  o f my  r e s u l t s i t i s s u r p r i s i n g t h a t H e s t e r r e p o r t e d no  between s i z e o f f r y and e i t h e r r a t i o n o r m a t e r n a l appears t h a t he measured o n l y 79 i n l e n g t h t h a n t h o s e o f my  study.  specific  relation  s i z e ; however, i t  f r y and t h a t t h e y were much more v a r i a b l e I t i s possible that with a larger  sample he might have found a r e l a t i o n s h i p between f r y s i z e and r a t i o n or maternal s i z e .  In  either  With r e g a r d t o g e s t a t i o n p e r i o d I have no  e x p l a n a t i o n o f t h e c o n s t a n c y r e p o r t e d by H e s t e r as opposed t o  t h e l e n g t h e n i n g o f the p e r i o d under poor c o n d i t i o n s which I I n g e n e r a l i t s h o u l d be emphasized t h a t t h e two  observed.  s t r a i n s of P e o c i l i a  compared here have p r o b a b l y been under q u i t e d i f f e r e n t  selective  regimes  66  for some time, and i t would be unrealistic to expect them to respond i n the same way even to i d e n t i c a l situations.  Bagenal (1969a) reported on laboratory experiments on the relationship between feeding l e v e l and fecundity i n Salmo t r u t t a . In the f i r s t experiment f i s h were kept i n two large tanks and fed on commercial dry-food - one tank at f u l l rations and the other at half that l e v e l . At the end of eight months fecundities of the two groups of ripening f i s h . were compared. Fish from the high food treatment had grown faster during the experiment, but when fecundities were adjusted f o r length there was no difference between the treatments. The slope of the regression of fecundity on length was significantly steeper i n the low food group; the larger low food individuals were about as fecund as individuals of the same size i n the high food treatment, but the smaller low food f i s h had very few eggs f o r t h e i r size.  This i s essentially the same situation  that I observed i n comparing my Experiments 1 and 3« Bagenal hypothesized that i n the low food treatment the large individuals were securing a disproportionate share of the food; he set up a second set of experiments to explore t h i s idea.  In h i s second experiments Bagenal set up four separate treatment groups of f i s h ; two of these were fed at f u l l and the other two at onet h i r d rations.  After five months of t h i s regime growth depensation was  evident i n a l l tanks; to reduce the size range i n each tank, the f i s h i n the two tanks within each food l e v e l were sorted by size and reassigned to tanks so that each food l e v e l after the sorting consisted of one tank of larger f i s h and one of smaller ones.  After four more months  67  the f i s h from the high food tanks were more fecund f o r t h e i r length than the low food f i s h , but t h i s time there was no difference between treatments i n the slope of fecundity on length.  In both sets of experiments Bagenal found that well-fed f i s h produced smaller eggs by dry weight than the food-deprived ones.  Scott  (1962) i n a similar study on Salmo gairdneri found no difference i n egg size by wet weight between food levels.  Bagenal (1969a) pointed out  that the water content of eggs increases as they mature, and that therefore the appropriate measure of egg size for comparative purposes i s dry weight.  Scott's well-fed f i s h were i n a more advanced state of  maturity than the starved ones; consequently, the s i m i l a r i t y i n wet weight between treatments suggests that the low-food f i s h had larger eggs i n terms of dry weight.  Studies on Exploited Marine Populations Several f i e l d studies have documented year-to—year changes i n fecundity within populations: Bagenal (1963b, 1965) for three species of f l a t f i s h i n the Clyde Sea, Hodder (1963) for haddock (Melanogrammus aeglefinus) of the Grand Banks, and Raitt (1968) for Norway pout (Trisopterus esmarkii) i n the North Sea.  In a l l the above cases  fecundity increases were apparently associated with either lowered population density or increases i n available food per individual.  In  the absence of data on egg size or some other measure of quality i t seems most probable that these cases represent Gadgil and Solbrig's (1972) population dynamic effect rather than any change i n reproductive  68  strategy ( i . e . change i n magnitude or distribution of reproductive effort).  Bagenal (1966) gives a comprehensive review of his own and others' f i e l d studies on the fecundity of plaice (Pleuronectes platessa) throughout i t s range i n the eastern North Atlantic and adjacent seas. Populations i n different l o c a l i t i e s vary considerably i n age composition and growth rate; since fecundity of plaice increases exponentially with length, different populations are compared i n terms of the expected fecundity of a 37 cm. f i s h (F^y) as calculated from the equation for t  the regression of fecundity on length for*a given area. (84,000  F^y i s lowest  eggs) i n the Southern Bight of the North Sea; fecundity increases  f a i r l y evenly with distance i n a l l directions from t h i s area.  Thus  F^  i s high i n the Bay of Biscay ( 1 8 4 , 0 0 0 ) , the west coast of Ireland ( 1 5 3 , 0 0 0 ) , and northern Norway ( 1 5 6 , 0 0 0 ) . Iceland and the Barents' Sea, F^ respectively).  In two l o c a l i t i e s s t i l l farther away, drops again ( 1 0 6 , 0 0 0 and 107,000  Fish from inside Trondheim Fjord i n Norway and from the  Baltic are much more fecund than those from other l o c a l i t i e s and do not f i t the general pattern described above; i n both cases, however, there i s evidence from marking experiments and meristic studies to suggest that the populations involved are r a c i a l l y d i s t i n c t from the other populations considered.  Bagenal (1966) postulates that over most of the area of plaice distribution, with the exception of Trondheim Fjord and the B a l t i c , there exists a single basic fecundity type and that differences i n fecundity  69  are caused by l o c a l differences i n food supply.  In support of t h i s  hypothesis he notes that populations with high fecundity also tend to have the following characteristics: 1)  low population density,  2)  fast growth, and  3)  high gutted weight at a given lengths  I t i s consistent with the above analysis to say that most populations of plaice follow the same reproductive strategy and that variations i n fecundity are due to the population dynamic effect.  In addition to his basic point about food supply, Bagenal (1966) notes that fecundity i s higher i n areas where the prevailing currents are l i k e l y to carry the planktonic plaice eggs and larvae f a r from suitable nursery grounds, and conversely, that low fecundity i s associated with favorable currents.  Bagenal points out that the survival  value of high fecundity i s obvious where mortality of young stages i s l i k e l y to be high; however, the selective mechanism favoring lower fecundity under apparently better conditions i s not immediately clear, since even under these circumstances more fecund individuals should on average make a larger contribution to the next generation.  This i s the  same problem which Svardson confronted i n 1949»  Bagenal considers Svardson's (1949) hypothesis that reduced fecundity can result from selection for larger eggs, which produce larvae, of greater competitive quality. Unfortunately, Bagenal's plaice samples were taken at different times of year i n different l o c a l i t i e s , and therefore f i s h from different areas were at different stages of maturity.  70  Because o f t h i s , d i r e c t comparison o f egg possible.  s i z e s among p o p u l a t i o n s was  He approached t h e q u e s t i o n i n d i r e c t l y by comparing  p o p u l a t i o n s on t h e b a s i s o f d e n s i t y o f o v e n - d r i e d  hot  two  eggs, which does not  v a r y much w i t h i n a p o p u l a t i o n o v e r a c o n s i d e r a b l e range o f d e v e l o p m e n t a l stages.  He  found t h a t eggs from t h e more f e c u n d p o p u l a t i o n were a l s o  more dense; s i n c e a l l p l a i c e eggs a r e p o s i t i v e l y buoyant, he t h a t t h e denser eggs c o n t a i n e d a h i g h e r p r o p o r t i o n o f f a t . c o n s i d e r e d t h a t t o be it  seems t o me  absolute  evidence  Bagenal  a g a i n s t Svardson's h y p o t h e s i s ; however,  t h a t Bagenal's p o i n t i s i r r e l e v a n t t o the q u e s t i o n of  s i z e o r energy c o n t e n t  o f an i n d i v i d u a l egg  hypothesis i s s t i l l v i a b l e i n t h i s  There i s another  and t h a t Svardson's  case.  p o s s i b l e explanation of the v a r i a t i o n i n p l a i c e  f e c u n d i t y r e p o r t e d by B a g e n a l . have noted,  reasoned  As Murphy  (1968)  (1972)  and S c h a f f e r  i f l a r v a l m o r t a l i t y v a r i e s a p p r e c i a b l y from y e a r t o y e a r ,  t h e f i t n e s s o f an a d u l t i s i n c r e a s e d i f r e p r o d u c t i v e e f f o r t can be over more y e a r s , even a t t h e c o s t o f reduced Data on a n n u a l v a r i a t i o n i n l a r v a l of the p l a i c e populations crude estimate age c o m p o s i t i o n  fecundity i n a given  year.  s u r v i v a l a r e not a v a i l a b l e f o r most  s t u d i e d ; however, i t i s p o s s i b l e t o make a  o f r e p r o d u c t i v e span i n d i f f e r e n t p o p u l a t i o n s from t h e of d i f f e r e n t  stocks.  I n T a b l e I I a r e assembled  o f r e p r o d u c t i v e l i f e s p a n f o r most o f t h e l o c a l i t i e s s t u d i e d by  (1966),  spread  t o g e t h e r w i t h t h e i r r e s p e c t i v e F^  from h i s T a b l e  2.  estimates Bagenal There i s  c o n s i d e r a b l e u n c e r t a i n t y about t h e v a l u e s f o r r e p r o d u c t i v e span, due s m a l l sample s i z e s i n many c a s e s as w e l l as t o t h e f a c t t h a t  to  different  p o p u l a t i o n s a r e s u b j e c t t o d i f f e r e n t l e v e l s o f f i s h i n g m o r t a l i t y , which  Table I I .  The c a l c u l a t e d f e c u n d i t y o f female p l a i c e 37 cm. i n l e n g t h (F^y) and e s t i m a t e s o f reproductive l i f e  span a t d i f f e r e n t l o c a l i t i e s .  Data on  F^r, from Bagenal (1966).  Other d a t a from sources i n d i c a t e d .  F Locality  37 (thousands)  Age range  Reproductive span (years)  107  V-XXII  18  S. B i g h t , N. Sea  84  I-XVII  Flamborough Gd., N. Sea  96  C l y d e Sea Sea  B a r e n t s Sea  Sample size  Source of data  (1953)  7  Wimpenny  .17  223  Simpson  (1959)  II-XV  14  33  Simpson  (1959)  137  III-X  8  55  Bagenal  (1958)  159  II-XI  10  156  Bagenal  (1963c)  127  n-xra  16  84  Bagenal  (1960a)  Plymouth  137  III-X  8  60  Bagenal  (1960a)  Wexford, S. .Ireland  139  II-IV  3  30  Bagenal  (1960b)  S c h u l l , S. I r e l a n d  150  II-IV  3  9  Bagenal  (1960b)  D i n g l e , W.  Ireland  153  II-VTII  7  47  Bagenal  (1960b)  Galway, W.  Ireland  146  II-V  4  20  Bagenal  (1960b)  132  II-VII  6  31  Bagenal  (1960b)  Clyde Rye  Bay  K e l l y b e g s , W.  Ireland  •  Table I I .  The c a l c u l a t e d f e c u n d i t y o f female p l a i c e 37 cm. i n l e n g t h (^7) reproductive l i f e  span a t d i f f e r e n t l o c a l i t i e s .  Other d a t a from sources i n d i c a t e d ,  Locality  F  37  (thousands)  Data on  estimates of  F^r, from Bagenal (1966).  (continued)  Age range  Sample size  Source of data  9  12  Bagenal  (1962)  Reproductive span ( y e a r s )  A r e n d a l , S. Norway  HI  III-XI  Bergen, W. Norway  134  IV-XV  12  24  Bagenal  (1962)  Trondheim Coast  150  IV-IX  6  11  Bagenal  (1962)  Troms<z(, N. Norway  156  iv-xra  14  37  Bagenal  (1962)  Bay o f B i s c a y  184  II-VII  6  47  Bagenal  (1963a)  Faxa Bay, I c e l a n d  106  III-X  8  many  Taning  (1929)  eoo  R E P R O D U C T I V E  F i g u r e 17.  SPAN  (YEARS)  The r e g r e s s i o n o f f e c u n d i t y o f p l a i c e from d i f f e r e n t a r e a s on r e p r o d u c t i v e l i f e span. The d a t a a r e shown i n Table I I . '  74  reduces the age range of the exploited stock. • My estimate of reproductive span i s simply the difference i n age between the youngest and the oldest observed mature female.  The regression of F^^ on reproductive span  (Figure 17) has a negative slope significantly different from zero (p = .01) and accounts for 34$ of the observed variation i n fecundity among areas.  The herring (Clupea harengus)of the northeast Atlantic and adjacent seas, because of t h e i r long-standing economic importance, are one of the world's most thoroughly studied f i s h groups. Fecundity and reproduction i n general have received considerable attention, due partly to the fact that the reproductive biology of different subgroups varies considerably and the differences have proved useful i n i d e n t i f i c a t i o n of genetically separate stocks.  Parrish and Saville (1965) define three major sub-groups, which themselves can be further subdivided; here, I w i l l term the major subgroups tribes and t h e i r further subdivisions stocks. The Oceanic Tribe (=Atlanto~Scandian) comprises stocks of large, long-lived herring which feed i n the open Atlantic and spawn i n winter and early spring i n r e l a t i v e l y deep water off Norway, Iceland, and the west coast of the B r i t i s h I s l e s . The Shelf Tribe are smaller and shorter-lived than the f i r s t group; they feed i n the North Sea and over the continental shelf to the west of the B r i t i s h I s l e s .  Different stocks of Shelf herring spawn from late  summer to early winter on offshore banks throughout the feeding area; most stocks exhibit pronounced seasonal migration patterns within the .  75  area.  The  N o r t h Sea,  C o a s t a l T r i b e l i v e i n t h e i n s h o r e waters o f t h e B a l t i c where t h e y spawm i n s p r i n g i n v e r y  Table HE o f the t h r e e  i s a summary o f i n f o r m a t i o n  herring t r i b e s ;  and  shallow water.  on t h e  reproductive  several s t r i k i n g trends  are  biology  evident.  s i z e v a r i e s q u i t e r e g u l a r l y from group t o group a c c o r d i n g  Egg  t o month o f  spawning ( F i g u r e 18); t h e r e i s a smooth i n t e r g r a d a t i o n between t h e eggs o f the w i n t e r spawning Oceanic T r i b e t h r o u g h t h e v e r y spawned i n l a t e  s p r i n g and  summer by B a l t i c  small  large  eggs  stocks of the C o a s t a l  Tribe.  I t i s noteworthy t h a t the t r e n d h o l d s between s t o c k s w i t h i n t r i b e s and even w i t h i n s t o c k s : f i r s t - t i m e older f i s h ,  spawners, which produce s m a l l e r eggs t h a n  spawn l a s t i n Norway ( l a t e r i n t h e  southern N o r t h Sea the population  s p r i n g ) , whereas i n t h e  r e c r u i t s t o t h e Downs Stock spawn b e f o r e  ( e a r l i e r i n the w i n t e r ) .  In general,  eggs are l e s s f e c u n d t h a n t h o s e which produce s m a l l e r  B l a x t e r and  Hempel  (1963)  the  stocks l a y i n g large eggs ( F i g u r e  19).  r a i s e d h e r r i n g l a r v a e from d i f f e r e n t  stocks i n the laboratory i n order to quantify  some o f t h e consequences o f  d i f f e r e n t egg  sizes.  with a higher  r a t i o o f yolk, weight t o body weight a t h a t c h i n g .  hatching  r e s t of  They found t h a t l a r g e r eggs produced l a r g e r l a r v a e After  t h e s e l a r g e r l a r v a e grew more r a p i d l y t o a l a r g e r maximum s i z e  t h a n d i d l a r v a e from s m a l l e r l o n g e r without f o o d ; to locate t h e i r f i r s t  eggs.  Larvae from l a r g e eggs a l s o  survived  such l a r v a e i n n a t u r e would have more time i n which food  items.  T a b l e I I I R e p r o d u c t i v e parameters o f A t l a n t i c h e r r i n g t r i b e s . from C u s h i n g (19&7) u n l e s s o t h e r w i s e n o t e d .  -Data  Tribe  Shelf  Shelf  Oceanic  Spawning time  Winter  Autumn  Spring  Fecundity at mean l e n g t h  38  80  51  F e c u n d i t y a t 28 cm. l e n g t h  37-52  (Downs, Dunmore S t o c k s )  75-85  18-23  (Buchan Stock)  (Norwegian Stock)  Egg d i a m e t e r (mm.)  1.4 - 1.5  1.1 -  1.2  1.5 - 1.7  Egg d r y weight, (mg. p e r 100)  35-37  (Downs)  16-18 29.4  (Buchan (Dogger  33-35  Reproductive span ( y e a r s )  9-10  12-14  17-20  Reproductive effort  greatest  intermediate  least  (1000's)  (1000's) 1  1  P a r r i s h and S a v i l l e  (1965)  77  40r  oo o  12  30r Ol  cn  (U  O O  < au cn E 20 j  5 Q  10  J  J  F  L  M  A  M  Jn  Jl  J___J  I  O  I  N  !  D  1  Month  Figure 18. Average dry weight of ripe eggs of various stocks^of Clupea harengus of the northeast Atlantic plotted against month o i spawning. From Hempel and Blaxter (1967).  78  40  Downs w  Q N o rway w - s p  Clyde w - s p 30  Dunmore w - sp  01  o o  a. oi  E  c—^ V.  /Manx s u m - a u t  Q  20  C^^)  ich s u m - a u t  M i n c  Buchan s u m - a u t  10  20  60  40  F  F i g u r e 19.  2 8  80  100  120  in 1 0 0 0 s  F e c u n d i t y - e g g s i z e diagram f o r some w i n t e r - s p r i n g and summerautumn spawning s t o c k s o f C l u p e a harengus. F^g i s t h e expected f e c u n d i t y o f a female 28 cm. i n l e n g t h . P a r r i s h and S a v i l l e (1965).  From  79  Within a stock fecundity increases exponentially with length of the spawning female (Baxter, 1959; many others).  However, egg size does  not vary i n a given population with age or size of the spawner, except that first-time spawners tend to spawn smaller eggs than repeat spawners of the same stock (Hempel and Blaxter, 1967).  The particular mix of fecundity and egg size adopted by a particular stock i s seen by most workers as an adaptation to the ecological situation which the larvae most commonly encounter. High fecundity (and small egg size) i s seen as an adaptive response to high predation rates on the larvae; large egg size (with concomitant lower fecundity) should increase the average number of surviving larvae per adult when low or uncertain food a v a i l a b i l i t y i s the major d i f f i c u l t y facing the young (Hempel, 1965)•  Cushing (1967) supported and extended the above point by examining the reproductive strategies of different herring stocks i n relation to the planktonic production cycle, (timing, amplitude, v a r i a b i l i t y ) i n t h e i r respective nursery areas.  and  Three autumn-spawning  Shelf stocks and three spring-spawning Oceanic stocks a l l time t h e i r reproduction so that the period during which t h e i r larvae must begin to feed coincides with the average time of occurrence of the annual peak of phytoplankton abundance (which i s taken as an indicator of abundance of the l a r v a l copepods eaten by young herring). Three winterspawning Shelf stocks, on the other hand, time t h e i r reproduction so that the larvae must begin feeding i n mid-winter, when plankton abundance  80  i s at i t s annual low point or i s just beginning t o increase towards a spring maximum. Other environmental factors considered by Cushing are summarized i n Table IV; they are abundance of planktonic predators of herring larvae (medusae, ctenophores, chaetognaths) and v a r i a b i l i t y i n timing and amplitude of the production cycle (talien t o be proportional to the depth of water i n the nursery areas).  Two patterns emerge from Tables XII and IV.  F i r s t , the trade-  off between egg size and fecundity seems to be consistent with Hempel's (1965) explanation: high predation favors increased fecundity at the expense of individual egg size, and low or highly variable food availabi l i t y , favors large egg size.  Cushing's(1967) arguments can be extended  to cover the problem of distribution of reproductive effort over the adult lifespan; reproductive effort expended i n a single season — t h e ratio of gonad weight to body weight — decreases with increasing v a r i a b i l i t y of the production cycle. As reproductive effort i n a single season increases, reproductive span (here measured as the difference between the maximum observed age, T  , and age at f i r s t maturity)  decreases. . These trends are further confirmed by consideration of the reproduction of the Baltic stocks of the Coastal Tribe (Parrish and S a v i l l e , 1965).  These stocks spawn i n very shallow water i n spring and  summer, and t h e i r larvae l i v e i n a situation of high food and high predation which i s quite stable from year t o year.  Baltic herring repre-  sent the extreme cases of high fecundity, small eggs, high reproductive effort, and short reproductive span.  TABLE IV.  Environment encountered b y l a r v a l A t l a n t i c h e r r i n g . From C u s h i n g (I967).  Tribe Spawning t i m e  Shelf winter  Shelf autumn  Oceanic spring  Average f o o d availability  low  high  high  Variation i n food a v a i l ability  low  moderate  high  Abundance o f predators  low  high  low  82  Reproductive Strategy Theory and Stock/Recruitment Cushing (1969, 1971? 1973) and Cushing and Harris (1973) have developed a new approach to the problem of the relation between spawning stock size and resultant recruitment.  Using data from 31 different stocks  Cushing plotted the recruitment per unit spawning stock against spawning stock on logarithmic axes.  The slope of t h i s regression he regarded as  an index of density dependence, i . e , more negative values indicate-a greater degree of density dependence. Cushing's index i s related t o the parameter a of Ricker's (1954» 1958) equation; increasing density dependence i n Cushing's terms implies a larger value of a, which means that the peak of the dome of the Ricker curve i s higher and occurs at a lower l e v e l of spawning stock.  The novel aspect of Cushing's work i s  that he finds a linear relationship between his index of density dependence and the cube root of average fecundity of a given population. More fecund populations show more density dependence, have a more pronounced dome to the stock/recruitment curve, and can sustain themselves under heavier fishing pressure.  Cushing's work i s open to c r i t i c i s m on several accounts.  First,  a l l the stocks examined by him are under exploitation at different intens i t i e s , and therefore average stock sizes are d i f f e r e n t i a l l y reduced from t h e i r v i r g i n levels.  I f a given stock actually follows a dome-shaped  stock/recruit curve, Cushing's technique w i l l assign i t a high index of density dependence when stock levels are high and a low index when stock i s low; the calculated index i s different according to whether most of the points l i e on the ascending or descending limb of the stock/recruit  83  curve*  T h i s e f f e c t i s i l l u s t r a t e d i n Cushing and  21B) : two  (1966:  s e r i e s o f d a t a f o r Norwegian h e r r i n g , c o r r e s p o n d i n g  d i f f e r e n t time p e r i o d s and the index  Bridger  Figure  to  stock l e v e l s give very d i f f e r e n t values f o r  o f d e n s i t y dependence.  A second c r i t i c i s m i s t h a t t h e  data  f o r i n d i v i d u a l s t o c k s show a g r e a t d e a l o f v a r i a t i o n around t h e l i n e s best f i t ;  i t i s c l e a r t h a t f o r many s t o c k s t h e r e g r e s s i o n i s not  ificant.  Therefore,  and  A t h i r d d i f f i c u l t y i s t h a t C u s h i n g compares  e c o l o g i c a l l y d i f f e r e n t groups o f f i s h e s on t h e s i n g l e  c r i t e r i o n of absolute  f e c u n d i t y without  v a r i a b l e s l i k e body s i z e and c o n s i d e r s two the  sign-  many o f t h e d e r i v e d i n d i c e s o f d e n s i t y dependence  are of d o u b t f u l v a l u e . taxonomically  of  c o n s i d e r a t i o n of the e f f e c t s  reproductive behavior  on f e c u n d i t y ,  s p e c i e s Of d i f f e r e n t body s i z e whose eggs a r e  same s i z e , t h e  same e x p e n d i t u r e  i f one  approximately  of reproductive e f f o r t w i l l  i n a higher f e c u n d i t y f o r the l a r g e r s p e c i e s .  of  result  Also, i n interspecies  comparisons f e c u n d i t y t e n d s t o d e c r e a s e as t h e amount o f p a r e n t a l c a r e f o r eggs and young i n c r e a s e s .  When Cushing's work i s viewed i n t h e c o n t e x t reproductive o n l y one  of the theory  s t r a t e g y d i s c u s s e d above, i t i s c l e a r t h a t he i s c o n s i d e r i n g  of s e v e r a l v a r i a b l e s of i n t e r e s t .  In order t o p r e d i c t the  o f a f i s h p o p u l a t i o n t o e x p l o i t a t i o n , one would want t o c o n s i d e r s p e c i f i c r e p r o d u c t i v e e f f o r t , r e p r o d u c t i v e l i f e s p a n , age and  egg  of  size, i n addition to fecundity.  at f i r s t  response  agematurity  However, i n t h e absence o f more  comprehensive d a t a , C u s h i n g ' s c o n c l u s i o n i s c o n s i s t e n t w i t h what would p r e d i c t e d from c o n s i d e r a t i o n s o f r e p r o d u c t i v e stocks w i l l  support  s t r a t e g y : t h a t more  a higher r a t e of f i s h i n g m o r t a l i t y .  My  be  fecund  reasoning  84  i s that, a l l other things being equal, high fecundity indicates high reproductive effort, which should be accompanied by high levels of adult mortality, short reproductive span, early maturity, and r e l a t i v e l y stable recruitment.  A population with the above characteristics should be  r e l a t i v e l y insensitive to increased adult mortality due to f i s h i n g ; i n addition, high fecundity and early maturity mean that the population has a high capacity for increase and that under appropriate environmental conditions r e l a t i v e l y small spawning stocks should be able to provide substantial numbers of recruits.  The picture as i t stands i s largely* qualitative and quite incomplete. However, the existing fisheries l i t e r a t u r e probably contains sufficient data for a more quantitative examination of the relations between reproductive strategy and the response of a f i s h population to harvesting. Such an approach might enable us to construct approximate stock and recruitment curves and to estimate sustainable yields on the basis of data collected during just one or a few seasons.  85  CONCLUSIONS  1)  The guppy population used i n the present study, as w e l l as  natural P o e c i l i i d populations investigated by other workers, appears to have evolved the a b i l i t y to modify reproductive strategy i n response to environmental conditions. Increasing intensity of competition for food appears to put a premium on large size of f r y which brings about a decrease i n fecundity, at least i n the smaller females. 2)  Measurement of the relevant reproductive parameters of the  adult members of a natural population i n just one or a few years should give insight into both the shape and the v a r i a b i l i t y of the stock and recruitment relationship for that population. Among these relevant parameters are egg size, age at f i r s t maturity, reproductive lifespan, and reproductive effort at each spawning.  SUMMARY  1)  A central problem of fishery management i s the determination  of the relationship between the abundance of a stock of spawning adults and the resultant recruitment of young f i s h .  I t i s of interest to  determine both the shape of curve relating stock to recruitment and the year to year variation about the curve. 2)  Theoretical stock/recruit curves imply the existence of density  dependent population regulation; recruitment per unit stock i s expected to decline at high stock densities. 3)  A density dependent relationship between absolute fecundity and  86  population density has been demonstrated i n many f i e l d and laboratory studies.  I t has been suggested that competition for food may be the  mechanism producing the relationship. 4)  The objectives of t h i s study are to investigate the effects of  competition for food on reproductive parameters — including size of f r y and time between broods as well as fecundity —  and to relate the findings  to the stock and recruitment problem. 5)  Laboratory experiments were performed on guppies descended from  natural stock collected i n Trinidad, West Indies. Two series of data on reproduction were collected using female guppies selected from large stock tanks and maintained i n i s o l a t i o n .  The f i s h of the f i r s t series had been  raised under conditions of r e l a t i v e l y low intensity of competition f o r food; the second group of f i s h had experienced more intense competition. 6)  In both series there was a significant positive regression of the  logarithm of the number of f r y per brood on maternal length. The slopes of the two regression lines were significantly different; the smaller females i n the high competition group were less fecund than f i s h of the same size i n the low competition treatment, but the larger females produced about the same numbers of f r y i n both treatments. 7)  Under low competition the average size of f r y increased significantly  with maternal length, but when competition was more intense, the f r y produced by the smaller females were larger, and a l l sizes of females produced f r y as large as those of the largest females of the low competition treatment. 8)  In both groups the weight of the entire brood increased  significantly with maternal length; the regressions for the two groups were not different.  Thus the smaller individuals of the high  87  c o m p e t i t i o n group m a i n t a i n e d  t h e t o t a l weight o f t h e i r b r o o d s b u t  changed t h e d i s t r i b u t i o n o f weight among i n d i v i d u a l f r y , p r o d u c i n g s m a l l e r broods o f l a r g e r f r y . 9)  The g e s t a t i o n p e r i o d was s i g n i f i c a n t l y l o n g e r f o r females o f  the high competition 10)  group.  The s e t o f r e p r o d u c t i v e c h a r a c t e r i s t i c s m a n i f e s t e d  b y an i n d i v i d u a l  organism—amount and d i s t r i b u t i o n over t h e organism's l i f e t i m e o f energy devoted t o r e p r o d u c t i o n , and d i s t r i b u t i o n o f energy among many o r few gametes o r progeny i n a s i n g l e r e p r o d u c t i v e a t t e m p t — c a n be viewed as t a c t i c s which have e v o l v e d i n c o n c e r t as p a r t o f a r e p r o d u c t i v e s t r a t e g y whose o b j e c t i v e i s t o maximize t h e i n d i v i d u a l ' s c o n t r i b u t i o n t o succeeding 11)  genetic  generations.  The a b i l i t y t o s w i t c h r e p r o d u c t i v e s t r a t e g y i n r e s p o n s e t o  environmental  c o n d i t i o n s would seem t o be o f p a r t i c u l a r  adaptive  v a l u e t o p o e c i l i i d s , which t y p i c a l l y have s h o r t g e n e r a t i o n t i m e s and life  spans and o f t e n i n h a b i t r a t h e r u n s t a b l e  s t o c k used i n t h e p r e s e n t  environments.  study c l e a r l y p o s s e s s e s  some  f l e x i b i l i t y f o r a d j u s t i n g s t r a t e g y t o t h e environmentj  The guppy  phenotypic i t i s suggested  t h a t t h e r e d u c t i o n i n f e c u n d i t y and i n c r e a s e i n s i z e o f i n d i v i d u a l f r y produced b y s m a l l females under i n t e n s e c o m p e t i t i o n would maximize t h e number o f progeny s u r v i v i n g t o r e p r o d u c e under t h o s e c o n d i t i o n s . Field  s t u d i e s b y o t h e r i n v e s t i g a t o r s on r e l a t e d s p e c i e s i n t h r e e  areas p r o v i d e some evidence 12)  other  of s i m i l a r switching of strategy.  The d i f f e r e n t i a l e f f e c t o f r a t i o n on s m a l l and l a r g e females i n  t h i s s t u d y i s a t t r i b u t e d t o c o n t e s t c o m p e t i t i o n i n which l a r g e r i n d i v i d u a l s were a b l e t o secure  a d i s p r o p o r t i o n a t e share  o f scarce food  resources.  88  13)  Other laboratory studies on guppies and trout (Salmo, spp.) have  also demonstrated a reduction i n fecundity with reduced rations. In at least one case the reduction i n fecundity was accompanied by an increase i n egg size. 14)  Several f i e l d studies have provided evidence of significant  year to year variation i n fecundity within exploited marine f i s h populations.  In these cases fecundity appears to be inversely related  to population density, and the effect may be mediated by competition for food. 15)  An examination of data on geographical variation i n fecundity of  plaice (Pleuronectes platessa) reveals a significant tendency for fecundity to be inversely related to length of reproductive lifespan i n different populations.  I t i s suggested that populations whose  l a r v a l mortality varies considerably from year to year tend to distribute reproductive effort over more years than populations for which l a r v a l survival i s r e l a t i v e l y constant. 16)  A comparison of reproductive parameters of different stocks of  herring (Clupea harengus) shows that winter and early spring spawners tend to produce r e l a t i v e l y few and large eggs whereas summer-autumn spawners are more fecund and have smaller eggs. There i s some evidence that both patterns represent adaptations to maximize the number of surviving larvae under the specific conditions which each spawning group faces. Large egg size (and low fecundity) i s seen as an adaption to starvation as a major l a r v a l mortality source; small eggs and high fecundity occur i n situations where predation i s a major cause of l a r v a l deaths. The distribution of reproductive effort over the adult l i f e span seems  89  to be related to the year to year v a r i a b i l i t y i n l a r v a l survival, as i n plaice. 17)  Cushing (1973) has demonstrated a rough relationship between the  average fecundity of a f i s h population and the shape of i t s stock/recruit curve; more fecund species tend to follow curves having a more pronounced dome, which implies that these populations can sustain themselves under.a higher rate of exploitation.  I suggest that i t should be  possible to make more precise predictions about a population's stock/recruit curve by considering other reproductive parameters i n addition to absolute fecundity—e.g. egg size, age at f i r s t maturity, and reproductive l i f e span.  90  BIBLIOGRAPHY B a g e n a l , T. B. 1958. b i o l . A s s . U. K.  The  f e c u n d i t y of Clyde p l a i c e .  J.  mar.  1960a. The f e c u n d i t y o f E n g l i s h C h a n n e l p l a i c e . b i o l . A s s . U. K. 39: 249-254.  J.  mar.  37: 309-313-  1960b. The f e c u n d i t y o f p l a i c e from t h e s o u t h and west o f I r e l a n d . J . mar. b i o l . A s s . U. K. 39: 255-262.  coasts  1962. The f e c u n d i t y o f p l a i c e from t h e c o a s t s o f Norway. b i o l . A s s . U.K. 42: 105-112.  1963a. The f e c u n d i t y o f p l a i c e from t h e Bay b i o l . A s s . U.K. 43: 177-179.  of Biscay.  1963b. biol.  V a r i a t i o n s i n p l a i c e fecundity in.the Clyde area. A s s . U. K. 43: 391-399.  J.  J.  mar.  mar.  J.  mar.  1965. The f e c u n d i t y o f l o n g rough d a b s * i n t h e C l y d e Sea a r e a . mar. b i o l . A s s . U. K. 45: 599-606.  J.  1966. The e c o l o g i c a l and g e o g r a p h i c a l a s p e c t s o f t h e f e c u n d i t y o f the p l a i c e . J . mar. b i o l . A s s . U. K. 46: 161-186.  1969a. The r e l a t i o n s h i p between f o o d and Salmo t r u t t a L. J . F i s h B i o l . 1: 167-182.  f e c u n d i t y i n brown t r o u t  1969b. R e l a t i o n s h i p between egg s i z e and f r y s u r v i v a l i n brown t r o u t Salmo t r u t t a L. J . F i s h B i o l . 1: 349-355. 1973. F i s h f e c u n d i t y and i t s r e l a t i o n s w i t h s t o c k and r e c r u i t m e n t . Rapp. P.-v. Reun. Cons. perm. i n t . E x p l o r . Mer. (In p r e s s ) . B a x t e r , I . G. 1959h e r r i n g spawners.  F e c u n d i t i e s o f w i n t e r - s p r i n g and J . Cons. perm. i n t . E x p l o r . Mer.  sumer-autumn 25: 73-80.  B e v e r t o n , R. J . H. and S. J . H o l t . 1957. On t h e dynamics o f e x p l o i t e d f i s h populations. F i s h e r y I n v e s t . , Lond. ( l l ) 19: 533 P» B l a x t e r , J . H. S. and G. Hempel. 1963. h e r r i n g l a r v a e (Clupea harengus L . ) .  Mer 28: 211-240. . ' ,  The i n f l u e n c e o f egg s i z e on J . Cons. perm. i n t . E x p l o r .  Bowers, A. B. and F. G. T. H o l l i d a y . 1961. H i s t o l o g i c a l changes i n t h e gonad a s s o c i a t e d w i t h t h e r e p r o d u c t i v e c y c l e o f t h e h e r r i n g (Clupea harengus L . ) . Mar. Res. 1961(5) : 1-16. Counts, R. C. I 9 6 I . A d e v i c e i o r measuring l a r v a l and C o p e i a 1961: 224-226.  juvenile fishes.  91  Cushing, D. H. 19°7» Th grouping of herring populations. b i o l . Ass. U. K. 47: 193-208. e  J . mar.  1969. The fluctuation of year-classes and the regulation of f i s h e r i e s . Fisk. D i r . Skr., Serie Havunderskelser 15: 368-379. 1971. The dependence of recruitment on parent stock i n different groups of fishes. J . Cons. perm. i n t . Explor. Mer 33: 340-362. 1973* The dependence of recruitment on parent stock. tech. Conf. f i s h . Mgmt. Devel., Vancouver, Can: 23 p.  F. A. 0.  Cusing, D. H. and J . P. Bridger. 1966. The stock of herring i n the North Sea, and changes due to f i s h i n g . Fishery Invest., Lond. (11)25: 1-123. Cushing, D. H. and J. G. K. Harris. 1973* Stock and recruitment and the problem of density-dependence. Rapp. P.-v. Reun. Cons. perm. i n t . Explor. Mer (In press). Fisher, R. A. 1930 (2nd Ed., 1958). The genetical theory of natural selection. Dover, New York, N. Y. x i v + 291 p. Gadgil, M. and ¥. H. Bossert. 1970. L i f e h i s t o r i c a l consequences of natural selection. Am. Nat. 104: 1-24. Gadgil, M. and 0. T. Solbrig. 1972. The concept of r - and K-selection: evidence from wild flowers and some theoretical considerations. Am. Nat. 106: 14.-31. Gulland, J . A. I965. Survival of the youngest stages of f i s h , and i t s relation to year class strength. Int. Comm. Northw. A t l . Fish. Spec. Pub. 6: 363-371. Hempel, G. 1965. On the importance of l a r v a l survival for the population dynamics of marine food f i s h . Rep. C a l i f , coop, oceanic Fish. Invest. 10: 13-23. Hempel, G. and J. H. S. Blaxter. 1967. Egg weight i n Atlantic herring. J . Cons. perm. i n t . Explor. Mer 31: 170-195. Hester, F. J . I964. Effects of food supply on fecundity i n the female guppy, Lebistes reticulatus (Peters). J . Fish. Res. Bd. Can. 21: 757-764. Hodder, V. M. 1963. Fecundity of Grand Bank haddock. J . Fish. Res. Bd. Can. 20: I465-I487. Kipling, C. and W. E. Frost. 1969. Variations i n the fecundity of pike (Esox lucius L.) i n Windermere. J . Fish B i o l . 1: 221-237.  92  Krumholz, L. A. 1948. R e p r o d u c t i o n i n t h e Western m o s q u i t o - f i s h , Gambusia a f f i n i s a f f i n i s ( B a i r d and G i r a r d ) , and i t s use i n mosquito c o n t r o l . E c o l . Monogr. 18: 1-43. 1963. R e l a t i o n s h i p s between f e r t i l i t y , s e x - r a t i o , and exposure t o p r e d a t i o n i n p o p u l a t i o n s o f t h e m o s q u i t o - f i s h Gambusia manni Hubbs at B i m i n i , Bahamas. I n t . Revue ges. H y d r o b i o l . Hydrogr. 48: 201-256.  1948.  Lack, D.  The  s i g n i f i c a n c e of c l u t c h s i z e .  Ibis  90: 25-45*  L a r k i n , P. A. 1973. Some o b s e r v a t i o n s on models o f s t o c k and r e c r u i t m e n t r e l a t i o n s h i p s f o r f i s h e s . Rapp. P.-v. Reun. Cons. perm. i n t . E x p l o r . Mer-(In p r e s s ) . MacArthur, R. H. and E. 0 . W i l s o n . 1967. The t h e o r y o f i s l a n d b i o g e o g r a p h y . P r i n c e t o n U n i v . P r e s s , P r i n c e t o n , N. J . 203 P* Magnuson, J . J . 1962. An a n a l y s i s o f a g g r e s s i v e b e h a v i o r , growth, and c o m p e t i t i o n f o r f o o d and space i n medaka ( O r y z i a s l a t i p e s ( P i s c e s ,  Can. J . Z o o l . 40:  Cyprinodontidae)). 1961.  M i l n e , A.  D e f i n i t i o n o f c o m p e t i t i o n among a n i m a l s .  exp. B i o l . 15:  Symp.  Soc.  40-61.  1968.  Murphy, G. I .  313-363.  P a t t e r n i n l i f e h i s t o r y and t h e environment.  Nat. 102: 391-403. 1954.  N i c h o l s o n , A. J .  A u s t . J . Z o o l . 2:  An o u t l i n e o f t h e dynamics o f a n i m a l  Am.  populations.  9-65°  P a r r i s h , B. B. and A. S a v i l l e . I 9 6 5 . The b i o l o g y o f t h e N o r t h e a s t Atlantic herring populations, Eceanogr. mar. b i o l . arm. Rev. 3' 323-373* Pianka,  E . R.  1972.  106: 581-588.  R a i t t , D.  F. S.  11:  1968.  E.  1954.  Am.  Nat.  The p o p u l a t i o n dynamics o f t h e Norway pout i n t h e  Mar. Res. 1968(5) :  North Sea. R i c k e r , W.  r ~ and K - s e l e c t i o n o r b - and d - s e l e c t i o n ?  1-24.  Stock and r e c r u i t m e n t .  J . F i s h . Res.  Bd.  Can.  559-623.  1958o Handbook o f computations f o r b i o l o g i c a l s t a t i s t i c s o f populations. B u l l . F i s h . Res. Bd. Can. 119: 300 p . R o s e n t h a l , H. L. 1952. Lebistes reticulatus  Hole 102:  30-38.  fish  O b s e r v a t i o n s on r e p r o d u c t i o n o f t h e p o e c i l i i d ( P e t e r s ) . B i o l . B u l l . mar. b i o l . Lab., Woods  93  Scott, D. P. 1962o Effect of food quantity on fecundity of rainbow trout Salmo gairdneri. J . Fish. Res. Bd. Can. 19: 715-731. Schaffer, W. M. Ph.D. Thesis.  1972. The evolution of optimal reproductive strategies. Princeton Univ., Princeton, N. J . 129 p.  Seghers, B. H. 1973* An analysis of geographic variation i n the antipredator adaptations of the guppy, P e o c i l i a r e t i c u l a t a . Ph.D. Thesis. Univ. of B r i t i s h Columbia, Vancouver, B. C. 238 p. Simpson, A. C. 1959° The spawning of the plaice i n the North Sea. Fishery Invest., Lond. (II)22(7): 1-111. Snedecor, G. W. 1956. S t a t i s t i c a l methods. 2nd Ed. College Press, Ames, Iowa. 503 p°  Iowa State  Stoik, A. 1951« Histo-endocrinological analysis of gestation phenomena i n the cyprinodont Lebistes reticulatus (Peters). Proc. K. ned. Akad. Wet.-(C)54: 550-578. Svardson, G. 1943° [Studies on the relationship between sexual, maturation and growth i n Lebistes]. Meddn. St. Unders. -o. ForsAnst. SotvattFisk. 21: I-48. (Transl. from Swedish by Fish. Res. Bd. Can. Transl. Ser. No. 126). 1949. Natural selection and egg number i n f i s h . Rep. Pap. Inst. Freshwat. Res. Drottningholm 29: 115-128.  short  Taning, A. V. 1929. Plaice investigations i n Icelandic waters. Rapp. P.-v. Reun. Cons. perm. i n t . Explor. mer 57(8): 1-134* Tinkle, D. W. and H. M. Wilbur. 1973• Environmental certainty, trophic l e v e l , and successional position i n l i f e history evolution. Unpublished manuscript. 23 p. Turner, C. L. 1937. Reproductive cycles and superfoetation i n p o e c i l i i d fishes. B i o l . B u l l . mar. b i o l . Lab., Woods Hole 72: 145-164. Verhulst, P. F. I838. Notice sur l a l o i qui l a population suit dans son accroissement. Corresp. Math. Phys. 10: 113-121. Wimpenny, R. S.  1953.  The p l a i c e . E. Arnold, London.  145  P«  94 Appendix T a b l e I .  Standard l e n g t h o f female (mm.)  D a t a from Experiment  Wet weight o f female p o s t partum (mg.)  1.  . Number o f f r y i n brood  14.5  60  3  17.5  130  4  18.9  160  3  22.5  310  9  22.4 25.2  260  25.3 26.7 27.1 27.3 27.8 30.0  32.3 33.7 39.7 45.2  Standard d e v i a t i o n o f f r y l e n g t h s (mm.)  Estimated dry weight o f brood (mg.)  Estimated index of reproductive effort  .21  2.26  .26 0.00  3.03 3.12  .15 .30  9.04 3.22  .029 .012  400  4 16  6.37 6.05 6.36  .18  16.00  .040  330  12  6.38  .16  12.12  .036  430 500  21  6.50  .24  23.28  .054  25  6.47  .30  27.42  .054  500  18  6.39  18.54  .037  490 660  21  6.45  .25 .22  22.51  .045  20  6.34  .20'  19.77  .029  870  44 42  6.66  40.07  .046  1650  39  6.54  .30 .21  44.54  .026  2470  175  - 6.75  •3S  177.59  .071  30.5 31.7  Mean s t a n d a r d length o f f r y w i t h i n brood (mm.)  5.97 . 5.98 6.40  ;  ,  .037 .023 .019  9 ' 740 840  19  Appendix T a b l e I I .  Numbers o f embryos c a r r i e d by f e m a l e s d y i n g d u r i n g Experiment 1.  S t a n d a r d l e n g t h o f famale (mm.)  Number o f embryos  15.5  8  22.5  8  24.4  18  26.0  13  26.0  23  26.9  24  27.0  35  28.9  30  29.7  44  30.3  56  33.6  28  34.5  36  39.7  58  43.6  108  96  Appendix Table III.  Data from Experiment 2.  Wet weight of female (mg.)  Weight of dry food consumed (mg.)  40  4  60  2  90  3  100  2  130  4  160  3  200  9  230  7  290  9  740  22  780  23  880  26  97 Appendix Table IV.  Standard length of female (mm.)  Data from Experiment 3<  Wet weight of female post partum (mg.)  Number of fry i n brood  Dry weight of brood (mg.)  Index of reproductive effort  .07  1.2 1.8  .017  6.22  .05  5.0  .18  5.0  ll  6.43 6.49  .033 .016  .15  I5.2  .044  12  premature  23  6.22  .19  20.1  .038  .22  5.2  HIGH  13.9 16.5  70 100  1 2  19.1  130 150  4 5 6  19.4 23.5 25.0 25.6 27.5  310 340 520  Mean standard length of fry within brood (mm.)  FOOD  6.15  FOOD  150  5  6.50  180  20.3  170 170  3 2  5.5 7.05 - 6.60  26.8  27.2  340 340 470 510  3 14 17 10  .018  premature  19.3 20.3  4  T R E A T M E N T  6.6  LOW  20.5 24.1 24.1  Standard deviation of fry lengths (mm.)  1  TREjATJMENT  .034  only one fry weighed and measured  .07  2.8  .016  .18  6.90  .10  4.3  .012  6.28  .20  6.28  .20  14.5 14.2 14.0  .042 .030  6.88  ! I  I  .19  .027  

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