Longevity and Mortality Rates of Fish in Nature 171 



being highest just before spawning. In the plaice it is apparent 

 that this variation is very much greater in older than in 

 younger individuals, and it seems that as the fish gets older 

 (or, perhaps, merely bigger) the strain of meeting the repro- 

 ductive demand increases to a point at which recovery is not 

 possible. It seems that this kind of effect is most evident in 

 species with high K, which mature at an early age but at a 

 size which is rather large in relation to the asymptotic length, 

 L^. Fish, such as sturgeon, with a low K, which mature when 

 relatively rather small, do not show a decline in their repro- 

 ductive capacities; Gamhusia affinis, on the other hand, 

 exhibits an absolute decrease in brood size w^ith increasing 

 age (Krumholz, 1948) and indeed this species seems to have a 

 true post-reproductive phase, which is rather rare in fish. 

 Further evidence on the decline in reproductive powers with 

 age in fish is presented in the contribution by Gerking to this 

 colloquium. 



The complexity of the relations between the growth and 

 reproduction parameters and mortality rates is illustrated by 

 Svardson's (1943) review of data for the guppy, Lehistes 

 reticulatus. Male guppies mature before females, and die 

 younger. Under experimental conditions of low food supply 

 they grow slowly to a low asymptotic size, and mature late; 

 with a medium food supply the final size is greater and 

 maturity somewhat earlier, but with an abundant food supply 

 the final body size is again lowered although the onset of 

 maturity is still further accelerated. 



Other observations concerning the relations between 

 growth, reproduction and death have to be fitted into the 

 picture outlined in this paper, but pending a detailed survey 

 of the known facts, can only briefly be mentioned here. It 

 has been thought for many years that the onset of sexual 

 maturity in fish is a function of their size rather than of their 

 age but like most such generalizations this is only partly true 

 since, within a species population, individuals reach maturity 

 over a considerable range of both age and size. There is, in 



