COMPARATIVE POPULATION STUDIES IN FISHERY BIOLOGY 65 



THE RELATION BETWEEN GROWTH AND NATURAL 

 MORTALITY RATES 



Beverton & Holt (1959) have examined the relation between values of K and 

 of M in wild fish populations for which sufficient pubhshed data were 

 available, and suggested as a result of their study, that within a particular 

 taxonomic group M and K were positively correlated, so that the ratio 

 M' = M/K is less variable than either M or K, and might even be considered 

 to have a characteristic value for each group. This seems to offer the possibility 

 of using only taxonomic criteria to set limits to the possible values of M' for 

 making stock assessments in the absence of other data, and even of making 

 simultaneous assessments for a number of populations of fish of the same 

 group inhabiting a certain area. This would alleviate one of the serious 

 difficulties of applying analytical stock assessment methods in tropical waters 

 in which not only may it be difficult to determine age, but in which there 

 are a very large number of species, often very similar, caught simultaneously 

 by the same gear. 



It is very commonly reported that most big fish in catches are females. 

 This can be a result of the growth of females to greater asymptotic sizes, but 

 where the older fish in catches are also females, it means of course that the 

 mortahty rate of males is higher than that of females. Beverton & Holt (i959) 

 suggested that this is true particularly where there are sexual differences in 

 growth rate. It may be noted that even though both values of K and of M 

 or Z may be higher in males than in females, the ratios M' or Z' may be 

 nearly the same in the two sexes, which need not therefore be treated 

 separately in making assessments, it being sufficient to use a value of Loo or 

 W^ which is an appropriately weighted mean of the differing values for the 

 two sexes. 



It is tempting to ascribe the relative constancy of the M/K ratio to the 

 similarity, or even the identity, of physiological processes which determine 

 the rates of natural mortality and growth. This explanation does not, how- 

 ever, appear to be satisfactory for situations in which mortahty is largely due 

 to predation. The existence of a mechanism relating maturity size with K 

 would, however, ensure that in a population subjected to a high extrinsically 

 determined natural mortality, and containing individuals with a genetically 

 determined range of K values, the fish with high K values, which mature 

 earlier, would have a selective advantage over those with low K values, so 

 that an adequate number would reach maturity in each generation to ensure 

 its survival. Such 'fast growing' individuals would have a much shghter 

 advantage if M were low, and their advantage might be offset by other 



