166 R. J. H. Beverton and S. J. Holt 



Because both T^^^ and M, and L^ and K, are themselves 

 fairly closely correlated (inversely), it is not unexpected that 

 if a relationship is found between Tj^ax ^^^ -^oo ^^^ should 

 also appear between M and K. What is perhaps significant 

 is that the clearly established exceptions to the general positive 

 correlation between longevity and size (which undoubtedly 

 emerges from the above analysis) are those species which have 

 a much higher value of K than would be expected from their 

 L^. This suggests that the "rate of completion of the growth 

 pattern" is more closely related to longevity than is size and 

 some further evidence on this point is discussed below. Be 

 that as it may, we have not yet come across a slow-growing 

 species which dies from natural causes when only a small 

 fraction of its growth pattern has been covered, or the con- 

 verse — a species whose natural lifespan extends far beyond 

 the point at which the limiting size is reached (as in man); and 

 it seems that for a wide range of fish species the natural span 

 of life is nicely adjusted to the time needed to complete, or 

 nearly to complete, the growth pattern. 



Some intra -specific comparisons 



We have so far looked at the relations between growth and 

 mortality in different species, but intra-specific comparisons 

 also throw some light on this question. If, firstly, we consider 

 differences between sexes we notice that, when the growth 

 rates are clearly different, L^ for males is usually less than for 

 females in the same population {Dasyatis akajei, Gadus 

 minutus, Ganibusia, Coitus, Isopsetta, Pleuronectes). In these 

 cases K for males is greater than for females, and the male 

 mortality rate is higher. When the growth rates do not differ, 

 or differ only slightly, the natural mortality rates are also 

 similar (Leucichthys artedi, L. kiyi, Salvelinus, Mallotus, 

 Leuresthes, Citharichthys). The chum salmon (Oncorhynchus 

 keta) of the Columbia River is exceptional : the male natural 

 mortality is greater than that of the female, and the male has 

 a higher L^. In Callionymus, also, the males approach a 



