Longevity and Mortality Rates of Fish in Nature 169 



growth did not indicate any marked difference in longevity 

 compared with those in which growth was normal. The ex- 

 perimental studies being carried out by Comfort (personal 

 communication) on growth and longevity in guppies {Lebistes 

 reticulatus) appear so far to be giving the same result, although 

 severe underfeeding during the early life of trout kept in 

 tanks has been shown to delay maturity and actually prolong 

 life (McCay, Dilley and Crowell, 1929). Again, the association 

 between high values of M and of K noted above may not hold 

 for comparisons between populations of the same species in 

 closely adjacent waters, as in the case of the bullhead {Coitus 

 gobio; Smyly, 1957) in Lake Windermere and the River 

 Brathay (see Table I). 



Mortality, growth and metabolic rate 



To understand the relations tentatively identified above, 

 it is necessary to extend our studies to include comparative 

 physiology and behaviour, and at this stage w^e can do little 

 more than indicate the lines of comparison that might profit- 

 ably be pursued. One of these follows from the fact that the 

 growth parameter K is predictable from the rate of endogen- 

 ous nitrogen excretion by a starved animal (von Bertalanffy, 

 1938), and it would be expected that this is also closely 

 related to metabolic rate and to activity, as Edmonds (1957) 

 has shown in a comparative study of some invertebrate 

 groups. For fish, the available data seem to confirm the 

 relation of K to metabolic rate. Thus the oxygen consumption 

 of Gadus callarias at 7-11° is 0-33-0-35 O2 ml./g.^^^/hr. and 

 that of Gadus virens at the same temperature is similar, 

 0-36-0 -47 O2 ml./g.2/3yhr. (Sundnes, 1957). These two species 

 have the same K values (0-2) though the natural mortality of 

 G. callarias is possibly rather higher than that of G. virens. 

 On the other hand, Leucichthys sardinella has, at about the 

 same temperature (7-9-4°), an oxygen consumption of 0-55- 

 0-75 O2 ml./g.2/3yhr., corresponding with a higher K value 

 (0-4) and much higher M (0-6). The cyprinids Labeo rohita, 



