The Biology of Senescence 



the late-developing females would very probably have a longer 

 total life-cycle. 



Studies of wild populations are almost always conducted 

 under conditions where the standing force of mortality through- 

 out life is very high, and they therefore give little information 

 about mortality trends in the latter part of the life-cycle of the 

 longer-lived forms. Excluding the very high larval mortality, 

 populations of many species of fish, studied in the wild, show 

 an age structure and a pattern of death similar to that found 

 in birds, i.e. a high constant mortality unrelated to age and a 

 virtually constant expectation of life (Frost and Smyly, 1952 

 (Fig. 16); Deevey, 1947). Substantial differences in life-span may 

 be dictated by availability of food organisms of a size suited to 

 adult feeding, and by competition between the fry of the 

 observed species and adults of other species. Some populations 

 of minnows show apparent specific age which is exceeded in 

 other populations of a closely-related species by a very large 

 factor (Frost, 1943; Tack, 1940). In Pimephales promelas, Markus 

 (1934) observed apparent specific size and specific age in all 

 but a few exceptionally large individuals. This was apparently 

 due to the fact that there was an overall mortality of 80 per cent 

 following spawning; the survivors, and individuals which took 

 no part in breeding during their first year of maturity, con- 

 tinued growth until the next breeding season. If reproduction 

 is avoided, life may be prolonged — Bidder (1932) points out 

 that eels, which, it is believed, normally die after spawning, live 

 many years in captivity (Flower, 1925). Frost and Smyly (1952) 

 found considerable differences in growth rates and in the form 

 of the growth curve between brown trout inhabiting tarns and 

 those inhabiting Windermere (Fig. 16). The age structure of 

 the tarn population agreed well with a steady annual survival 

 rate of 35 per cent between the second and eighth year of life. 

 In these fish growth had become very slow, whereas in the 

 Windermere population individual fish were still growing at 

 7 years upon an approximately linear scale. The ability to con- 

 tinue growth may depend on attaining a size which makes it 

 possible to prey on smaller fish. Long-lived fish such as pike 

 certainly continue to grow measurably for very long periods 

 (Schloemer, 1936) but the increase in size is associated with an 



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