Longevity and Mortality Rates of Fish in Nature 145 



different year classes of fish whose initial abundance varies 

 considerably. The perch data are unique because they show 

 the survival_of a known number of fish introduced into 

 experimental ponds — in the one case as fry {a) and in the 

 tJtheF'asrfive-year-old fish (b) — and the n sample d regularly, 

 for 15^ years and 17 years respectively; within these spans 

 of age there is ho ev ideiice of a varying age-specific mortality 

 rate. 



Of the shorter-lived species shown in Fig. 1, nearly all have 

 a survival curve with some degree of downward curvature 

 over nearly the whole range. This is seen particularly clearly 

 Tnjthe herring data (Clupea spp.), of which those for the 

 Norwegian herring (C. harengus; Lea, 1930) are the combined 

 data for a period of twenty years in which year-class fluctu- 

 ation has been largely eliminated. The data for the bullhead 

 (Cottus gobio; Smyly, 1957) and the dragonet (Callionymus 

 lyra; Chang, 1951) are included to show the difference be- 

 tween the survival curves for males and females. In both of 

 these the males have a higher mortality rate and a shorter 

 lifespan, and this is indeed what is usually found where 

 there is any difference between the sexes; we have, however, 

 come across one or two exceptions which are referred to later. 



The tendency for the natural mortality rate to increase 

 with age, which is noticeable in several of the examples shown 

 in Fig. 1, is, indeed, found more often than not, and for other 

 instances the reader is referred to papers by, for example, 

 Kennedy (1954) on the Lake trout {Cristivomer namaycush), 

 by Wohlschlag (1954a) on the Alaskan whitefish (Leucichthys 

 sardinella), and by Ricker (1949) on several species. This 

 variation of the mortality rate with age reaches an extreme 

 form in species where all or nearly all individuals die at, or 

 soon after, spawning for the first time. The best known 

 instance of this is in the Pacific salmon (Oncorhynchus spp.), 

 which migrates up-river from the sea at between three and 

 five years of age, spawns and then dies. The immature phase 

 of the life-cycle spent in the sea has only recently been studied, 



