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



but there is no reason to believe that the mortahty rate 

 during that time is abnormally high. Other examples of a 

 catastrophic mortality at, or shortly after spawning include 

 the Tasmanian whitebait {Lovettia seali; Blackburn, 1950); 

 the capelin {Mallotus villosus; Templeman, 1948); the small 

 freshwater atherinid Labidesihes sicculus (Hubbs, 1921) 

 which spawns at about one year of age and then dies off 

 within a further two or three months; and, probably, the 

 dragonet {Callionymus lyra; Chang, 1951). In most of these 

 it is the males which suffer the most severe mortality, the 

 evidence being that a proportion of the females spawn more 

 than once, even though that proportion may be small. 



Survival curves in fish thus range from effective linearity 

 over the whole of the observed range of age to sharp dis- 

 continuity at the onset of maturity j^ with a wide range of 

 intermediates in which the mortality rate increases steadily 

 with age without obvious discontinuity. This makes it dif- 

 ficult to adopt any single numerical index as an index of life::, 

 span, or of force of mortality, for all species. Thus the maxi- 

 mum age recorded in samples is satisfactory for the species in 

 which the mortality rate increases fairly sharply with age, but 

 is less so in the long-lived species, where it becomes rather 

 critically dependent on the size of the samples and on the 

 accuracy of the age-determination technique. Conversely, 

 the average mortality rate is not a particularly useful measure 

 where the mortality rate is highly age-specific, but is satis- 

 factory in the long-lived species with nearly linear survival 

 curves. 



For the time being we have therefore tabulated wherever 

 possible both the maximum age recorded in the sample (T^^ax) 

 and the average instantaneous coefficient of natural mortality 

 (M) over the range of age groups which, as far as we could 

 judge, were fully represented in the same samples. These are 

 given in Table I, from which it will be seen that the lifespan of 

 fish can range from little more than a year in several quite 

 unrelated species including Labidesihes, of the mullet family, 



