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



juveniles is seldom possible. It is known, however, that in 

 many species, and especially in those which lay large numbers 

 of eggs, there is a very high mortality during the first weeks 

 of life ; in North Sea plaice, for example, only about one in ten 

 thousand survive the first few months. Thus the survival 

 curve for fish characteristically descends very rapidly at first 

 and then flattens out, though in viviparous species, and 

 species which lay a small number of eggs but afford the newly- 

 hatched fry some degree of parental care, this initial descent 

 is probably less marked. 



Even after the early phase of heavy mortality some con- 

 siderable time may elapse before the fish have grown large 

 enough to be retained by the fishing gear, so that a representa- 

 tive survival curve has to begin at some later age when the 

 individuals are first properly represented in the samples. The 

 survival curves shown in Fig. 1 therefore start at the age 

 group which is most abundant in catches, and for comparison 

 all the data have been adjusted to a peak number of 1,000. 

 The numbers are plotted on a logarithmic scale, so that a 

 linear survival curve indicates a constant natural mortality 

 rate independent of age, whereas a downward curve shows 

 that the mortality rate is increasing with age. The broken 

 lines are drawn purely to assist the eye in detecting linearity 

 or departures from it, and where the survival curve is not a 

 straight line the broken lines are drawn through the first and 

 last points. 



In the long-lived species, of which the examples shown in 

 Fig. 1 are sturgeon (Acipenser fulvescens ; Probst and Cooper, 

 1954), whitefish (Coregonus clupeaformis ; Hart, 1931) and 

 perch {Perca fluviatilis ; Aim, 1952), the r aortality r a,te- seems 

 to be effectively constant over a considerable span of age at 

 about 5 to 10 per cent per year, although in the age groups of 

 sturgeon beyond about 30 years the mortality rate appears 

 to increase. The fluctuations in the data for sturgeon and 

 whitefish are partly due to the fact that sampling was possible 

 for a limited period only and that the age groups refer to 



