REPRODUCTION AND RECRUITMENT IN FRESHWATER FISH 287 



course of this survival can be made: (i) to assume that the rate of survival is 

 constant from egg to three years old (at a rate of Z = i • 48), or (2) to assume 

 that there is very heavy mortahty very early in life (before any growth) and 

 that mortahty subsequently is the same in the first three years as in the fourth 

 (Z = 0-63), or (3) to assume that mortahty in the second and third years is 

 the same as in the fourth (Z = 0-63) but that the first year starts with a 

 heavy mortahty which soon becomes much less. These three possibilities are 

 shown in Fig. i. 



10,000 



4 5 



in years 



Fig. I.— Numbers of survivors plotted on a logarithmic scale against age for brown trout in 

 Three Dubs Tarn (data from Frost & Smyly, 1952). Three possible curves of survival from 

 eggs to three-year-old fish are show^n. 



The growth made in each year of hfe can be estimated from the scales by 

 back-calculation (as has been done by Frost & Smyly (1952)) and the weight 

 calculated from a length/weight relationship. The production can be calcu- 

 lated in various ways (Allen, 1951; Ricker, 1958^. In the present example it 

 has been assumed that both growth and mortality are equally distributed 

 throughout the year and production has been estimated from the product of 

 the mean biomass and the specific growth-rate for the year. In fact this 



