288 



E. D. LE CREN 



Table I. Estimates of the production of trout in Three Duhs Tarn 



summer months and mortality more evenly distributed, but there is no real 

 information about the seasonal variation in mortality, and the estimates are 

 adequate for the present purpose. The estimates are set out in Table I, giving 

 three estimates for the first three years, based on the three assumptions about 

 the course of early mortality. The third, intermediate, estimate assumes 

 (from independent information) that soon after they hatch most of the 

 alevins migrate down the spawning becks into the tarn, where they grow 

 rather fast but suffer a heavy mortahty. They are supplemented in mid- 

 summer by a further immigration of those fry that have remamed in the 

 streams and survived better but grown slower. 



It will be seen from Table I that, although different assumptions about 

 early mortality can have a profound effect upon the estimates of production, 

 whatever assumption is adopted the rate of production is very much greater 

 in the young fish than in the older. On a similar basis estimates can be made 

 for the production of perch in Windermere. Here, with a pelagic larval 

 phase, early mortality is much greater and a wide range of possible rates 

 could be used, but again the same three assumptions are made and minimal, 

 probable, and maximal estimates calculated (Table II). The maximal estimates 

 are obviously too large and give very high figures for production and sur- 

 viving biomass, but it is difficult to calculate probable figures without 



Table II. Estimates of the production of perch in Windermere before 1941 



[metric tons) 



