284 E. D. LE CP^N 



tion of population dynamics in terms of numbers alone of limited value, and 

 growth and biomass must also be taken into account. A useful parameter to 

 estimate, as well as number, biomass, mortality rate, and growth rate, is the 

 ^production , in the sense of the sum of the growth made by all members of 

 the population regardless of their fate and whether they survive to the end 

 of the period being considered. The production can be used as a yardstick 

 for estimates of the efficiency of the yield' (whether it be the catch of a 

 fishery, recruits or smolt migrants), and for estimating the food demands of 

 the population. Information on the latter may be of particular relevance to 

 some fresh waters where the size and productivity of the environment may 

 be the main factor limiting the yield offish. 



The following paper discusses aspects and examples of the production of 

 gonad products and the growth, survival and production of young stages, in 

 a few freshwater species, and draws tentative conclusions about the action 

 of some of the controlling factors and their relevance to an understanding 

 both of natural population mechanisms and practical fishery management. 



THE PRODUCTION OF EGGS AND MILT 

 Although growth studies of freshwater fish are numerous, few authors have 

 studied the relationship between gonad weight and body weight, or taken 

 into account the annual production of gonad products by fish as part of their 

 production , or as a drain, together with increase in body size, on their 

 assimilatory processes. It is interesting therefore to compute rough approxi- 

 mations of the total weights of gonad products shed by fish and to compare 

 these with the total production in the form of growth in body size. 



In Windermere before the fishing experiment started in 1941 the average 

 annual increment in weight for adult perch {Perca Jluviatilis) was 5 g (Le 

 Cren, 1958). The rate of natural mortahty (M) for males was approximately 

 0-44 and for females 0-32. The eggs just before shedding are 20 per cent of 

 the total body weight and the milt 8 per cent (Le Cren, 1951). The fishing 

 mortality (F) due to traps in 1941 can be estimated as roughly i • 52 for males 

 and the catch for the North Basin is known as about 780,000 males. The 

 average weight of adult males was 26 g and females 3 5 g, and, allowing for 

 the proportions of II group males that are mature, and the probable relative 

 populations in the North and South basins of Windermere, and assuming, 

 which is reasonable, that growth and mortality are distributed through the 

 year in an approximately similar way, rough estimates can be made of 

 production. These computations give a production of adult perch for the 

 whole of Windermere in 1940 of 23,700 kg, and about 21,000 kg of perch 

 eggs and milt were shed. The total area of Windermere is 14-82 km^ 



