THE EFFICIENCY OF REPRODUCTION AND 

 RECRUITMENT IN FRESHWATER FISH 



E. D. Le Cren 



Freshwater Biological Association, Ambleside, Westmorland 



INTRODUCTION 



In the major marine fisheries it has been traditional to regard the relationship 

 between the number of parents and subsequent number of young that are 

 available for recruitment into the fished stock to be sufficiently tenuous to be 

 ignored. In the management of freshwater fisheries, however, conservation 

 of spawners has always been considered of great importance, and restrictions 

 upon fishing have often been imposed with the object of protecting the 

 spawning stock rather than preventing overfishing in the sense usually 

 applied to marine fisheries (i.e. catching the stock before they have had time 

 to grow to the optimum size). It is true that legal restrictions on fishing have 

 often aimed at sharing the catch more equitably or preventing fishing by 

 unauthorized persons; but in the background all the time has been the idea 

 that conservation of adult stock is of fundamental importance, and sometimes 

 this idea has become a sacred obhgation. 



In fact, very little indeed is known about the dynamics of the whole 

 reproductive process in fish populations. Although there is some knowledge 

 of the egg stage in salmonid fish and a considerable literature on the technol- 

 ogy of fish culture, few observations have been made on the ecology of the 

 young stages in the wild. Ricker (1954, 1958^) has reviewed the theory and 

 literature on the relationships between number of parents and number of 

 progeny and presented data from year-to-year variations in the runs of 

 various Pacific salmon {Oncorhynchus spp.). Beverton & Holt (1957) bave 

 discussed this subject again and presented alternative theoretical curves. Real 

 data, however, are still scanty and even where information on the numbers 

 of eggs and resultant recruits are known, information from the intervening 

 period on rates and causes of mortality is usually absent. An understanding 

 of the dynamics of the whole reproductive and recruitment process is urgently 

 needed, and is probably the largest gap in our knowledge offish population 

 dynamics. 



The plasticity of growth rate and adult size in fish renders any considera- 



