REPRODUCTION AND RECRUITMENT IN FP^SHWATER FISH 295 



is correlated with size on migration to the sea, so it may not always be 

 advantageous to produce more numerous but smaller migrants. 



The biological significance of the normal high levels of spawning and 

 heavy juvenile losses is doubtless connected with the operation of density- 

 independent mortality which tends to cause large fluctuations in survival. 

 The variation in larval and juvenile survival that occurs among many marine 

 fish is well known; freshwater species also show such variations in year-class 

 strength, and these can be correlated with climatic factors (e.g. Le Cren, 

 1955). These density-independent mortality factors will normally obscure 

 the operation of density-dependent factors, but in the long-run the latter 

 will be important for the maintenance of stable populations, both in natural 

 and fished stocks. The data described above suggest that the 'efficiency' of 

 reproduction and recruitment is greater in less dense populations, and this will 

 tend to be compensatory. The normal overproduction of gonad products 

 and young is an insurance against the heavy independent mortality which 

 may occur on some occasions. It seems that this 'insurance' makes con- 

 siderable demands upon the production of the species, and often, as well, on 

 the productivity of the environment. It might, therefore, be the aim of 

 fishery managers to reduce the need for such a large insurance. This could 

 be done by trying to mitigate the effects of extremely unfavourable environ- 

 mental conditions, and using facihties for artificial propagation more for 

 resuscitating stocks that had been decimated by exceptionally unfavourable 

 events in the environment than for adding a small contribution to normal 

 natural propagation. Much could be done as well by measures to reduce 

 juvenile mortality similar to the predator control in Cultus Lake. Every 

 fish that is encouraged to spawn, or allowed to die a natural death, over 

 and above the minimum number necessary for the long-term maintenance 

 of the stock is a fish wasted by the fishery. Above all there would seem to be 

 a great need to reach a real understanding of the processes of propagation 

 and recruitment from the point of view of population dynamics. 



SUMMARY 



A consideration of some estimates of production from a few species of 

 freshwater fish shows that the production of gonad products can be as large 

 as the growth production of new flesh, and is probably usually a significant 

 proportion of the total production. Little of the production of juvenile fish 

 survives to recruitment and so the overall reproductive and recruitment 

 process is inefficient. However, this efficiency is inversely related to popula- 

 tion density and so is a precaution against the occasional catastrophic density- 

 independent mortality. Fishery management should aim to understand these 



