Some factors regulating the size of populations of freshwater fish

Summary

The unusual adaptabilities of fish populations in numbers, size, and mobility, and their comparatively long generation times with repeated annual breeding are emphasised. Fish fecundity is affected by size and thus growth, and by other factors, and can be compensatory to some degree. Egg survival can be high in salmonids but can be influenced by density of spawners in relation to suitable spawning area. The survival and growth of fry and juvenile fish is variable. Climatic-induced factors can be very important in determining survival rates in the early stages but in salmonids territorial behaviour acts as a density-determining mechanism. Adult mortality rates tend to be constant regardless of age; causes of natural mortality are obscure but predation must be significant in many instances; stream and littoral species are often territorial. In unusual habitats and towards the limits of geographical ranges, physical factors may limit abundance, but otherwise population density is probably determined by biotic and especially density- related factors. More detailed and accurately quantitative data on all aspects of the population dynamics of fish are needed and a comparative study of population parameters and quantitative trophic dynamics would be rewarding.