ON COMPETITION AND TEMPORAL VARIATION IN TEMPERATE FRESH-WATER FISH POPULATIONS

Recruitment in fish populations is characterized by a high temporal variability and a considerable stochasticity. It has therefore been suggested that fish populations should be likely candidates for coexistence based on the storage mechanism. Many characteristics of fish populations such as high variability in year class strength and mortality biased towards smaller (younger) individuals are also elements that make coexistence based on the storage mechanism likely. At the same time, considerable experimental evidence usually associated with deterministic competition models, such as density dependence in individual growth rates, density dependence in populations densities, and competition-related niche dynamics, is also present. We use temporal variation in temperature, an environmental variable involving both a stochastic and a regular (seasonal) component, to illustrate how both coexistence based on a storage mechanism and patterns expected from deterministic competition models may co-occur. Temperature is as important in fish as in other poikilothermic organisms, since the carrying capacity of the population will vary with temperature through the latter's effect on the metabolic requirements of individuals. Seasonal variation in temperature will, through its effect on metabolic demands, also cause favourable periods to alternate with unfavourable periods, i.e. environmental effects covary with competition effects as assumed in lottery models. We suggest that seasonal variation in temperature will promote coexistence and at the same time allow competition to be a regular event with significant evolutionary potential. Experimental and field evidence that supports the idea of a seasonal variation in resourse limitation strongly coupled to temperature is provided.