Fish population dynamics in the River Frome, Dorset

This paper examines the influence of biotic and abiotic factors on the population dynamics of salmonid and non‐salmonid (coarse) fish in a chalk river. The naturally regulated flow regime, allied to a supply of water rich in inorganic salts, gives rise to high levels of primary and secondary production. Fish growth rates are also high, but they vary with relatively small changes in the summer water temperature regime. Hence the survival of newly‐hatched fish varies and this is shown up in later years by variations in year‐class strengths. Anadromous salmonids show a distinct seasonality in their upstream spawning migration, and many fish to not migrate until after the end of the salmon angling season. However, river discharge influences migration and in years of very low discharge the seasonal pattern can be disrupted. Little is known concerning species interactions, or of the effect of man‐made adjustments to the river environment, on fish behaviour and distribution patterns. Information on pike predation shows that apparently common‐sense regimes of fishery management, designed to protect salmon populations, may be counterproductive. Traditional times for cutting aquatic macrophytes as a flood alleviation measure may be detrimental to the spawning success of roach. A switch to removing the weed in the autumn would have less impact on fish populations and is more cost‐effective. Information derived from short‐term investigations can be extremely informative, but results need to be set against a background of long‐term investigations.