Craig Goch Report No. 10 The effect of density on mortality in juvenile Atlantic salmon (Salmo salar L.)

Le Cren (1973) suggested that the most likely population-regulating process in salmonid populations is density-dependent territorial behaviour. Thus, in trout (Salmo trutta L.) populations aggressive territorial behaviour was manifested in density-dependent mortality and the size of the adult stock was largely dependent on the area of suitable rearing ground for the fry (Le Cren, 1973). Ricker (1954) also concluded that in most fish populations compensatory mortality in the juvenile stages, rather than in the mature stock, was of greatest importance as a regulator of population size. The nature of such regulating mechanisms in juvenile populations is, therefore, of fundamental importance in the understanding of the dynamics of fish populations and the implementation of management policies. This paper considers mortality rates of young Atlantic salmon (Salmo salar L.) in the upper catchment of the R. Wye over a two year period. The work forms part of a broader study of fish populations in the R. Wye