Effects of Brook Trout Predation on a Crayfish Population

Abstract

A study was made of the effect of trout predation upon the crayfish population of a marl lake. The study provided a measurement of the efficiency of energy transfer between trophic levels in the lake and the assessment of the relative importance of fish predation as an interspecies interaction. Trout fed only on crayfish less than one year old. Only trout greater than 229 mm in length fed extensively on crayfish. Trout predation accounted for the removal of less than 3% of the population of age-0.0 to 0.5-year crayfish during the period from June to January, and 16% of the population of age-0.5 to 1.0 crayfish from January to May. Crayfish were an important part of the trout diet in midwinter and late summer. Age-0 crayfish which were much larger in midwinter than in summer constituted a greater percentage of the fish diet at these times. On an annual basis trout utilized only 2.9% of the total net production and only 4.2% of the available net production of crayfish biomass in the lake. Substituting the trout predation rate for fishing mortality in Rickerˈs equilibrium yield equation, showed that a maximum yield of crayfish would be attained at two times the current rate of trout predation. Crayfish yield to the trout might be increased by introducing another smaller crayfish, O. propinquus, with the same reproductive potential as O. virilis. Predation was not an important population control mechanism.