Food interrelationships between brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), in a small put-and-take stillwater fishery

Abstract. The food of brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum). in a small stillwater put-and-take fishery in Surrey, England was examined by stomach content analysis of fish caught by angling between the months of April and September 1985. Overall, brown and rainbow trout tended to utilize different food sources. Mann-Whitney U-tests showed most major food items to be eaten in significantly different amounts, the most important exceptions being chironomid pupae and adult Diptera. In all months except April, rainbow trout utilized mainly midwater food, particularly Cladocera. Brown trout fed mainly on benthic food organisms and fish. Spearman rank correlations showed diets to be dissimilar during this period, with significant negative correlations in May and August. These results indicate an absence of interspecific competition for food, and imply spatial separation of brown and rainbow trout. In April both brown and rainbow trout fed extensively on the temporary bottom fauna, mainly trichopteran and megalopteran larvae, and diets were significantly positively correlated.