Effects of Forest Spraying with DDT in New Brunswick on Food of Young Atlantic Salmon

Food in stomachs of young Atlantic salmon from the Northwest Miramichi River, N.B., changed following aerial DDT spraying of the surrounding watershed. Before spraying, young salmon typically ate immature aquatic insects, fry (underyearlings) concentrating on Diptera and small Ephemeroptera, and parr (overyearlings) on Diptera, Trichoptera, and all sizes of Ephemeroptera. Reduction of all aquatic insects by DDT was soon followed by resurgence of Chironomidae and other Diptera, and surviving fry fed heavily on these; parr fed on Diptera but also ate snails, worms, and fish, previously unimportant in their diet. Five years after the last spraying the pre-spray complexity of young salmon food was being approached. Trichoptera were slowest of the major fish-food types to reappear in stomachs.In fry stomachs Ephemeroptera, and in parr stomachs Ephemeroptera and Trichoptera, were relatively more abundant than in the surrounding stream fauna. Greater availability of these insects and active selection by fish are discussed as possible causes of these relationships.