SEASONAL CHANGES AND SELECTIVITY IN THE DIET OF BROWN TROUT IN THE RIVER NERA. (CENTRAL ITALY)

ABSTRACT The stomach contents of brown trout from the middle region of the Nera River, Central Italy, were compared with the composition of the macroinvertebrate community of the same river stretch. The epibenthic fauna, identified mostly at the species level, was composed mainly of Ephemeroptera (Serratella ignita, Baetis spp., Rhithrogena semicolorata), Plecoptera (Dinocras cephalotes), Trichoptera (Hydropsyche sp., Micrasema spp.) and Diptera. We found that the dietary composition of brown trout was not simply proportional to the environmental density of the prey. Ephemeroptera dominated the spring and summer diets; Trichoptera prevailed in winter. The electivity index showed a negative selection towards Baetis spp. and chironomids and positive values of selection for Trichoptera. Dimensions, mobility, degree of exposure, and visibility of prey, were important factors in shaping brown trout's diet. Nevertheless species of the genera Micrasema and Lepidostoma, despite their reduced dimensions and scarce...