Diet of 0+ brown trout (Salmo trutta L., 1758) from the river Erro (Navarra, north of Spain)

Abstract

The diet composition of 41 0+ brown trout (Salmo trutta L., 1758) (33-97 mm TL) captured in August 2002 in the Erro River (North of Spain) is described. The diet was mainly composed by aquatic invertebrates. Excluding nematodes because they are possible parasites, the most consumed prey items were mayflies, dipterans, and crustaceans. Fingerling brown trout refused Chironomidae, Elmidae, and terrestrial invertebrates from the drift, and Elmidae, Heptageniidae, and Leuctridae from the benthos, while they showed preference for Rhyacophilidae from the benthos. Fingerling brown trout seems to act as an opportunistic predator, and the consumption of different preys seems to be influenced by their accessibility, predation risk, and their energetic value.