Diet of brown trout in relation to variation in abundance and size of pelagic fish prey

Abstract

The seasonal diet of a predator, brown trout Salmo trutta[total length (LT) 17–69 cm] and simultaneous density and size‐structure of prey populations, vendace Coregonus albula and smelt Osmerus eperlanus(4–16 cm LT), in a large boreal lake were analysed and compared in 2001 and 2002. The upper LT limit for consumed prey was c. 40% of the predator LT. All brown trout, however, preferred small (<10 cm LT) and avoided large (≥10 cm LT) prey. The results also suggested that equal densities of similar‐sized (4–10 cm LT) fish of the two prey species led to random foraging on these species by brown trout, but if either one of the prey species predominated (>50%) in the lake, brown trout shifted to foraging on this species almost exclusively. Brown trout diets thus reflected the density dynamics of the two alternative prey species.