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

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 (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.