Prey-size selection by brown trout (Salmo trutta L.) in a stream in northern Spain

Brown trout in the River Negro in northern Spain preferentially ate larger aquatic prey items (found throughout the water column). A model based on size-dependent prey encounters was able to account for this trend and to generate accurate predictions of the consumption of aquatic prey of different sizes. In contrast, the same model failed to predict the size composition of terrestrial prey (restricted to the upper layers of the water column) eaten by the trout. Trout ignored the larger (more profitable) terrestrial prey, and the consumption of prey of a given size class was more dependent on their relative abundance than on their size. However, the smallest prey were rejected. We suggest that trout were switching, i.e., overexploiting the most abundant prey, because of perceptual limitations mediated by large differences in relative abundance of the different size classes of terrestrial prey. The size-frequency distributions of the available terrestrial prey were always greatly dominated (75-90%) by the two smallest size classes (1-2 and 2-3 mm long), prey over 4 mm long being extremely scarce, while size distributions of aquatic prey were less skewed. Overall, active choice guided by energetic; optimization criteria appeared to be of limited importance in determining the size composition of prey eaten by this population of brown trout Our results also indicate that the operating mechanisms of prey-size selection are probably not independent of the characteristics of the size-frequency distribution of the available prey.