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 si ze-dependent prey encounters was able to account for this trend and to gene rate accurate predictions of the consumption of aquatic prey of different s izes. In contrast, the same model failed to predict the size composition of terrestrial prey (restricted to the upper layers of the water column) eate n 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 t heir relative abundance than on their size. However, the smallest prey were rejected. We suggest that trout were switching, i.e., overexploiting the m ost abundant prey, because of perceptual limitations mediated by large diff erences in relative abundance of the different size classes of terrestrial prey. The size-frequency distributions of the available terrestrial prey we re 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 di stributions of aquatic prey were less skewed. Overall, active choice guided by energetic; optimization criteria appeared to be of limited importance i n determining the size composition of prey eaten by this population of brow n trout Our results also indicate that the operating mechanisms of prey-siz e selection are probably not independent of the characteristics of the size -frequency distribution of the available prey.