Stream salmonids as opportunistic foragers: the importance of terrestrial invertebrates along a stream-size gradient

Terrestrial invertebrates have been reported to be positively selected by stream salmonids. We assessed the impor- tance of terrestrial and aquatic invertebrates to salmonid diets in 25 streams in Finland, with the hypothesis that terrestrial prey would be important in only the smallest forest streams. Several measures of prey availability were used, including pro- portional abundance in benthic or drift samples, compared with a trait-based approach, to predict diet composition. Across all 25 streams in autumn, blackfly and caddis larvae were the most important prey items. Terrestrial invertebrates were of moderate importance in all streams, including the smallest. Pure availability predict…