The influence of large wood on brown trout (Salmo trutta) behaviour and surface foraging

1. Changes in riparian vegetation owing to forest harvesting may affect the input of large wood, a major structural element, to streams. Studies of large wood impacts on stream fish have focused on population‐level responses, whereas little attention has been given to how wood affects fish behaviour. 2. In a laboratory stream experiment, we tested how two size classes of brown trout, Salmo trutta, (mean size of 85 and 125 mm), alone and together, responded to a gradient of large wood in terms of activity, foraging on terrestrial drift and interactions between conspecifics. 3. The results showed that the presence of large wood significantly reduced the overall activity of the fish, the number of agonistic interactions between individuals and the proportion of captured prey. However, activity decreased relatively more than the proportion of captured prey, resulting in a significant positive net effect of wood on the number of prey captures per time spent active (PTA). This indicates that trout living in habitats with high wood density may have a higher net energy gain than trout living in habitats with less wood. 4. There were no observable size‐class differences in the benefits of large wood or in the utilisation of surface‐drifting terrestrial prey. 5. These results suggest that the presence of large wood may be an important factor shaping stream communities and that a lack of structural complexity may decrease energy gain, increase agonistic interactions and, consequently, lower the production of brown trout.