Effect of in-Stream Wood on Juvenile Trout Growth and Behaviour

Woody debris in streams provides shelter for fish and a substrate for stream invertebrates, an important prey resource for stream-dwelling fish. Most studies have focused on the effects of large woody debris, largely ignoring small woody debris. I used a combination of laboratory and semi-natural experiments to explore how small in-stream wood affects the behavior, diet and growth of young-of-the-year (YOY) brown trout (Salmo trutta). A laboratory experiment, performed at 13° C, showed that YOY brown trout spend less time feeding and cruising in the presence of in-stream small wood, and that the effect is more pronounced at an intermediate density of wood than at a high density. A semi-natural experiment revealed that trout growth rates from August to December (water temperature decreasing from 17° to 1° C) were lower in the presence of small wood than in its absence. Trout in both studies were most often found sheltering underneath the wood. My results suggest that small in-stream wood is used as a shelter by juvenile trout, in spite of its potential negative effect on trout foraging and growth.