Stream Foraging Brown Trout: Responses to Woody Debris and Terrestrial Invertebrates

Trout foraging in small forested streams is largely affected by the surrounding forest, both by affecting habitat structure and prey availability. In this presentation we focus on trout responses to woody debris and terrestrial invertebrates, two factors influenced by riparian zone structure, potentially affecting streams and foraging of brown trout (Salmo trutta). Using a combination of, laboratory and field experiments, we examined the effects of woody debris and terrestrial invertebrates on prey availability and on the growth rates, diets and behavior of different size-classes of trout. The field experiments showed that addition of high densities of large wood affected trout growth in a positive way. This positive effect of large wood on trout growth may be related to prey abundance, as indicated by the high standing crop of aquatic macroinvertebrates on the wood. The positive effects may also be related to decreased energy expenditures by the trout, as trout increased their feeding efficiency and decreased their swimming activity and level of aggression as wood densities were increased in a laboratory experiment. Terrestrial invertebrates are generally assumed to be a high quality prey resource for fish, and our field experiments showed that reduction of terrestrial invertebrate inputs had a negative effect on trout growth. The availability of terrestrial prey in the stream was also coupled to trout diet and linked to growth, as fish with high growth rates had high proportions of terrestrial prey in their diets. Many of our results were also dependent on fish-size. We observed, for example, that large trout had higher capture rates on surface-drifting terrestrial prey than small trout when prey densities were intermediate or high, but at low prey densities, capture rates by large and small trout were similar. Moreover, both large wood and terrestrial invertebrates affected growth of small and large trout, but the effects were generally stronger and more consistent for large trout.