Effects of peatland drainage on the size and diet of yearling salmon in a humic northern river

In the humic River Nuorittajoki, northern Finland, the recapture rates of s tocked yearling salmon, Salmo salar L., were lower in riffles affected by l oading from peat mining and forest drainage areas than in the reference rif fles receiving a considerably smaller loading. The salmon caught from the a ffected riffles were also smaller and thinner by the end of the summer than those from the reference riffles. The size was inversely related to the es timated particulate matter load imposed on the area. The salmon at the affe cted riffles appeared to feed less but there was no difference in the niche width between the two types of riffle. Hydropsyche and Rhyacophila larvae occupied significantly larger proportions of the stomach volume in the refe rence than in the affected riffles whereas Ephemerella larvae had an opposi te trend. These observations suggest changes in the foraging of yearling sa lmon in riffles affected by increased particulate matter loading from drain ed peatlands.