Effects of beaver dams on the fish fauna of forest streams

Abstract

The beaver ( Castor fiber) was exterminated in Sweden, but now, after re-introduction, again occurrs in high densities. The activities of beaver – felling of trees and construction of dams – are important ecological factors in forest streams. The proportion of wetlands and open water in the forest landscape is increased, and new stream sections with slow-running water are created. This change of habitat characteristics is predicted to have consequences for the composition of stream fish communities. We studied the fish fauna in seven small to moderate-size streams with beaver in central Sweden, using electro-fishing. Beaver ponds and adjacent riffle sections were compared to unaffected reference sections. The two most common species were minnow ( Phoxinus phoxinus) and brown trout ( Salmo trutta). Three additional species occurred in lower numbers: Bullhead ( Cottus gobio), burbot ( Lota lota) and pike ( Esox lucius). Brown trouts were more common in reference sections than in beaver-affected sections, while the opposite tended to be the case for minnows. The numbers of brown trouts and minnows in separate sections were negatively correlated. The size composition of the two species also differed between sections. Brown trouts captured in the ponds were larger than those in the riffle sections, while the opposite was true for the minnows. Shallow areas of the beaver ponds were important habitat for minnow fry. Beaver ponds are likely to serve as habitat for larger trouts in small streams during drought periods. Minnows appeared to spread from ponds to surrounding riffle areas. We conclude that beaver-mediated habitat changes have important effects on the relative abundance of fish species in Swedish forest streams, and that they may enhance fish species diversity.