Adverse impact of forestry on fish and fisheries in stream environments of the Isojoki basin, western Finland

The effects of forestry on fish and fisheries were studied in the Isojoki basin, western Finland. The most abundant fish species in the study area, brown trout (Salmo trutta L.), was found in 27 of 50 brooks studied. Enzyme electrophoresis revealed at least five genetically differentiated stocks of brown trout in the brooks. Regression analysis indicated that the population density of brown trout was positively dependent on the abundance of pools, stony bottom material (2 to < 10 cm in diameter), undercut banks in the streams, and water pH, while the abundance of ditches in the catchment area and shading by the tree canopy affected the density negatively. Incubation experiments showed that sedimentation with mineral and humus material impaired the survival of trout eggs in the rapids. The results suggest that dredging of brooks and increased soil erosion due to drainage may threaten the reproduction and genetic diversity of trout stocks in the brooks and main river.