EFFECTS OF PULP-MILL EFFLUENT ON A BALTIC COASTAL FISH COMMUNITY

Abundance and species composition of fish, including early stages, were studied along a gradient from a Bothnian pulp mill producing bleached kraft. Close to the outlet, the density of fish was very low and the recruitment drastically reduced. Further away, biomass increased to a high level in a zone characterized by dense populations of cyprinids, mainly roach, and ruffe. Other common Baltic species, e.g. herring, perch and sand-goby, occurred in subnormal densities. Compared to the cyprinids, the recruitment of these species was disturbed in much wider areas, which is considered to be a consequence of their larval biology. The populations of at least perch and sand-goby must largely be based on immigration. The effluent impact on the fish communities seems to be due to a combination of its eutrophicating, toxic and repelling properties; the low transparency of the water may also be important.