A decade of research on the environmental impacts of pulp and paper mill effluents in Canada: development and application of fish bioassays.

Laboratory tests have been used to assess the regulatory and research questions related to the effects of pulp mill effluents on aquatic biota. Acute, short-term laboratory tests have clearly shown the improvement in final effluent quality following installation of secondary treatment at Canadian pulp mills. In an effort to predict and investigate impacts on wild fish, laboratory bioassays were developed to examine sublethal endpoints: induction of hepatic mixed function oxygenase activity and reduction of sex steroid concentrations. These laboratory assays have been used to assess whole effluents, specific chemicals, and components of pulp mill processes, and to discriminate between historical and present-day effluent discharges. These tests have shown that induction of mixed-function oxygenase activity and reduction of sex steroid concentrations are produced by effluents from a variety of mill types, with and without chlorine bleaching, in hardwood and softwood pulping facilities, and before and after effluent treatment. These short-term bioassays have enabled reductions in sex steroid concentrations to be linked to mill process streams, and have provided information on effective waste stream treatment. Longer term, life-cycle fish bioassays have shown that chronic exposure to pulp mill effluents commonly results in growth enhancement, liver enlargement, and decreases in gonad size, secondary sex characteristics, and fecundity. These long-term laboratory exposures are able to mimic the most commonly observed alterations of wild fish exposed to pulp mill effluents: increases in condition factors, increases in liver-somatic indices, and decreases in gonadosomatic indices. This pattern of response is a combination of nutrient enrichment with metabolic disruption. The most sensitive and biologically meaningful endpoint is decreased reproduction in fish life-cycle exposures. As the laboratory tests move forward into the next decade, attention will focus on the reproductive endpoints and on the possibility of shortening the fish bioassays while still maintaining sensitivity and biological relevance.