Parasite frequency and liver anomalies in three-spined stickleback,Gasterosteus aculeatus (L.), after long-term exposure to pulp mill effluents in marine mesocosms

Model ecosystems, simulating a Baltic Sea littoral habitat, were used for evaluating the effects of different effluents from kraft pulp mill industries on fish. Populations of newly hatched three-spined stickleback (Gasterosteus aculeatus) were continuously exposed to four different types of pulp mill effluents in a flow-through sea water system. After 5 1/2 months of exposure, the sticklebacks were collected for a skin parasite count and histological examination of the gills and liver. The frequencies of two types of skin-dwelling ciliates of the genera Trichodina and Apiosoma were higher in the exposed fish populations, particularly in those exposed to the high effluent doses (diluted 400 times). Anomalies in the liver cell structure were observed as necrotic cells, nuclear pyknosis, vacuolation, and fat accumulation. The two effluents from the factories producing unbleached softwood and bleached hardwood without oxygen pre-bleaching affected the sticklebacks most, both in respect to ciliate abundance and to liver anomalies.