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This report describes the results from studies on the temporal development of fish and zoobenthos communities in the vicinity of the nuclear power plant in Simpevarp in the Baltic Sea over the past 40 years.

Fish communities have been monitored using fyke nets and survey nets. The results indicate that the number of species and the abundance of the dominating species, perch (Perca fluviatilis) has increased in the vicinity of the power plant. Negative trends in abundance were observed in only a few fish species. The marine species cod (Gadus morhua) and herring (Clupea harengus membras), which occur predominantly in coastal areas during the colder season, showed strong variation over time, and dominated the catches during different time periods. Cod abundance decreased significantly over the years. Strong temporal variation was also seen in some other species during the colder season, probably following the concentration of herring. However, monitoring during the cold water period was affected by disturbance from seal in more recent years.

Monitoring of fish diseases and parasites showed no effect of the cooling water on their prevalence.

Water temperatures in the Baltic Sea show an increasing trend during the studied time period. This increase has been related to a generally increased growth rate of perch in the Baltic Sea, which was also seen in Simpevarp. However, the increase was stronger in areas affected by cooling water than in the other areas. Studies of perch young life stages showed a positive relationship between the abundance of adult perch in the Simpevarp area and the productivity of perch recruits in the Hamnefjärden area, which is strongly affected by cooling water. However, it is not clear whether the recruits originate from the Hamnefjärden area or if they have immigrated from adjacent areas at an early stage. Additionally, indications of a negative relationship between high winter temperatures and the development of perch gonads were seen.

Fish losses due to cooling water entrainment were low compared to mortality caused by the commercial fishery, but effects on the local coastal fishery cannot be excluded.

Catches per unit effort of eel in nearby commercial fisheries showed a small but decreasing trend since the 1970's.

As for zoobenthos, species composition and total abundance showed strong changes over time since the 1960's. These were mainly related to large scale changes in the Baltic Sea. However, some effects of the cooling water were seen in the site located closest to the effluent.