Biologisk recipientkontroll vid Forsmarks kärnkraftverk: årsrapport för 2011

This report describes surveys conducted in 2011 within the biological surveillance program of the Forsmark nuclear power plant, aiming to monitor the effects of cooling water intake and discharge on fish, zoobenthos, and sea birds. In addition to the regular surveillance program, a set of supplementary studies were conducted in order to provide more refined baseline data before a planned increase in cooling water usage. Primary results of the studies are given in relation to previous years’ results. Losses at the cooling water intake were dominated by stickleback (Gasterosteus aculeatus), as in previous years. The greatest abundances occurred in the fall and main part of the sticklebacks was young individuals. Losses of herring (Clupea harengus membrans) were high in the autumn in comparison to previous years. Their low average weights indicated a predominance of young individuals, and hence, a strong recruitment of herring in the Forsmark area in 2011. Losses of eel (Anguilla anguilla) were higher than in previous years. Their mean weights indicated that the majority of the trapped individuals were silver eels. The results of gill net surveys in the Biotest basin showed high catches both in the spring and autumn, dominated by perch (Perca fluviatilis) and roach (Rutilus rutilus). Catches of northern pike (Esox lucius) were low, and the species was nearly absent. The results indicated that spawning perch and roach are attracted to the Biotest basin during spring, probably due to the elevated temperature. Despite the large number of potentially spawning individuals, catches of young-of-the-year perch in late summer were low. However, the incidence of young roach was higher than in many previous years. Surveys with fyke nets showed that the abundance of eel in the Biotest basin has decreased since 2004, when fish migrations in and out of the basin were made possible by removing the grating that was previously positioned at its opening to the sea. The size of the eels in the catches has increased since the 1990's. A large number of eels, 500000, were restocked into the Biotest basin in 1989.The results suggests that the restocked eels are still present in the area. They are probably also a large part of the eels that are trapped in the cooling water intake. Smelt (Osmerus eperlanus) was observed for the first time since 2004 during the surveys with fyke nets and gill nets in the spring. The species may be attracted to the area of elevated water temperatures for spawning. However, its reproductive success in the Biotest basin is expected to be low, due to the strong temperature difference between water in the Biotest basin and the sea outside, which would probably have detrimental effects on the survival of larvae when they are washed out into the sea after hatching. It is possible that herring, which has a similar spawning habit as smelt, is also attracted to the Biotest basin in spring. During the surveys of fish larvae in the Forsmark archipelago, relatively high abundances of young-of-the-year perch were observed, in comparison to the very low abundances observed in previous years. The perches were larger than expected for the time of year, indicating that some of the individuals may originate from the Biotest basin, where growth rates are faster due to the elevated temperature. Overall, gill nets catches in the Biotest basin during fall were the highest recorded since 1990. The trend over time shows that catches of fish have increased in the Biotest basin since 2004. The condition index of roaches from the Biotest basin was, however, poorer than for individuals from the reference area. This could indicate food deficiency for roach inside the basin, and potentially also for other benthivorous fish species. After increasing abundance and biomass of benthic macrofauna in the 1980's and 1990's, biomasses remained high until 2010 at the station temporarily affected by cooling water (Länsman). Similar changes also appeared at stations with corresponding depth and sediments in the reference area, but differed during the seven last years with decreasing biomasses. Sampling 2011 however showed extremely low abundance and biomass in the Forsmark area ( Länsman), whereas number of species were quite similar to recent years. The deeper station in the Forsmark area (Engelska Grundet), which not is affected directly by cooling water and corresponding station in the reference area showed similar developments in benthic macrofauna. The bird surveys showed that the Biotest basin and its adjacent areas are important resting and overwintering sites for birds. Most birds were observed in the actual Biotest basin and in adjacent areas in the outlet.