Biologisk recipientkontroll vid Oskarshamns kärnkraftverk: årsrapport för 2025

The Oskarshamn nuclear power plant affects the surrounding environment primarily using seawater as a coolant. Because of the cooling system and its discharge, the temperature in the recipient bay, Hamnefjärden is normally higher on average than the nearby reference area, Borholmsfjärden. The operating reactor was shut down for revision from end of March to early November, which was much longer than scheduled. The average water temperature in Hamnefjärden during the revision period of 2025 was 0.2 °C colder than it was in Borholmsfjärden. which has not been recorded in several decades. No targeted studies on fish loss in the cooling water intake management system were conducted 2025, and no deviations were reported from the power plant’s intake station. The prolonged maintenance outage meant that there was no additional heating of the water in Hamnefjärden for almost half of 2025, which most likely affected catches throughout the sampling season. Thus, a higher abundance of coldwater species (i.e. species with low temperature optima) occurred in the “biological gillnets” during spring fishing in Hamnefjärden. Furthermore, this could also be the main explanation as to why European eel catches in Hamnefjärden’s fyke net fishing were extraordinarily low. However, catches of the invasive alien species round goby in the fyke nets were the largest on record. Perch, common roach, and Eurasian ruffe were highly abundant in the sampling conducted during summer with standardized biological gillnest. During the autumn sampling, this shifted, to perch, white bream, rudd, and common roach being the most abundant species in the catches. The average bottom water temperature during summer was the highest it has been in five years, which would explain why more warmwater species (i.e. with high temperature optima) were caught during these months (even though it was in the middle of revision). In addition to the aforementioned species, bleak, herring, and Eurasian ruffe were abundant in the Nordic coastal survey nets. Any discrepancies in catches between the recipient and reference area can partly be explained by natural differences in abiotic factors, rather than any impact from the power plant’s warm water outlet. Several of the known coldwater species, such as herring, shorthorn sculpin and cod were caught during spring, in the coastal survey nets set in the coastal area just outside the mouth of Hamnefjärden. Meanwhile, catch rates for many of the commonly caught species remained low in 2025’s coastal survey nets. Seal disturbance was recorded in 81 percent of the sampling effort. Perch and roach fry were either absent or recruited in low numbers during the underwater detonation’s surveys conducted in Hamnefjärden 2025. According to Fulton´s condition factor, female perch and roach in Simpevarp and Kvädöfjärden were of good condition or slightly above the threshold. None of the sampled female perch had deformed gonads. Most fish with identified disease symptoms were caught in the recipient area. Only two fish with identified disease symptoms were recorded in the reference area. The commercial silver eel fishing near Simpevarp yielded lower catches in 2025 than in the past three years. In fact, these catch quantities were one of the lowest they have ever been since recordings started in 1972. During the benthic fauna survey 2025, a total of 21 species were recorded in Simpevarp and 15 in Kvädöfjärden. Blue mussel, followed by the Baltic macoma mussel were the most abundant species on shallow bottoms in both areas. On deep bottoms of Simpevarp, the tube-building marine bristle worm Pygospio elegans was the most common species, while the most abundant species in Kvädöfjärden were the Baltic macoma mussel and the benthic amphipod Monoporeia affinis.