Biologisk recipientkontroll vid Forsmarks kärnkraftverk: årsrapport för 2025

Forsmark Nuclear Power Plant is Sweden’s largest electricity producer. The intake and discharge of seawater used to cool the reactors affect the surrounding coastal marine environment. These effects are monitored within the ongoing biological recipient monitoring program. This annual report presents results from the 2025 monitoring activities, focusing on fish, benthic fauna (bottomdwelling invertebrates), and seabird communities. In 2025, Forsmark Nuclear Power Plant produced 23.7 terawatt-hours (TWh) of electricity, corresponding to approximately one-sixth of Sweden’s electricity consumption, and resulting in a high turnover of cooling water. In the cooling water discharge area for reactors 1 and 2 (F1 and F2), Biotestsjön, water temperatures exceeded 25 °C for 26 days during summer, almost entirely in July. In the discharge area associated with the third reactor (F3), temperatures exceeded 30 °C for 11 days, with a maximum recorded temperature of 34 °C. Fish losses at the coolant water intake screening stations consisted primarily of small-bodied species and young-of-the-year individuals, as in previous years. Approximately 25 million individuals were impinged at the intake screens during the 2025 sampling periods, slightly more than in 2024 but still lower compared with the past decade. This likely reflects reduced flow velocities caused by maintenance and repair work during much of the sampling period. Three-spined stickleback dominated in the samples, accounting for approximately 86% of total fish losses, likely reflecting the species’ high abundance in surrounding coastal waters. The number of European eels captured at the intake screens was lower in 2025 than in 2024, and overall eel losses remained very low in the long-term time series. Losses were highest in autumn, mainly involving mature silver eels migrating toward spawning areas. Development of a camera AIbased eel detection system was successful with over 90% positive recognition; however, work on an automated system to divert eels before they reach the collection sump and cleaning pumps appears to have stalled. Resuming this work is important to improve eel survival and allow the release of individuals in good condition. Spring surveys in Biotestsjön showed that fish densities remain extremely high. The most abundant species were roach, perch, and white bream, as in previous years. White bream catches were higher than in 2024 and relatively high in the long-term perspective. As previously observed, large numbers of fish appear to be attracted to the warm water in Biotestsjön for spawning. In contrast, eel abundance remains low compared with earlier years, although relatively high compared with the past decade. For warm-water species such as roach and perch, conditions in Biotestsjön during 2025 appeared favourable, with growth and body condition close to long-term averages. Recruitment of young- ofthe -year was lower than in the previous year in Biotestsjön but remained favourable in the Forsmark archipelago, where perch larvae occurred in high numbers for the second consecutive year after several years of low abundance. During autumn 2025, only a few older perch individuals than X years were recorded in survey catches in Biotestsjön. In 2025, only three of five sampling stations were surveyed during spring and winter, as the removal of the pier has made two stations too shallow to sample. In the Forsmark archipelago, monitoring surveys indicated that fish densities remained high relatively catches from recent years. Perch and roach continued to be the most abundant species. The surveys of benthic fauna on soft bottoms showed variation in species composition between sites outside the Forsmark Nuclear Power Plant and the reference area in Finbofjärden. Three species Summary were found at all stations—the American polychaete (Marenzelleria sp.), the Baltic clam (Limecola balthica L.), and the amphipod Monoporeia affinis—all occurring at relatively high densities. The indicator species M. affinis was the third most abundant species in the samples, reaching 100 individuals per square metre, which is higher than the average density across the entire time series. Surveys of hard-bottom fauna in the Forsmark area showed substantial differences between sites in both species occurrence and individual density. Densities were higher in Biotestsjön than at sites less affected by the warm-water discharge. Comparing species distributions across the Forsmark area clearly illustrates the effects of cooling-water use. In Biotestsjön, species that benefit from elevated water temperatures occur at high densities. Several non-native species are also present, including the bloody-red mysid (Hemimysis anomala), Conrad’s false mussel (Mytilopsis leucophaeata), and the New Zealand mud snail (Potamopyrgus antipodarum). In Asphällafjärden (cooling water inlet), species richness is higher than at the Biotest and has remained so throughout the monitoring period. In this area, benthic communities are strongly influenced by power plant operation through the intake of cooling water, where filter-feeding species such as the brackish-water cockle (Cerastoderma glaucum) and the blue mussel (Mytilus edulis) benefit from the continuously high flowing water. The number of waterbirds decreased again in 2025 to 37 740 counts, which is below the average for all years since the survey began in 2002. Out of the seven subareas Biotestsjön and Asphällafjärden were the subareas with more than half of all recorded bird observations in 2025. However, more observations were made in Biotestsjön than Asphällafjärden. For the first time, common goldeneye was the most abundant species in the survey area, followed by tufted duck and cormorant. Various activities in Asphällafjärden are believed to have affected bird occurrence here, both in terms of number of individuals per species and their spatial distribution within the greater survey area. For example, Biotestsjön became a more important subarea for the benthivorous species tufted duck and common goldeneye than Asphällafjärden, which is unusual. This indicates that the functional groups utilizing the different subareas were also affected by the activities and differed from previous years.