Hydroakustiska undersökningar i Stockholms och Södermanlands skärgårdar

The purpose of thisstudy was to collect data on coastal pelagic fish stocks and on herring recruitment in the southern Stockholm and northern Södermanland archipelagos to contribute with knowledge to the follow-up on the relocated trawl limit from 1 February 2025 to 30 April 2027. To cover large areas with varying depths data was collected using hydroacoustics, supported by short midwater trawls at representative depths, to assign species composition and size distribution. In focus for this study were the pelagic species herring, sprat, stickleback, and smelt. Where data permitted, the proportion of young-of-the-year fish was calculated. Comparisons were made with results from a corresponding survey in 2021, as well as with results from similar, older surveys. In late summer 2025, the number of fish in Himmerfjärden, Askö-/Hållsfjärdarna, and Gälö- /Svärdsfjärdarna was dominated by young-of-the-year herring, unlike in 2021 when stickleback was most abundant. In Askö-/Hållsfjärdarna in 2025, large numbers of herring larvae were caught which were not verified in the hydroacoustic data, probably because these larvae were mostly too small to be registered by the hydroacoustics as individual fish. In most areas, the biomass was dominated by yearling and older herring. Two-year-old herring were most common in all areas except Himmerfjärden, where both yearling and two-year-old herring were equally common. Three-yearSummary old and older herring were caught in smaller numbers than one- and two-year-olds in all surveyed areas. Sprat was most abundant in open areas such as Askö-/Hållsfjärdarna and Nämdö, while smelt was most abundant, as young-of-the-year, in the inner archipelago areas. Three-spined stickleback was abundant in all areas. The largest occurrences of stickleback in 2025 were around Nämdö, while it had lower abundance in Himmerfjärden, Askö-/Hållsfjärden, and Gälö-/Svärdsfjärden compared to 2021. Recruitment of herring shows large variation over the years. The fact that young-of-the-year herring was estimated to be significantly more abundant in 2025 than in 2021 was probably due to 2021 being an exceptionally weak year for herring recruitment in the study area. Three-spined stickleback was absent in the trawl catches in Himmerfjärden in years 2002–2004 but was abundant in both 2021 and 2025 and can thus be considered established in the area.