Pelagisk fisk i Mälaren 2024

The Institute of Freshwater Research at the Department of Aquatic Resources, SLU, conducted a hydroacoustic survey with pelagic trawling in Lake Mälaren in September 2024. Fieldwork took place between September 16 and 19, 2024, as part of the annual hydroacoustic surveys carried out in lakes Mälaren, Vättern, and Vänern. The areas surveyed in Mälaren were Ekoln, Granfjärden, Görväln, and Prästfjärden. The 2024 data analyses focused on the pelagic fish species smelt (nors) and vendace (siklöja), divided into age classes: young-of-the-year (0+) and yearling-and-older individuals (>0+), as well as pikeperch, young-of-the-year pikeperch, bream, and three-spined Summary stickleback. Other species were not caught in sufficient numbers to allow analysis. To assess the development of fish stocks in terms of recruitment, abundance, and changes in fish length, two types of analyses were carried out: Short-term trends were evaluated by comparing the mean fish densities from the last two years (2023 and 2024) with the average of the five preceding years (2018–2022). Long-term trends (since 2012) were analyzed using linear regression, where fish abundance was explained by year. In addition to abundance, the development over time using indices of size, age, and condition was presented, as well as total biomass and the number of fish divided into size classes. The results were: • In the short term, the number of fish under 17 cm increased in Görväln, while the number of larger fish decreased in Prästfjärden. In Prästfjärden and Ekoln, there were nonsignificant trends indicating a decrease in total biomass per hectare. During the same period, bream decreased in all areas except Granfjärden, and stickleback declined in Görväln and Prästfjärden. Vendace also showed a decline in Prästfjärden. In Granfjärden, there were tendencies toward increases in the number of adult smelt and adult pikeperch. In Görväln, both adult smelt and young-of-the-year smelt increased. • Over the longer term (since 2012) the number of fish per hectare in the 4–8 cm size range showed a trend to increase in Ekoln, as well as fish in the 8–17 cm range in Görväln. At the species level, adult smelt in Görväln have steadily increased since 2012, while pikeperch show a declining trend in Prästfjärden. • Adult pikeperch length has increased in eastern Mälaren (Görväln and Prästfjärden) and the length of the largest individuals have increased in both eastern Mälaren and Granfjärden. Growth in young-of-the-year pikeperch appear to have increased in Granfjärden. The length of adult smelt has decreased in Ekoln, while the length of smelt young-of-the-year has increased in both Ekoln and eastern Mälaren. Vendace young-ofthe-year have also increased in length in eastern Mälaren. However, the condition of adult vendace has declined in eastern Mälaren.