Where can the Vendace (Coregonus albula) live during the least favourable period of the summer stratification in Lake Mälaren?

Sammanfattning: Temporal stratification in lakes due to high air temperatures and dynamic wind conditions changes the environment for organisms in the depth gradient. For fish that have narrow requirements for temperature and oxygen conditions, stratification can lead to a significant reduction in habitat volume during the summer when surface temperatures are high. In Sweden, temporal stratification occurs between July and October. The vendace (Coregonus albula) is a cold-water fish that is found in several waters around Sweden. In Lake Mälaren, there is targeted commercial fishing for the species, but the total annual catch has drastically decreased in recent decades. Shrinking habitats and longer periods of temporal stratification during the summer could be a partial explanation. Understanding and predicting changes of the vendace populationin Lake Mälaren requires mapping of available habitat during the summer temporal stratification. This study has investigated the worst habitat conditions in terms of habitat thickness for vendace during the years 2017-2022 in Lake Mälaren based on the habitat requirements temperature ≤18 °C and DO concentration ≥2 mg/L. The spatial distribution in two basins where analysed. The results showed that the vendace had limited available habitat in three of seven water bodies investigated (habitat thickness ≤2.5 m). The spatial analysis of the two basins showed that habitat volume represented 41 % and 11 % of the total volume of water in each basin. The available habitat of the vendace during the summer temporal stratification in Lake Mälaren is thus limited in parts of the lake.