Are warm temperatures the key factor to prevent winter swarms of

The invasive bloody red mysid, Hemimysis anomala, has recently colonised peri-alpine lakes in Western Europe. During the last decade, scuba divers have regularly reported observations of substantial winter swarms of this animal in Lakes Bourget (France) and Geneva (Switzerland-France). During the last four years, we have regularly dived and observed such phenomena in a variety of habitats, revealing thousands to millions of individuals during the period between December and March, with maximal densities occurring in the water column and small open wrecks in January or February. Surprisingly, however, no swarm was observed during the winters of 2022–2023. This led to the hypothesis that temperature could be the main direct or indirect factor responsible for this lower abundance, resulting in the absence of significant gatherings. Some possible explanations are discussed.