Adaptations acquired during their evolution mean that some species are unsuited to physiological challenges that rising temperatures pose.  A group of fish species that fit this description are the coldwater, stenothermic coregonines (Salmonidae; Coregoninae): glacial relict species well adapted to cooler conditions. In Lake Constance, coregonines have been the mainstay of the local fishery for centuries; they are a local cultural good, as well as keystone species in the lake. One recent striking observation concerning Lake Constance coregonines is that adult fish lose weight during summer (DeWeber et al. 2022): historically they would gain weight during summer. Is it possible that the interaction between rising water temperatures in the epilimnion of Lake Constance, and recent changes in the zooplankton community, is the cause of this summertime malnourishment?

In recent years, there has been a persistent trend of temperatures exceeding historical norms. Higher temperatures increase metabolic demands on cold-water adapted species. Hence, Lake Constance coregonines may avoid warming surface waters, simply because temperatures there exceed their preferred range. Recent changes in the zooplankton community of Lake Constance may compound the stressor of rising temperatures by reducing the amount of food available to coregonines. Since 2016, the biomass of Daphnia cucullata has increased dramatically. Other previously dominant Daphnia species exhibited vertical diel migration through the epilimnion, so even if coregonines did avoid the warmest water near to the surface, they would be able to access Daphnia as food when these species descended within the water column. However, the now dominant Daphnia cucullata does not migrate vertically over the diel cycle, and tends to inhabit the warmest surface water. Ergo, coregonines may be avoiding the highest densities of an important food source.