Exploring ultimate hypotheses to predict diel vertical migrations in coregonid fish

Evolutionary hypotheses for diel vertical migrations (DVM) of aquatic animals include foraging opportunity, predator avoidance, and bioenergetics efficiency. Here we test which hypothesis predicts DVM in the small planktivorous coregonids vendace, Coregonus albula, and Fontane cisco, Coregonus fontanae, in a deep oligotrophic lake. Densities and population depths of young-of-the-year and larger coregonids were determined by hydroacoustics during day and night over 10 consecutive months. Depth distributions of predator-like fishes and zooplankton resources were recorded as well. Furthermore, Secchi depth, water temperature, oxygen concentrations, and pH values were determined at each sampling month. A DVM of the coregonids was observed in all months. Population depths during the night were significantly correlated to water temperatures, oxygen concentrations, and pH values. In contrast, the vertical distributions of predators or resources were not correlated with the coregonid depth distribution. These results do not correspond to the feeding opportunity or predator avoidance hypotheses of DVM, but support in part the bioenergetics efficiency hypothesis. However, the stable migration pattern of fishes over all months despite substantial changes in biotic and abiotic conditions suggests that diel migrations in the coregonids are a genetically fixed behavioral trait to minimize the anticipated potential predation risk in the illuminated water layers during daytime.