Escaping malnutrition by shifting habitats: a driver of three-spined stickleback invasion in Lake Constance.

Fatty acids, and especially long-chain polyunsaturated acids, are biologically important components in the metabolism of vertebrates including fish. So-called essential fatty acids are those which in a given animal cannot be synthesised or modified from precursors and must therefore be acquired via the diet. Because EFA are often unevenly distributed in nature, this requirement may drive species to make behavioural or ecological adaptations in order to avoid malnutrition. This is especially true for fish like the three-spined stickleback of Upper Lake Constance (ULC), whose recent marine ancestors evolved with access to EFA rich prey (Gasterosteus aculeatus), but which found themselves in an EFA-deficient habitat. An unexpected and unprecedented ecological shift in the ULC stickleback population from the littoral to pelagic zone in 2012 might be linked to EFA availability, triggering ecological release and enabling them to build a hyperabundant population while displacing the former keystone species, the pelagic whitefish Coregonus wartmanni. To test this hypothesis, sticklebacks from the littoral and pelagic zones of ULC were sampled seasonally in two consecutive years, and their stomach contents and fatty acid profiles were analysed. Pelagic sticklebacks were found to possess significantly higher values of an important EFA, docosahexaenoic acid (DHA), especially during autumn. Evaluation of the DHA supply suggests that sticklebacks feeding in the littoral zone during autumn could not meet their DHA requirement, whereas DHA availability in the pelagic zone was surplus to demand. During autumn, pelagic sticklebacks consumed large amounts of DHA-rich prey, i.e. copepods, whereas littoral sticklebacks relied mainly on mostly cladocerans, which provide much lower quantities of DHA. Access to pelagic zooplankton in 2012 was possibly facilitated by low densities of previously dominant zooplanktivorous whitefish. The present study offers a convincing physiological explanation for the observed expansion of invasive sticklebacks from the littoral to the pelagic zone of Lake Constance, contributing to a phase shift with severe consequences for fisheries. This article is protected by copyright. All rights reserved.