Feeding ecology of Coregonus albula and Osmerus eperlanus in the limnetic waters of Lake Mälaren, Sweden

A large block of unpublished information from the late 1960s on the interactive ecology of two highly zooplanktivorous fishes - vendace (Coregonus albula) and smelt (Osmerus eperlanus) - in Lambarfjärden of Lake Mälaren, Sweden, was used to demonstrate quantitatively diel and seasonal changes in their limnetic feeding distribution, to compare their trophic structures, and to examine their selective use of some 20 different zooplanktonic taxa with those pelagically present by species, size, abundance and vertical distribution. Although there were significant differences in gillraker number, as well as spacing and structure between vendace and smelt, these did not seem to be strongly reflected in selection or use of major zooplanktonic prey taxa during the summer-autumn seasons. While small cladocerans dominated the prey choice of both species during June-September, large-sized calanoid copepods dominated in October. Schoener's overlap indices were high for both vertical distribution and prey taxa use by the two species, suggesting the possibility of strong competitive interaction between their planktivorous stages, though alternative explanations are explored. Their joint use of pelagic space and prey, changing seasonally, may offer a partial explanation for the marked year class fluctuations and periodic failures of vendace recruitment known to occur in Lambarfjärden, and other lakes where the two species coexist. In addition, the comparisons support the image of the environmental cul-de-sac position of vendace as an extreme zooplankton specialist, with a very narrow diet spectrum and habitat range, whereas smelt seem to express a more opportunistic ecology.