Ontogenetic variation in lacustrine European smelt (Osmerus eperlanus) populations as a response to ecosystem characteristics : an indicator of population sensitivity to environmental and climate stressors

Smelts play a key role in the pelagic ecosystem of large lakes in northern Europe and North America. In numbers, they often dominate the open water. In large lakes in Scandinavia (including Finland), European smelt (Osmerus eperlanus L.), a cold-water glacial relict, is commonly the most important prey for piscivorous fish species, but also acts by ontogenetic shifts as a predator on zoo-plankton, small crustaceans, fish larvae, mysids and occasionally – with increasing size - fish. Furthermore, the large numbers of smelt in the open water are important competitors to other planktivorous fish. Due to the diverse life histories and biological interactions of smelt in large lakes, its role in the food-web structure is expected to be variable. Smelt population dynamics, recruitment, size and age structure, growth, life history and mortality were analysed and compared for five Swedish lakes that varied in size, depth, morphology, trophic status and latitude to understand the varying life histories and roles in lake food-webs. The results showed that in shallow, eutrophic lakes smelt stayed small and short-lived, and populations experienced high mortality. In deeper, colder and less nutrient-rich lakes, smelts grew larger and older, and might shift to a piscivorous trophic level. By ontogenetic adaptions smelt seems to uphold high abundance and recruitment over a wide range of ecosystems, but in shallow lakes without cold water refuges smelt populations run the risk of collapsing on the occasion of extremely warm summers with drastic consequences for their predators and lake ecosystems.