Smelt (Osmerus eperlanus): Glacial relict, planktivore, predator, competitor, and key prey for the endangered Arctic char in Lake Vättern, southern Sweden

Smelts are crucial forage prey for many piscivorous fish species and have, as such, been frequently introduced outside their indigenous ranges, a management measure causing controversy, as smelts are also efficient predators and competitors. In Lake Vättern, relict Arctic char, fourhorn sculpin, smelt and seven species of crustaceans form a deep-water food-web, and interact closely as competitors, predators and prey. Here, we analyzed new and archived data back to the late 1960s in order to understand and relate observed life-history shifts in the smelt population, to concurrent adverse life-history changes observed in the endangered Arctic char population. Smelt growth was sigmoidal and the size frequency distribution dominated by fish up to 110 mm and age 0-4, although smelt occasionally reached 250 mm. Smelt diet shifts as the fish grow, from zooplankton, to Mysis relicta, to piscivory for individuals larger than 150 mm. Recruitment commonly demonstrates regular inter-annual pulses of YOY abundance although strong year classes produced during years with higher spring temperature cause post-annual irregularity of year class strength. Young smelt reside in surface water and older fish in deeper water. This ontogenetic shift in distribution depends on temperature occupied, prey availability and risk of inter- and intra-specific predation. Interestingly, even though very small Arctic char feed on smelt, most small Arctic char are found in deep water together with large smelt, and most large Arctic char are found in shallow water with small smelt.