Food of brook charr in extreme oligotrophic conditions of an alpine lake

Abstract

A stunted form of Salvelinus fontinalis has persisted since its introduction in 1948 in ultra oligotrophic Zielony Lake in the Tatra Mountains, southern Poland, despite extremely poor available food. The charr shifts from feeding on airborne insects to benthic chironomid larvae when the lake is frozen in October, and to the sole planktonic crustacean survivor (Cyclops abyssorum tatricus) when access to the sediments becomes difficult due to oxygen depletion in April–May. The plankton food resource enables the charr to survive till June, when the ice-cover breaks up. Due to charr predation the old Cyclops generation becomes extinct in May, but Cyclops eggs hatch after passing through the charr intestine unharmed. Each new Cyclops generation explodes due to the absence of cannibalistic adults, which have been removed by the charr, and persists in high densities until the next April–May when the egg-carrying females (highly selected by the charr) appear again to focus the charr attention on planktonic food resources.