Regional Mountain Conference

The introduction of fish into fishless alpine-type lakes resulted in radical alterations to the recipient ecosystem, associated usually with cascading effects across food webs. Most alpine-type lakes in the Tatra Mountains have been stocked with salmonid fish over the past two centuries, which triggered profound ecological shifts. The partial removal of fish from one of the lakes: Przedni Staw Polski (PSP) of the Five Lakes Valley, between 2021-2023, led to a significant improvement in water transparency, increasing from 7 m to 17 m. This was linked to a reduction of chlorophyll a concentrations in the upper water strata and an increased abundance of Daphia galeata. Following the reduction of fish biomass, the vertical distribution of zooplankton in PSP changed. Contrary to the expectations, the daytime residence depth of D. galeata and Cyclops abyssorum, which are dominant zooplankters in the lake, shifted downwards. No such changes were observed in the other lakes of the valley (Czarny Staw Polski and Wielki Staw Polski) where the fish populations were not manipulated.