Juvenile fish expansion following biomanipulation and its effect on zooplankton

In Lake Ringsjon, southern Sweden, we estimated yearly variations in both 0+ and older fish abundances, following the fish reductions in 1988-1992. Between 1987 and 1994, the total planktivore biomass decreased fourfold whereas the abundance of 0+ fish increased twenty times. The resulting change in predation pressure from planktivorous fish on zooplankton increased up to 2-3 times, after the fish reduction. We conclude that after an initial decrease, the predation pressure on zooplankton increased considerably due to the increase in 0+ fish abundance. The expansion in juvenile fish abundance may therefore have been an important factor behind the apparent lack of top-down effect on the zooplankton community following biomanipulation of Lake Ringsjon. The horizontal distribution of 0+ fish in Lake Ringsjon is also discussed.