On the food competition between stickleback and jellyfish in the Baltic

Abstract

The threespine stickleback Gasterosteus aculeatus and the common jellyfish Aurelia aurita are keystone species in many marine ecosystems, including the shallow cove Kertinge Nor, in Denmark. Both species feed on zooplankton, raising the potential for competition between them. While jellyfish are tactile filtering planktivores, sticklebacks are visual feeders that actively detect, attack and capture prey. The study compared clearance rates (Cl) and tested the hypothesis that jellyfish are more efficient in feeding on small prey and sticklebacks on larger prey animals. Individual (Cl_ind) and population (Cl_pop) feeding characteristics were studied under good visual conditions. Individual sticklebacks (TL = 44 mm) demonstrated 14–51-fold higher Clind than jellyfish (d = 27 mm) when feeding on small (< 1 mm) and medium (1–4 mm) sized prey and threefold higher Cl_ind when feeding on larger prey (4–11 mm). Cl_pop was calculated for both species based on their densities in the cove. When consuming small- and medium-sized prey in May–July, Cl_pop for stickleback was 2–20-fold higher than for jellyfish, but in August following a decrease in fish density, Cl_pop was higher for jellyfish. This may imply higher predation pressure from stickleback on zooplankton in Kertinge Nor at the beginning of the season, though the common jellyfish was considered earlier as a species controlling zooplankton there. The two competing species likely coexist in the cove due to different seasonal cycles of abundance and thus different seasonal patterns of plankton consumption.