Impact of coelenterate predation on larval plaice Pleuronectes platessa and flounder Platichthys flesus stock in the western Wadden Sea

Abstract

During the seasonal immigration of plaice and flounder larvae into the Dutch Wadden Sea in spring, the coelenterates Pleurobrachia pileus L. and Aurelia aurita L. have been positively identified as predators of both flatfish species. Evidence for a causal relation between outbursts of the predatory coelenterates and the observed abrupt termination of larval flatfish immigration is found in (1) the coincidence of increases in coelenterate numbers and decreases in larval densities, (2) positive observations of larval flatfish in coelenterate stomach contents, (3) a comparison between food demand of the coelenterate population and standing stock of the pelagic food resource at that time. Impact of this predation on plaice recruitment is expected to be small, since most larvae have already completed their immigration into the Wadden Sea when the coelenterate outburst starts. Although the coelenterate outburst occurs at about the same time every year, their predation may have a density-dependent component, since the timing of plaice immigration seems to be related to abundance, with larger yearclasses arriving later than smaller ones. For flounder larvae the impact will be greater, because of the shorter period of immigration and the coincidence between coelenterate outburst and maximum flounder abundance.