Predation by crustaceans on a newly settled 0-group plaice Pleuronectes platessa population in the western Wadden Sea

Predation by the brown shrimp Crangon crangon causes a significant density-dependent mortality of 0-group plaice during and shortly after settling on tidal flats in the western Wadden Sea between February and May. Evidence was obtained from (1) laboratory experiments indicating that larger shrimps are able to prey on plaice S 3 0 mm; (2) field observations showing that each year the period of density-dependent mortality ends in May-June when the plaice population has reached a mean size of about 30 mm; (3) the occurrence of fin damage in the field, characteristic of unsuccessful attacks by shrimps, according to laboratory experiments; and (4) shrimp stomachs containing significant numbers of plaice otohths. The shrimp predation is density-dependent and accounts for the total mortahty in plaice 6 3 5 mm as estimated in a previous paper (van der Veer 1986). The observed dependence of mortality on density appears to be caused by the combined effect of a functional and numerical response of the predator to fluctuations in plaice density. The number of fin damages of a plaice does not seem to influence its survival. Predation by larger crabs seems to be of minor importance and occurs only after May. Shrimp predation appears to act as a fine control mechanism reducing between-year variations in year-class strength generated in the egg or larval stages in the open sea.