Immigration, settlement, and density-dependent mortality of a larval and early postlarval 0-group plaice (Pleuronectes platessa) population in the western Wadden Sea

Mortality factors operating in a temporarily isolated 0-group plaice population in the western Wadden Sea have been examined from information on larval immigration, subsequent settlement and changes in demersal densities. Larvae entered the western Wadden Sea every year from the end of February to the beginning of May. In years of strong larval abundance the main perlod of arrival seemed somewhat delayed from March to April. As a result, between-years mean length and density of the settled population showed an inverse relation on June 1. Density-dependent mortality was observed among plaice up to about 35 mm, with instantaneous mortality rates ranging up to 0.06 d-l during and shortly after settling of the larvae. Arguments are given that, although in the coastal zone pelagic predators are present (several coelenterate species), the main dens~ty-dependent mortality processes operate in the demersal phase. The larval data further showed that level of yearclass strength was established early in life, during the planktonic stage in the open sea. Observed density-dependent mortality in the Wadden Sea reduced the between-year variation in year-class strength generated in the open sea egg and larval stages from a coefficient of variation of 62 % during larval immigration to 30 % after settling in June.