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.

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.

A test of the 'maximum growth/optimal food condition' (MG/OFC) hypothesis is described for 0-group plaice Pleuronectes platessa L. in the western Dutch Wadden Sea. The MG/OFC hypothesis assumes optimal food conditions (i.e. no competition for superfluous food and hence no negative density-dependent growth) and consequently a maximum growth rate only determined by prevailing water temperature conditions. Growth of 0-group plaice subpopulations was determined for a number of areas differing in food abundance and composition (benthic biomass of potential food items) and density of 0-group plaice in both the intertidal and the sublittoral. Growth showed a positive relationship with food abundance, in both the intertidal and the sublittoral, falsifying the MG/OFC hypothesis. At similar food abundance, growth in the intertidal was always higher than in the sublittoral due to the presence of the lugworm Arenicola marina in the intertidal only. In the intertidal, growth was positively related to density of A. marina. The assumption of the MG/OFC hypothesis of no negative densitydependent growth was confirmed both in the intertidal and in the sublittoral, although it did not result in maximum growth rate for 0-group plaice everywhere in the field Differences in growth rate therefore do not necessarily reflect competition for food and density-dependent growth, but they might reflect differences in food composition and availability (searching and handling time).

Larval immigration and subsequent settlement, abundance fluctuations, growth, and mortality of a 0-group flounder population were studied in a tidal flat area, the Balgzand, in the western Wadden Sea. Larvae entered the area from early April to early May. The first settled flounder were found about 1 mo after larval immigration started. Settled flounder were restricted to the siltier areas of the tidal flats. Growth did not seem to be food-limited within the density range observed over the years and depended only on prevailing water temperatures. The life history pattern of flounder was similar to that of plaice in the same area: year-class strength was generated in the open sea and after settlement a period of density-dependent mortality followed, most probably due to predation by crustaceans. The instantaneous mortality rate Z during summer seemed to be higher (0 08 d-l) than in plaice (0.03 dC1), probably due to a combination of higher water temperature and higher abundance of predators in summer than during the settlement of plaice in spring. No relationship between the year-class strengths of plaice and flounder was found

Recruitment mechanisms in a number of 0-group plaice nurseries in northwestern Europe were analysed. In all areas the same pattern of abundance of 0-group plaice emerged: increasing densities during the period of settlement followed by a continuous decrease during the rest of the season. Differences in hming of settlement in combination with differences in water temperature resulted in a wide range of mean length (30 to 78 mm) in the various areas by the end of August. Growth always appeared to be optimal within the constraints of the prevailing water temperature. Predation is evidently the main mortality factor and the same 2 types of predators were found everywhere: crustaceans in spring and migratory fish in summer and autumn. The impact of predation by crustaceans in spring varied among areas and appeared to depend on local temperature conditions. In areas characterized by low temperatures in spring no impact on the year-class strength was found, whereas in areas with high temperatures in spring, crustaceans affected recruitment. Either a reduction or a generation of variation in recruitment of plaice was observed, depending on the minimum temperature in the nursery in winter, which affects migration and/or mortality of the shrimps. Fish predation in summer and autumn appeared to have little impact on the recruitment of plaice, because variations in year-class strength were not affected by this type of predation.