Recruitment mechanisms in North Sea plaice Pleuronectes platessa

Abstract

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 timing 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.