Recruitment in flatfish, with special emphasis on North Atlantic species: Progress made by the Flatfish Symposia

Abstract

In summarizing the main results on recruitment that emerged from the series of Flatfish Symposia, two aspects were distinguished: mean level and interannual variability. Recruitment to a stock appears to be related to the quantity of juvenile nursery habitats, suggesting that either larval supply or the carrying capacity of the nurseries is the limiting factor. However, available information on growth of 0-group flatfish suggests that the carrying capacity of nursery areas is never reached. Variability in year-class strength is generated during the pelagic egg and larval stage, probably by variations in the hydrodynamic circulation and in the mortality rates of eggs and larvae. Density-dependent processes seem to occur only during the juvenile stages, particularly in respect of growth. However, no impact on recruitment variability has been found. Density-dependent mortality during the phase shortly after settlement dampens the interannual recruitment variability. There is no evidence of density-dependent effects in the adult phase at present, but these may have been important at lower levels of exploitation. The importance of the factors determining recruitment vary not only among species, but also over the species' range. It is suggested that damping processes can only occur in the demersal stage, implying that variability in year-class strength can only decrease in fish species with a demersal stage. If true, ultimate variability in recruitment in fish species will be related to the relative duration of the pelagic and demersal stages.