Recruitment mechanisms in flatfish: What did we learn and where do we go?

This paper summarizes the developments in flatfish recruitment studies over the last decade with emphasis on the general patterns that have emerged from the contributions to the two Flatfish Symposia of 1990 and 1993. Recruitment variability is largely generated by density-independent factors acting during the pelagic egg and larval phases. Effects of variability generating processes tend to be amplified towards the edges of the distribution range and appear to be related to abiotic conditions. Density-dependent feedback processes occur in the demersal juvenile phase, when flatfish become highly specialized benthic feeding fish. There is evidence that density-dependent feedback processes may also occur during the adult phase in not or lightly exploited populations. Areas of future research that emerge from this symposium are: 1. the classification of flatfish populations in 'ecological equivalents', including the habitat requirements of the successive life history stages; 2. comparative studies among species and populations of recruitment processes; 3. analysis of environmental factors determining the survival of pelagic eggs and larvae; 4. density-dependent habitat selection of demersal juveniles in relation with growth and mortality; 5. analysis of the habitat characteristics of species for pelagic eggs and larvae, demersal juveniles and adults, including drift of eggs and larvae, transport from spawning towards nursery areas and adult migration; and 6. simulation studies of relevant population dynamical processes to explore quantitatively the importance of the various processes and the necessary precision with which these should be known.