POPULATION-REGULATING PROCESSES DURING THE ADULT PHASE IN FLATFISH

Flatfish support major fisheries and the study of regulatory processes are of paramount importance for evaluating the resilience of the reso urce to exploitation. This paper reviews the evidence for processes op erating during the adult phase that may 1. generate interannual variab ility in recruitment; 2. contribute to population regulation through d ensity-dependent growth, density-dependent ripening of adults and dens ity-dependent egg production. With regard to (I), there is evidence th at in the adult phase processes do occur that may generate recruitment variability through variation in size-specific fecundity, contraction of spawning season, reduction in egg quality, change in sex ratio and size composition of the adult population. However, time series of rec ruitment do not provide support for this hypothesis. With regard to (2 ), there is ample evidence that exploitation of flatfish coincides wit h an increase in growth, although the mechanisms involved are not alwa ys clear. The presence of density-dependent growth in the adult phase of unexploited populations appears to be the most likely explanation i n some cases. From the early years of exploitation of flatfish stocks inhabiting cold waters, evidence exists that adult fish do not spawn e ach year. Fecundity schedules show annual variations, but the availabl e information suggests that size-specific fecundity is stable over a b road range of population abundance and may only decrease at high popul ation abundance. The analysis is complicated by the possibility of a t rade-off between egg numbers and egg size. Nevertheless, a density-dep endent decrease in growth will automatically result in a decrease in a bsolute fecundity because of the reduced body size. The potential cont ribution of these regulatory effects on population regulation is explo red. Results indicate that density-dependent ripening and absolute fec undity, mediated through density-dependent growth, may control recruit ment at high levels of population abundance. The effect of a density-d ependent decrease in size-specific fecundity seems to play a minor rol e, although this role may become important at extremely high levels of population abundance.