IMPACT OF JUVENILE GROWTH ON RECRUITMENT IN FLATFISH

In this review, the impact of juvenile growth on subsequent recruitmen t in flatfish is discussed. Recruitment is defined as the number of sp ecimens of a specific year class that survives to attain sexual maturi ty and joins the reproductive population. Theoretically, variability i n growth rate can have an impact on recruitment either by means of siz e-selective mortality during juvenile life and/or by means of size-dep endent onset of maturation. In flatfish up to about 10 cm, growth depe nds on size in such a way that variability in size within a population increases during the first year of life, and decreases again in the s ubsequent part of juvenile life. Temporal variability in size within l ocal populations appears to be lower than spatial variability. Due to the prolonged spawning period, and hence period of settlement, variabi lity in size of juvenile flatfish increases with decreasing latitude. As a consequence of these patterns, size-selective mortality appears t o be mainly restricted to the O-group and to gain importance with decr easing latitude. A literature search for field data yielded only a few references suggesting size-selective mortality. In none of the studie s was any relationship with ultimate recruitment studied or even sugge sted. Size-dependent onset of maturation has been found in some flatfi sh species, with slow-growing individuals or cohorts showing delayed m aturation. Size-dependent onset of maturation has a clear effect on th e level of recruitment. However, in the species studied, the main trai ts in year-class strength still existed at the moment of recruitment t o the reproducing stock. Size-dependent onset of maturation also appea red to affect the year-to-year variability in recruitment, but differe nt effects were observed among species. It is argued that both size-se lective mortality and size-dependent onset of maturation are more like ly to dampen than to generate variability in recruitment. The study of the impact of juvenile growth on recruitment in flatfish is hampered by the absence of long-term data sets on recruitment. Especially compa rable series of (sub)tropical species and of populations covering the total range of distribution of a species are lacking.