Reproductive variability in North Sea plaice, sole, and cod

Abstract

The between-year variability in reproductive potential of the stocks of North Sea plaice, sole, and cod is investigated using time series of growth, maturation, and fecundity. Special attention is given to possible density-dependent effects. The growth rate of both plaice and sole increased in the 1960s. Evidence of density-dependent effects was observed only in the somatic growth of juvenile plaice and in the condition factor of sole. However, these results can also be explained by concurrent time trends. The age, but not length, of plaice at first maturity was correlated with juvenile growth. Fecundity of cod and ovary weight of sole were positively correlated with spawning biomass, whereas a weak positive correlation between fecundity and growth was observed in plaice. In general, there was very little evidence that variability in the reproductive parameters of these species is related to population density. The data suggest that conditions for growth vary substantially between years, but are independent of stock size at the present level of exploitation. Nevertheless, the observed changes in growth, maturation, and fecundity appear to have compensated for the losses in total egg production due to increased exploitation by about 25% for all species. It is suggested that effects of reproductive variability which are related to the age composition of the spawning population, and which may affect the quality of eggs (e.g. proportion of recruit spawners), could be more important than the density-dependent effects.