Population biology of the Norway lobster, Nephrops norvegicus (L.) in the Firth of Clyde, Scotland II: fecundity and size at onset of sexual maturity

Abstract

The fecundity and size at onset of sexual maturity of Nephrops norvegicus was estimated at different locations within the Firth of Clyde, S.W. Scotland. The “potential” fecundity was estimated from oocyte counts in mature ovaries, while estimates of “actual” fecundity were made from counts of the eggs attached to the pleopods of ovigerous females. Size at onset of maturity of Nephrops norvegicus was estimated from the study of primary (ovary maturity and eggs on pleopods) and secondary (allometric) sexual characteristics. Relationships between potential fecundity and female carapace length were found to conform to a power function (F=aLb). Comparison of the logarithmic formulations of the relationships for seven locations revealed no differences in power terms, but there were significant differences in the constants. Results suggested that differences in fecundity mainly reflected a geographical variation in oocyte volume (expressed in terms of mean dry weight) which appeared to be related to growth. Where growth rate was characterized by a low value for the asymptotic length of the carapace (L∞), females appeared to have smaller oocytes and larger size-specific fecundity. Estimates of size at onset of sexual maturity varied over small geographic scales (tens of km) and ranged between 21–34 mm carapace length for females and 29–46 mm for males, the estimate obtained from the different approaches being similar. Size at onset of maturity was positively related to L∞(p<0.05) and negatively related to adult density (p<0.05). Age at onset of maturity appeared relatively constant geographically but varied between the sexes (males, 4–4.5 years; females, 3–3.5 years).