Geographical variation in size at the onset of maturity of male and female Norway lobster Nephrops norvegicus (L., Homarida: Decapoda) in Scottish waters

Geographical variation in the size at the onset of maturity (‘SOM’) of male and female Norway lobster (Nephrops norvegicus) in Scottish stocks was assessed using estimates based on eleven morphometric measurements analysed using segmented regression analysis. Significant differences were found in SOM in males, with lower values found in areas characterized by high population densities. Specifically, estimates of male SOM for the stocks in the Firth of Forth, Clyde and Sound of Jura were lower, than those estimated for the stocks of the North Minch, which in turn were lower than those of Noup, Moray Firth and Fladen. Differences were consistent across a range of morphometric measurement pairs. In high density areas, males mature at the same carapace length as females and at smaller body sizes than males in low density areas. Estimates of SOM in females, based on morphometric analyses, did not vary significantly across the analysed stocks, and were higher than those derived using traditional maturity assessment methods (maturity ogives). The potential for behavioural, density dependant and environmental modulation of SOM in the Norway lobster is discussed. Improved understanding of the maturation process and associated growth stanza would improve estimation of SOM in Nephrops and inform fisheries management measures aimed at protecting stock reproductive potential.