Maturation differences between sub-stocks of haddock, Melanogrammus aeglefinus

Contemporary maturation schedules of North Sea haddock, Melanogrammus aeglefinus, differ between spawning centres east and west of the Greenwich meridian. In this study, young-of-the-year haddock from these two sub-stocks were raised under a common environment in order to test whether this sub-stock difference reflected temperature exposure during maturation or an intrinsic (presumed genetic) effect. Maturity–size relationships differed between sub-stocks for the same temperature, indicating an intrinsic component to the contemporary differences now found in the wild. Relative liver mass additionally explained some variation in the maturity–length relationships. Despite inhabiting a thermal regime more favourable for early maturation, west North Sea haddock had the lowest body size and liver mass at maturity for a given temperature. However, historic fishing effort was much higher in the west North Sea, suggesting that contemporary differences may reflect long-term differences in sub-stock mortality.