Assessing nursery contribution to recruitment: relevance of closed areas to haddock Melanogrammus aeglefinus

Closing areas where juveniles aggregate has often been assumed to benefit recruitment since it reduces mortality of undersized individuals. However, the number of juveniles per unit area that recruit to an adult population may vary due to a combination of factors. Consequently, more appropriate nursery definitions have been based on measures of juvenile contribution to an adult population, either as overall contribution (essential juvenile habitat hypothesis) or recruits per area (the nursery role hypothesis). Nonetheless, these definitions do not explain why nursery contribution differs. In the present study, the nursery contribution of areas in the North Sea and west of Scotland was examined for haddock Melanogrammus aeglefinus. Elemental concentrations in the equivalent parts of the otolith of juvenile and adult fish were compared from the same year-class. Discriminant analysis indicated that 4 discrete 0-group regions around the Scottish coast could be distinguished with a classification accuracy of 83 to 89%. In contrast, the elemental signatures from adult cores sampled in adjacent spawning areas indicated high levels of mixing throughout the study region. The Scottish east coast region accounted for the highest overall contribution in 2003, and this was related to the high 0-group density in that region. While 0-group densities in this area have been consistently high since 2001, previous large year-classes have been characterised by high densities further north and east. Therefore, if 0-group density is always a good indicator of nursery contribution, temporary area closures that vary in relation to juvenile settlement concentrations may have more benefit to recruitment than permanent closures.