Evidence for substock dynamics within whiting (Merlangius merlangus) management regions

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Whiting in the North Sea and Eastern Channel is currently assessed as a single management unit. However, several studies suggest that this stock may be comprised of more than one subpopulation within a larger metapopulation. A key characteristic of metapopulations is asynchrony in the dynamics of component subpopulations. In this study, indices of recruitment and spawning–stock biomass (SSB) were developed to test for asynchrony across putative subpopulations in the North Sea and west of Scotland. Differences in SSB and recruitment trends were detected, consistent with expectations from metapopulation dynamics. At least three different subpopulation components (southern and northern North Sea, and west of Scotland) were indicated on the basis of differing trends. Analysis of spatial distribution suggested that the boundary between the northern and southern North Sea subpopulations was associated with the change in bathymetry that extended from the coast of Norfolk in England to the southern tip of Norway. The current management system for whiting in the North Sea assumes a unit stock, which is contrary to current sources of biological evidence and seems inappropriate. Consideration of a north–south split along the boundary detected should be beneficial for both assessment and management of the resource.

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