Evidence from survey data for regional variability in cod dynamics in the North Sea and West of Scotland

Although cod (Gadus morhua) in the North Sea and ICES Division VIa are assessed as single units, recent research suggests that the stocks consist of reproductively isolated subpopulations within a metapopulation. We investigate whether temporal trends in stock indicators are asynchronous across subpopulations, which would support the metapopulation hypothesis. First quarter trawl survey data for the years 1983 -2005 were aggregated into putative areas of high spawner fidelity (three in VIa, seven in the North Sea) to obtain indices of spawning-stock biomass (SSB) and recruitment (numbers-at-age 1). Asynchrony was investigated by fitting a smoother to the data for each of the ten spawning areas and testing whether the smoothers were parallel. Trends in SSB differed between spawning areas in both VIa and the North Sea. In VIa, SSB collapsed in the most southwesterly area, but remained more constant elsewhere. In the North Sea, there was a general decline in SSB, but areas thought to contain resident inshore populations showed more rapid declines than those in adjacent offshore areas. Recruitment results offered less support for a metapopulation, although recruitment in the southern North Sea declined rapidly before any trend was seen for the North Sea as a whole.