Long-term fluctuations in cod and pollack along the Norwegian Skagerrak coast
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We tested whether the long-term fluctuations in abundance of 0-group cod Gadus morhua and pollack Pollachius pollachius sampled from 1919 to 1994 at 38 stations along the Norwegian Skagerrak coast might be related to the North Atlantic Oscillation (NAO). The short- and long-term fluctuations of both species exhibited different spatial structures and were probably of different origins. The former, which were previously shown to be related to biotic interactions, displayed signifcant highest similarities across the smallest scales (0t o 21 km). In contrast, the long-term fluctuations were similar at a larger scale (equal to or larger than the Norwegian Skagerrak coast) and were probably due to extrinsic factors. We have shown that the climatic conditions along the Norwegian Skagerrak coast and the fluctuations of Calanus finmarchicus in the Skagerrak were closely associated with the NAO. Long-term fluctuations of cod and pollack were, however, not associated linearly or non-linearly with the NAO or C. finmarchicus. Trends of cod and pollack appeared to be related to the long-term fluctuations in the bottom sea grass coverage, mainly constituted by Zostera marina, and it cannot be dismissed that fishing has contributed to the recent decline of these fish populations. We also conclude that the postsettlement period is crucial for these populations because of competition for space.
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