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Do haddock select habitats to maximize condition?

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Haddock Melanogrammus aeglefinus in the North sea increased their distributional range when more abundant, but this density dependent habitat selection (DDHS) explained only a small part of the year-on-year variation in distribution patterns. The condition of haddock was examined at 24 sites in the North Sea in August and September 2004 and related to their abundance, to examine if the ideal free distribution theory (IFD), which assumes that organisms select habitats that maximize their rate of food intake, can be used to explain this variation in large scale distribution patterns. At a given temperature, condition (hepato-somatic index, IH) was better at stations where haddock were most abundant. Therefore, haddock were not distributed perfectly according to the IFD in 2004. The positive correlation between abundance and IH, however, indicated there was some habitat selection by haddock, as in the total absence of habitat selection no correlation between IH and abundance, and no spatial variation in abundance was expected. DDHS may only explain a small part of the yearly variation in the distribution because haddock did not equalize and maximize their fitness at the scale of the North Sea. In addition, stable isotope analysis of muscle samples showed that haddock did not avoid competition for food when at high abundance by feeding at a lower or wider range of trophic levels.

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