A central problem when using commercial catch per unit effort (CPUE) as an index of fish stock abundanceis that fishing vessels search for concentrations of fish. For a given stock abundance, CPUE may become high if thevessels succeed in finding patches of fish and low if the vessels distribute their catching operations more randomly. Inthis work, the relationship between catchability and two measures of the degree of spatial concentration of a trawl fleet(the fleet’s spatial extent and the fleet’s degree of spatial patchiness) is investigated for four different fish stocks. Thecatchability of northeast Arctic cod (Gadus morhua) is strongly related to the fleet’s degree of spatial concentration,but the relationship is weaker for northeast Arctic haddock (Melanogrammus aeglefinus), and no relationships appearfor two saithe (Pollachius virens) stocks. Our findings suggest that adjusting CPUE with a measure of the fleet’saverage degree of concentration relates CPUE more strongly with abundance for migratory stocks.
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