The genetic population structure of the pelagic gadoid blue whiting Micromesistius poutassou throughout its east Atlantic distribution range was explored using polymorphisms at the tissue enzyme loci IDHP-2* and PGM-1*. The study included 5025 individuals from 65 locations in the east Atlantic and the Mediterranean. Significant geographic heterogeneity in allele frequencies was demonstrated at both loci (IDHP-2*: P=0·030, PGM-1*: P=0·005). The degree of genetic differentiation (F ST =1·1%) in blue whiting appears to be at the same level as for the demersal gadoids cod and haddock. Several separate reproductive units were indicated at the fringes of the distribution range, i.e. in the Barents Sea, and in one Norwegian fjord (Romsdalsfjord). 1998 The Fisheries Society of the British Isles

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