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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

Genetic variation in blue whiting Micromesistius poutassou was investigated by starch gel electrophoresis of enzymes in tissue extracts. A total of 130 specimens from the spawning areas west of the British Isles were collected from trawl catches in 1990 (n=30) and 1992 (n= 100). In 25 tissue enzyme loci screened for genetic variants in the 1990 sample, polymorphisms (0.95 criterion) were found at IDDH-2*, IDHP-2* and PGM-I *, giving a frequency of polymorphic loci of P0.,,=0~12. In pooled samples the average heterozygosity per locus was estimated at H,=0.043 and the effective number of alleles per locus at 2.20, 1.45 and 1.53, respectively, for IDDH-2*. IDHP-2* and PGM-I *.

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