Population Differentiation in Blue Whiting (Micromesistius poutassou) in the North East Atlantic Ocean
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Blue whiting is a commercially important gadoid fish found throughout the Atlantic Ocean and the Mediterranean. It is a pelagic, migratory species with feeding grounds in the Norwegian Sea and in the Bay of Biscay. The main spawning area is west of Ireland and takes place in early March and April. Currently, for management purposes, ICES considers the blue whiting population in the NE Atlantic as a single stock, although there is genetic evidence that this grouping may be divided into northern and southern populations. The aim of this project was to elucidate the stock structure of blue whiting in Irish waters using allozyme and microsatellite molecular markers. Seven samples were collected during the spawning season in April 2003 and March 2004 from the Faroes/Shetlands southwards to the Celtic Sea, a latitude range of ~50°N to 60°N. At the allozyme locus, PGM (Phosphoglucomutase), where four alleles were observed, significant differences were observed between two samples at the extremes of the range. The three microsatellite loci, MpouBW7, MpouBWS and MpouBWI3, exhibited 16, 17 and 15 alleles, respectively. Results from the two most polymorphic loci (MpouBW7 and MpouBWS), indicated that samples collected in the Celtic Sea in March 2004 were significantly different from all other samples, including each other. The third locus, MpouBW13, demonstrated significant heterogeneity between two samples at the extremes of the range. The results indicate that the significant genetic differentiation between Celtic Sea and other samples could either be due to temporal variation between year classes, or to genetic differences between northern and southern samples. Direct comparison between allozymes and microsatellites showed that microsatellites have greater potential to detect population differentiation than less variable allozyme markers. Fst estimates (a measure of population differentiation) for allozymes were low (0.004) in comparison to estimates for microsatellites (mean 0.019) on the same samples but neither Fst estimate differed significantly from zero.
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