| Summary: | The long-term sustainability of fish stocks is dependent upon the identification of biologically meaningful demographic boundaries and, subsequently, appropriate assessment figures and management strategies. In the case of the horse mackerel, Trachurus trachurus, stock management in the North-east Atlantic area is currently based on stock identities derived from multidisciplinary data integration, but including genetic markers that might lack statistical power, hence potentially misrepresenting the units of assessment. For this reason, and based on multiannual field observations, the boundaries of the vast 'western stock', whose distribution spans from the Bay of Biscay to the Norwegian coast, were reassessed. The preliminary results of a population genetics survey of samples collected over three consecutive years off the western Irish coast (areas VI and VII) and Norway (IVa), and screened by employing a suite of thirteen microsatellite loci are presented. Results indicate that the core of the putative stock in the west of Ireland is temporally stable, but its spatial extent and connection with Norwegian waters varies between years. Interannual variation in nutrient transport and oceanographic regimes may explain unstable patterns of genetic variation west of Norway, where local, smaller, previously unknown populations might exist.
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