Stock identification of horse mackerel (Trachurus trachurus) through the analysis of body shape

To identify the horse mackerel (Trachurus trachurus) stock units by the analysis of body shape, most of the species distribution range was sampled, and the morphometric measurements were analysed using discriminant analysis and Procrustes techniques. The results from both analyses showed a clear distinction between Atlantic and Mediterranean samples. For the Atlantic there is a latitudinal gradient, but with a clear boundary between three clusters corresponding to the North Sea, north of the Galician coast through the west of the British isles, and to the Portuguese coast and Gulf of Cádiz. The relationship between the North African coast and these Atlantic stock units could not be fully clarified, because of a gap in the sampling coverage, corresponding to the Moroccan coast. The differences found between the sample collected in Mauritania and those from the Iberian coasts are not as marked as those between the Iberian samples and those to the north of Galicia. However, given the great distance between Mauritania and Iberia, it is necessary to analyse samples from contiguous locations in the Moroccan coast to clarify this question. Regarding the Mediterranean, three groups appeared to be distinct: a western one corresponding to the Balearic Islands, Alborán Sea and Catalonia, an eastern one including the Ionian and Aegean Seas, and a central one with the Tyrrhenian and Adriatic Seas, Sardinia and Sicily. The sampling scheme did not cover the Mediterranean coasts of North Africa and Middle-East, which can correspond to stock units unidentified in this study. The new perception of the stock units coming from this study gives stronger biological support to the routine assessments of the horse mackerel stocks.