Stock discrimination of horse mackerel (Trachurus trachurus) in the Northeast Atlantic and Mediterranean Sea: integrating the results from different stock identification approaches

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The goal of horse mackerel stock discrimination was tackled by integrating both established and innovative approaches such as genetic markers (allozymes, mtDNA, microsatellite DNA and SSCP), morphometry and parasites, tagging experiments and life history traits (growth, reproduction and distribution), within the EU-funded HOMSIR project. The sampling covered almost the whole distribution range of horse mackerel through 20 sampling localities. 200 fish per sampling site were collected taking in consideration the spawning season and temporal variation (two years of sampling). The results from genetic approaches (Multilocus Allozyme Electrophoresis, mtDNA and microsatellite DNA) showed that there was only slight genetic differentiation among sampling sites. Another genetic technique (SSCP), was successful in finding a genetic marker that demonstrated substructuring in horse mackerel populations. Several approaches (SSCP, morphometrics, parasites) support the separation between the Atlantic Ocean and the Mediterranean Sea in horse mackerel populations, although the most western Mediterranean area could also be connected with the Atlantic populations. In the Northeast Atlantic, various stocks can be distinguished: The west Atlantic coast of the Iberian Peninsula (southern stock); the west coasts of European countries from northern Spain to Norway (western stock) and the North Sea stock. This information implies the revision of the boundaries of the so-called southern and western stocks as currently defined. Various results suggested that adult horse mackerel could migrate through different areas/or stocks in the Northeast Atlantic. Horse mackerel from the Mauritanian coast is distinguished by its high production parameters (growth and batch fecundity). The population of horse mackerel in the Mediterranean Sea is substructured roughly in at least three main areas: western, central and eastern Mediterranean. In this contribution, we try to integrate the fundamental findings of differente approaches showing that the holistic approach is the appropriate way to identify stocks in horse mackerel.