Stock identification of Mediterranean horse mackerel (Trachurus mediterraneus) through the analysis of morphometric characters in the Adriatic Sea.

Phenotypical differentiation among stocks of Mediterranean horse mackerel Trachurus mediterraneus in the Adriatic Sea was investigated through the analysis of several individual morphometric characters. Overall, 426 individuals of Mediterranean horse mackerels were sampled from the northern, central and southern Adriatic Sea in 2012 and 2013, and then compared using multivariate techniques (linear discriminant analysis). Discriminant analysis suggested a scarce intermingling between the northern and central Adriatic with the southern part. The former two areas showed more stable populations, overlapping both in space and time. The latter seemed to be more variable over the years. A possible hypothesis for this, to be investigated, is the flow of individuals from the Ionian and Aegean Seas populations through the Otranto Channel. When the morphometric variables were investigated, the results showed that most of the variability was associated with the head characters. In particular, the northern and central Adriatic Sea individuals had shorter and thicker heads than the southern ones. This could be due to different feeding habits: the former mainly feed on small fishes, the latter mainly on euphausiids. A short mouth could reduce the power of suction of bigger preys, while a long mouth could increase the volume of water to be filtered to feed on small planktonic crustaceans. From this study, it comes clear that the Mediterranean horse mackerel populations should not be managed as a single stock in the Adriatic Sea as it was evident that morphological two different populations are present in the basin.