Un approccio multidisciplinare per l’identificazione dello stock del nasello (Merluccius merluccius, Linnaeus 1758) nei mari sardi

The “stock” concept is fundamental to fisheries management. To manage a fishery responsibly it is important to understand its stock structure and how fishing effort and mortality are distributed (Begg e Waldman, 1999). The failure to comply the true stock structure and the ineffective fisheries management of an exploited species can lead to overfishing and depletion of the stock (Begg et al., 1999a). In the last years, scientific community has pointed out that the most successful way to address stock identification studies is the application of a holistic approach, thus involving a broad selection of complementary techniques (Begg et al., 1999a; Abaunza, 2008a; Cadrin et al., 2013). The European hake (Merluccius merluccius, Linnaeus 1758) represents a basic component of the demersal communities and one of the target species in the Mediterranean Sea (Lo Brutto et al., 2004; Ligas et al., 2011). For this reason it has been extensively studied. Furthermore, hake stocks are generally considered over-exploited both in the Atlantic Ocean (ICES, 2011; 2012) and in the Mediterranean Sea (STECF, 2013; GFCM, 2014). At present, European hake populations in the Mediterranean Sea are managed considering several relatively small geographical sub-areas (GSAs), which are identified mainly on the basis of political considerations (Ragonese et al., 2012; Vittori et al., 2013). In this partition, Sardinian seas are represented by GSAs 11.1 and 11.2 and are split between two FAO Statistical Divisions (Balearic Islands and Sardinia). It is known that the complexity of many marine species stocks is often underestimated in modern stock management processes (Begg et al., 1999a). As regards the Mediterranean sea, various studies suggested a geographical distribution of the stocks that appears considerably different from the current management units partition (Abaunza et al., 2000; Abaunza et al., 2008c; Murta et al., 2008), and this fact have been observed also for the European hake (Castillo et al., 2004; Mattiucci et al., 2004; Tanner et al., 2012; Fiorentino et al., 2014). The need of the achievement of a comprehensive knowledge about the stock structure of a commercially important species like M. merluccius, has led to the development of this study, which aims, through the employment of a multidisciplinary approach, at a better geographical definition of this species stock in the waters around Sardinia. A first insight into the complexity of the European hake population in the area has been supplied by the analysis and comparison of its length structure in the western and eastern coasts of Sardinia, showing a degree of differentiation that has been subsequently deepened through the application of various techniques. The analysis of the variability in the growth of the species hasn’t indicated a meaningful differentiation of the populations in the two sides of the island, but has suggested that the presence of many spawning areas in the western coast could be the cause of the difference in the 0 age-class mean length obtained, as well as in the length structure above-mentioned. On the contrary, the implementation of a geometric morphometric technique to study the shape of individuals has led to the discovery of a significant, even though of moderate extent, differentiation between the two coasts. However, as remarked by Tudela (1999) and Levi et al. (2004) in other case-studies, these differences were believed to be linked only to environmental influences, without a connection to those biological parameters that are fundamental in the definition of a stock. This hypothesis has been made also considering that the observed difference in shape didn’t seem to affect individuals’ growth. Lastly, the parasitological exam of specimens collected in the whole area of Sardinian seas, has supported the hypothesis of homogeneity of the populations in the area. Indeed, no significant differences have been found in the relative abundance and prevalence of the various parasite taxa examined. Comparing these results with those obtained in a similar study carried out in the Mediterranean Sea (Mattiucci et al., 2004) it has been possible to highlight a considerable similarity of our study area with the central area of Mediterranean sea, and, conversely, a clear differentiation between Sardinian seas and the eastern part of Mediterranean basin. Similar considerations has been made in the context of StockMed project (Fiorentino et al., 2014). This study has therefore provided a solid evidence of the homogeneity of the European hake stock in the waters around Sardinia, leading to the conclusion that this population should be managed jointly. Moreover, it has supported the recently expressed opinion that its stock (as well as those of other important species) could be wider than expected, underlining the need of improving current management units boundaries.