Applicazione delle chiavi età lunghezza allo stock assessment per l'identificazione di linee gestionali idonee alla tutela di alcuni raidi mediterranei

Elasmobranchs, due to their K-selected life strategy, represent a taxonomic group particularly susceptible to fishing mortality (Cahmi et al., 1998). Although the increasing number of studies that shows a global collapse of their populations (Dulvy & Reynolds, 2002; Baum et al., 2003; Cortés, et al., 2002, 2006; Dulvy et al., 2010), our knowledge about this organisms life-cycle seems surprisingly still scarce (Frisk et al., 2010; Cortés et al., 2012). Among elasmobranchs the Rajidae family is generally identified as one of the most vulnerable and impacted (Dulvy & Reynolds, 2002). Given the wide-ranging decline in terms of abundance, diversity and distribution observed, the skates stocks appear even in more critical condition in the Mediterranean sea (Walker et al., 2005). For this reason the need for management plans seems clear. However this kind of measures necessitate a precise evaluation of the population‟s condition, through a stock assessment process. Nevertheless such analysis require a huge amount of data regarding some peculiar aspects of the studied species life-cycle, and among these, especially growth parameters. This necessity often represented a problem for stock assessment studies. Indeed the estimation of age and growth for elasmobranchs is considered generally more challenging than in teleosts (Campana, 2014). Therefore the first step of this work was represented by the estimation of age and growth, through annuli counts in the vertebral centra of the three selected skates species: Raja brachyura, Raja polystigma, Dipturus oxyrinchus. Four different models were applied on length at age data: the common von Bertalanffy growth function, the Exponential, and finally the Gompertz and the Logistic functions. According to the Akaike‟s Information Criterion, the classical von Bertalanffy growth function, showed the best fitting to the observed data for R. brachyura (L∞= 1113.4 mm, k= 0.10 e t0 = -1.19). On the other hand the growth of the other two species resulted better described by the S-shaped functions, respectively the Gompertz model for D. oxyrinchus (L∞= 1275.5 mm, k= 0.14 and 2 Inflection Point = 3.97), and the Logistic model for R. polystigma (L∞= 691.49 mm, k= 0.26 and Inflection Point = 4.03). The obtained growth parameter were employed in several analysis for stock assessment. Those analysis revealed a general high exploitation level for Sardinian skates populations. Indeed both species R. brachyura and R. polystigma seemed to be in an overfishing condition. These results, in addition with what already reported for other two raids, R. asterias and R . clavata, in the GFCM workshop on the stock assessment of selected species of elasmobranchs (GFCM:SAC14/ 2012/Inf.16) confirmed the critical situation of these organisms in the Mediterranean sea. D. oxyrinchus, instead, showed a healthy stock state not subjected to overexploitation. This result was also confirmed by the biomass and abundance trends, which proved to be stable for the entire time period investigated by MEDITS, in agreement with what reported by Follesa et al. (2013) in Sardinia and Relini et al. (2010) for the Italian coasts. In conclusion, this work represents a first step towards a better understanding of the growth and the life cycle of the Rajidae family in the Mediterranean Sea, these knowledge were then applied to the stock assessment of these organisms in order to provide several advices for a conscious and correct management of their fragile populations.