STOCK DISCRIMINATION OF TWO INSULAR POPULATIONS OF DIPLODUS ANNULARIS (ACTINOPTERYGII: PERCIFORMES: SPARIDAE) ALONG THE COAST OF TUNISIA BY ANALYSIS OF OTOLITH SHAPE

Background. The annular seabream, Diplodus annularis (Linnaeus, 1758), is the most common sparid fish in the Mediterranean Sea. This species is widespread in Tunisia. Despite their economic importance, due to their excellent white and delicate flesh (demersal fish) and their significant size, few studies have hitherto focused on stock discrimination and on the fisheries management. The goal of this research was to evaluate the stock structure of D. annularis for two Tunisian insular populations based on the otolith shape, using different statistical approaches. Materials and methods. The specimens of Diplodus annularis were collected during five months ranging from May through September 2014 at two sites: Djerba and Kerkennah. In total, 120 fish were collected from the two sites: 60 specimens of each population (30 males and 30 females) and were included in different statistical analyses. The Fourier coefficients were determined to evaluate the degree of similarity in the otoliths and detect the reciprocal variability. Results. By comparing the two studied populations, we revealed statistical significant differences, an asymmetry (left-right), of otoliths (P < 0.05). In the same population, symmetry (left-right) was detected for the males, females of Kerkennah, and females of Djerba. However, an asymmetry was detected only for the males of Djerba. Also, sexual dimorphism was observed only for the Djerba population. Conclusion. The comparison of the otolith morphology of the two populations showed a clear difference in shape and a left-right asymmetry of otoliths. This asymmetry indicates that the two populations belong to different stock of fish. The stocks characterization is considered as an important tool for fisheries management. Further studies are necessary to determine the origin of the otolith shape differences and whether they are related only to environmental conditions or a genetic basis may also be responsible.