Good practices for common sole assessment in the Adriatic Sea: Genetic and morphological differentiation of Solea solea (Linnaeus, 1758) from S. aegyptiaca (Chabanaud, 1927) and stock identification

In the Adriatic Sea two cryptic species of sole coexist, the common and Egyptian sole. Soles are one of the most valuable demersal fishery resources in the Adriatic Sea, so a correct species identification is crucial in order to perform stock assessment and implement effective management measures based on reliable and accurate data. In this study specimens collected during fishery independent and fishery-dependent activities in the Adriatic were analysed and identified coupling morphological and genetic approaches. A comparison of these two methods for the sole species identification was carried out to assess the most effective, accurate and practical diagnostic morphological key-character(s). Results showed that external characters, in particular features of the posterior dorsal and anal fins, are valid and accurate morphological markers. Based on these traits, a practical identification key of the two sibling species was proposed. Moreover, it was possible to estimate the extent of the error due to species misidentification introduced in the common sole stock assessment carried out in the Northern-central Adriatic Sea (GSA17). A 5% bias in the correct identification of common sole specimens was detected. However, this bias was shown not to affect the common sole stock assessment. Moreover, the genetic profiling of the Adriatic common sole allowed estimating genetic diversity and assessing population structure. Significant divergence between common soles inhabiting the eastern part of the Southern Adriatic Sea and those collected from the other areas of the basin was confirmed. Therefore, the occurrence of genetically differentiated subpopulations supports the need to implement independent stock assessments and management measures.