Population differentiation in megrim (Lepidorhombus whiffiagonis) and four spotted megrim (Lepidorhombus boscii) across Atlantic and Mediterranean waters and implications for wild stock management

Megrim, Lepidorhombus whiffiagonis, and four spot megrim, Lepidorhombus boscii, are two marine fish species of high commercial interest. Despite their quite heavy exploitation little is known on the genetic structure of their populations. The present work aimed at characterizing the first seven microsatellites markers available for the two megrim species. These new markers were in a second step employed to describe the population structure of the two species among their almost entire habitat range (Atlantic and Mediterranean samples). Our study confirmed the existence of a strong genetic difference between Atlantic and Mediterranean megrim species already described in the literature for L. whiffiagonis on the basis of variations at ribosomal genes. Additionally our analysis gave the first evidences of a strong genetic differentiation among Atlantic populations in both megrim species (within Atlantic global FST in L. whiffiagonis and L. boscii were respectively 0.158 and 0.145). When describing megrim population structure, the comparison between allele-frequency-based tests (FST comparisons) and genotype-based inferences (Bayesian approach) gave evidences of a hierarchical structure of the populations. In conclusion, our work enlighten the existence of two different stocks within the Atlantic Ocean and one in the Mediterranean Sea that will clearly need to be managed separately. As the present results do not fully support the current megrim stock boundaries they will surely help to rethink megrim management policies in the future.