Insights into the stock structure of Scomber colias Gmelin, 1789 along the Northwest African coast inferred from otolith shape analysis

Scomber colias Gmelin, 1789 is of growing interest as a commercial species, making it one of the most demanding fish on the northwest coasts of Africa (NWA). However, relevant data on the stock structure and connectivity of S. colias along the NWA is scarce. The Eastern Central Atlantic Fisheries Committee (CECAF) assumes the existence of two stocks (a northern and a southern stock) between Morocco and Senegal, separated by Cap Boujdour. However, given the absence of relevant biological information on the migration and connectivity of the species, CECAF suggests a joint assessment of the two stocks. Otoliths can be used as natural tracers to study the population structure, fish movements, and habitat connectivity of fish. Otolith morphology, for instance, is a phenotypic trait with a genetic background that usually responds to environmental variables. This study used otolith shape analysis (wavelets) to investigate S. colias stock structure along the NWA, from Morocco to Senegal. Fish were collected between September and December of 2019 by a scientific survey in ten sites (Larache, Casablanca, Safi, Agadir, Tantan, Tarfaya, South of Boujdour, North of Cape Barbas, south Mauritania and Senegal) using a purse seine. The hereby results showed a highly significant regional differentiation among the sites, suggesting the existence of two population-units, with an overall correct re-classification of 91% to the two clusters. This dataset indicates the existence of an oceanographic barrier at the level of 28 degrees N (Tarfaya), which limits the connectivity between the two stocks. Moreover, the mean otolith shape based on wavelet reconstruction for the two population-units revealed high phenotypic plasticity for S. colias otoliths. The broad variance between the two morphotypes highlights the environmental heterogeneity of the Atlantic Northwest African waters. The variation in the rostrum and anti-rostrum of S. colias otoliths is most likely due to genetic diversity or long-term isolation of the two population-units. The hereby data support the hypothesis that S. colias in the NWA is divided into two stocks with limited connectivity. In terms of fisheries management, this study recommends a fine-scale stock assessment.