Does the Atlantic horse mackerel Trachurus trachurus (Teleostei: Carangidae) differentiate morphologically within the putative Moroccan-Saharan stock?

To discriminate potential stock units of the Atlantic horse mackerel Trachurus trachurus within the putative Moroccan–Saharan stock, we investigated the morphological variation of this species from three fishing areas (from the north to the south: Larache, Safi and Dakhla) in the Moroccan Atlantic coastal waters. We used a geometric morphometric approach based on 30 truss measures derived from 14 landmarks in 165 fish specimens. We compared fish from three areas using a multivariate analysis including discriminant analysis and Procrustes technique. We found indications of at least two subunits: a differentiated subgroup in the north off Larache, probably a self-recruiting population or a locally adapted morphotype, and two overlapped subunits, respectively in the mid and southern Moroccan Atlantic coast off Safi and Dakhla. Examination of the contribution of each morphometric variable to canonical functions indicated that differences among samples seemed to be associated with morphological changes in the anterior and posterior parts of body, which may indicate adaptations to local environmental conditions and habitat use. Further investigations using other methods are needed to confirm the identified stock units. The present study and the obtained results on such a vulnerable declining small pelagic species are of great importance for fish stock assessment management and practice, but also have ecological and evolutionary implications related to local adaptations in fish populations that are under intense fishing pressure as an induced evolutionary selective force, which affects several adaptive traits.