Stock Discrimination in the Horse Mackerel Trachurus trachurus (Teleostei: Carangidae) off the Moroccan Atlantic Coastal Waters using a Morphometric-Meristic Analysis

The Atlantic horse mackerel Trachurus trachurus, the family Carangidae, is a small pelagic with high commercial importance. This species is classified as globally vulnerable (IUCN) due to fishing pressure and low recruitment. It has drastically declined during the last three decades. In contrast, to the well-studied Euro-Mediterranean stocks, the putative Moroccan-Saharan stock has been poorly investigated. A first step is to discriminate potential subunits within this large stock for a better assessment management and practice. We examined variation in morphometric and meristic characters in this species from three fishing areas (from the North to the south: Larache, Safi and Dakhla) off the Moroccan Atlantic coastal waters. We used a linear morphometric approach based on body weight, 14 body dimensions and 5 meristic characters in 147 fish specimens. We compared fish from the three areas using a multivariate analysis including discriminant function analysis (DFA) and principal component analysis (PCA). Based on morphometric data, we distinguished three subunits: a first subgroup off Larache in the north, probably a self-recruiting population or a locally adapted morphotype, and a second and third units in the mid and southern Moroccan Atlantic coasts, respectively off Safi and Dakhla fishing areas. On the other hand, the result of DFA for meristic characters confirmed the differentiated southern subgroup off Dakhla, but two overlapped subunits including Safi and Larache areas. 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 lateral line. Further investigations using other methods such as genetics, otoliths, and parasites are needed to confirm the identified stock units. Our findings on such a threatened small pelagic species would be useful for the stock management in the Moroccan Atlantic coastal waters. They also have ecological and evolutionary implications related to local adaptations in fish populations that are under intense fishing pressure.