Linear Versus Geometric Morphometric Approaches for the Stock Discrimination in the Atlantic Horse Mackerel Trachurus Trachurus (Teleostei: Carangidae) within the Putative Moroccan-Saharan Stock
The horse mackerel Trachurus trachurus is a small pelagic with high commercial importance. This species is globally vulnerable (IUCN) due to fishing pressure and low recruitment. It has drastically declined during the last decades. In contrast to the well-studied Euro-Mediterranean stocks, the putative Moroccan-Saharan stock has been poorly investigated. Thus, for a better assessment management and practice, identification of potential subunits within this large stock is required. We examined variation in morphometric characters in samples from three distant fishing areas (from the North to the south: Larache, Safi and Dakhla) along the Moroccan Atlantic coast. We compared the linear (LMA) and geometric morphometric (GMA) approaches, based, respectively on 11 body dimensions, and 30 truss measures derived from 14 landmarks. We used a discriminant function (DFA) and/or principal component (PCA) analyses, and Procrustes technique for GMA. We found indications of at least two subunits based on GMA: 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. LMA revealed rather three distinct subunits. Other methods (e.g., otolithometry) are needed to confirm the identified stock units. Our findings on this vulnerable declining small pelagic species are of a 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 affect several adaptive traits.