Overlapping distribution of two sympatric species: the case of black hakes, Merluccius polli Cadenat 1950 and Merluccius senegalensis Cadenat 1950, off Mauritania

Abstract

We studied the decapods collected in four trawling surveys conducted in Mauritanian waters, in 281 hauls performed at depths between 81 and 1825 m, in November–December 2007–2010. A total of 214,982 specimens with a biomass of 1.6 tonnes was captured, Nematocarcinidae being the most abundant family, mainly due to one single species, Nematocarcinus africanus . Parapenaeus longirostris and Glyphus marsupialis were the species that most contributed to the total biomass, while Acanthephyra pelagica was the most common in the studied area. With a total of 118 species, belonging to 39 families , Mauritanian waters are more diverse in decapods than other comparable zones, probably due to the coexistence of tropical and temperate species. Two new species were described and some records increased the geographic range of certain species in the Atlantic . Five main assemblages were identified: shelf (< 100 m), deep shelf-upper slope (100–400 m), deep reef (400–550 m), middle slope (550–1400 m) and deep slope (1400–1800 m). Species of each assemblage are typified. Depth was the main factor structuring the assemblages, along with depth-dependant variables such as bottom temperature, longitude and organic matter content. Latitude also influenced assemblage structure. Greatest abundance and biomass occurred on the deep reef , in relation to minimum oxygen values (1.0–1.3 ml l−1), which may favour the abundance of certain species, such as N. africanus, but negatively affect other species, resulting in a diversity reduction. Diversity generally increased with depth, the highest values being registered on the deep slope.