Overlapping distribution of two sympatric species: The case of black hakes, Merluccius polli Cadenat 1960 and Merluccius senegalensis Cadenat 1960, off Mauritania

Abstract

Merluccius polli and Merluccius senegalensis are sympatric species distributed in waters off Northwest Africa, where they overlap. Mauritanian waters are central to this overlapping area where both species are landed jointly as Merluccius spp. and managed as a single species, as observed in other hake stocks. This study was conducted during the onset of the reproductive peak in both species. According to our modelling, the presence/absence and abundance of both hakes are significantly influenced by depth and temperature covariates, and are also affected by latitude and longitude in M. polli, the deep-water species. Merluccius polli largely dominates in this area with a global ratio of 4:1 and presents wider bathymetric (80-1100 m) and thermal (5.9-16.6 °C) ranges. However, the shallow-water species M. senegalensis was rarely caught down to 500 m, disappearing beyond 700 m, and presents a narrower temperature tolerance range (8.8-17.7 °C). The significant interannual variation in abundances suggests some influence of climatic variability. The low presence of M. senegalensis found in this study contradicts earlier references, which established the predominance of this species. Misidentification in catches, a northwards shift in distribution and overexploitation could explain the current scarcity of M. senegalensis. Despite co-occurrence, both species are ecologically different and are completely segregated on Mauritanian bottoms, although they eventually appear together in the trawl gear. Our survey results combined with further biological knowledge could allow a separate assessment of both species in order to improve the future management of these stocks.