Otolith microstructure analysis reveals differentiated growth histories in sympatric black hakes (Merluccius polli and Merluccius senegalensis)

This is the first study to provide age estimations of the two black hake species living in Mauritanian waters. Here we propose a new interpretation of microstructural growth increments that are assumed to be deposited daily. Our approach to age-reading combining the counts in both sagittal and transverse planes proved successful with black hake otoliths, including adults'. A newly developed image analysis software tool (OTOLab) was used to help in identifying and marking daily growth increments throughout the otoliths. Larval growth was analyzed from otolith sagittal sections and showed wider mean daily increments in Merluccius polli (3.32 μ) than in Merluccius senegalensis (2.85 μ). Conversely, once recruited to demersal life after metamorphosis, juvenile and adult growth was analyzed from otolith transverse sections, where the widest mean daily increments were found in M. senegalensis (4.19 μ) (M. polli, 3.19 μ). Although the growth models of both species were not statistically different, the two black hakes probably have alternate growth strategies and, like the European hake, belong to the group of fast-growing hakes. When both species were considered together, the mean length of one-year-old black hakes was 36.7 cm, while the averaged individual growth rate was 1.12 mm day−1. These growth values are the highest found for any hake species and are probably explained by the influence of one of the world's most important upwelling systems, off northwest Africa. Although environmental factors may influence individual growth, ontogeny is the main factor determining growth rates at any life stage. Our results support otolith microstructural analysis as a realistic choice to regularly age hakes.