Stock separation of the shallow-water hake Merluccius capensis in the Benguela ecosystem using otolith shape analysis

The fishing industry is an important economic sector in Namibia and South Africa, with the shallow-water hake Merluccius capensis being an important target species. Recent genetic studies of M. capensis found two stocks in the Benguela Current Large Marine Ecosystem, one in the north (17-29 degrees S) and one in the south (29-36 degrees S), and a proposed mixed stock in the Orange River area (around 29 degrees S). The present study investigated the use of otolith shape analysis for purposes of stock-structure analysis of M. capensis. Merluccius capensis otolith samples were collected during demersal-trawl surveys along the Benguela, for the years 1992, 2004 and 2005. Different years were selected to investigate temporal stability in otolith shape in the northern Benguela. A total of 1 628 otolith images were analysed using the shapeR library in R. Otolith shape was analysed using wavelet transformation, and ANOVA-like permutation tests indicated no significant differences between the northern (17 degrees 31 '-25 degrees 29 ' S) and central (25 degrees 30 '-29 degrees 05 ' S) Benguela for all years but showed significant differences between the northern and southern (29 degrees 05 '-35 degrees 50 ' S) Benguela. This study therefore demonstrated that otolith shape could be used for stock discrimination of M. capensis. It confirmed one stock of M. capensis in the northern and central Benguela and another in the southern Benguela, which supports the current, separate management approach for this species. It also showed some differences in otolith shape from the 1990s to the 2000s, which could be explained by increased movement of the southern Benguela stock to the northern Benguela and increased hybridisation in the later years.

Automated summary

The study demonstrated the use of otolith shape analysis for stock discrimination of M. capensis in the Benguela Current Large Marine Ecosystem, supporting separate management approaches for different stocks. It also showed some differences in otolith shape between different years, potentially due to increased movement of the southern stock to the north and increased hybridisation in later years.