Stock Separation Of The Shallow-Water Hake Merluccius Capensis In The Benguela Using Otolith Shape Analysis And Parasite Infestation

Abstract

The fishing industry is an important sector in Namibia with hake contributing about one third of the total commercial catch. Merluccius capensis, the shallow water hake, forms the bulk of this resource. Studies on the distribution of spawners and juveniles, spawning areas and genetics have proposed three stock structure hypotheses of M. capensis in the Benguela: (1) one stock throughout, (2) one in the northern and one in the southern Benguela or (3) three stocks: one in the northern, one in southern Namibia and the Orange River (border between Namibia and South Africa), and one in the southern Benguela. This study aimed to investigate these hypotheses of M. capensis stock structure within the Benguela using otolith shape analysis and parasite infestation, and to identify parasite species as potential biological tags. M. capensis otolith samples were collected during demersal trawl surveys along the Namibian coast in 1992, 2004 and 2005 and along the South African West coast in 2005. The Benguela ecosystem was divided into three areas, northern (17.00-25.29 S), central (25.30 - 29.05 S) and southern Benguela (South African waters, 28.86 - 35.81 S), for this study. A total of 1628 otolith images were analyzed using the ShapeR package in R. Otolith outlines from Wavelet transformations and ANOVA permutation tests indicated no significant differences between the northern and central Benguela for the years 1992, 2004 and 2005 (p > 0.05 for all) but showed significant differences between the northern and southern Benguela in 2005 (F = 14.81, p < 0.001). Comparing otolith size descriptors and otolith shape indices showed that otoliths from the southern Benguela were significantly more elongated (longer, narrower and less round) than those from the northern and central Benguela. 80 M. capensis were collected off Namibia and examined for parasites and compared with a study done on M. capensis parasites in South Africa in the previous year. A total of 10 parasite species were found on Namibian M. capensis. Prevalence (number of infected fish/number of fish examined)*100 and abundance (total number of parasites/total number of fish examined) were compared between areas. The following species have the potential to be used as biological tags (1) Chondracanthus merlucii indicating significant differences in abundance between the northern and central Benguela, and northern and southern Benguela; (2) Parabrachiella australis, only present in the northern and central Benguela, and (3) Stephanostomum multispinosum, only present in the southern Benguela. Otolith shape analysis and parasite infestation levels therefore revealed one M. capensis stock in the northern Benguela and another in the southern Benguela, with a possible separation further north than the Orange River, as shown by parasites, supporting stock structure hypothesis number (2). Further studies should include sampling for parasites conducted seasonally and adding stock identity methods such as otolith microchemistry along the Benguela.