Application of a multi-method approach to the stock identification and discrimination of kingklip, Genypterus capensis (Smith, 1847), off the South African coast

Mobara, Ayesha

Application of a multi-method approach to the stock identification and discrimination of kingklip, Genypterus capensis (Smith, 1847), off the South African coast

Thesis / Dissertation

2024

Abstract

Kingklip, Genypterus capensis, is a marine demersal fish distributed in shelf and slope waters of southern Africa, from Walvis Bay in Namibia to Algoa Bay in South Africa. This commercially important species is caught by demersal trawling and longlines. Whereas previous analyses of population structure in kingklip gave contrasting results, the most recent genetic and genomic analyses have indicated population structure in this species with three genomic clusters observed: one off the central Namibian coast, one off the southern Namibian coast and the South African West Coast, and one off the South African South Coast. The identification and delineation of sub-populations is necessary for effective management of exploited species, and determination of the stock identity of individual fish is most effectively and cheaply done using phenotypic characteristics. To assess whether the two genomically-differentiated stocks off South Africa are phenotypically differentiated, a holistic and integrated multi-method approach that analysed morphological (body and otolith), meristic, otolith elemental composition, and parasitological data of kingklip from the West and South Coast was applied. Nine body morphological (standardised for fish size) and three meristic variables were analysed. For both univariate and multivariate analysis, no significant differences between mature Genypterus capensis from the West and South coasts were observed in any of the 12 characteristics. For immature specimens, significant differences between regions were observed in four morphological (upper jaw, lower jaw, orbital diameter, and interorbital diameter), but not in any meristic characteristics. Univariate and multivariate analyses indicated that four (rectangularity, roundness, ellipticity, and aspect ratio) of the six otolith shape indices calculated showed significant spatial differences in mature specimens. In immature specimens, circularity, form factor, ellipticity, and aspect ratio, showed significant differences when using multivariate analysis, and roundness was also significantly different between regions when using univariate analysis. Elliptical Fourier Analysis of otolith shape was applied utilising 157 harmonics, of which seven in mature specimens, and nine in immature specimens were significantly different between regions. Seventeen elements were measured in kingklip otoliths and of these barium, phosphorus, and nickel showed significant differences between regions in immature but no differences were observed in mature fish. Regional differences in four kingklip parasites (Anisakis sp., Diphyllobothrium sp., Lecithochirium sp., Tentacularia sp.) previously identified as potential biotags were assessed, with univariate analysis indicating a significant difference for Tentacularia sp. for both mature and immature specimens. A Stock Differentiation Index (SDI) indicated weak (SDI = 0.26) spatial structure for mature, and a moderate (SDI = 0.33) spatial structure for immature Genypterus capensis in South African waters. These results indicate that the genomic differentiation of G. capensis off the South African coast does not translate into substantial and spatially differentiated phenotypic variation in mature specimens, however moderate stock delineation is observed in immature samples. This suggests that mixing between the two genomically-differentiated kingklip stocks is lowest for juvenile fish and increases with age, which accords with the hypothesis of separated and localised spawning locations for these two stocks off the West and South coasts of South Africa. That otolith shape indices had the strongest discriminatory power of the characteristics examined in this study suggests that analyses of otolith shape at an early stage inferred via measurement of the shape of the first otolith annulus (laid down when fish are 1-year old) may be a useful phenotypic characteristic for stock identity determination, and further research should be conducted in this regard.

Keywords

Biological Sciences

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PhD / Doctoral

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