Multidisciplinary investigation into stock structure of small pelagic fishes in southern Africa

Doctoral Thesis


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University of Cape Town

Three abundant small pelagic fish species co-occur in the Benguela upwelling ecosystem off southern Africa: sardine (Sardinops sagax), anchovy (Engraulis encrasicolus) and round herring (Etrumeus whiteheadi). It is hypothesised that populations of these species have a complex structure that reflects the complexity of their varying habitats. On the basis of the locality and timing of spawning, and morphological, meristic and parasite studies, it has been proposed that there are separate stocks of sardine in South African waters west and east of Cape Agulhas. Consequently, new operational procedures for managing the fishery are based on the premise of two such stocks, termed the west and south coast stocks respectively. In this study, it is hypothesized that the factors causing purported stock differences in sardine should also apply to anchovy and redeye. Three different approaches were used to further test the multi-stock hypothesis for sardine in southern Africa and to provide new information for understanding possible stock differences in anchovy and round herring. The population structure of sardine was investigated through studies on samples taken from the Cape west and south coasts as well as from Namibia and KwaZulu-Natal. Two dimensionless otolith shape indices, otolith elemental signatures, seven microsatellite loci and the mitochondrial DNA marker, ND2, were used to supplement existing information on sardine. Otolith shape, while influenced by fish length and season of capture, distinguished the east coast samples from those taken elsewhere, while the otolith elemental signatures indicated differentiation among sites on a small scale and between samples taken from the west and south coast for certain elements. Both genetic indicators showed high levels of genetic diversity and variation among individuals. There were some genetic differences among sites within a single year, but overall the results suggest that the South African sardine population is well mixed, with weak evidence of genetic patchiness that is not temporally stable. This genetic pattern is evidence of sweepstake recruitment. For anchovy, two spawning locations have been identified in South African waters, but there have been no previous investigations into stock structure. The current study aimed to apply different methods to anchovy samples to determine their usefulness for differentiating population structure,. An analysis of otolith shape revealed that it was primarily influenced by the length of the fish, with no differentiation among sites, whereas an investigation into genetic differentiation using five microsatellite markers revealed high levels of genetic diversity and differentiation among sites. Since there were relatively few samples for a study of this sort, further work is recommended to investigate stock structure in South African anchovy. For round herring, only otolith shape was analysed to test for stock structure. Otolith shape was found to be primarily related to the length of the fish (as in anchovy) and sex, but no significant difference was found between coasts. There were different magnitudes of variability in the results for the three species using different methods. It is likely that the complex, variable environment causes complex patterns of mixing and micro-structuring within all three species, linked to their different niches.

Includes bibliographical references.