Otolith chemistry of fishes from Kosi Bay, South Africa: A preliminary multiple analytical methods approach to reconstruct fish migrations

Over exploitation of fish stocks in coastal areas is a subject of global concern. Increasing numbers of traditional fish traps, recreational fishing effort and unknown extents of illegal gillnetting are blamed for the declining abundance of estuarine-dependent fish species in the Kosi Bay estuarine lake system in KwaZulu-Natal, South Africa. Otoliths from four important fishery species (Lutjanus argentimaculatus, Pomadasys commersonnii, Acanthopagrus vagus, and Rhabdosargus sarba) and water samples collected from throughout the Kosi Bay system were analyzed for trace element constituents and δ18O values, in an attempt to reconstruct fish movements. The applied microscopic X-ray fluorescence analysis proved valuable in detecting μm-scale spatially resolved Sr/Ca ratio distributions across otolith thin sections. Sr/Ca ratios and δ18O in otoliths correlated well with ambient water conditions and, thus, enabled us to reconstruct migratory histories of individual fish in the estuarine lake system. The findings are representative of the known biology of these species, although exact spatial and temporal ranges of migrations remain unclear. Interestingly, samples of two species (L. argentimaculatus and R. sarba) did not show any clue of migrations back to sea, which would be necessary for successful spawning. Further laboratory and field investigations with larger sample sizes are necessary to detect fishery impacts on migratory behavior at a population level.