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Report to IWC Annual Scientific Committee Meeting: Metal concentrations of three dolphin species incidentally caught in bather protection nets off KwaZulu-Natal, South Africa
Pl?n, S., Roussouw, N., Uren, R., Naidoo, K., Siebert, U., Bouwman, H.
Metal toxicity causes a myriad of sub-lethal effects in marine mammals. To investigate this burden in dolphins from South Africa, we used a comparative approach of examining a total of 36 trace elements from 76 muscle tissue samples using inductively coupled plasma mass spectrometery. The three species incidentally caught off the KwaZulu-Natal coastline included the Indian Ocean humpback dolphin Sousa plumbea (n=36), Indo-Pacific bottlenose dolphin Tursiops aduncus (n=32) and the Common dolphin Delphinus delphis (n=8). Na, Sb, Sr, and Zn concentrations were significantly higher in S. plumbea compared to T. aduncus. There were significant differences for Cd, Fe, Se, U and V between all three species; in all cases, S. plumbea had the lowest concentrations, and D. delphis the highest. There were no significant differences of Hg between any combinations of species. No influence of sex was detected within each species. Fe, Se, Cd, and Hg concentrations were significantly lower in T. aduncus juveniles compared to adults, while Rb was significantly higher. Linear regressions showed several significant positive and negative associations between concentrations and dolphin length and mass, suggesting allometric involvement. These differences are likely due to a combination of differing habitat, feeding ecology, age (mass and length), physiology and pollution. Of concern were the concentrations of mercury in all three species, which were generally higher than concentrations reported for similar species elsewhere. S. plumbea females were the only group that showed a decrease in mercury with length and mass, suggesting offloading of mercury to calves. Our results present the only recent study on metals in cetaceans from South Africa and, together with very high organic pollutant concentrations documented for dolphins from the same location, show that there is a likely threat needing long-term monitoring.