Trace metal pollution in Hong Kong: Implications for the health of Hong Kongʹs Indo-Pacific hump-backed dolphins (Sousa chinensis)

The territorial waters of Hong Kong are highly contaminated with anthropogenic pollutants, including trace metals. Hong Kongʹs population of Indo-Pacific hump-backed dolphins (Sousa chinensis) inhabits an area where a high volume of sewage waste discharge and the close proximity of contaminated mud pits mean a considerable potential for trace metal contamination. Trace metal concentrations in the muscle tissue of fish caught from areas of high dolphin abundance are notable. However, in order to gauge the amount of trace metal consumed by dolphins, whole fish must be analysed. Whole fish from areas of high dolphin abundance have considerably higher trace metal loads than muscle tissue alone. Considering the consumption of prey items by dolphins, a daily intake of trace metals was estimated as As, 410–518; Cd, 34–44; Co, 4–5; Cr, 40–50; Cu, 44–56; Hg, 51–66; Mo, 22–27; Ni, 19–24; Pb, 320–403; Se, 99–125 and Zn, 388–490 (μg kg−1 body wt. per day). Concentrations of arsenic, chromium, lead, molybdenum and nickel in dolphin tissues were an order of magnitude lower than in prey items, suggesting these elements may be excreted by this species. Mercury concentrations in dolphin tissues were, however, an order of magnitude higher than in prey items and could be considered potentially health threatening (max: 906 μg kg−1 dry wt.). Hong Kongʹs Indo-Pacific hump-backed dolphin population has a high dietary intake of trace metals which may, especially in the case of mercury, be a cause for concern.