Pollutants associated with mass mortality of Nile crocodiles (Crocodylus niloticus) in the Kruger National Park, South Africa

Abstract

The first of a series of mass mortalities of Nile crocodiles in the Olifants and Letaba rivers in the Kruger National Park (KNP) was reported in the winter of 2008. The present study investigated the levels and possible effects on eggshell thickness of inorganic elements and organic pollutants in Nile crocodile eggs from these rivers, and comparing them with eggs from a reference crocodile farm and a reference dam inside the KNP. The egg contents were analyzed for chlorinated organic compounds and brominated flame retardants. Eggshells and egg contents were analyzed for inorganic elements. The elemental concentrations in the eggshells and contents were low when compared with previous studies. The highest concentrations were found in the eggs from the reference crocodile farm. The eggs from the reference dam and the crocodile farm had thicker shells, and the eggs from the Olifants and Letaba rivers had thinner shells. Not all eggs in a female develop at the same rate, while eggshell formation presumably occurs at the same time for all eggs. As a result, the elemental profile of egg contents may differ between eggs of the same clutch, but less so for the shells. Weak or no associations were found between the elemental concentrations of the content and eggshells and eggshell thinning. A possible organic pollutant–induced eggshell thinning effect was found. The compounds found were not at levels that could have caused the mortalities, but may affect the sex ratios through endocrine disruption. Further studies are therefore required.