iSimangaliso – Crocodiles and the environment
The lakes and wetlands of the iSimangaliso Wetland Park World Heritage Site are synonymous with one of the oldest predators on Earth – the Nile crocodile. Over 1000 crocodiles inhabit iSimangaliso’s water bodies, making it one of the most important protected areas for the conservation of these reptiles in the country.
In South Africa, Nile crocodile populations are threatened by habitat removal, illegal killings, destruction of nesting sites and human disturbance. However, recent research conducted within iSimangaliso has also drawn attention to the impact that toxic compounds released into the environment may be having on crocodile populations in this region.
Using newly developed methods, a team of scientists led by Dr Marc Humphries, Dr Xander Combrink and Prof Jan Myburgh has been able to surgically extract and analyse fat tissues from live crocodiles within iSimangaliso. “These fat tissues provide valuable information on the type of toxins that crocodiles have been exposed to in the environment. As part of a pilot study, fat samples were surgically removed from the tails of 15 crocodiles captured within iSimangaliso and tested for a range of potential contaminants. The procedure causes no pain nor long term injury to the animals,” says Humphries.
Analysis revealed that all crocodiles sampled contained numerous toxic organochlorine compounds that were present in elevated levels relative to environmental background. The compounds detected belong to a family of environmentally persistent chemicals that have been used extensively around the world as agricultural insecticides and to combat diseases carried by insects. Their use in Maputaland became prevalent in the early 1950s.
Arguably the most well-known of these is DDT, which has been used to combat the spread of malaria in the region. However, the use of such chemicals presents numerous human health and ecotoxicological issues. Organochlorine compounds are highly toxic to aquatic life and may persist in the environment for several decades. Although these compounds degrade over time, their breakdown products themselves are often toxic and even more persistent in the environment. Over time, they tend to accumulate in biological tissues and be transferred within the food chain to organisms at higher trophic levels.
“Today the use of these chemicals has been phased out and completely banned in most countries. South Africa continues to employ certain compounds on a limited basis and the Department of Health regularly uses DDT in northern KwaZulu-Natal because of its effectiveness in reducing malaria infections. However, this remains a controversial issue because of the associated environmental and human health risks.”
“Chemicals used within the catchment areas of iSimangaliso ultimately find their way into the Park,” says Humphries. “Analysis of sediment samples collected from the lakes and wetlands within iSimangaliso show that these habitats act as sinks for organochlorine contaminants in the local environment. This exposes a variety of fish and invertebrate species to elevated levels of contamination. Crocodiles play a dominant role in the ecosystem and may live several decades, making them particularly susceptible to the accumulation of contaminants. The results of this study show that crocodiles accumulate toxins to high levels within iSimangaliso Wetland Park with some of the highest organochlorine concentrations ever recorded in crocodilian tissue from anywhere in the world.”
At present, very little is known about the potential ecotoxicological effects these compounds have on crocodiles living within iSimangaliso. Few studies have looked at contamination and its associated health effects in crocodilians. What is known, however, is that contaminants accumulated within the tissues of crocodiles can be maternally transferred to developing eggs, potentially affecting reproductive success. The high level of contaminants detected within the sample population from iSimangaliso therefore raises concerns regarding the influence of these toxins on reproduction, egg viability and hatching success. These results suggest that the accumulation of contaminants may be an important factor to consider when assessing the long term viability of crocodile populations in the region.
Says Nerosha Govender, iSimangaliso’s Manager of Development and Planning, “The persistence, accumulative potential and toxic nature of these chemicals highlights the need for a better understanding of the potential long-term reproductive impacts on local crocodile populations. Ecotoxicological research within iSimangaliso is on-going and hopes to shed further light on the risks associated with pesticide exposure, particularly in crocodile populations.”
The research formed part of Archibold Buah-Kwofie’s Ph.D. studies (University of the Witwatersrand). Further information on results from the study can be found here.
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