DDT Strikes Back: Galapagos Sea Lions Face Increasing Health Risks

Abstract

Dichlorodiphenyltrichloroethane (DDT) is an effective and relatively cheap anti-malaria pesticide which has saved human lives, but also brought Northern wildlife species to the brink of extinction due to its persistence, bioaccumulation in the food-web, and hormone-mimicking characteristics (Carson 1962; Vallack et al. 1998; UNEP 2001; ATSDR 2002; Blus 2003; WHO 2006; Colborn and Carroll 2007; Van den Berg 2008). Thirty-two years after its ban in the US and its inclusion on Schedule 2 of the UN Stockholm Convention on Persistent Organic Pollutants (POPs) because of its damaging health effects in human and wildlife populations (UNEP 2001; ATSDR 2002; WHO 2006; Vallack et al. 1998; Colborn and Carroll 2007; Blus 2003), the WHO has recommitted to the use of the malaria-fighting pesticide DDT in tropical countries because of an increase in malaria cases in tropical countries (e. g., Peru, Bolivia, Paraguay, Brazil, and African nations), where DDT use was halted or low application rates were used (Van den Berg 2008; WHO 2006; Roberts et al. 1997, 2000). Although evolutionary adaptation of the malaria vector has triggered insecticide resistance to DDT (Denholm et al. 2002), the return to an increased reliance on DDT use is testament to DDT’s effectiveness in combating malaria at a low cost. Large volumes of DDT (i.e., 4,000–5,000 tonnes of active ingredient per year) were used in India and Africa countries to overcome the malaria crisis (Van den Berg 2008). However, 50 years of research on DDT in the temperate and arctic regions of the Northern hemisphere have documented that human health benefits go hand in hand with detrimental effects on wildlife populations (Carson 1962; De Guise et al. 1998; Lahvis et al. 1995; Guillette et al. 1994; Blus 2003;). Since wildlife provides life support, sustenance, and economic opportunities through harvesting and ecotourism in many developing tropical countries, the health of wildlife cannot be viewed in isolation of human health. A balance needs to be struck between health benefits and health risks of DDT use. This requires an understanding of the fate, trends, and dose–response relationship for DDT in tropical wildlife. Such information does currently not exist and monitoring and source control programs that can help to document this relationship are as absent in tropical countries to date as they were in the Western world when DDT was first used in the 1940s and 1960s (Roberts et al. 2000; ATSDR 2002; Blus 2003; Colborn and Carroll 2007; Van den Berg 2008). From a global perspective, the protection of coastal food webs from contamination by chemical pollutants is critical to the long term conservation of the biodiversity and native inhabitants residing in unique places of the Earth, including UNESCO Global Heritage sites such as the Galapagos Islands. Coastal waters that are contaminated with persistent chemicals and pathogens can lead to human illness and adverse health, reduced fisheries quality and quantity, and impacts of the health of marine wildlife. This had obvious social and economic consequences. Conversely, coastal waters that are protected from chemical pollutants provide for an abundance of clean fisheries products and wildlife, and essential foundation for the well-being of the local biodiversity, human residents and the ecotourism sector. At the top of the marine-coastal food chain, marine mammals can provide an “integrated” overview of ecosystem health. As aquatic animals, they are also vulnerable to infection by pathogens of terrestrial origin. By documenting the presence of chemical pollutants in this species, we are able to deliver science-based advice to conservationists, managers, regulators, and stakeholders, on the implementation of best management practices. Equivalent to the role of killer whales as global sentinels of pollution in the Northeastern Pacific, the Galapagos sea lion has recently been used as a sentinel of marine pollution and a key indicator of the ecosystem functioning and oceanic-coastal environmental health in Galapagos Islands. Therefore, this synopsis aims to synthesize our recent findings on DDT contamination in Galapagos sea lions and its use as a sentinel-model species of environmental pollution by POPs.