ABSTRACT

Marine mammals of the order Sirenia differ greatly from cetaceans, pinnipeds, sea otters and polar bears. Sirenians have an evolutionary lineage distinct from these groups, and among living mammals are more closely related to the terrestrial elephants and hyraxes than to other aquatic species. Sirenians are herbivores that feed primarily on marine and freshwater plants, and are thus exposed to toxic substances through ingestion solely at lower levels of food webs. Unlike other groups of marine mammals, they occur only in warm, tropical or subtropical coastal waters and associated freshwater systems, primarily in the developing world (Marsh and Lefebvre, 1994). Their distribution is limited to areas where macroscopic aquatic plants occur, and by an intolerance to cool water dictated by low metabolic rates and high thermal conductance (Whitehead, 1977; Gallivan and Best, 1980; Irvine, 1983). The four species of extant sirenians are all protected worldwide by various laws and treaties because of concern about their population status. In many areas populations are considered declining or have been extirpated (Marsh and Lefebvre, 1994). Most of the negative human impacts on sirenian populations have been a result of hunting, incidental entanglement in artisanal fisheries, habitat change and accidental collisions with boats.