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Turbid and tannin-stained waterways are difficult habitats to study and present problems with respect to the management of aquatic fauna, and especially of endangered and cryptic animals such as manatees Trichechus spp. Linnaeus. Throughout much of the range of the extant trichechid species (T. inunguis, T. manatus, and T. senegalensis), the scientific approaches that are used successfully to document the status of the Florida subspecies (T. m. latirostris) are not feasible, due either to constraints associated with habitat or to the high costs involved. We examined the use of side-scan sonar as a novel way to acoustically detect free-ranging West Indian manatees (T. m. latirostris and T. m. manatus), and evaluated the effectiveness of the technology under various environmental conditions. Blind transects, where one observer only looked at the sonar, were conducted in Mexico and Florida to confirm that manatees could be detected and to determine a preliminary manatee detection rate for the sonar unit. Quality sonar images were produced under most environmental conditions examined, provided that ambient water movement was low. Manatees were successfully detected laterally up to 18 m, and the best acoustic images were produced when the animals were stationary and oriented parallel to the boat. Preliminary detection was 81 and 93% in Florida and Mexico, respectively. We conclude that over limited areas, and under certain environmental conditions, side-scan sonar is a cost-effective option to accurately detect manatees and provide important information for conservation decisions. Like all survey techniques, it has limitations, but its unique advantages will be a welcomed alternative to aerial surveys and visual boat surveys of opaque waterways.

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Endangered Species Research

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