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Habitat selection and abundance of West Indian Manatees Trichechus manatus at the margins of their expanding range

Carl S. Cloyed, Elizabeth E. Hieb, Kayla P. DaCosta, Monica Ross, Ruth H. Carmichael

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ABSTRACT: Habitat selection and abundances at range margins during geographic expansion may influence movement into new areas, shaping the trajectory of climate driven changes in species distribution. The West Indian manatee is an ideal species to study how habitat selection influences range expansion because their presence has rapidly increased during the past two decades in the northern Gulf of Mexico (nGoM), a region outside their historical range. We estimated habitat selection and abundances of manatees in coastal Alabama waters along the nGoM coast using resource selection functions and N-mixture models, respectively. Warm season (May-Nov) manatee abundances were estimated at 25 and 34 manatees at any given time in coastal Alabama waters in 2010 and 2019, respectively. Manatees primarily used the Mobile-Tensaw River Delta and Dog River areas, selecting nearshore, shallow water habitats proximate to submerged aquatic vegetation. Distance to boat ramp and human population density had stronger effects on opportunistic sighting data but remained important for the tagged data, indicating that manatee habitat selection overlapped with humans. Temperature strongly predicted manatee sightings; most sightings occurred when temperatures were > 20° C. Our data indicate that the key interacting factors likely to moderate manatee range expansion, and therefore be important to management and conservation of this species, include increased sea temperature, availability of nearshore habitat with submerged aquatic vegetation, and regional manatee population dynamics. As environmental conditions at the range margins continue to become more favorable to manatees and areas within the range core decline in quality, areas at the range margins may become increasingly important.