The long-term viability of the Florida manatee is threatened in part by mortality from boat collisions. This study investigated manatee behavior during boat approaches to better understand factors that lead to manatee-boat collisions. Digital acoustic recording tags (DTAGs) and Argos-linked GPS tags were deployed on 18 manatees in southwest Florida, and boat traffic around them was mapped. Suites of manatee behavioral, environmental, and boat-related factors were recorded during opportunistic boat passes. We built generalized linear mixed models to determine which factors accounted for variation in occurrence, number, and timing of manatee behavioral changes during boat passes. Manatees responded to boats, changing their orientation, depth, and fluking behavior most often when a boat approached closely (<10 m). Manatees were also more likely to change their depth when not on a seagrass bed and when actively fluking before a boat pass. Boat speed did not affect the occurrence or intensity of manatee response. Compared to fast approaches, however, slower passes allowed the manatee more time to respond, and behavioral change occurred earlier relative to the time of the boat's closest point of approach. We conclude that faster boats likely pose a greater risk of collision with manatees than do slower boats.

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Watercraft pose a threat to endangered Florida manatees (Trichechus manatus latirostris). Mortality from watercraft collisions has adversely impacted the manatee population’s growth rate, therefore reducing this threat is an important management goal. To assess factors that contribute to the risk of watercraft strikes to manatees, we studied the diving behavior of nine manatees carrying GPS tags and time–depth recorders in Tampa Bay, Florida, during winters 2002–2006. We applied a Bayesian formulation of generalized linear mixed models to depth data to model the probability (Pt) that manatees would be no deeper than 1.25 m from the water’s surface as a function of behavioral and habitat covariates. Manatees above this threshold were considered to be within striking depth of a watercraft. Seventy-eight percent of depth records (individual range 62–86%) were within striking depth (mean = 1.09 m, max = 16.20 m), illustrating how vulnerable manatees are to strikes. In some circumstances manatees made consecutive dives to the bottom while traveling, even in areas >14 m, possibly to conserve energy. This is the first documentation of potential cost-efficient diving behavior in manatees. Manatees were at higher risk of being within striking depth in shallow water (<0.91 m), over seagrass, at night, and while stationary or moving slowly; they were less likely to be within striking depth when ≤50 m from a charted waterway. In shallow water the probability of a manatee being within striking depth was 0.96 (CI = 0.93–0.98) and decreased as water depth increased. The probability was greater over seagrass (Pt = 0.96, CI = 0.93–0.98) than over other substrates (Pt = 0.73, CI = 0.58–0.84). Quantitative approaches to assessing risk can improve the effectiveness of manatee conservation measures by helping identify areas for protection.

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