[MARMAM] Influence of Manatees' Diving on Their Risk of Collision with Watercraft

Edwards, Holly Holly.Edwards at MyFWC.com
Wed May 11 08:04:26 PDT 2016

My co-authors and I are pleased to announce our recent publication entitled" Influence of Manatees' Diving on Their Risk of Collision with Watercraft" in the journal PLOS ONE.

This manuscript can be downloaded at: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0151450

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.

Holly H. Edwards Ph.D.
Florida Fish and Wildlife Conservation Commission
Florida Fish and Wildlife Research Institute
100 Eighth Ave. SE, St Petersburg, FL 33701
Phone:  727-896-8626 ext 4745, 727-502-4745
Fax: 727-893-9176

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