Quantifying sublethal Florida manatee-watercraft interactions by examining scars on manatee carcasses

A leading human-related threat to the Florida manatee Trichechus manatus latirostris is collisions with watercraft, which account for 20-25% of reported mortalities. Quantitative threat assessments do not include information on all known manatee-watercraft interactions. These interactions often result in sublethal wounding, usually leaving multiple fresh external wounds in a variety of patterns. These wounds then resolve into well-healed scars. We characterized and quantified watercraft-related scar patterns (1 pattern = 1 strike event) on 2935 nonperinatal carcasses (>150 cm total length) that were recovered from 2007 through 2016 to compare the number of patterns by life stage, sex, and population region and across years. We used generalized linear mixed models to examine the effects of several factors on the probability carcasses having scars and on the number of scar patterns per carcass. The models indicated that approximately 96% of adults, approximately 70% of subadults, and approximately 34% of calves had watercraft-related scars. The raw data showed that 1 in 4 adults had been hit 10 or more times; 5 adult carcasses bore evidence of 40 or more strikes. On average, adult females had more scar patterns than did adult males. Manatees on Florida’s west coast had more scar patterns than did those on the east coast, while carcasses from the less populated Everglades had significantly fewer scar patterns than did those from the rest of the state. These results improve our understanding of the extent of sublethal injury of the Florida manatee caused by boat strikes.