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

Abstract

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.

Increasing mortality of Endangered Antillean manatees Trichechus manatus manatus due to watercraft collisions in Belize

“…Taken together, these relationships indicate that in creasing the number of boats in an area or over time will likely result in more manatee watercraft collisions. Our estimate of watercraft collision being the cause of 27% of reported strandings is slightly higher than the 20−25% of reported mortalities in Florida manatees Trichechus manatus latirostris, where boat collision has been determined as a significant threat (Wright et al 1995, Lightsey et al 2006, Florida Fish and Wildlife Conservation Commission 2007, Runge et al 2015, Bassett et al 2020.…”

Evaluating wildlife tourism exposure to Florida manatees using citizen science, public interviews, and trail cameras

“…The integration of all these methodologies is particularly important for species such as the Florida manatee ( T. manatus latirostris ), where few studies have been implemented that address impacts of wildlife tourism on manatees. The Florida manatee is highly susceptible to various anthropogenic threats, including those stemming from boat traffic (Bassett et al, 2020; Edwards et al, 2016) and tourism (Sorice et al, 2003). They are drawn to the warm water refuges provided by power plants and natural springs during winter months when temperatures drop below their tolerance threshold of 20 °C (Laist et al, 2013; Laist & Reynolds, 2005).…”

The influence of variations in background noise on Florida manatee (Trichechus manatus latirostris) detection of boat noise and vocalizations

“…Endangered Species Act, the high mortality rate from watercraft collisions is a major concern for the species [ 9 ]. Sublethal wounds from watercraft collisions are also common with an estimated 96% of adult manatees having scars from a collision, and a high percentage with scars from multiple collisions [ 10 ]. The sound of an approaching boat is a vital cue manatees use to avoid collisions [ 11 ].…”