In a first-of-its-kind study evaluating patterns of mortality in Florida manatee calves, University of Florida researchers found the leading cause of death to be natural causes, but point to a concerning trend of parasitic infection impacted by red tide events.
Exposure to brevetoxins from red tide events could render manatees more susceptible to developing severe parasitic infections and is a discovery worthy of further study, the researchers said.
Published recently in Diseases of Aquatic Organisms, the study was a retrospective review of pathology data amassed from 2009 to 2017, and was conducted in collaboration with the Florida Fish and Wildlife Conservation Commission.
“Up to now, calves have been subsumed in population models and not evaluated independent of the population as a whole,” said Tatiana Weisbrod, D.V.M., a resident in aquatic animal health with the University of Florida College of Veterinary Medicine and the study’s first author.
The study also represents the first time that details about disease and deaths for perinatal Florida manatees has been described, she said.
Through the commission’s Florida Manatee Carcass and Recovery Program and Necropsy program, necropsies on manatee carcasses have been conducted throughout the state since the 1970s, Weisbrod said.
“The cause of death is assigned to each carcass based on the necropsy findings, but historically, unless there is overt trauma—from watercraft injuries, for example—very small carcasses of young calves are generally categorized broadly as perinatal deaths without further description,” Weisbrod said. “In this study, we reviewed the records of over 1,200 manatee calves and provided a detailed description for the causes of death in this size class.”
The researchers found that the leading cause of death for these calves was natural causes, specifically from presumed stillbirths or dystocia.
“Secondarily, an emerging concern was the significant number of deaths associated with severe parasitic enteritis from tramatodes in the southwestern portion of the state,” she said, adding that the finding was unexpected, since up to now, parasites have rarely been associated with death or significant pathological observations in Florida manatees.
“The percentage of deaths from parasitic enteritis appears to be increasing annually and brevetoxins from red tide events were often detected in infected carcasses,” Weisbrod said.
Although watercraft trauma has been a leading cause of death for Florida manatees as a whole historically, the researchers said their study indicated that in manatee calves, it was not.
“A great deal of effort has been focused on mitigating that threat since the adoption of manatee protection zones in the 1970s,” Weisbrod said. “In calves, watercraft remains a threat, but is not the leading cause of death, which indicates there may be additional opportunities for conservation efforts aimed at this class size.”
In long-lived species with high maternal investment, such as manatees, adult survival is the main determinant of population survival, but calf survival is essential for population growth.
“Currently, we are facing an unprecedented situation, with the ongoing unusual mortality event for Florida manatees along the Atlantic coast, with the loss of seagrass beds leading to starvation,” Weisbrod said, adding that seagrass loss is a complex issue, with many factors contributing to the problem.
“These deaths have a direct impact on manatee populations, but we also know from studying other mammals, like whales, that poor nutritional planes and poor body condition among surviving animals can have an indirect and negative effect on reproductive rates and overall success,” she said. “Thus, understanding contemporary threats to calves better informs and guides our strategy for conserving this vital size class in the face of an ever-changing environmental landscape.”