Abstract
High maternal investment and extended inter-calving intervals in Florida manatees Trichechus manatus latirostris make calf survivorship critical to overall population growth. However, detailed patterns of causes of mortality in calves have not been reported and state agency statistics report portions of perinatal mortality based on body length rather than actual cause of death (COD). The objectives of this study were to categorize COD based on necropsy data and geographical location in Florida for 1209 manatee calf carcasses (<236 cm total length) examined between January 2009 and December 2017 and to describe factors contributing to calf mortality. Results indicated COD was attributed to natural causes (47%, n = 573), cold stress syndrome (38%, n = 457), watercraft injury (13%, n = 155), or other human-related causes (2%, n = 24). Natural causes were the leading COD for small calves <151 cm, with death due to stillbirth or dystocia most frequent (48%, n = 273/573). Enteric trematodiasis contributed to a large proportion of deaths from natural causes in large calves within the southwest region of Florida, with an increasing annual trend. Brevetoxicosis contributed substantially to natural causes within the southwest region exclusively and was commonly comorbid with enteric trematodiasis. Cold stress syndrome was the leading cause of death for large calves (151-235 cm), with the Atlantic region having the highest proportion of cases. Watercraft injury was a sustained threat to large calves, especially within the southwest region. This report provides details on specific health threats and patterns of mortality among manatee calves.
Highlights
The Florida manatee Trichechus manatus latirostris, a subspecies of the West Indian manatee, is an herbivorous aquatic mammal native to the southeastern USA, coastal Florida
This study provides novel, detailed data on Florida manatee calf mortality, including patterns among cause of death (COD), geographical location, and size classes, covering almost one decade of pathology data collected at the Fish and Wildlife Conservation Commission (FWC) Mammal Pathobiology Laboratory (MMPL)
The manatee population has grown over time, including most of the study period (Hostetler et al 2021), and it is possible that an increased proportion of younger females led to more unsuccessful births (O’Shea & Hartley 1995)
Summary
The Florida manatee Trichechus manatus latirostris, a subspecies of the West Indian manatee, is an herbivorous aquatic mammal native to the southeastern USA, coastal Florida. Initial analysis of mortality data collected from the 1970s to the 1990s revealed WC injuries and other human activities as the largest single cause of manatee mortality (O’Shea et al 1985, Ackerman et al 1995, Lightsey et al 2006), leading to protective measures limiting boater speeds in certain areas (US Fish and Wildlife Service 2001, Calleson 2014) Despite these efforts, WC injury remains a significant cause of mortality; quantitative models have identified red tide events and loss of warmwater habitat as significant threats (Runge et al 2017). In addition to delineating small calves that did not die from human-related (HR) causes, this ‘perinatal’ category has historically represented dependent calves unlikely to survive if separated from their mothers and was defined by the upper size limit of the largest known in situ fetus at the time
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