Manatee Trichechus manatus latirostris calf mortality in Florida: a retrospective review of pathology data from 2009-2017.

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.