Abstract

A cooperative project between the Denver Wildlife Research Center, Gainesville, FL, and the College of Veterinary Medicine, University of Florida, Gainesville, FL, was established in 1980. The objective was to obtain detailed information of the natural causes of death in the West Indian manatee (Trichechus manatus). A standard protocol was devised which called for a complete necropsy of those manatees which were salvaged under fresh conditions. This presentation summarizes the pathologies findings obtained from 12 manatees necropsied between 1980 and 1983.

Introduction

The West Indian manatee (Trichechus manatus), listed as an endangered species since 1972, ranges from the Southeastern United States throughout the Caribbean to Northeastern Brazil. In Florida, it inhabits the coastal areas, lagoons, and certain rivers.

The present number is estimated at around 1,000 in the state of Florida that once had thousands of them. Considered a large, herbivorous aquatic mammal, it virtually has no natural enemies. Their slow speed and occurance in habitats heavily utilized by man make them very vulnerable to human related activities.

Materials and Methods

Mortality data have been reported periodically over the last several years. A Manatee Salvage Program was initiated in 1974 by the University of Miami and the Sirenian Project of the Denver Wildlife Research Center, U.S. Fish and Wildlife Service, stationed in Gainesville, FL. From the programs inception through March of 1981, 458 deaths were registered in manatees.

Major categories of mortality centered around human-related activities. 21% of deaths were attributed to boats or barge collisions. 8% included fatal crushing or drowning in flood control gates or canal locks; 5% of deaths were contributed to vandalism, poaching, or entrapment in fishing lines or nets. 15% of deaths occurred in the dependent calf category, here young manatees, measuring less than 150 cm in total length were orphaned or separated from their mothers and suffered from debilitating disease. Another 5% of deaths included natural causes such as disease, parasitism, or starvation. 46% of ammals vpre undetermined with regard to cause of death, in some instances resulting from severe decomposition.

In order to obtain more specific information on the natural causes of death in manatees, a cooperative project with the College of Veterinary Medicine, University of Florida and the Denver Wildlife Research Center was established in 1980 in Gainesville, FL. A standard protocol was devised that called for a complete necropsy of those manatees which were salvaged under fresh conditions. This presentation summarizes the pathologic findings obtained from 12 manatees necropsied between 1960 and 1983.

Results

The dead animals ranged between 4 weeks and 20 years of age. Four (4) were females and eight (8) were males. Death categories involved environmental and natural causes. The cause of death of 2 manatees could not be determined.

Environmentally induced mortalities included trauma, starvation, human-related factors, and an outbreak of red tide causing neurotoxicosis. The latter event involved a die-off of 41 manatees during February, March, and April of 1982. Circumstantial evidence indicated that death occurred as a result of (1) the seasonal dispersal of manatees in the area; (2) An unusual drought condition; (3) a red tide outbreak; and (4) a relatively dense population of ascidians or sea squirts. Ascidians ingested incidentally by manatees during feeding on seagrass provided a likely route of exposure to neurotoxins.

Natural disease causes included one manatee with encephalytic Toxoplasma gondii infection and a second animal with Gram-negative bacterial encephalitis. Each of the various death categories will be exemplified by a case report demonstrating gross pathologic and/or microscopic findings. In addition to the pathologic demonstrations, some unusual anatomical features of individual manatee organs will be highlighted.

Discussion

The survival chance of manatees may be bleak. Manatees have a low reproductive rate (one young every 2 or 3 years) and they appear to be especially susceptible to unexpected environmental changes in the winter months. The increasing human population in the Southeastern United States will not help to reduce manatee mortality, unless more educational programs, sanctuaries, and stricter law enforcements are created and implemented. The protection of Trichechus manatus in Florida depends on the absence of predation, on mankind, and the proper recognition of its problems.

References

1. Husar, S.L. The West Indian Manatee Trichechus manatus). U.S. Dept. of the Interior, Fish and Wildlife Service, Washington D.C., 1977.
2. Hartman, D.S. Ecology and Behavior of the Manatee (Trichechus manatus) in Florida. Amer. Sac. Mammal. Special Pub. No.5, 1979.
3. Bonde, R.K. Do manatees have a chance? J. Amer. Cetacean Sac. 16: 3­5 (1982).