Abstract

In the past 10 years, over 700 Florida manatees (Trichechus manatus latirostris) have been killed by watercraft collisions. Watercraft-related death is the most common anthropogenic cause of death in manatees, and it comprises approximately 25% of the annual manatee mortality. We are able to determine that mortality is a result of watercraft trauma because the wound patterns are recognizable and diagnostic. Watercraft can inflict both sharp- and blunt-force trauma to manatees. At the Marine Mammal Pathobiology laboratory, we characterize wounds and attempt to determine the cause of death of all of the carcasses that we receive. Although human-inflicted trauma to manatees is often discussed in the literature^2,3, this is the first time such a large-scale documentation of the consequences of watercraft strikes for manatees has been made.

Sharp-force trauma includes injuries from propeller blades and skegs, while blunt-force trauma includes injuries from hulls, keels, and rudders. Manatee skin is incredibly thick and is reinforced with densely woven collagen fibers.¹ Therefore, a sharp object is capable of causing a blunt-force trauma. Propeller wounds are usually found in a series, and may vary in appearance depending on the depth and angle of penetration and the pitch of the propeller. Individual, linear wounds are considered to be from skegs, keels, rudders, or other non-rotating features of the vessel. These lesions are generally superficial and are typically accompanied by deep tissue trauma. Both sharp- and blunt-force traumatic injuries cause mortality. This mortality may be a direct result of the sharp- and blunt-forces, or via chronic effects of both.

Consequences of trauma include both acute and chronic lesions. Acute lesions can be fresh, superficial, or deep skin lacerations. Fresh lacerations are open, with few signs of healing. They may be associated with inflammation of the surrounding tissues, hemorrhage leading to bruising, frank blood, or blood clots. Hemothorax, pyothorax, and/or pneumothorax are often found upon entry into the thoracic cavity. Lacerated muscle, hemidiaphrams, transverse septae, gastrointestinal tract, and lungs may be present. Often broken and/or luxated ribs and other broken bones, and lacerated kidneys and liver are apparent.

In cases where a chronic condition is generated by an incident causing a scar, the original incident is usually considered to be the cause of death. Healed scars can often be visualized on a manatee carcass. The scarred region is typically smoother than the surrounding skin. Healing scars may have rough margins and be pink, white, or yellow in color. Healed scars can be completely covered by dark, pigmented epidermis. Common, grossly evident chronic lesions include internal abscesses, organ adhesions, exsanguination, and sequelae to broken bones.

When manatee carcasses are fresh, samples of all major organs, appropriate lesions, and all major lymph nodes are taken for histopathological analysis. The most common histopathological diagnosis as to the cause of death of a watercraft-impacted manatee is severe blood loss secondary to traumatic impact, leading to hypovolemia and death. This blood loss can be secondary to rib and kidney fractures, perforated bowel and lung, abdominal hemorrhage, and hemorrhage from subdural and subdermal spaces. Pleuropneumonia and serositis of the diaphragm is often seen in animals with grossly apparent acute or chronic rib fractures.

Acknowledgements

The authors wish to thank the staff and volunteers at the MMPL for their invaluable assistance with manatee necropsies and Dr. Greg Bossart and associates for their histological diagnoses. This work was jointly supported by the Save the Manatee Trust Fund and the Florida Fish and Wildlife Conservation Commission Marine Mammal Training grant to the University of Florida.

References

1.  Kipps EK, WA McLellan, SA Rommel, DA Pabst. 2002. Skin Density and its Influence of Buoyancy in the Manatee (Trichechus manatus latirostris), harbor porpoise (Phoca phocoena), and Bottlenose Dolphin (Tursiops truncatus). Marine Mammal Science, 18(3): 765-778.

2.  Pitchford TD, ME Pitchford, SA Rommel. 2001. Characterizing Watercraft from Watercraft-induced mortalities in Florida Manatees. In: Proceedings of the Fourteenth Biennial Conference on the Biology of Marine Mammals, Vancouver, BC, November 28-December 3, 2001.

3.  Wright SD, BB Ackerman, RK Bonde, CA Beck, D J Banowetz. 1995. Analysis of Watercraft-related Mortality of Manatees in Florida 1979-1991. In: O'Shea, T.J. and B.B. Ackerman, H. F. Percival (eds.). Population Biology of the Florida Manatee. U.S. Department of the Interior, National Biological Service, Washington, D.C. Pp. 259-268.