Virtual library - PhD theses
- Doctoral thesis title: Age determination and population biology of the Florida manatee, Trichechus manatus latirostris
- Institution: University of Florida
- Supervisor: Drs. Stephen R. Humphrey e Thomas J. O’Shea
- Year of defense: May 1993
Florida manatees (Trichechus manatus latirostris) are at risk due to modern stresses on the population, primarily from boat traffic and habitat alteration. Life history and population studies to understand the consequences of such stresses have been hampered by the lack of a method of age estimation. Skeletal material of 16 known-age, minimum known-age, or tetracycline-marked Florida manatees was prepared using modifications of histological techniques employed in age-determination studies of dolphins. Results consistent with known age, minimum known age or tetracycline-labeling were obtained only from the dome region of the periotic bone. Age-specific aspects of mortality and reproduction were evaluated for 1,212 manatee Specimens collected between 1976 and 1991. Approximately 59 layers were found in the periotic dome of one manatee of unknown age, and several had growth-layer-group counts ranging from 21-39. Sexual maturation can occur between 3 and 4 years of age. At any one time 33% of the mature females were pregnant, indicating an average calving period of 3.0 years. Fecundity remained relatively unchanged (0.24 female offspring/adult female/year) from age of first parturition throughout life. Half the carcasses belonged to age classes 0, 1 and 2, and average age was low (5.7 years). Survival rate was low among the very young, increased up to age class 4, and remained constant (89.6%) from age class 4 to at least 39. The manatee survivorship curve is consistent with the type expected in a long-lived mammal, but lacks a plateau through middle age. The steeper gradient is attributed to exacerbated levels of adult mortality represented by the constant threat of collision with watercraft. Death from intense cold affected mostly juveniles, but the effect of boat strikes was constant across age classes. Computer simulation (vortex51) projected a 44% chance of persistence over 1,000 years for a stationary (rate of increase = 0) manatee population. The current situation allows no margin of error. If increasing numbers of boats result in more deaths, the manatee population will tend toward extinction. Only cultural change can prevent this outcome.