Applying Life History Traits of the Florida Manatee, Trichechus Manatus latirostris, to Future Conservation Strategies
M.S. Marine Biology
This is a literature review of the biology of the Florida manatee, Trichechus manatus latirostris. Manatee populations can be surveyed in several ways, including aerial surveys (which is the primary means of obtaining data), photo-identification (formed the Manatee Individual Identification database, MIPS), and radio-tagging (determines migration routes and movement patterns to identify high-use areas). By gathering life history parameters, such as the average lifespan, the age of sexual maturity, fecundity (number of births), interval between births, and calf dependency, we can apply population modeling to determine survival rates.
A comparison of life history parameters was made between three areas: Southeast Florida, Big Bend Coast-NW Florida, and Sarasota Bay. Manatees found in Southeast Florida had a reported gestation period of 12-14 months, where pregnancy was highest in the spring/winter, calf dependency was 1-2 years, interval between births was an average of 3 years, sexual maturity was reached as early as 3 years old, but on average 3-4years of age. Results in the Big Bend Coast yielded a gestation period of 12 months with pregnancy highest in the spring, 77% of calves were dependent for at least one winter season, but less than 2 seasons; whereas 23% were dependent for at least 2, but less than 3 winter seasons. Intervals between births was reported as 2.48 winter seasons, sexual maturity was reached between the ages of 2.5-4.0 years of age (average is 3.2 years of age). In Sarasota Bay, the gestation period was 12-13 months long, where most births occurred during the non-winter season, calf dependency lasted for a mean of 1.8 seasons, with the interval between births at 2.2 seasons and reaching sexual maturity at 3-4 years of age.
Aerial surveys were conducted at power plants to obtain an estimate of the numbers of manatees in those areas. The first study, from 1984-1985 revealed a total of 3,804 manatees found throughout the five FPL power plants (Cape Canaveral Plant-PCC, Riviera Plant-PRV, Port Everglades-PPE, Lauderdale Plant-PFL, and Fort Myers Plant-PFM) with a total of 394 calves observed. The highest counts occurred at PRV, PPE, and PFM in winter. The second study occurred from 1982-1992. Manatees were sighted on every survey at Cape Canaveral Plant, Riviera Plant, and Port Everglades Plant, which typically had over 200 manatees. The Lauderdale Plant attracted few manatees toward the latter end of the study because the plant was not a stable source of warm-water. The Fort Myers Plant provided shelter for up to 338 manatees and had the highest percentage of calves every winter.
Abundance and distribution data was applied to population modeling for probabilities of survival. The first model was based upon the classic approach of Lotka done by Eberhardt and O’Shea (1995) who applied information from the Crystal River, Blue Spring, and Atlantic Coast regions. Through this model they supported efforts to reduce mortality as the most effective means to promote the increased growth for the eventual recovery of the Florida manatee. Parameters used for population modeling included 1) the maximum age used was 50 years, 2) age at first parturition was 4 years, 3) rates of reproduction calculated using data from Blue Springs (43 calves in 144 manatee-years) and Crystal River (147 calves born in 389 manatee-years), and 4) female adult survival-37 of 45 calves to the next winter calculated to be 0.82. Photo-identification was incorporated into models developed by federal and state management agencies to assess the status and recovery of the Florida manatee in four regions: the Northwest region, Upper St. Johns River, the Southwest region and the Atlantic Coast. In order to determine adult survival rates in these four regions, Program MARK was applied. In the Northwest, the highest ranked model showed that survival was constant over the 20-year study (exception of three years with intense hurricanes over Category 3), with survival between sexes at 0.01, male survival was slightly lower than females. The Upper St. Johns River was the only area with known ages of individuals to estimate calf and subadult survival. The model found a considerable difference in survival between 1st (0.810) and 2nd (0.915) year calves and 0.969 for 3 years and older. A minute difference was found between subadult survival (0.96) and adult survival (0.97). Modeling for the Atlantic Coast showed that the highest ranked model was biased due to data collection, therefore data from the second best model provided an adult survival rate of 0.940. The two best models for the Southwest region included survival probabilities that were constant among years and equal for the groups, with adult estimated survival probabilities of 0.903 and 0.902. The lowest survival of manatee calves was estimated for the Atlantic Coast and Southwest region where there is greatest human development, the largest proportion of watercraft related deaths, and areas used for warm water sites. Another model discussed was the capture-recapture analyses, which found that the breeding probability was similar between the Northwest, (0.41) and Atlantic Coast (0.34). A stage based model, which is a population model to assess whether manatees have met the demographic criteria for recovery (inspired by suggestion of downlisting manatees), found that the Upper St. Johns River region and the Northwest region had the highest adult survival rates, whereas the Southwest region had the lowest adult survival rates, and the Atlantic region had controversial growth rates, where the 10 year study showed a gradual increase of individuals, but the 5 year study (most recent years) showed that survival rates have been declining. The Bayesian approach focused on the Atlantic coast manatee population (since not much information has been gathered here) and found that populations increased at a rate of 3%-6% annually since 1994.
Mortality factors were divided into 6 categories from 1974-1992: watercraft (21% in 1976-80 to 28% in 1986-1992), gate or lock (9% in 1976-80 to 3% in 1986-92), other human related (4.9% per year until 1986-92 where it was 3% of all deaths), perinatal (including neonates and stillbirths) (14% in 1976-80 to 25% in 1986-92, other natural (3% in 1976-80 to 17% to 1986-92), and undetermined (49% in 1976-80 to 24% in 1986-92), which were then correlated with regions and seasons. Watercraft mortality is the number one cause of death by human-related activities. The annual number of deaths have risen each year from 1974-1991, with the highest recorded death total occurring in 1991 of 53 and then a spike of watercraft related mortalities in the year 1999 with 82 deaths, in the year 2000 with 78 deaths, in the year 2001 with 81 deaths, and in the year 2002 with 95 deaths.
In 2001, the Florida manatee was assessed to determine if it still qualified as endangered in the State of Florida. Criterions A-E were discussed and it was concluded that the Florida manatee did not meet any of the criterion for the classification as endangered. However, it does meet Criterion A as a threatened species based on a projected population decline of at least 50% over the next 45 years and also met the requirements under Criterion C for the threatened status based on population size (< 2500 mature individuals) and a projected decline of at least 20% over the next 30 years. A new method was used to assess the manatee status in 2005. Criterions A-E were discussed. This re-evaluation also determined that the Florida manatee should be downlisted to threatened status, plus manatee subpopulations were evaluated in the four regions in Florida to determine necessary management and conservation plans for the future. These assessments demonstrated that both the Southwest and Upper St. Johns subpopulations would qualify as endangered, while the Atlantic and Northwest subpopulations would qualify as threatened.
Current management of the Florida manatee includes manatee protection plans (MPPs) which are enforced in 13 key counties, manatee boating speed zones, boat facility permits, education for the public, licensing of boaters including boating classes at nearby colleges, strict law enforcement, boat design (such as propeller guards), and most importantly, the continuance of warm water sites during the winter. Gerstein (2002) researched the hearing abilities of the Florida manatee and concluded that manatees have a functional hearing range of 400-46,000 hertz with peak sensitivity at 16,000-18,000 hertz (not 1,000-5,000 hertz as previously thought). Using this information, Gerstein and colleagues devised an environmentally friendly acoustic alerting device that projects very low intensity and highly directional acoustic signals that manatees could quickly learn to associate with boats, ships, and barges. Since the device is considered highly directional, manatees would only hear the signals when they are in the direct line of an approaching boat and are in danger of a collision. Unfortunately this device has not been allowed to be tested in the wild since manatees may need to be injured before associating the signals with the device. Another viable option in the protection of manatees from boats is infrared technology. Keith (2002) found that it is possible to passively detect manatees through the use of thermal imaging technology and other infrared imaging systems, but manatee exhalations were not visible, although the nostrils themselves emitted a heat signature that was detected. The thermal infrared imaging camera yielded the best images of manatees at the surface, but the major drawback is its cost ($15-30,000). The night vision and night-shot video systems were able to detect manatees beneath the surface and have a lower cost ($300-500 each), but the major disadvantage is poorer image quality. Keith (2005) evaluated the ability of new underwater infrared cameras: the Sea View BW Cam 150 and the Atlantis AUW-525C. The SeaView camera performed more poorly in all tests; at close range it allowed for visual identification of the targets, but the targets were unfocused and had a more limited range. The SeaView camera was more likely to lose resolution as depth increased. The Atlantis camera was much better under poor lighting conditions and bright light conditions with a significantly better range. This camera achieved higher resolution at greater distances, and maintained high resolution as depth increased. Further research is needed to devise a system either using the acoustical device or infrared technology to reduce the number of watercraft mortalities each year and to maintain the current population of the Florida manatee.
Corinne Annunziata. 2006. Applying Life History Traits of the Florida Manatee, Trichechus Manatus latirostris, to Future Conservation Strategies. Capstone. Nova Southeastern University. Retrieved from NSUWorks, . (54)
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