|Scientific Name:||Trichechus manatus|
|Species Authority:||Linnaeus, 1758|
|Infra-specific Taxa Assessed:|
|Taxonomic Notes:||The West Indian Manatee is currently divided into the Florida (T. m. latirostris) and Antillean (T. m. manatus) subspecies (Hatt 1934, Domning and Hayek 1986). Recent mtDNA data (García-Rodríguez et al. 1998, Vianna et al. 2006) indicate three distinctive lineages corresponding geographically with: (1) Florida and the Greater Antilles; (2) Western and Southern Gulf of Mexico, Central America, and NW South America west of the Lesser Antilles; and (3) NE South America east of the Lesser Antilles. Evidence exists for viable hybridization with T. inunguis near the mouth of the Amazon, in Guyana, French Guiana, and possibly Suriname.|
|Red List Category & Criteria:||Vulnerable C1 ver 3.1|
|Assessor(s):||Deutsch, C.J., Self-Sullivan, C. & Mignucci-Giannoni, A.|
|Reviewer(s):||Reynolds III, J.E. & Powell, J.A. (Sirenia Red List Authority)|
Listed as Vulnerable because the number of mature individuals is currently estimated to number less than 10,000 (based on combined population estimates for the Florida and Antillean subspecies) and is expected to decline at a rate of at least 10% over the course of three generations (given a generation time of ~20 years) as a result of both habitat loss and anthropogenic factors.
Trichechus manatus latirostris: EN C1:
The Florida manatee subspecies is listed as Endangered on the basis of a population size of less than 2,500 mature individuals and the population is estimated to decline by at least 20% over the next two generations (estimated at ~40 years) due to anticipated future changes in warm-water habitat and threats from increasing watercraft traffic over the next several decades.
Trichechus manatus manatus: EN C1:
The Antillean manatee subspecies is listed as Endangered because the current population is estimated at less than 2,500 mature individuals and is predicted to undergo a decline of more than 20% over the next two generations (estimated at ~40 years for an unexploited population, based on T. m. latirostris data) without effective conservation actions, due to current and projected future anthropogenic threats (habitat degradation and loss, hunting, accidental fishing-related mortality, pollution, and human disturbance).
|Range Description:||Florida manatees (T. m. latirostris) are found only in the United States, although a few vagrants have been known to reach the Bahamas. Their year-round distribution is restricted to peninsular Florida because they need warm water to survive the winter. During the non-winter months (March to November), some manatees disperse to adjoining states. Along the Atlantic coast these states include Georgia (highest manatee use outside of Florida), South Carolina, North Carolina, and Virginia; one satellite-tagged manatee traveled as far north as Rhode Island (Deutsch et al. 2003), and another manatee was observed in New York (Long Island). Along the Gulf coast west of Florida, manatees are occasionally sighted in Alabama, Mississippi, Louisiana, and Texas. The source (Florida or Mexico) of the Texas manatees is not always clear, but the weight of recent genetic and other evidence suggests most are from the Florida subspecies. Major freshwater bodies utilized by manatees in Florida include Lake Okeechobee, St. Johns River, Suwannee River, Caloosahatchee River, among others.
During the warm season (March or April through October or November, depending on latitude and year), manatees disperse throughout the coastal waters, estuaries, and major rivers of Florida and some migrate to neighboring states, particularly south-eastern Georgia. Their range constricts dramatically in the winter season (December to February) when manatees seek shelter from the cold at a limited number of warm-water sites or areas in the southern two-thirds of Florida. These sites include 10 principal power plant thermal outfalls (seven on the Atlantic coast, three on the Gulf coast) and four major artesian springs (Blue Spring, springs at the head of Crystal River, Homosassa Spring, and Warm Mineral Spring) that are frequented by a large proportion of the manatee population during winter.
The Antillean Manatee (T. m. manatus) inhabits riverine and coastal systems in the tropical and subtropical Western Atlantic Coastal Zone from the Bahamas to Brazil, including the Caribbean Sea and Gulf of Mexico. Although at least one individual in the Bahamas is a known migrant from Florida (Reid 2000, 2001), the Bahamas is detailed in this T. m. manatus assessment rather than the T. m. latirostris assessment. During the past decade, populations have been confirmed in the coastal waters and/or rivers of at least 19 of the 37 countries with historical records (Table 1); a population may be extant in Haiti (Ottenwalder 1995), although in very reduced numbers if at all. Rare sightings, categorized as vagrants, have been documented in five additional countries (Debrot Gore pers. comm).
Follow the link below for Table 1: Summary of reported data by country for extant manatee populations.
Native:Bahamas; Belize; Bonaire, Sint Eustatius and Saba (Saba - Regionally Extinct, Sint Eustatius - Regionally Extinct); Brazil; Cayman Islands; Colombia; Costa Rica; Cuba; Curaçao; Dominican Republic; French Guiana; Guatemala; Guyana; Honduras; Jamaica; Mexico; Nicaragua; Panama; Puerto Rico; Suriname; Trinidad and Tobago; United States; Venezuela, Bolivarian Republic of; Virgin Islands, British; Virgin Islands, U.S.
Possibly extinct:Turks and Caicos Islands
Regionally extinct:Anguilla; Antigua and Barbuda; Aruba; Barbados; Dominica; Grenada; Guadeloupe; Martinique; Montserrat; Saint Barthélemy; Saint Kitts and Nevis; Saint Lucia; Saint Martin (French part); Saint Vincent and the Grenadines; Sint Maarten (Dutch part)
|FAO Marine Fishing Areas:||
Atlantic – northwest; Atlantic – southwest; Atlantic – western central
|Range Map:||Click here to open the map viewer and explore range.|
Trichechus manatus latirostris
The Florida Fish and Wildlife Conservation Commission (FWC) recently conducted two Biological Status Reviews of the Florida manatee that involved comprehensive compilation and synthesis of population and range data, along with extensive modeling of population projections (FWC 2002, 2003; Haubold et al. 2006). In addition, the Manatee Population Status Working Group of the Florida Manatee Recovery and Implementation Team has produced a comprehensive Biological Population Assessment (MPSWG 2005).
There are no statistically based estimates (with variance) of abundance for the entire Florida manatee population. The minimum documented abundance is determined from range-wide synoptic surveys which have been conducted nearly every winter since 1991. The surveys are timed to coincide with periods of extreme cold weather when manatees aggregate at a limited number of warm-water sites. The highest count obtained during these surveys was 3,300 manatees in January 2001 (Haubold et al. 2006); this is presumed to be a minimum count, but the fraction detected is unknown. Based on the assumption of a stable age distribution, and estimating proportion mature to be 0.70 from the core biological population model, the estimated number of mature individuals in the population is 2,310. Because detection probability varies greatly with weather conditions and across sites, population experts have consistently cautioned against using these data for trend analyses.
Long-term studies of the Florida manatee indicate that there are four relatively distinct regional subpopulations, as recognized in the Florida Manatee Recovery Plan (USFWS 2001). Each subpopulation comprises individuals that tend to return to the same warm-water refuges each winter and have similar non-winter distribution patterns. Exchange of individuals among subpopulations is considered to be relatively limited, based on data from telemetry (Rathbun et al. 1990, Weigle et al. 2001, Deutsch et al. 2003) and photo-identification (Rathbun et al. 1990, Reid et al. 1991, FWC and USGS, unpubl.). The four subpopulations differ in abundance, demographic rates, habitats, and major threats. What follows is a summary of our understanding of their rates of population growth over the past decade and very brief statements on the status of each subpopulation. Minimum population sizes are provided by the high synoptic survey count of 5 to 6 January 2001 (FWC unpubl.). Population growth rates cited below were estimated by Runge and colleagues (2004) using a stage-based model that integrated mark-recapture estimates of survival and reproduction (Kendall et al. 2004, Langtimm et al. 2004). The estimates of vital rates were made possible through a long-term, multi-agency effort to photographically identify individual manatees based on their distinct patterns of scars and mutilations (Beck and Reid 1995).
The Atlantic Coast subpopulation extends along the entire east coast of Florida (including the Florida Keys and Florida Bay), coastal states northward along the Atlantic seaboard, and the lower St. Johns River north of Palatka. A total of 1,444 manatees were counted in the Atlantic region during the 2001 synoptic survey. Craig and Reynolds (2004) estimated that the population size of manatees using power plants on the Atlantic Coast during winter 2001 was 1,607 (95% Bayesian credible interval: 1,353 to 1,972). Over the most recent 10-year period, Runge et al. (2004) estimated that the Atlantic subpopulation had grown at an annual rate of 1.0% per year (95% CI: -1.2 to 2.9%), but this was not statistically different from zero. The subpopulation may have increased slowly or it may have declined slightly over this time period. Craig and Reynolds (2004) used a Bayesian approach to model growth in aerial counts of manatees at several major aggregation sites along the Atlantic coast (primarily power plants). This model took into account manatee movement between surveys and variation in detection rates with ambient temperature. The trends in counts suggest the population of animals using Atlantic power plants increased at a rate of 5 to 7% per year from 1982 to 1989, leveled off (growth rate 0 to 4%yr-1) between 1990 and 1993, and has been increasing at about 4 to 6% per year since 1994. The discrepancy in growth rates of these two modeling approaches needs further investigation to provide a better understanding of population trends along the Atlantic coast.
Upper St. Johns River Subpopulation:
The Upper St. Johns River subpopulation occurs in a much smaller area in the river south of Palatka. A total of 112 manatees were counted during the 2001 synoptic survey, but intensive observations that facilitate identification of nearly every individual using Blue Spring, the main overwintering site in the region, indicate that at least 141 different manatees visited the spring during that winter. A total of 190 manatees were counted at Blue Spring during a synoptic survey in 2006. The subpopulation occupying the Upper St. Johns River has shown strong growth over the past decade, increasing at an annual rate of 6.2% (95% CI: 3.7 to 8.1%) based on the stage-based model. This growth rate is supported by high survival and reproductive rates. This is the smallest of the four subpopulations, contributing less than 5% to the maximum synoptic count, but it is growing the fastest.
The Northwest subpopulation extends from the Pasco-Hernando County line along the central Gulf coast northward through the Florida Panhandle and including the coastal areas of adjoining states at least as far as Louisiana. A total of 377 manatees were counted in the Northwest region during the 2001 synoptic survey. This subpopulation has grown at an annual rate of 3.7% (95% CI: 1.6 to 5.6%) over the past 10 years. This is the second smallest subpopulation, accounting for about 11% of the highest synoptic count. Its dynamics are similar to those of the USJ, with a high adult survival rate, except reproduction seems to be lower.
The Southwest subpopulation extends from the Pasco-Hernando County line (north of Tampa) southward to Whitewater Bay (part of Everglades National Park) in Monroe County. Minimum subpopulation size from the high synoptic survey count in 2001 was 1,367 individuals for the Southwest region. This subpopulation has declined at an estimated rate of -1.1% per year (95% CI: -5.4 to +2.4%) over the most recent eight-year period. The relatively wide confidence interval reflects greater uncertainty about survival and reproductive rates in this region, in part due to a shorter time series of sight-resight data. Estimates of adult survival are lower than those of all other subpopulations, probably due to the combined effects of chronic human-related (watercraft) mortality and episodic mortality events caused by red tide. It should also be noted that manatees in the Southwest subpopulation are found in a broad diversity of habitats from the more developed Tampa Bay to the more pristine reaches of Everglades National Park; demographic data are lacking for individuals in the southernmost parts of the region. Temporary emigration outside of the study areas may result in a downward bias in survival rates.
FWC has a wide-reaching manatee carcass salvage and necropsy programme that recovers nearly every dead manatee reported in the state of Florida. Consequently, we have a long time series of excellent data on causes of death. In all of the four subpopulations, adult mortality is mostly attributable to human-related causes, primarily watercraft collisions. However, because of the lower survival rates in the Atlantic and Southwest subpopulations, the impact of this anthropogenic mortality on population growth is much greater than in the other two regions. For the immature age class, perinatal mortality is the most common “cause” and watercraft collisions is the next highest known cause of death for all subpopulations (see Major Threats).
It is clear that the two smallest subpopulations have been growing over the past decade, while the two largest subpopulations have grown slightly, not at all, or may be declining. Available data suggest that the Southwest subpopulation is in decline. One way to estimate the overall growth rate for the Florida manatee is to take an average of the four subpopulation's growth rates, weighted by relative size. This yields a point estimate of 0.6% per year. Confidence intervals do not incorporate our uncertainty in the relative sizes of the subpopulations. That is, this point estimate assumes that the sighting detection rate in the synoptic survey was the same in each region in 2001, which is very unlikely, and that weights have remained constant over time. So, while the available data suggest that the point estimate of population growth for this subspecies may be close to zero or slightly positive, the associated uncertainty means that the population could be declining or increasing slightly.
Population Projections: FWC used population viability analysis models to address the three criteria that involved the probability of future population decline or extinction (FWC 2002, 2003; Haubold et al. 2006). The models were based on vital rate estimates for each of four subpopulations and they incorporated demographic and environmental stochasticity. The models simulated scenarios based on plausible future threats to manatees and their habitat, including expected declines in carrying capacity through loss of warm-water refugia (loss of power plant discharges, declines in spring flow), potential increases in mortality (mostly due to watercraft collisions) associated with projected human population growth, and natural catastrophic mortality events (red tide, cold, disease). The outcome of the most recent PVA model was that there is a 12.1% chance of a 50% decline in the next three generations (generation time estimated between 16.8 and 22.6 years), and a 55.5% chance of a 20% reduction in the next two generations (Criterion C1) (Haubold et al. 2006). For details, peer reviews, and a fuller discussion of these results, assumptions, limitations, and conclusions, please see the FWC’s Biological Status Reviews of the Florida manatee.
Trichechus manatus manatus
There are no statistically derived population estimates for T. m. manatus within its range, which historically covers 41 countries in the Wider Caribbean Region. Peer-reviewed publications are sparse, but there has been a significant increase in research on Antillean manatees over the past 10 years, resulting in many theses, presentations at regional meetings, local reports, and adoption of management plans.
We compiled data gathered through an extensive literature review and personal communication with 45 scientists representing expertise in 29 countries, but exclusive of the Florida manatee population (which was assessed independently). Antillean manatee populations occur in 20 of the 37 countries assessed, with sightings of vagrants in additional countries. However, distribution is not continuous and populations are patchy and fragmented. Using these documents, supplemented by the questionnaires completed by local experts, we derived a very rough minimum population estimate for the subspecies. Our confidence in population estimates varies dramatically from country to country, depending on the nature and extent of recent research efforts. For that reason, we used orders of magnitude in an effort to establish some baseline population estimates for each of the 20 countries where populations are most probably extant and viable. Although the quality and quantity of data vary from country to country, there is consistency regarding a continuing decline in both manatees and the quality and quantity of available habitat.
Estimates by country, based on the best available data, range from less than 10 up to ~1,000 total animals, with the largest populations reported from Mexico and Belize (Table 1). No country reports an increasing country-wide population (except Bahamas with less than 10 animals). All publications and personal communications indicate country-wide populations are declining, stable, or unknown. In a few countries, localized populations may be increasing; in most countries, reports indicate a significant decline over the past 30-50 years, but this is based solely on anecdotal evidence and/or interviews with local people.
While the data in Table 1 suggest that approximately 2,600 individuals exist, scattered widely through the Caribbean region, optimistic "estimates" of the size of the manatee population (also based on interviews with experts in different countries, and not necessarily on empirical data) suggest that it may actually be in the range of 5,600 individuals. The age structure of the various manatee subpopulations of T. m. manatus is unknown, but the percentage of mature animals in Florida was estimated to be 70% through population modeling (Haubold et al. 2006) and 46% through carcass recovery (Hernandez et al. 1995, Marmontel 1995, Marmontel et al. 1997). The figure based on carcasses is biased low due to the presence of a large proportion of calves in the sample. We note that hunting and other threats in the Caribbean may lead to a very different age structure for T. m. manatus than for T. m. latirostris; in fact, if hunters target large (i.e., mature) manatees, the percentage of mature individuals for T. m. manatus could be substantially lower than is the case for T. m. latirostris. Therefore, we consider that the percentage of mature individuals for the Antillean subspecies is likely to lie somewhere between 46% and 70%.
Based on the above information, we feel it is likely that the actual population size is intermediate between the number counted (2,600; minimum population size) and the more optimistic suggestions (5,600); an average between those two numbers would place the actual population size at approximately 4,100 manatees. Using this average value, and the two percentages given above for mature animals, the number of mature individuals would therefore lie between 1,886 and 2,870. Selecting the average value between these numbers produces an estimate of 2,378 mature individuals. While we do not place much significance in these average values, it is reasonable to conclude - based on the available evidence and taking the precautionary approach - that the overall number of mature individuals in this subspecies is likely to be less the 2,500.
|Habitat and Ecology:||
Trichechus manatus latirostris
The following text was excerpted directly from the Florida Manatee Recovery Plan (USFWS 2001), with some minor revisions, condensation, and updates from the assessors.
Manatees undertake extensive seasonal migrations with seasonal distribution determined by water temperature. When ambient water temperatures drop below 20°C (68°F) in autumn and winter, manatees aggregate at natural and artificial warm-water refuges or move to southern Florida (Lefebvre et al. 2001, Weigle et al. 2001, Deutsch et al. 2003). Most artificial refuges are created by warm-water outfalls from power plants. The largest winter aggregations (maximum count of 100 or more animals) are at refuges in central and southern Florida. The northernmost natural warm-water refuges used regularly by manatees are at Crystal River on the west coast and at Blue Spring in the St. Johns River on the east coast. Most manatees return to the same warm-water refuges each year; however, some use different refuges in different years and others use two or more refuges in the same winter (Rathbun et al. 1990, Reid et al. 1991, Deutsch 2000, Deutsch et al. 2003). Many minor aggregation sites are used as temporary thermal refuges. Most of these refuges are deeper canals or boat basins where warmer water temperatures persist as temperatures in adjacent bays and rivers decline. Manatees using thermal refuges move to nearby grassbeds to feed (generally within 30 km), and may even return to a more distant warm season range during mild periods in mid-winter (Deutsch et al. 2003).
As water temperatures rise manatees disperse from winter aggregation areas. While some remain near their winter refuges, most undertake extensive travels along the coast and some move far up rivers and canals. On the east coast, summer sightings drop off rapidly north of Georgia (Lefebvre et al. 2001) and are rare north of Cape Hatteras (Rathbun et al. 1982, Schwartz 1995); the northernmost published sighting is from Rhode Island (Reid 1996, Deutsch et al. 2003). On the west coast, sightings drop off sharply west of the Suwannee River in Florida (Marine Mammal Commission 1986), although a small number of manatees are seen each summer in the Wakulla River at the base of the Florida Panhandle. Rare sightings also have been made in the Dry Tortugas (Reynolds and Ferguson 1984) and the Bahamas (Lefebvre et al. 2001; Odell et al. 1978; Reid 2000, 2001). As in winter, manatees show strong site fidelity to the same summer habitats year after year (Reid et al. 1991, Koelsch 1997, Deutsch et al. 2003).
In recent years, the most important spring habitat along the east coast of Florida has been the northern Banana River and Indian River Lagoon and their associated waters in Brevard County; more than 300 to 500 manatees have been counted in this area shortly before dispersing in late spring (Provancha and Provancha 1988, FWC unpubl.). A comparable spring aggregation area does not appear to exist on the west coast, although Charlotte Harbor was visited in the spring by almost half of the 35 manatees radio-tagged at the Fort Myers power plant in Lee County (Lefebvre and Frohlich 1986). During summer, manatees may be commonly found almost anywhere in Florida where water depths and access channels are greater than 1 m (O’Shea 1988). Manatees can be found in very shallow water. Hartman (1979) observed manatees utilizing waters as shallow as 0.4 m with their backs out of the water. In warm seasons they usually occur alone or in pairs, although interacting groups of five to ten animals are not unusual.
Migration corridors and responses by individual animals have been elaborated by long-term telemetry studies (Weigle et al. 2001, Deutsch et al. 2003). Scientists have demonstrated site-fidelity in manatees, but have also noted that individual animals adjust their behaviours to take advantage of protected areas or changes in availability of resources. For example, Buckingham et al. (1999) confirmed increased manatee use of selected sanctuary areas during times when surrounding disturbance by boats was high.
Shallow grass beds with ready access to deep channels are preferred feeding areas in coastal and riverine habitats. Manatees often use secluded canals, creeks, embayments, and lagoons, particularly near the mouths of coastal rivers and sloughs, for feeding, resting, cavorting, mating, and calving. In estuarine and brackish areas, natural and artificial fresh water sources are sought by manatees. Although manatees can tolerate a wide range of salinities (Ortiz et al. 1998), they prefer habitats where osmotic stress is minimal or where fresh water is periodically available (O’Shea and Kochman 1990). Ortiz et al. (1998) report that “manatees may be susceptible to dehydration after an extended period if freshwater is not available”.
Manatees are herbivores that feed opportunistically on a wide variety of submerged, floating, and emergent vegetation. Because of their broad distribution and migratory patterns, Florida manatees utilize a wider diversity of food items and are possibly less specialized in their feeding strategies than manatees in tropical regions (Lefebvre et al. 2000). Seagrasses appear to be a staple of the manatee diet in coastal areas (Smith 1990, Provancha and Hall 1991, Lefebvre et al. 2000). Packard (1984) noted two feeding methods in coastal seagrass beds: (1) rooting, where virtually the entire plant is consumed; and (2) grazing, where exposed grass blades are eaten without disturbing the roots or sediment. Manatees may return to specific seagrass beds to graze on new growth (Koelsch 1997, Lefebvre et al. 2000). In the upper Banana River, Provancha and Hall (1991) found spring concentrations of manatees grazing in beds dominated by manatee grass (Syringodium filiforme). They also reported an apparent preference for manatee grass and shoalgrass (Halodule wrightii) over the macroalga Caulerpa spp. Along the Florida-Georgia border, manatees feed in salt marshes on smooth cordgrass (Spartina alterniflora) by timing feeding periods with high tide (Baugh et al. 1989, Zoodsma 1991).
Breeding takes place when one or more males (ranging from 5 to 22) are attracted to an oestrous female to form an ephemeral mating herd (Rathbun et al. 1995). Mating herds can last up to four weeks, with different males joining and leaving the herd daily (Hartman 1979, Bengtson 1981, Rathbun et al. 1995). Permanent bonds between males and females do not form. During peak activity, the males in mating herds compete intensely for access to the female (Hartman 1979). Successive copulations involving different males have been reported. Some observations suggest that larger, presumably older, males dominate access to females early in the formation of mating herds and are responsible for most pregnancies (Rathbun et al. 1995), but males as young as three years old are spermatogenic (Hernandez et al. 1995). Although breeding has been reported in all seasons, Hernandez et al. (1995) reported that histological studies of reproductive organs from carcasses of males found evidence of sperm production in 94% of adult males recovered from March through November. Only 20% of adult males recovered from December through February showed similar production.
Females appear to reach sexual maturity by about age five but have given birth as early as four (Marmontel 1995, Odell et al. 1995, O’Shea and Hartley 1995, Rathbun et al. 1995), and males may reach sexual maturity at 3 to 4 years of age (Hernandez et al. 1995). Manatees may live in excess of 50 years (Marmontel 1995), and evidence for reproductive senescence is unclear (Marmontel 1995, Rathbun et al. 1995). Wild females have been documented continuing to calve into their late 30s (USGS-Sirenia, unpubl.), and a captive animal gave birth in her mid-40s (FWS, unpubl.). The length of the gestation period is uncertain but is thought to be between 11 and 14 months (Odell et al. 1995, Rathbun et al. 1995, Reid et al. 1995). The normal litter size is one, with twins reported rarely (Marmontel 1995, Odell et al. 1995, O’Shea and Hartley 1995, Rathbun et al. 1995).
Calf dependency usually lasts one to two years after birth (Hartman 1979, O’Shea and Hartley 1995, Rathbun et al. 1995, Reid et al. 1995). Calving intervals average about 2.5 years (when the calf survives to weaning), but they vary greatly among individuals and may be considerably longer depending on age and perhaps other factors (Marmontel 1995, Odell et al. 1995, Rathbun et al. 1995, Reid et al. 1995). Females that abort or lose a calf due to perinatal death may become pregnant again within a few months (Odell et al. 1995), or even weeks (Hartman 1979).
Trichechus manatus manatus
The habitat and ecology of the Antillean manatee is thought to be similar to its sister subspecies. However, given that the Florida manatee inhabits the most northern extent of the species range, behaviour and life history characteristics may be dissimilar in this tropical subspecies. Antillean manatees use rivers, lakes, coastal and inland lagoons, and coastal marine environments, including seagrass, mangrove, and coral reef ecosystems. Although they are able to survive in fresh, estuarine, or marine environments for extended periods of time, several lines of evidence indicate a dependence on periodic access to fresh water for osmoregulation (Ortiz et al. 1998, 1999; Lefebvre et al. 2001; Reid et al. 2003). As herbivores, access to aquatic vegetation is necessary for survival; seagrasses (Thalassia, Halodule, Halophila and Syringodium) appear to be favored in estuarine and marine areas. Osmoregulation and thermoregulation are behaviorally controlled by movements between activity centers. Quiet protected areas are necessary for resting and for cows with young calves; connections (travel corridors) between feeding, drinking, nursery, mating, and resting areas are very important. A discussion of the environmental factors influencing seasonal movements and migratory behavior in both subspecies is provided in Deutsch et al. (2003).
Trichechus manatus latirostris
Threats to the Florida manatee encompass catastrophic natural events and anthropogenic factors that could cause declines in reproductive and survival rates or declines in the carrying capacity of the environment. Much of the following text is modified from the Biological Status Reviews for the Florida manatee (FWC 2002, Haubold et al. 2006).
About half of adult mortality rangewide is attributable to human-related causes, primarily watercraft collisions (Ackerman et al. 1995, Deutsch et al. 2002). This is significant because the manatee population growth rate is highly sensitive to changes in adult survival rate (Eberhardt and O’Shea 1995, Marmontel et al. 1997, Runge et al. 2004). The future of the Florida manatee is also jeopardized by the predicted loss and deterioration of warm-water habitat, including retirement or deregulation of aging power plants and reduction in natural spring flows.
Watercraft collisions: Watercraft collisions account for approximately 25% of all manatee deaths and 35% of documented deaths of known cause, and are the single greatest cause of human-related mortality (Ackerman et al. 1995, Wright et al. 1995, FWC-FWRI unpubl.). In 2005, there were over one million registered vessels in Florida (FHSMV, http://casey.hsmv.state.fl.us/Intranet/dmv/TaxCollDocs/vesselstats2005.pdf), and many more out-of-state boaters visit Florida annually. The number of registered vessels in Florida has increased by an average of 2.9% per year over the past 25 years, doubling since 1980 (FWC, unpubl.). Given that about 97% of registrations are for recreational watercraft (Wright et al. 1995), it can be expected that there will be a continued increase in recreational vessels plying the waterways of Florida with a concomitant increase in the human population. In addition to the expected increase in boat numbers over the coming century, there are other factors that may act synergistically to increase the risk of collisions between manatees and watercraft. Modifications to the design of vessel hulls and engines are allowing boats to travel at higher speeds in shallower waters (Wright et al. 1995), thus threatening manatees and scarring seagrass beds. Boater compliance with existing slow speed zones is inconsistent (Gorzelany 1998, Shapiro 2001).
Sub-lethal effects on manatees of increased vessel traffic and a growing human population in the coastal zone are cause for concern. Most adult manatee carcasses bear scars from previous boat strikes and the healed, skeletal fractures of some indicate that they had survived previous traumatic impacts (Wright et al. 1995, Lightsey et al. 2006). Of over 1,000 living individuals in the manatee photo-identification database (Beck and Reid 1995), 97% had scar patterns from multiple boat strikes (O’Shea et al. 2001). Approximately one-third of these individuals were severely mutilated, especially on the tail and the dorsum. It should be noted that the photo-identification database contains only animals with scars or other identifiable features. Non-lethal injuries may reduce the breeding success of wounded females and may permanently remove some animals from the breeding population (O’Shea 1995, Reynolds 1999). Vessel traffic and recreational activities that disturb manatees may cause them to leave preferred habitats and may alter biologically important behaviours such as feeding, suckling, or resting (O’Shea 1995, Wright et al. 1995).
Loss of Warm-water Habitat: Expected changes in the network of warm-water refuges over the next several decades present the most serious long-term threat to manatees in Florida, as noted in the federal Recovery Plan: “one of the greatest threats to the continued existence of the Florida manatee is the stability and longevity of warm-water refuges” (USFWS 2001, p. 28). Ultimately, the discharges from power plants provide unreliable warm-water habitat when viewed over the long term (i.e., next 20 to 100 years) because the once-through cooling technology that creates the large thermal plumes is being replaced by more efficient and alternative cooling technologies (Laist and Reynolds 2005a). Short-term threats to the network of warm-water sites also loom on the immediate horizon. Some aging power plants may be shut down and potential deregulation of the electric utility industry may eliminate or reduce the reliability of warm-water effluents that large numbers of manatees depend on to survive winter cold periods (Rose 1997, U.S. Fish and Wildlife Service 2000). Temporary disruptions in heated effluents during winter have caused changes in local manatee distribution (Packard et al. 1989) and have been implicated in elevated numbers of deaths from cold stress (Campbell and Irvine 1981, Ackerman et al. 1995). The complete elimination of a secondary warm-water refuge in northeastern Florida through diffusion of the heated effluent resulted in a shift in manatee distribution within the area and in substantial mortality of manatees that remained in the region (Deutsch et al. 2000, Laist and Reynolds 2005a). Loss of certain key warm-water sites could result in catastrophic mortality and would likely reduce the environmental carrying capacity for manatees in Florida.
The long-term reliability of artesian springs that provide natural warm-water refuges for manatees is also in doubt because human demand for ground water and loss of recharge areas through development will likely result in diminished spring flows (Reynolds 2000, Laist and Reynolds 2005a). According to the U.S. Census Bureau (2001), Florida’s human population increased by about 23% to 16 million between 1990 and 2000, and projections suggest that the number of people living in Florida will increase by another 10 million people by 2025. In order to meet the increased demand for water that a growth in human population will entail, it is likely that spring flows and water quality will decline, further reducing natural warm-water habitat for manatees. This natural habitat will become even more important in the future as existing industrial sites disappear.
Other Direct Threats to Manatees from Human Activities: Other threats from human activities include entanglement (in fishing gear or debris), entrapment in water-control structures and pipes, exposure to contaminants, incidental ingestion of debris, and crushing (in flood-control structures, in canal locks, or between large ships and docks) (Beck and Barros 1991, Ackerman et al. 1995). Indirect effects from increased vessel traffic include increased water turbidity from wake action and scarring of seagrass beds by propellers (Sargent et al. 1995).
Indirect Threats to Manatees from Human Activities: There is no commercial or subsistence utilization in the USA. However, manatees have become the centre of a large ecotourism industry at certain winter aggregation sites, such as Crystal River. Tens of thousands of people visit these areas to observe and swim with manatees. No-entry sanctuaries provide manatees with havens to avoid swimmers and boats at these sites. Manatees do, in fact, increase their use of these sanctuaries when more boats and swimmers are present (Buckingham et al. 1999, King and Heinen 2004). Manatees also have been found to alter their behaviour in response to the presence of human swimmers, including decreased resting and suckling and increased swimming (King and Heinen 2004).
The tremendous growth in the human population in coastal Florida over the past half century has resulted in drastic losses of coastal wetland habitats. Seagrass distribution and abundance in many estuaries have declined as the result of direct human impacts (dredging and propeller scarring) and indirect effects of development (declining water quality and nutrient loading). Within Tampa Bay, for example, an estimated 80% of the seagrass present in the early 1900s was lost by 1980 (Kurz et al. 2000). This decline in seagrass coverage is slowly being reversed through actions to reduce nitrogen loading in the regional watershed, which have improved water clarity in much of Tampa Bay (Johansson and Greening 2000, Kurz et al. 2000). Non-point-source runoff is difficult to control, however, so water clarity declines in years of above-average precipitation. Reductions in optical water clarity cause declines in the health and abundance of submerged aquatic vegetation (Stevenson et al. 1993). Indirect effects from increased vessel traffic include increased water turbidity from wake action and scarring of seagrass beds by propellers (Sargent et al. 1995). It will be particularly important to protect, restore, and maintain aquatic vegetation communities in the vicinity of warm-water aggregation sites. Without conservation measures to secure these winter habitats, manatees would have to travel greater distances, concentrate into smaller areas, and forage in sub-optimal environments.
Naturally-occurring catastrophic threats to manatees include prolonged periods of very cold temperatures, hurricanes, harmful algal blooms (i.e., “red tide”), and the potential for a disease epizootic. The threat from extended periods of cold weather relates to the availability and quality of warm-water habitat, which has already been discussed above.
Hurricanes: Hurricanes are another type of weather-related phenomenon that can potentially impact manatee populations. In the Northwest subpopulation, Langtimm and Beck (2003) found that adult survival rate was depressed in years with severe storms or hurricanes. The mechanism(s) underlying the lower survival probabilities are unknown, as there has not been a corresponding elevation in the number of reported carcasses. Such events could also result in large-scale emigration out of the affected region. In eastern Australia, for example, the simultaneous occurrence of flooding and a cyclone, combined with poor watershed management practices, resulted in the loss of 1,000 km² of seagrass beds and in the mass movement and mortality of dugongs (Dugong dugong; Preen and Marsh 1995). Given the notice from meteorologists that we have entered a new 25- to 50-year cycle of greater hurricane activity and intensity (Landsea et al. 1996), as well as possible longer-term changes associated with global climate change (McCarthy et al. 2001), storm activity may have a greater impact on manatee populations in the future.
Red Tide: Manatees on Florida’s Gulf coast are frequently exposed to brevetoxin, a potent neurotoxin produced by the dinoflagellate Karenia brevis during red tide events. In 1996, 151 manatees were confirmed or suspected to have died in southwestern Florida from brevetoxicosis (Landsberg and Steidinger 1998, Bossart et al. 1998). This epizootic was particularly detrimental to the manatee population because more adults were killed than any other age class. Other red tide mortality events in 1982, 2002, 2003, and 2005 resulted in the confirmed red tide-related deaths of 37, 33, 86, and 68 manatees, respectively, with an additional 35 carcasses suspected to have died from red tide during those years (O’Shea et al. 1991; FWC unpubl.). Recent studies have determined that brevetoxin can exist outside of the algal cells (e.g., on seagrass) for extended periods of time, thus further increasing the threat to foraging manatees (Flewelling et al. 2005). Red tide represents a major natural source of mortality for manatees in the southwest region. There is no clear evidence that these events have been increasing in frequency along Florida’s coast, but certainly the impact on the manatee population has increased over the past two decades. The role of nutrient loading to coastal systems relative to the intensity and duration of inshore red tides is an active area of research for the FWRI harmful algal blooms group.
Pathogens: Manatees could potentially be exposed to pathogens. Spread of such pathogens could be particularly rapid during winter when manatees are concentrated in warm-water refuges. Large-scale mortality events caused by disease or toxins have decimated other populations of marine mammals, including seals and dolphins, removing 50% or more of the individuals in some events (Harwood and Hall 1990). Manatees have robust immune systems that have, through the present time, provided disease resistance. Since 1997 papilloma virus has been found in captive Florida manatees and there is some evidence that it may also be present in the wild population in northwest Florida (Bossart et al. 2002a, Woodruff et al. 2005). While the consensus is that this virus probably does not present a serious threat to manatees at this time, managers are proceeding cautiously (e.g., by establishing a quarantine on exposed captives) and surveillance for papilloma lesions in wild manatees continues.
Trichechus manatus manatus
Major threats to survival of the Antillean manatee include habitat degradation and loss, hunting, incidental catch/accidental take, watercraft collisions, entanglement in fishing gear, pollution, natural disasters, and human disturbance. Although threats due to hunting are diminishing in some areas, all other threats are increasing in most areas. Illegal hunting for subsistence and profit was reported as a significant threat in Brazil, Colombia, Costa Rica, Cuba, Dominican Republic, French Guiana, Guatemala, Honduras, Mexico, Suriname, Trinidad and Tobago, and Venezuela. Pollution from agriculture and mining was consistently noted in reports from South American countries. Intrinsic factors that limit the population’s ability to withstand these anthropogenic impacts include low fecundity, slow growth, limited dispersal, and restricted range.
Of particular note were: (1) In Belize, watercraft related mortality was reported as the major threat, followed by illegal hunting and entanglement in fishing gear; and (2) In north-eastern Brazil, the stranding of live-orphaned calves was identified as the main recent threat to the species (Parente et al. 2004). Between 1981 and 2002, 74 stranded manatees were reported on the north-eastern coast of Brazil with 58% (n=43) being live dependent calves. This high percentage suggested that that the main threat may be human disturbance resulting in mother-calf separation. Disturbance in the area is due to shrimp farms, salt farms, a general increase in human activities in the coastal zone, and uncontrolled tourism. Additional major threats include hunting, entanglement in fishing nets, watercraft collisions, indiscriminate development of the coast, and degradation of aquatic environments.
Trichechus manatus latirostris
The following text was excerpted directly from the Florida Manatee Recovery Plan (USFWS 2001), with condensation and some minor revisions from the assessors. The state of Florida’s Manatee Management Plan is due to be completed by June 2007 and will provide further details of planned management and conservation actions.
Efforts to Reduce Watercraft-Related Injuries and Deaths
The largest identified cause of manatee death is collisions with watercraft. Many living manatees also bear scars or wounds from vessel strikes. Because watercraft operators cannot reliably detect and avoid hitting manatees, federal and state managers have sought to limit watercraft speed in areas where manatees are most likely to occur to afford both manatees and boaters time to avoid collisions. In 1989, the Florida Governor and Cabinet approved a series of recommendations by the former FDNR to improve protection of manatees in 13 key counties. Since then state and local governments have cooperated in the creation and implementation of county Manatee Protection Plans and county-wide manatee protection speed zone rules. Two types of manatee protection areas also have been developed by FWS: (1) manatee sanctuaries, areas in which all waterborne activities are prohibited; and (2) manatee refuges areas where certain waterborne activities are restricted or prohibited. FWS and FWC continue to evaluate needs for additional protection areas that may be necessary to achieve recovery. The goal is to consider the needs of the manatee at an ecosystem level and to establish regulations to ensure that adequate protected areas are available throughout Florida to satisfy habitat requirements of the Florida manatee population with a view toward recovery.
In recent years, both the FWS and FWC have been using targeted enforcement strategies in an attempt to increase boater compliance with speed zones and ultimately reduce manatee injuries and death. FWS’ strategy has been to allocate significant enforcement manpower to specific areas on designated weekends. These enforcement teams travel to various locations around the state, with particular emphasis given to those zones within counties where there is a history of high watercraft-caused manatee deaths. FWC has increased its emphasis on enforcement and compliance with manatee speed zones by adding new officers, conducting law enforcement task force initiatives, increasing overtime, and increasing the proportion of law enforcement time devoted to manatee conservation.
Managers, researchers, and the boating industry have investigated the use of various devices to aid in the reduction of watercraft-related manatee deaths. For example, the State of Florida funded an evaluation of propeller guards (Milligan and Tennant 1998). The state’s evaluation concluded that these devices would reduce cutting damage associated with propellers when boats were operating at low speeds. However, when boats (including boats equipped with propeller guards) operate at high speeds, guards would be of little benefit because animals would continue to be killed by blunt trauma associated with impacts from boat hulls, lower units, and other gear. The U.S. Coast Guard (USCG) identified additional concerns, stating that propeller guards on small recreational vessels “may create more problems than they solve” and does not support their use on recreational vessels at this time (Carmichael 2001). There are propeller guard applications, however, that appear to work for certain large, commercial vessels; for example, the use of guards on C-tractor tugs has eliminated this specific source of manatee mortality at the Kings Bay Naval Submarine Base in St. Marys, Georgia. To prevent injuries to manatees, propeller guards are used on some rental and sight-seeing boats at Blue Spring and Crystal River.
Researchers have also begun to investigate the manatees’ acoustic environment to better evaluate the animal’s response to vessel traffic. This line of research needs to be thoroughly assessed for its potential as another management tool to minimize collisions between manatees and boats. Results from Gerstein et al. (1999) indicate that manatees hear in the range from 500 Hz to 46 kHz and that inadequate hearing sensitivity at low frequencies may be a contributing factor to the manatees’ ability to effectively detect boat noise to avoid collisions. One technology often discussed is an acoustic deterrence device mounted on a boat. Conceptually, this technological approach may sound like an answer to the manatee/watercraft issue. A number of problems have been defined with the use of acoustic deterrents. No alarm/warning device has yet been demonstrated to adequately protect wildlife or marine mammals. Additionally, concern has also been stated regarding the increase in background noise that these deterrents would add to an already noisy marine environment. It has not been determined what negative impacts this device would have on marine life and what effects it would have on animals that use acoustic cues for a variety of purposes. For these reasons, this technology needs to be thoroughly researched and assessed and managers need to evaluate the MMPA and ESA “take” issues related to implementing such technology.
Current research into the sensory capabilities of manatees is being supported at both the state and federal levels. One study assessed the effects of boat noise in a more controlled environment, by recording the physical and acoustic reaction of a manatee to a pre-determined acoustical level. This study design will allow the development of a relationship between acoustic amplitude and behavioural responses. Another study examined acoustical propagation over various types of marine topography. In cooperation with Mote Marine Laboratory and the Woods Hole Oceanographic Institution, the FWC is also examining manatee behavioural response to watercraft using new technology, the DTAG, a digital acoustic tag which records acoustic attributes of the environment and detailed manatee movement simultaneously. A study to assess manatee behaviours in the presence of fishing gear and their response to novelty and the potential for reducing gear interactions also has an acoustic component. The FWC is also supporting the development and implementation of technological solutions for reducing the risks that watercraft pose to manatees.
Additional priority actions to better protect manatees include boater education, maintenance of signs and buoys, compliance assessment, and periodic re-evaluation of the effectiveness of the rules. Such work requires close cooperation between FWC Imperiled Species Management, FWC’s Division of Law Enforcement, county officials, the Inland Navigation Districts, FWS, USCG, and, of course, boaters.
Efforts to Reduce Flood Gate and Navigation Lock Deaths
Entrapment in water-control structures and navigational locks is the second largest cause of human-related manatee deaths. In some cases, manatees appear to have been crushed in closing gates; in others, they may have been drowned after being pinned against narrow gate openings by water currents. Water-control structures implicated in manatee deaths in Dade and Broward counties are operated by the South Florida WMD. From 1976 through 2000, 166 manatees have been killed in water control structures in Dade County alone, accounting for 33% of all manatee deaths in this county. In the early 1980s, steps were taken to modify gate-opening procedures to ensure openings were wide enough to allow a manatee to pass through unharmed. Steps were also initiated to fence off openings and cavities in gate structures where manatees might become trapped. Manatee deaths subsequently declined and remained low for much of that decade. Much progress has been made toward identifying, testing, and installing manatee protection devices (e.g., pressure sensors) at water control structures. The Army Corps of Engineers (ACOE) has also installed removable barriers on the upstream side of the Ortona and St. Lucie Lock spillway structures. The large difference in the upstream and downstream water levels at these structures compromises the effectiveness and use of pressure sensor devices. Locks frequented by manatees have been retrofitted with an acoustic array on the gates; this device detects the presence of a manatee during lock closing to avoid pinning or crushing. An interagency task force, established in 1991, continues to monitor, examine and make recommendations to protect manatees at water control structures and navigational locks.
Intensive coastal development throughout Florida poses a long-term threat to the Florida manatee. There are three major approaches to address this problem. First, FWS, FWC, Georgia Department of Natural Resources (GDNR), and other recovery partners review and comment on applications for federal and state permits for construction projects in manatee habitat areas and to minimize their impacts. Under section 7 of the Endangered Species Act, FWS annually reviews hundreds of permit applications to the ACOE for construction projects in waters and wetlands that include or are adjacent to important manatee habitat. FWC and GDNR provide similar reviews to their respective state’s environmental permitting programs.
A second approach is the development of county manatee protection plans. The provisions of these plans are implemented through amendments to local growth management plans under the Florida’s Local Government Comprehensive Planning and Land Development Regulation Act of 1985. In addition to boat speed rules, manatee protection plans are to include boat facility siting policies and other measures to protect manatees and their habitat.
A third approach to habitat protection is land acquisition. Both FWS and the State of Florida have taken steps to acquire and add new areas containing important manatee habitat to federal and state protected area systems. The State of Florida has acquired important areas through several programs, most notably the Florida Forever Program. In Florida, the Governor and Cabinet have included special consideration for purchase of lands that can be of benefit to manatees and their habitat. Over $500 million has been spent to acquire 250,000 acres, whose importance included, but was by no means limited to, protection of manatee habitat. FWS has also acquired and now manages thousands of acres of land important to manatees and many other species in the NWR System. In addition to these efforts, FWS’s initiative to propose new manatee refuges and sanctuaries factors into habitat protection. Both the State of Florida and FWS are continuing cooperative efforts with a view towards establishing a network of important manatee habitats throughout Florida.
Manatee Rescue, Rehabilitation and Release
Thousands of reports of distressed manatees purportedly in need of assistance have been made to the state wildlife enforcement offices and other resource protection agencies by a concerned public. While most of the manatees do not require assistance, dozens of manatees are rescued and treated each year. A network of state and local agencies and private organizations, coordinated by FWS, has been rescuing and treating these animals for well over 20 years.
Manatees are brought into captivity when stressed by cold weather, when struck and injured by watercraft, when injured because of entanglements in crab traps and monofilament fishing line, when orphaned, and when compromised by other natural and man-made factors. Programme veterinarians and staff have developed treatments and protocols for these animals and have been remarkably successful in their efforts to rehabilitate compromised individuals. Since 1973, over 180 manatees have been treated and returned to the wild (FWS unpubl.).
Media coverage of manatee rescues, treatments, and releases helps to educate millions of people about manatees, the life-threatening problems that they face, and actions that can be taken to minimize the effect of anthropogenic activities on this species. In addition, more than 18 million visitors a year see manatees at rehabilitation facilities and participate in manatee education programs sponsored by several parks. The publicity and outreach inherent in this program provide significant support to efforts to recover the manatee.
Public Education, Awareness, and Support
Government agencies, industries, oceanaria and environmental groups have all contributed to manatee public awareness and education efforts that were initiated in the 1970s. These efforts have expanded in scope and increased in quantity since that time. Some key counties in Florida have also started the education component of their manatee protection plans. These public awareness and education efforts encourage informed public participation in regulatory and other management decision-making processes and provide constructive avenues for private funding of state manatee recovery programs, research, and land acquisition efforts through programmes such as the specialty automobile license tag for manatees.
The public has been made aware of new information on the biology and status of manatees, urgent conservation issues, and the regulations and measures required to assure their protection through the production of brochures, posters, films and videos, press releases, public service announcements and advertisements, and other media-oriented materials. Outdoor signs have been produced that provide general manatee information and highlight the problems associated with feeding manatees.
Manatee viewing opportunities have also been made available to the public. In addition, volunteers from several organizations annually give presentations to schools and other groups and distribute educational materials at festivals and events. Such efforts are essential for obtaining public compliance with conservation measures to protect manatees and their habitats.
Many public awareness materials have been developed specifically focusing on boater education. Public awareness waterway signs are produced and distributed alerting boaters to the presence of manatees. Brochures, boat decals, boater’s guides, and other materials with manatee protection tips and boating safety information have been produced and are distributed by law enforcement groups, through marinas, and boating safety classes. Educational kiosks have been designed and installed at marinas, boat ramps, and other waterfront locations. Monofilament fishing line collection sites and cleanup efforts are being established. Several agencies and organizations provide educator’s guides, posters, and coloring and activity books to teachers in Florida and across the United States. In addition, Save The Manatee Club (SMC) and FWC Advisory Council on Environmental Education have produced a video for distribution to schools throughout Florida and the United States. SMC and FWC also provide free manatee education packets to students and staff interviews for students. Agencies and organizations help to educate law enforcement personnel about manatees and inform them about available outreach materials that can be distributed to user groups.
Trichechus manatus manatus
Conservation of the Antillean manatee at the regional level has been driven by the SPAW Protocol to the Cartagena Convention (Freestone 1991), resulting in the Regional Management Plan for the West Indian Manatee, Trichechus manatus (UNEP 1995). In all countries with extant populations there is protective legislation with some effort towards conservation through governmental agencies and/or non-governmental organizations. In a few countries efforts have increased significantly over the past decade. Conservation measures include: (1) policy-based actions such as protective legislation, management plans, recovery plans, and community management; (2) educational outreach programs and awareness activities; (3) research actions such as site specific and country-wide surveys, behavioural studies both in captivity and in situ, remote sensing projects, health assessments, and genetic studies; (4) habitat and site-based actions such as protected areas and community-based initiatives; and (5) species-based actions such as re-introductions, stranding networks, and rehabilitation programs.
The species is listed on CITES Appendix I.
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