This taxon is a subspecies of the West Indian Manatee, Trichechus manatus (Linnaeus, 1758).

The Florida Manatee subspecies is listed as Vulnerable under criterion C1 on the basis of a population size of less than 10,000 mature individuals and there being a reasonable chance that the overall population could decline by at least 10% over the next three generations (estimated at ~60 years). Applying the estimate of the proportion of mature animals in the population (0.73) to the estimated total abundance in 2022 yields a point estimate of 7,158 adult Florida Manatees (minimum of 6,106 based on the lower 95% CRI for abundance), including a small number in The Bahamas. Results from the most recent state-wide population modelling work (Runge et al. 2017) would point to Near Threatened as the appropriate listing category for the Florida Manatee, but that does not tell the whole story. Much has changed since that time, indicating a worsening prognosis for manatees along the east coast of Florida, as described below. We have adopted a precautionary approach in selecting the Red List category that best fits the current status of this subspecies.

Population dynamics were projected over the coming century using a stage-based, stochastic population viability analysis (PVA) model (see Population Trends and Projections under the Population section below). Under what was considered to be the most plausible ‘baseline’ scenario at the time, the chance of the adult manatee population in Florida declining by at least 10% over 60 years was calculated to be 1.5% (Runge et al. 2017). The chance that such a decline would occur on either coast of Florida was an order of magnitude higher at 15.5%. Runge et al. (2017) also considered the impact on the population trajectory of a possible worst-case scenario (deemed unlikely but possible) that involved multiple emerging or strengthening threats occurring concurrently (see Population Trends and Projections under the Population section below for details). The statistic needed for evaluating criterion C1 (i.e., probability of 10% decline over 60 years) was not provided for this scenario, but they found that the probability of quasi-extinction (below 500 adults) of the population on either coast over 100 years was 0.4% under the baseline scenario and 6.0% under the scenario with multiple emerging threats. The metric of 10% decline over 60 years for this multiple-threats scenario should be much more sensitive than a quasi-extinction metric. It is noteworthy that the metrics of future population status at the state-wide and coastal scales mask much higher risks of decline at the regional scale. The two largest management units (comprising about 80-85% of the state-wide population) were shown to have a high chance of at least 30% or 50% population declines over the next 100 years under the baseline scenario: 0.50 and 0.37 probability, respectively, in the Atlantic region and 0.46 and 0.29 probability, respectively, in the Southwest region (Runge et al. 2017). Factors leading to these potential future declines in regional abundance include expected loss of warm-water habitats (and hence carrying capacity), due primarily to retirement of power plants that discharge thermal effluents, along with increased frequency of mortality events caused by red tide.

The 2017 PVA model may have underestimated the chance of a 10% decline over three generations, however, because it did not incorporate changes in some potentially serious emerging or future threats that were difficult to project in a defensible, quantitative way, including: (a) the long-term effects of sea level rise on foraging and warm-water habitats; (b) the long-term effects of climate change on winter severity and seagrasses; and (c) the change in watercraft-related mortality associated with an increasing human population. With respect to the last point, sensitivity analyses found that a doubling of mortality rate from boat strikes would increase the probability of quasi-extinction many-fold (Runge et al. 2017); a 10% population decline metric should also be much more sensitive than the quasi-extinction metric to these effects. Short-term and long-term changes in submerged aquatic vegetation (SAV), especially seagrass habitat, are of great concern, as that provides the principal source of forage for manatees in most regions. As sea level rises, net seagrass area is expected to contract in water bodies with armoured shorelines (such as in most of the developed portions of Florida’s shores) because the habitat cannot migrate inland with rising waters (Doody 2004). In addition, the model likely underestimated cold-related mortality for manatees overwintering in low-quality warm-water habitats (which will be increasingly used as power plant thermal discharges disappear or become less reliable), especially in the northern half of their range; this would have the effect of reducing the projected population impacts stemming from the loss or contraction of key warm-water habitats (see Runge et al. 2017).

The vast majority of seagrass habitat has been lost over the past decade in the Indian River Lagoon (IRL) (Morris et al. 2018, 2022; Adams et al. 2019; see Threats to Forage Habitat under the Threats section below), which holds the prime foraging grounds for manatees along the Atlantic coast of Florida (e.g., Deutsch et al. 2003b, Martin et al. 2015). Since 2011, repeated and prolonged phytoplankton blooms, fed by excessive nutrient loading from external and legacy internal sources, have blocked light from reaching SAV, killing vast areas of seagrass; seagrass area, density, and cover in the IRL has been dramatically reduced. Other potential forage sources (e.g., drift macroalgae) have also been scarce in most years. The largest manatee unusual mortality event (UME) to date began in December 2020 in the northern IRL and has extended throughout nearly the entire Atlantic coastal region. Over the winters (December through March) of 2020–21 and 2021–22, 1039 manatee carcasses were verified in the Atlantic region (from all causes of death), including a relatively high number of adults. The primary cause of the UME is starvation (see Unusual Mortality Event in the Atlantic Coast Region in the Threats section below). The ecosystem in the IRL has undergone a regime shift of dominant primary producers from macrophytes (seagrass, macroalgae) to phytoplankton. Time will tell whether this can be reversed through dedicated efforts to improve water quality and restore habitat. 

Furthermore, there have been marked declines of seagrass and freshwater SAV in many other water bodies inhabited by manatees on the east coast of Florida. This not only reduces the availability of forage during the winter, which is the most energetically stressful time of year for manatees, but also during the warm season when manatees would normally breed and recover body condition. Entering the winter in depleted condition increases susceptibility to cold stress and mortality. It is too early to be able to quantify the impact of this crash in environmental carrying capacity on manatee population dynamics, but preliminary indications are that it is likely to be large. We expect that deterioration in body condition and health will result in lower reproductive rates, as has been observed in dugongs after extensive loss of seagrass (Preen and Marsh 1995). Some evidence suggests that a large drop in reproduction has occurred. Unlike other UMEs caused by red tide or cold, this event is expected to continue to affect manatee health, survival, and/or reproduction until water quality is improved and a sufficient amount of forage habitat is restored; it will likely take many years of dedicated efforts to restore seagrass to pre-bloom levels. The number of verified manatee carcasses in Florida during calendar year 2021 was a record 1,100, nearly double the recent 5-year average. Given the high level of manatee mortality caused by loss of habitat, the lack of assurance of successful habitat restoration, the uncertain but potentially detrimental long-term impacts of climate change and sea level rise on coastal habitats, and the fact that SAV has also been lost outside of the IRL in southeast Florida, in the lower St. Johns River, in much of the upper St. Johns River (i.e., the other subpopulation region on the east coast), and even in estuaries on the west coast, a 10% or greater loss of the state-wide population seems realistic over the next three generations.

Florida Manatees are found in the United States of America (USA), but a small number have recently reached the Bahamas and have taken up residence there. Their year-round distribution in the southeastern USA 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 other southeastern coastal states. Along the Atlantic coast these states include Georgia, South Carolina, North Carolina, and Virginia; a small number of manatee sightings have occurred along the mid-Atlantic coast as far north as Massachusetts. Along the Gulf of Mexico coast west of Florida, some manatees regularly migrate to Alabama during the warmer months and others are occasionally sighted in Mississippi, Louisiana, and as far west as Texas.  In recent years, a few vagrants—identified through photo-identification as known Florida Manatees—have shown up in Cuba (Alvarez-Alemán et al. 2010, 2018b) and Mexico’s Yucatán Peninsula (Morales-Vela et al. 2021, Castelblanco-Martínez et al. 2021). 

FRESHWATER BODIES: Not confined to any but include the following:

• MAJOR LAKES: Lake Okeechobee and Lake George, Florida
• MAJOR RIVERS: St. Johns River, Suwannee River, Manatee River, Caloosahatchee River, St. Lucie River, and Crystal River – all in Florida

During the warm season (March or April through October or November, depending on latitude and weather patterns in a given year), manatees disperse throughout the coastal waters, estuaries, and major rivers of Florida and some migrate to neighbouring states, particularly Georgia and Alabama. Manatee sighting frequency declines to the north of Georgia and to the west of Alabama; manatees have been occasionally sighted as far north as Massachusetts along the mid-Atlantic coast of the USA and as far west as western Texas along the Gulf of Mexico coast. One manatee was even documented near Memphis, Tennessee, over 1,000 km up the Mississippi River from New Orleans, Louisiana (Deutsch et al. 2022a). The exact delineation of range limits is somewhat arbitrary, but based on available sighting reports (e.g., Fertl et al. 2005, Cummings et al. 2014, USGS unpublished data) we have placed them in the mid-Chesapeake Bay on the Atlantic coast and Corpus Christi, Texas on the Gulf of Mexico coast.

Florida Manatee 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 seven principal power plant thermal outfalls (four on the Atlantic coast, three on the Gulf of Mexico coast) and four major artesian springs (Volusia Blue Spring, Kings Bay springs at the head of Crystal River, Homosassa Springs, and Warm Mineral Spring) that are frequented by a large proportion of the manatee population during winter. In addition, manatees use several other springs and a number of passive thermal basins where warmer water temperatures persist as ambient temperatures in adjacent bays and rivers decline during cold fronts.

A very small resident population of West Indian Manatees has become established (including reproduction) in the northern islands of The Bahamas. Knowles et al. (2016) determined that there were at least 15 manatees among the islands based on photo-identification and sighting patterns. Individuals have been documented through photo-identification and, in one case, from satellite telemetry to have come from both coasts of Florida (Reid 2000, Lefebvre et al. 2001, Melillo-Sweeting et al. 2011, Rood et al. 2020). We consider this to be a range extension of the Florida subspecies. There is no history or known origin for other manatees sighted in this vast island nation and there are likely individuals from the Antillean (Greater Caribbean) subspecies present. In general, manatee sightings are most common north of 24 degrees latitude (except for the Exuma Islands and San Salvador Island), especially including Grand Bahama, Abaco, the Berry Islands, New Providence, Andros, and Eleuthera; sightings are much less frequent in the southern portion of The Bahamas, but there are some regular sightings at Long Island (James Reid, USGS, pers. comm.). The subspecies' boundaries shown on the range maps in The Bahamas are therefore somewhat arbitrary; genetic and photo-identification studies will be needed to elucidate the origins of manatees found in these islands.

There is a wealth of knowledge about the life history and population biology of the Florida Manatee, built upon long time series of data collected using photo-identification, satellite telemetry, aerial surveys, carcass salvage and necropsy, health assessments, and genetic analyses. Manatees can live to at least 60 years of age, become sexually mature at about 4–7 years old, have a low reproductive rate (one calf every ~2.5–3 years), and a calf dependency period lasting ~1.5–2 years. This life-history strategy requires a high survival rate among adults for the species to persist.

Abundance: Since the time of the last Red List assessment (Deutsch et al. 2008), an innovative approach has been taken to estimate the abundance of Florida Manatees, along with a quantitative measure of uncertainty. Martin et al. (2015) integrated multiple sources of information from a stratified random sampling design, double-observer protocol, repeated passes around survey plots, and manatee dive data to account for spatial variation and imperfect detection of manatees. This approach has been further improved upon by Hostetler et al. (2018) and, most recently, by Gowan et al. (2023), whose state-wide estimate of abundance in 2021–2022 was 9,790 manatees (95% credible interval (CI): 8,350–11,730), of which 4,630 (3,960–5,420) were on the west coast of Florida and 5,160 (3,940–6,980) were on the east coast. Gowan et al. (2023) caution against inferring trends from the abundance data alone. Rather, these abundance estimates are most appropriately used in the context of the Integrated Population Model (see below), which incorporates multiple data sources to generate a more accurate assessment of population size and trends (Hostetler et al. 2021). The estimated proportion of the population that is mature (i.e., at least 4.5 years old) is 0.73 (Supplementary Data SD3 in Lonati et al. 2019), resulting in a point estimate of 7,147 mature individuals in the United States in 2021–2022 (6,096 for the lower 95% credible limit of abundance). Adding in a small number of manatees from The Bahamas (15 total, 11 mature) results in a total for the Florida subspecies as follows: best estimate of 9,805 total and 7,158 mature; and a minimum reasonable estimate of 8,365 total and 6,106 mature (see Supplementary Information Table S1). Thus, abundance for the subspecies meets the first part of criterion C (i.e., <10,000 mature individuals). Additional information on manatee abundance in Florida comes from near-annual synoptic surveys conducted through 2019, which are timed to coincide with periods of very cold weather when most manatees aggregate at a limited number of warm-water sites in their winter range. The highest state-wide count obtained during these surveys was 6,620 manatees in 2017 (FWC 2018). This represents a minimum bound on abundance because the surveys were not designed to estimate detectability, so the fraction detected is unknown. The highest synoptic counts to date for the west and east coasts of Florida are 3,339 (Jan–Feb 2019) and 3,731 (Jan 2018), respectively (FWC 2021).

Subpopulations:  For population assessment purposes, the Florida Manatee population has been divided into four geographic regions (Northwest, Southwest, Atlantic, and upper St. Johns River), sometimes referred to as subpopulations (USFWS 2001) or ‘management units’ (FWC 2007). Radio-tracking and photo-identification studies have shown that manatees within each region tend to use the same network of warm-water sites during winter and have similar, often overlapping, distribution patterns outside of winter (Bengtson 1981, Rathbun et al. 1990, Reid et al. 1991, Weigle et al. 2001, Deutsch et al. 2003b). These studies have also found that movement of individuals between east and west coasts is limited, but dispersal between regions within a coast is somewhat more frequent (FWC, USGS, and Mote Marine Laboratory, unpublished data). Regardless of whether the degree of genetic or demographic exchange warrants the subpopulation label, these regions differ in habitats, major threats, and (in some cases) vital rates; as such, it has proven useful to analyse population status and trends separately for the four regions.  The Northwest region (NW) extends from the Pasco-Hernando County line along the central Gulf coast northward through the Florida Panhandle and including the coastal areas of adjoining Gulf of Mexico states. The Southwest region (SW) extends from the Pasco-Hernando County line southward to Cape Sable in Monroe County. The Atlantic region (ATL) 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 the Clay-Putnam County line. Manatees in the Upper St. Johns River region (USJ) live in a much smaller area in the river, lakes, and tributaries south (upstream) of the Clay-Putnam County line. The proportion of the population within each region was estimated from the 2011/2012 surveys by Runge et al. (2017) as follows: 10% in NW, 34% in SW, 51% in ATL, and 5% USJ. The relative proportions of manatees on the west and east coasts shifted in the 2015/2016 surveys (west 55%, east 45%), but the region-specific estimates were not considered comparable (in part due to a change in seasonal timing of surveys) (Hostetler et al. 2018).

Population Trends and Projections: Adult survival and female reproductive rates have been determined with high precision through application of state-of-the-art, mark-recapture analytical methods to sight-resight data acquired through photo-identification of distinctly marked individuals (Kendall et al. 2004, Langtimm et al. 2004, 2016). Mean annual adult survival rates, excluding additional mortality due to cold and red tide events, are high (0.97–0.98) in all four regions (Runge et al. 2017). Given the sensitivity of manatee population growth rate to adult survival (Eberhardt and O’Shea 1995, Runge et al. 2004), this has allowed the population to grow, and it provides resilience in the face of current and future threats. Population growth is reflected in increasing counts of manatees at a number of primary warm-water sites (e.g., Kleen and Breland 2014, Reynolds and Scolardi 2016).

Population Viability Analysis (PVA): A custom stage-based PVA model, termed the manatee Core Biological Model (CBM), was used to address the three IUCN criteria (A, C, E) that included metrics on the probability of future population decline or extinction. The CBM projected the population dynamics of manatees in each of the four regions independently (i.e., four separate models), based on region-specific estimates of vital rates and life history (Runge et al. 2017). The last year in which vital rates were estimated for the CBM varied across regions from 2009–2012 for adult survival and from 2010–2014 for reproductive rate. The CBM incorporated demographic and environmental stochasticity, as well as uncertainty in parameter estimates and, in some cases, uncertainty in model structure. The ‘baseline’ scenario included plausible future threats to manatees and their habitat, including expected declines in carrying capacity through loss of warm-water habitat (i.e., loss of power plant thermal discharges, declines in spring flow), increases in cold mortality as warm-water habitat declines and carrying capacity is approached, and increases in the frequency of unusual mortality events due to red tide.

With respect to criterion A, the probability of the adult manatee population in Florida (i.e., essentially, the subspecies) declining by at least 30% over three generations (taken to be 60 years, Haubold et al. 2006) was calculated to be 0.3% under the CBM ‘baseline’ scenario (Runge et al. 2017). The probability that such a decline would occur on either the east or the west coast of Florida was estimated to be 6.6%. Regarding criterion C, the probability of the adult manatee population in Florida declining by at least 10% over 60 years was estimated to be 1.5% under the CBM ‘baseline’ scenario (Runge et al. 2017). The probability that such a decline would occur on either coast of Florida was an order of magnitude higher at 15.5%. The metric in criterion E is the probability of extinction in the wild in 100 years. Runge and colleagues (2017) preferred a quasi-extinction metric for several reasons, including that population dynamics become challenging to forecast and unpredictable at extremely low population sizes. They calculated that the probability of the adult manatee population falling below a quasi-extinction threshold of 500 animals (i.e., an effective population size of 250 adults, based on Tucker et al. (2012)) on either coast of Florida over 100 years is only 0.4% under the ‘baseline’ scenario .It was higher under two other scenarios considered: 6.0% under a scenario of multiple emerging threats; and 8.2% under a hypothesis of density-dependent mortality underlying two cold-related unusual mortality events. These probabilities would be lower if applied at the state-wide (subspecific) scale.

As a consequence of expected losses of industrial warm-water habitat, the CBM forecasts major shifts in the population distribution among regions—from the two southern regions, where most manatees currently rely on power plant thermal discharges for warmth (Laist et al. 2013), to the two northern regions, where manatees use artesian spring systems (Runge et al. 2017). Odds are nearly 50:50 that the SW and ATL subpopulations (where most manatees currently reside) will decline by 30% or more over the coming century. Runge et al. (2017) noted that: “… the relatively low probabilities of quasi-extinction at the [state-wide and] coastal scale mask higher risks of decline at the regional level” (p. 37). They concluded that “… the risk of major population decline is very low at the state-wide (subspecies) level, moderate at the coastal level, and high at the regional level for the two currently largest management units. This seeming paradox stems from the fact that one regional population is expected to decline and one to increase on each coast” (p. 34). Finally, additional scenarios were run to explore sensitivity of population metrics (particularly quasi-extinction) to various parameters and to evaluate the impacts of potential emerging threats. The scenario in which multiple emerging threats co-occur would have the greatest impact on the population. Relative to the baseline scenario, this scenario added: (a) a 50% increase in the watercraft-related mortality rate, (b) immediate loss of warm-water carrying capacity associated with coal-fired power plants, (c) long-term 50% reduction of carrying capacity at natural springs (vs. 26% reduction for mean baseline), (d) increased frequency of cold winters, and (e) a 2% reduction in survival in the ATL region due to a chronic source of mortality. Under this scenario, the probability of quasi-extinction (500 adult threshold) on either coast over 100 years was estimated to be 6.0% (Runge et al. 2017).

Integrated Population Model (IPM): Hostetler et al. (2021) built an IPM that reconstructed historical population dynamics for manatees in the SW region of Florida from 1997–2016. This IPM integrated multiple sources of population data, including two estimates of abundance from aerial surveys (Hostetler et al. 2018), number of verified carcasses by size/age class, and estimates of adult survival and female reproductive rates from sight-resight analyses based on photo-identification. The population in the SW region was estimated to have increased at an average rate of 2% (95% CRI, 1–3%) per year over this 19-year period, from 2,014 manatees (95% CRI, 1,861–2,229) in 1997 to 2,966 manatees (95% CRI, 2,551–3,434) in 2016 (Hostetler et al. 2021). The model generated estimates of parameters that had been missing, such as survival of calves and subadults, and population size in years before and between abundance surveys. They found that an intense red tide bloom in 2013 caused an 11% decline (95% CRI, 7–15%) in the regional population, with particular impacts on calf and subadult mortality. This modeling approach provided estimates of population parameters that were more accurate or more precise (or both) than existing estimates. For example, the baseline survival probabilities for calves and subadults in the CBM were derived from a small, old study in the USJ region and extrapolated based on ratios to adult survival probability (Runge et al. 2017). For the SW region, survival rates of immature classes (first 4 years) estimated from the IPM were substantially lower than the extrapolated estimates used in the CBM. The IPM approach to evaluating current and past population dynamics and status will be extended to the other three regions and also applied at coastal scales.

Unusual Mortality Event in the Atlantic Coast Region: Starting in early December 2020, an unprecedented number of manatees died in the Atlantic coast region of Florida during the winter and spring of 2020–2021, and a high level of mortality occurred again in the subsequent winter (Deutsch et al. 2021, FWC 2023a,b). The number of carcasses documented in the Atlantic region during the months of December through March of 2020–21 (582) and 2021–22 (457) was 5.0 and 3.9 times higher, respectively, than the prior five-year average for those months (116.4, December 2015–November 2020). Over the course of these two winters, 59% of carcasses from the Atlantic region were found in the waters of Brevard County (northern IRL); the number of carcasses and live manatees in distress were substantially elevated in southeast Florida during the first winter as well. The state-wide number of verified manatee carcasses hit a record-setting 1100 in 2021, nearly double the recent five-year average of 579 (calendar years of 2015–2019); 800 carcasses were documented in 2022, and a preliminary total of 555 in 2023 (https://myfwc.com/research/manatee/rescue-mortality-response/statistics/mortality/).

The investigation into this UME is ongoing, but it is clear that the primary cause of mortality was starvation. Poor body condition of malnourished animals also made them more vulnerable to the stress of cold winter temperatures. Externally, emaciation presented as a distinct dip between the head and the rest of the body (“peanut head”), longitudinal ventral folds, flattening of the body, appearance of an elongated peduncle area, and sometimes visible outline of skeletal structures such as the skull. Necropsied carcasses had findings of significant emaciation and profound atrophy of fat, muscle, and other internal organs, especially liver (FWC 2023b). Often there was little or no filling of the gastrointestinal tract. Health impacts of chronic malnutrition appear to be long-lasting, showing up in the warm season as well (FWC 2023b). There is preliminary evidence suggesting a large decline in reproduction during the UME. Manatees were malnourished and starving due to the massive loss of seagrass in the IRL that had been steadily worsening since 2011 due to repeated algal (phytoplankton) blooms (Morris et al. 2018, 2022); SAV has been declining in other areas along the east coast as well (see Threats to Manatee Forage Habitat under Threats section below).

The impact of this crash in environmental carrying capacity on manatee population dynamics has not yet been determined, but preliminary indications are that it is likely to be large. Carcass counts in the Atlantic region during winters 2022–23 and 2023–24 were at or below baseline levels, apparently because manatees were in better body condition than the prior two winters. Recovery of seagrass in the IRL will be vital to improvement in manatee health and survival, and seagrass is showing some signs of recovery in limited areas. Updates on the UME are posted by the Florida Fish and Wildlife Conservation Commission here: https://myfwc.com/research/manatee/rescue-mortality-response/ume/.

Current Population Trend: The ‘current population trend’ field is meant to convey a snapshot of the probable status of a population within the IUCN assessment cycle, but it is not part of the criteria used to determine the appropriate Red List category. The term “refers to trends over a period of ca. three years around the present” (IUCN 2013, p. 21). Clarification on the meaning and determination of current population trend was provided by IUCN staff: “the period under consideration should be six years: based on knowledge of population trend over the three years immediately before the assessment date and the projected trend over the next three years, is the population likely to be stable, declining, increasing, or is the current population trend unknown?” (C.M. Pollock pers. comm. to Benjamin Morales, 6 January 2022). In our case, the period under consideration is from August 2020 to August 2026.

Population models over longer time periods indicate that the Florida Manatee population has generally increased over a period of decades (Runge et al. 2017). Manatees in the southwest region of Florida have shown a slow but variable increase in population, averaging 2% per year from 1997 to 2016, with occasional drops due to red tide events (Hostetler et al. 2021). Given the recent high mortality and apparently low reproduction in the Atlantic region, it seems likely that this subpopulation incurred a substantial decline in abundance from 2020 to 2022. In order to assess current population trend for the Florida Manatee as a whole, however, we would need information on actual and projected population change from all four regions for the 2020–2026 period. Since this information is not available at this time, we must rate current population trend as ‘unknown.’

Population Genetics: Genetic diversity within a population can affect the organism’s ability to adapt to long-term environmental change. In this regard, the Florida Manatee may have limited capacity for adaptation through genetic evolutionary change, as it has very low genetic diversity. This is borne out through analyses of mitochondrial DNA (only one haplotype) and nuclear DNA (relatively low levels of polymorphism and allelic diversity in microsatellites) (Garcia-Rodriguez et al. 1998, Vianna et al. 2006, Tucker et al. 2012). Either a founder effect or a population bottleneck, or both, could account for the current situation. Tucker et al. (2012) found little genetic differentiation among regions or coasts, but effective population size was higher on the west coast than the east coast.

Much has been learned about the ecology, behaviour, and habitat needs of the Florida Manatee over the past several decades of focused research, starting with the treatise by Hartman (1979). Florida Manatee biology has been summarised nicely by Reep and Bonde (2006, 2021), and Marsh et al. (2011) have written a comprehensive scholarly review and synthesis of sirenian ecology. This section summarises some of the highlights of this body of knowledge for the Florida subspecies.

The movements of Florida Manatees are largely influenced by their need for food, thermal shelter in winter, freshwater for drinking, resting sites, and breeding. See Deutsch et al. (2022a, 2022b) for reviews of movement behaviour in Florida Manatees and other sirenians across a range of spatio-temporal scales. Manatees in the United States typically undertake long-distance migrations, resulting in seasonal shifts in geographic distribution of the population (Weigle et al. 2001, Deutsch et al. 2003b, Cloyed et al. 2021, Slone et al. 2022). These seasonal migrations are mostly driven by fluctuations in water temperature, with 20° C being a critical limit (Hartman 1979, Bengtson 1981, Shane 1984, Deutsch et al. 2003b). The physiology of manatees is adapted to tropical waters; consequently, their very low metabolic rate and high thermal conductance make them vulnerable to illness and death when exposed to cold water temperatures (Irvine 1983, Bossart et al. 2002, Hardy et al. 2019). Individuals show strong fidelity (returning year after year) to seasonal ranges, including to specific warm-water sites used during cold weather (Rathbun et al. 1990, 1995; Reid et al. 1991; Koelsch 1997; Deutsch 2000; Deutsch et al. 2003b). These winter and warm season ranges are typically separated by lengthy travel corridors used during migration. There is evidence for natal philopatry, with movement patterns and seasonal ranges being passed on from mother to calf during the long dependency period (Deutsch et al. 2003b, 2022a; O’Shea et al. 2022). Manatees are individualistic in their movement behaviour, with considerable variation among individuals in range size, seasonal movement patterns (from residents to long-distance migrants), trigger temperature for migratory timing, degree of range and site fidelity, and time spent outside of warm-water refuges (Deutsch et al. 2003b, 2022a; Deutsch and Barlas 2016).

Another key characteristic of the Florida Manatee is that it is a generalist in many aspects of its ecology. Manatees can and do thrive in a large diversity of coastal and inland water bodies, including those fringed with mangroves or salt marshes and those in urban environments with dense human populations. They live in waters that range from freshwater to brackish to marine, although they regularly seek out freshwater sources to drink in estuarine and marine environments, where they are commonly found near the mouths of rivers (Lefebvre et al. 2001). Like all sirenians, Florida Manatees are herbivorous, but on rare occasions they have been observed to scavenge on dead fish or consume sessile aquatic invertebrates (Courbis and Worthy 2003). The varied environments that manatees occupy present them with a range of potential types of vegetation—benthic, emergent, floating, and bank—and, as generalist herbivores, they have a very broad diet (Etheridge et al. 1985, Smith 1993, Keith-Diagne et al. 2022). This includes seagrasses (especially Halodule wrightii or shoal grass, and Syringodium filiforme or manatee grass) and drift macroalgae in estuarine habitats, smooth cord grass (Spartina alterniflora [now Sprobolus alterniflorus]) in salt marshes, and floating plants such as water hyacinth (Eichhornia crassipes) and submersed native or exotic vegetation (such as Hydrilla verticillata) in slow-moving freshwater bodies. Manatees and the habitats upon which they depend have certainly suffered a number of direct and indirect adverse impacts from human activities in the coastal zone (see Threats). But this aquatic mammal has also demonstrated an ability to adapt to and even take advantage of some human alterations to their natural habitats. Feeding on abundant, non-native invasive freshwater vegetation such as Eichhornia and Hydrilla is one example. Manatees often use dredged channels as travel corridors or to gain access to shallow feeding areas. Residential canals and other dredged basins are frequently used as quiet resting sites, and these sites sometimes even provide thermal benefit over nearby ambient waters during cold periods.

There are two exceptions to the manatee’s generalist habits. First, as noted above, manatees are restricted by water temperature, unable to tolerate prolonged exposure to low water temperatures, with 20° C being the lower limit of their thermoneutral zone (Worthy et al. 2000). Second, their coastal habitat is limited to a relatively narrow strip close to shore where they can find food, freshwater, and shelter; they are generally found in riverine or coastal waters within the 12-foot (3.7-m) depth contour. Even though manatees have been documented diving to depths of up to 16 m, a recent study has found that, on average, 78% of the time the manatee was no more than 1.25 m from the surface (Edwards et al. 2016).

Regional networks of warm-water habitats that provide sufficient thermal shelter to manatees during cold weather are critical to manatee overwinter survival in Florida (Flamm et al. 2012). These habitats can be classified into essentially three main types: (1) discharges of artesian groundwater from natural springs; (2) discharges of thermal effluent from industrial outfalls, primarily power generating stations; and (3) thermal basins that retain heat due to thermal inertia associated with depth (e.g., dredged basins) or to temperature-inverted haloclines (Stith et al. 2010), and that sometimes gain heat through groundwater seeps, stormwater inputs, or possibly microbial degradation of organic material in the sediment (Laist and Reynolds 2005a). The spatial extent, thermal quality, and reliability of these habitats vary within and across types, and this is most obvious during severely cold winters (e.g., Barlas et al. 2011, Stith et al. 2012). Natural springs are generally the most reliable, consistently discharging water at temperatures of 22–23° C. Manatees aggregate at warm-water refuges in large numbers during cold weather, often several hundred animals at a given site (Laist and Reynolds 2005a, 2005b; Laist et al. 2013), preferring sites that provide the best combination of thermal shelter and proximity to forage. Manatee dependence on these two key resources during winter results in a repeated movement pattern of central-place foraging; manatees make feeding forays from the warm-water site (the central place) to seagrass beds or other areas with aquatic vegetation for periods of hours to days and then return to that site for warmth (Bengtson 1981; Deutsch et al. 2003a, 2006; Deutsch and Barlas 2016; Haase et al. 2017, 2020).

Manatees are considered semi-social, usually found alone or in small groups where they rest, travel, socialise, or forage together (O’Shea et al. 2022). The only stable bond between individuals is that between mother and calf. Oestrous females can attract up to 20 or more males, forming a mating herd that lasts up to a few weeks and whose composition changes daily (Hartman 1979, Bengtson 1981, Rathbun et al. 1995). Most mating activity and births occur during the spring and summer (March to September) (Ackerman et al. 1995, O’Shea and Hartley 1995, Rathbun et al. 1995, Reid et al. 1995, Schwarz 2008); during this time, males search for oestrous females, resulting in a much higher movement rate for males than females (Bengtson 1981, Deutsch et al. 2003b). Breeding is much less common in winter, as males undergo a period of spermatogenic quiescence during that season (Hernandez et al. 1995).

Threats encompass extreme natural events and anthropogenic factors that can reduce reproduction, survival rates, or the carrying capacity of the environment. Manatees occupy habitats in the same coastal zone and inland waterways where humans recreate and carry out other activities, a fact that contributes to manatee vulnerability to human pressures. According to the U.S. Census Bureau (2020), Florida’s human population grew by 66% to an estimated 21.5 million from 1990 to 2020; projections suggest that the human population of Florida will increase to over 26 million people by 2040 and 34 million by 2070 (Carr and Zwick 2016, Rayer and Wang 2020). This is likely to increase risk to manatees from human-related activities. Potentially catastrophic threats to manatees include exposure to cold temperatures, harmful algal blooms, seagrass loss, hurricanes, and emergent disease. Climate change may also threaten Florida Manatees over the long term (Edwards 2013) by exacerbating the above threats or creating others. Here we group threats into direct ones that result in mortality or injury and indirect ones that degrade important manatee habitat.

Direct Threats to Manatees: Manatees are injured or killed by several types of human-related activities. Besides collisions with vessels, described below, another documented threat is entanglement in fishing gear (crab pot line, monofilament line) or debris and incidental ingestion of marine debris that injures or blocks the gastrointestinal tract (Beck and Barros 1991, Adimey et al. 2014, Reinert et al. 2017). Entanglement rarely results in mortality but often causes disfiguring injuries, even amputation of a pectoral flipper. Manatees also die from entrapment in water-control structures and stormwater pipes, and from crushing in flood-control structures, in canal locks, or between large ships and docks (Ackerman et al. 1995, FWC 2007).

Boat Strikes: Watercraft collisions have accounted for an average of 21.1% (min – max = 8.8 - 31.1% across years) of all reported manatee deaths and 29.9% (16.0 – 39.9%) of deaths of known cause from 2000–2019, and this is the single greatest cause of human-related mortality (Deutsch and Reynolds 2012; FWC manatee mortality data, https://myfwc.com/research/manatee/rescue-mortality-response/statistics/mortality/). The proportion of adult deaths due to boat collisions is much higher than in immature age classes (Deutsch and Reynolds 2012, Runge et al. 2017), which is significant because adult survivorship is the principal driver of manatee population dynamics (Runge et al. 2004). The number of registered vessels in Florida increased by an average of 2.9% per year over 25 years, topping 1 million boats by 2007 (FLHSMV, https://www.flhsmv.gov/motor-vehicles-tags-titles/vessels/vessel-owner-statistics/). Those numbers declined over the subsequent seven years due to an economic recession, but they have resumed a gradual increase since 2013, reaching 1,029,993 vessels by 2022. In addition, thousands more visitors ply Florida’s waterways with out-of-state vessels. Given that about 97% of registrations are for recreational watercraft (Wright et al. 1995; FLHSMV, https://www.flhsmv.gov/motor-vehicles-tags-titles/vessels/vessel-owner-statistics/), it is reasonable to expect a continued increase in recreational vessels on Florida's waterways 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 lethal collisions between manatees and watercraft. Modifications to the design of vessel hulls and engines have allowed boats to travel at higher speeds in shallower waters (Wright et al. 1995), thus threatening manatees and scarring seagrass beds. The weight of evidence indicates that faster boats pose a greater risk of collision with manatees than do slow-moving boats, because manatees have less time to respond to boats moving at planing speeds (Calleson and Frohlich 2007; Rycyk et al. 2018, 2022). Boater compliance with posted speed zones has averaged about 50–60% in a few studies, but it varies greatly across sites, vessel type and size, and other factors (Shapiro 2001, Gorzelany 2004). Some waterways experienced 85% compliance rates and others as little as 14% (Gorzelany 2013). Increased law enforcement patrols and establishment of a general boater licensing programme would likely increase the effectiveness of the regulations adopted to protect manatees from collisions.

Sub-lethal effects on manatees of increased vessel traffic and a growing human population in the coastal zone are also cause for concern. A detailed study of Florida Manatee carcasses recovered in Florida over a 10-year period (2007–2016) by Bassett et al. (2020) found that 96% of adult carcasses bore scars from previous boat collisions and about ¼ of adult carcasses showed evidence of 10 or more separate watercraft strikes. This level of sublethal trauma from boat strikes is greater than in any other marine mammal than has been studied. The healed, skeletal fractures in some carcasses indicate that the animals 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). Many of these individuals were severely mutilated, especially on the tail and the dorsum. 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), although that effect has not been investigated. Vessel traffic and recreational activities that disturb manatees may cause them to leave preferred habitats and may alter biologically important behaviours such as feeding, nursing, or resting (O’Shea 1995, Wright et al. 1995). This may explain why manatees preferentially select seagrass habitats with lower levels of low-frequency (<1 kHz) ambient noise (Miksis-Olds et al. 2007).

Cold Events: Manatees seek warm-water sites when temperatures drop below 20^o C and are unable to tolerate prolonged exposure to temperatures below about 16^o C (Irvine 1983). Major spikes in cold-related manatee deaths have been documented during cold winters numerous times (O’Shea et al. 1985, Ackerman et al. 1995, Hardy et al. 2019). An unusual mortality event (UME) during winter 2009–10 was unprecedented in its scale and spatial scope, with 480 manatee carcasses reported state-wide during its three-month timeframe; 89% of deaths for which cause could be determined were due to cold stress (Barlas et al. 2011). Death from exposure to cold can occur acutely, from hypothermia, or from chronic exposure. Manatees chronically exposed to water temperatures below 20^o C display a range of clinical and pathological signs such as emaciation, oedema, serous atrophy of fats, and dehydration (O’Shea et al. 1985, Bossart et al. 2002). Calves and subadults are the most vulnerable to cold-related death (O’Shea et al. 1985, Ackerman et al. 1995). Unless management is proactive about replacing lost industrial warm-water refuges or in otherwise mitigating the negative impacts resulting from loss of natural and human-made warm-water habitats, we can expect higher cold-related mortality during cold winters in the future (Laist and Reynolds 2005b).

Red Tide: Manatees in Florida’s southwest region are frequently exposed to and die from brevetoxin, a potent neurotoxin produced by the dinoflagellate Karenia brevis, during “red tide” blooms (O’Shea et al. 1991, Bossart et al. 1998, Landsberg and Steidinger 1998, Flewelling et al. 2005). Red tide blooms have resulted in major manatee mortality events 12 times between 1982 and 2022 (all except one since 1996), likely killing over 1,400 manatees (FWC, unpublished data). The largest mortality events to date occurred during red tide blooms in the calendar years of 2013 and 2018, resulting in the deaths of 277 and 288 manatees, respectively, within the boundaries of the blooms in southwest Florida (FWC, unpublished data, https://myfwc.com/research/manatee/rescue-mortality-response/statistics/mortality/red-tide/). These mortality events can have noticeable impacts on survival rates, and hence, on population growth (Runge et al. 2017, Hostetler et al. 2021). For example, the large red tide mortality event of 2013 was estimated to cause abundance in the SW region to decline by 11% (95% CRI, 7–15%), in contrast to an increase of 2% (1–3%) in an average year (Hostetler et al. 2021). Over the past quarter century, manatee mortality events due to red tide have occurred in the SW region about every other year (Martin et al. 2017; FWC, unpublished data). Forecasts from an expert panel suggest that manatee die-offs from harmful algal blooms will probably become more frequent in the future (Martin et al. 2017) because of ocean warming due to climate change, coastal nutrient loading, and other factors (e.g., Hallegraeff 2016, Gobler 2020, Griffith and Gobler 2020, Medina et al. 2022).

Hurricanes: Hurricanes are another type of weather-related catastrophe that can potentially impact manatee populations and their habitats. Manatees can be stranded or entrapped in small ponds by surge waters. In the northwest region, apparent adult survival rate was lower in years with severe storms or hurricanes (Langtimm and Beck 2003). Such events could also result in permanent, large-scale emigration. 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 dugon) (Preen and Marsh 1995), a sirenian relative of the manatee. The increased runoff associated with hurricanes in Florida has also been shown to reduce water visibility and salinity, and to result in declines in submerged aquatic vegetation (SAV) coverage (e.g., Ridler et al. 2006, Carlson et al. 2010).

Disease: Large-scale mortality events caused by disease have decimated other populations of marine mammals, including seals and dolphins, often removing 50% or more of the individuals (Harwood and Hall 1990). No such epizootics have been documented in manatees, but the population has been exposed to pathogens—such as Toxoplasma gondii (Buergelt and Bonde 1983; Smith et al. 2016) and morbillivirus (Duignan et al. 1995)—that have been responsible for mortality events in other marine mammal species (e.g., Lipscomb et al. 1994, Dubey et al. 2003). Spread of such pathogens could be particularly rapid during winter when manatees are densely concentrated in warm-water refuges and when their immune systems may be compromised by exposure to cold (Walsh et al. 2005). Thus, the emergence of a serious infectious disease poses a potential threat to the population (Runge et al. 2017).  Threats from continuing or emerging diseases are monitored through a comprehensive manatee carcass salvage and necropsy programme carried out by the Florida Fish and Wildlife Conservation Commission (https://myfwc.com/research/manatee/rescue-mortality-response/), through rescue and rehabilitation efforts (see section on Conservation), and through periodic health assessments of free-ranging wild manatees (Bonde et al. 2012). 

Threats to Manatee Habitat: Manatees require forage, thermal refuges in the winter, quiet resting areas, and freshwater. Here we focus on threats to habitats that provide warm-water shelter and forage.

Threats to Warm-water Habitat: Expected changes to the warm-water network over the next several decades likely present the most serious habitat threat to manatees in Florida over the long term; if unmitigated, these changes will likely result in higher cold-related mortality and lower carrying capacity (Laist and Reynolds 2005a, 2005b; Runge et al. 2017). Manatees have used the thermal effluents of power plants in winter for over six decades and now a large proportion of the population in the SW and ATL regions rely on them for warmth (Laist et al. 2013). Eventual retirement of these power plants or future elimination of the once-through cooling technology permitted by regulatory agencies threatens the reliability and existence of these warm-water sites (Laist and Reynolds 2005b). The increasing establishment of utility-scale clean energy sources, such as solar farms, are accelerating these trends and will likely lead to less reliable thermal discharges sooner than previously expected. Several industrial thermal outfalls used by manatees, including those from power plants, have already been eliminated. Furthermore, artesian spring flows have declined, and water quality has been degraded (high nitrogen), leading to overgrowth of noxious algae and loss of native vegetation, in many spring runs; these trends will likely continue as demand for water increases with continued growth in the human population (Florida Springs Task Force 2000). Declining flows will provide less warm-water habitat for wintering manatees, particularly in the NW and USJ regions where they are nearly entirely dependent on spring flows for thermal refuge. In addition, dams and other structures currently impede or prevent manatee access to a number of spring systems (Taylor 2006).

Threats to Forage Habitat: Human development in the coastal zone over the past century has often negatively impacted manatee foraging habitat, due to loss of seagrass beds directly from dredge-and-fill activities and indirectly through nutrient loading and reduced water clarity (Fonseca et al. 1998). Declines in water quality (e.g., increased nitrates) due to sewage releases during storms or other events, non-point source runoff (e.g., from agriculture, urban areas), and groundwater nutrient inputs (e.g., from septic tanks) can promote the growth of undesirable macroalgae, such as Anadyomene spp. in estuarine systems (Santos et al. 2020) and the unpalatable blue-green alga Lyngbya sp. in freshwater systems (Hudon et al. 2014), which can smother food plants used by manatees (Florida Springs Task Force 2000). Such increases in nutrients also promote dense, single-celled algal blooms, which can shade out seagrasses and other submerged aquatic vegetation (Phlips et al. 2015, 2021; Trefry and Fox 2021). Vessel traffic can also degrade SAV through increased water turbidity from wake action and scarring of shallow seagrass beds by propellers and anchors (Sargent et al. 1995).

Unprecedented seagrass losses in the Indian River Lagoon—a 250-km long biodiverse estuary along the central-east coast of Florida—have been occurring since 2011 due to prolonged and repeated algal blooms that block sunlight from reaching benthic plants, resulting in seagrass mortality (Phlips et al. 2015, 2021; Morris et al. 2018, 2022; Lapointe et al. 2020). Between 2009 and 2021, the IRL lost about 24,000 hectares of seagrass habitat, a 75% reduction in areal extent; and by 2020, the mean percent seagrass cover within that footprint had declined by 89%, from ~20% to ~2%, on average (St. Johns River Water Management District (SJRWMD), unpublished data). So over 95% of seagrass biomass in the IRL has been lost during this period. Seagrass has essentially disappeared in many areas, leaving mostly bare sandy substrate where there had once been lush meadows of seagrass and drift macroalgae. These catastrophic losses of SAV have dramatically reduced the forage-based carrying capacity of this vital lagoon system for manatees and have resulted in mass starvation and mortality of manatees (FWC 2023a; see Unusual Mortality Event in the Atlantic Coast Region under the Population section above). The northern IRL is the major year-round hub of manatee activity on the east coast (Deutsch et al. 2003b), including for migrants both north and south of the region, which is why the consequences of seagrass loss here for manatees have extended throughout the entire Atlantic coast region (Deutsch et al. 2021, FWC 2023b). Limited availability of forage resulted in manatees that summered in the IRL entering winter in depleted poor body condition (FWC 2023b). This made the animals more susceptible to the effects of cold, including cold stress syndrome and mortality.

Loss of seagrass has been documented to result in manatee mortality through a more indirect route as well. After the “superbloom” of phytoplankton in the Indian River Lagoon in 2011, the area and biomass of seagrass declined sharply. In 2013 a variety of macroalgae became abundant and many manatees in the region shifted to feeding on these algae (Allen et al. 2022). An unusual mortality event soon followed, with apparently healthy manatees in good body condition dying suddenly from an unknown cause. After years of exhaustive investigations, it was determined that this dietary shift led to dysbiosis (disruption of the gut microbiome) and clostridial infection that resulted in acute death of some manatees (Landsberg et al. 2022).

Furthermore, seagrass has also declined along nearly all of Florida’s east coast, from the lower St. Johns River in the north to Biscayne Bay and Florida Bay in the south and many intracoastal waterways in between (e.g., Collado-Vides et al. 2013, Hall et al. 2016, Orlando et al. 2016, Marine Resources Council 2018, Morris et al. 2018, Kahn 2019). The substantial loss of beds of Vallisneria and other native freshwater SAV in the lower St. Johns River, extending well into the upper St. Johns basin at least as far as Lake George (SJRWMD, unpublished data), is also concerning, as that stretches into another manatee management unit (subpopulation). Although seagrass beds have generally been expanding over the years in estuaries along Florida’s Gulf coast (Tomasko et al. 2018), recently the trend has reversed in a number of important areas. In some cases, this has been associated with replacement by the rooted macroalga Caulerpa prolifera (e.g., parts of Tampa Bay) and in other cases by extensive blooms of drift macroalgae (e.g., Charlotte Harbour) (see presentations from 2021 Florida Macroalgae Workshops, https://sarasotabay.org/2021-macroalgae-workshops/). These developments serve as a cautionary note about how quickly positive habitat trends can shift in worrisome directions.

Climate Change Impacts to Habitats: The potential effects of climate change on sirenians are myriad (Edwards 2013), but there have been no analyses to test potential hypotheses. Effects on the occurrence and severity of harmful algal blooms are noted above. If the overall warming trend leads to consistently warmer winters in Florida, then we might expect a reduction in cold-related mortality. Rising water temperature poses a direct threat to seagrasses if it reaches the plant’s physiological tolerance or otherwise reduces productivity (Barber and Behrens 1985, Short et al. 2016). Increased frequency of extreme weather events is also likely to reduce forage habitat in coastal areas (Babcock et al. 2019), with consequent impacts on sirenians (e.g., Preen and Marsh 1995, Preen et al. 1995). Of concern in Florida, sea level rise will cause the shrinkage of seagrass beds at their outer depth limits in areas where armouring of the shoreline prevents shoreward migration, in what is known as ‘coastal squeeze’ (Short and Neckles 1999, Doody 2004). This situation will likely occur along most of the developed shorelines in Florida and may get worse as state and local governments and residents attempt to protect infrastructure, homes, and commercial developments from rising seas. Finally, higher water levels may suppress flow of coastal springs, further reducing warm-water habitat. Many other cascading impacts on manatees from disruption of the climate system are possible (Edwards 2013, Marsh et al. 2022), but the uncertainties are great.

There is no commercial or subsistence utilisation.

Non-consumptive use
Florida 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 annually to observe and swim with manatees, creating challenges for management (Sorice 2003).  No-entry sanctuaries provide manatees with havens to avoid swimmers and boats at some of these sites (Buckingham et al. 1999).  At other sites, groups of paddlers (e.g., on kayaks) or swimmers can cause considerable disturbance to manatees trying to rest and conserve energy.

The text in this section was originally excerpted and condensed from the Florida Manatee Recovery Plan (USFWS 2001), but it has since been considerably revised and updated by the assessors. Numerous actions by state and federal agencies related to research and monitoring are not included in this section but can be found elsewhere (USFWS 2001, FWC 2007, Deutsch and Reynolds 2012). The West Indian Manatee, including both subspecies, is protected under United States federal legislation through the Endangered Species Act (ESA) of 1973 and the Marine Mammal Protection Act of 1972. At the state level, the Florida Manatee Sanctuary Act of 1978 provides the framework for the establishment of a number of important regulatory protections for manatees, such as boat speed rules.

The Florida Manatee is a conservation-reliant species, which means that the sustainability of the population is supported by active conservation programmes. There is a high degree of interaction (both direct and indirect) between manatees and a variety of human activities in a state where coastal development and human population density are high and increasing. Large and active research and management programmes at federal, state, and county levels have been implemented to reduce watercraft-related and other human-caused mortality (e.g., through speed restriction zones and sanctuaries), to protect and restore key warm-water habitats, and to rescue, rehabilitate and release injured or sick manatees (USFWS 2001, FWC 2007). The PVA described in the population section of this Red List assessment assumes that existing protections, regulations, rescue and rehabilitation programmes, and enforcement continue into the future (Runge et al. 2017). Reduction or elimination of such conservation efforts and protections would likely halt or reverse the trend in population recovery that has occurred over the past several decades.

Efforts to Reduce Watercraft-related Injuries and Deaths

The largest cause of manatee death over which managers have some control is watercraft collision. Many living manatees bear scars or wounds from vessel strikes (Bassett et al. 2020). 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 give both manatees and boaters more time to avoid collisions (Calleson and Frohlich 2007). Speed zones can be quite effective at reducing the risk of lethal collisions (Udell et al. 2018). Since 1989, state and local governments have cooperated in the development and implementation of county manatee protection plans—which, among other things, affect vessel traffic patterns through boat facility siting—and manatee protection speed zone rules (FWC 2007). Two types of manatee protection areas also have been established by the federal U.S. Fish and Wildlife Service (USFWS): (1) manatee sanctuaries, areas in which all waterborne activities are prohibited, typically placed in critical warm-water sites; and (2) manatee refuges, areas where certain waterborne activities (e.g., operation of motorised vessels) are restricted or prohibited. USFWS and FWC use these regulations as key management tools to ensure that adequate protected areas are available throughout Florida to meet manatee habitat requirements with a view toward recovery. Both agencies employ targeted enforcement strategies in an attempt to increase boater compliance with speed zones and, ultimately, to reduce manatee injuries and deaths.

Managers, researchers, and the boating industry have investigated the use of various devices to aid in the reduction of watercraft-related manatee deaths. Propeller guards, for example, would reduce cutting damage associated with propellers, but they are of much less benefit when boats operate at high speeds (e.g., on a plane) because manatees would still be killed by the blunt trauma from impacts of boat hulls, lower units, and other gear (Wright et al. 1995; Milligan and Tennant 1998). There are propeller guard applications, however, that work to prevent sharp trauma by propellers of certain large, slow-moving commercial vessels, such as tugs. Where manatees aggregate in large numbers at or near warm-water refuges, boats are required to move at idle or slow speeds; in these areas, propeller guards are also sometimes used on sight-seeing and tour boats to prevent cutting injuries.

Priority actions related to minimising manatee injury and death from boat strikes include boater education, enforcement of speed zones and refuges/sanctuaries, maintenance of signs and buoys, compliance assessment, and periodic re-evaluation of the effectiveness of the rules. Such work requires close cooperation between FWC and USFWS managers and law enforcement, county officials, the U.S. Coast Guard, and boaters.

Efforts to Reduce Flood Gate and Navigation Lock Deaths

Entrapment in water-control structures and navigational locks has historically been the second largest cause of human-related manatee deaths. In some cases, manatees appear to have been crushed in closing gates; in others, they have drowned after being pinned against narrow gate openings by water currents rushing through. In Florida, water-control structures and navigation locks are largely operated by State of Florida Water Management Districts and the US Army Corps of Engineers (ACOE); a few structures are operated by private interests.

In the early 1980s, gate-opening procedures were modified to ensure openings were wide enough to allow a manatee to pass through unharmed. Openings and cavities in gate structures where manatees might become trapped were fenced off. Manatee deaths subsequently declined. Much progress has been made since then toward identifying priority water control structures and testing and installing manatee protection devices at those structures. This includes pressure sensor devices, which have been installed at water control structures frequently visited by manatees. An acoustic array that detects the presence of a manatee when it interrupts the sonic signal has been installed on some navigation lock structures. When a manatee is detected near the gate during the last 52 inches of closure, an alarm sounds; the gate stops closing and is then re-opened back to 52 inches. Manatee protection devices (including acoustic arrays, pressure-sensitive piezoelectric strips, grates, and bars) have been installed on all major structures known to crush or entrap manatees. The incidence of deaths in locks and structures is now quite low. An interagency task force continues to monitor structure-related mortality, examine site-specific problems, and make recommendations to protect manatees at water control structures and navigational locks.

Habitat Protection and Restoration

Intensive coastal development throughout Florida poses a long-term threat to the Florida Manatee. Three major approaches have been taken to address this broad problem. First, USFWS, 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 in order to minimise and mitigate their impacts. Under section 7 of the ESA, USFWS has annually reviewed hundreds of permit applications to the ACOE for construction projects in waters and wetlands that include or are adjacent to important manatee habitat.

A second approach is the development of county manatee protection plans, in coordination with and approval from FWC. 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. Manatee protection plans include components on boat facility siting policies, law enforcement, education/outreach, and habitat protection (FWC 2007).

A third approach to habitat protection is acquisition of environmentally sensitive land or conservation easements to limit development and permitted uses on those lands. Both the USFWS and the State of Florida have acquired new areas containing important manatee habitat, adding to federal and state protected area systems. Both the State of Florida and USFWS are continuing cooperative efforts with a view towards establishing a network of important manatee habitats throughout Florida.

As noted above, seagrass as well as freshwater SAV have suffered declines in many regions of Florida. Efforts are underway to improve water quality by reducing nutrient inputs from land-based sources, as well as to remove legacy nutrient loads by dredging muck deposits in some waterways (e.g., IRL). Summarizing these efforts is beyond the scope of this assessment. It is well-recognised, however, that improving water quality is key to successful forage habitat restoration (e.g., planting seagrass or freshwater SAV). A sampling of current manatee habitat restoration projects can be found here: https://myfwc.com/wildlifehabitats/habitat/ahcr/manatee-projects/.

To address concerns about the long-term security of manatee warm-water habitats, efforts have been and continue to be made to protect Florida’s springs and spring runs, where feasible. Many of the springs used by manatees are now in public ownership. Seasonal use restrictions regulate human activities at many, but not all, important sites. Spring flows are supposed to be protected through the State of Florida’s adoption of minimum flow regulations, which (theoretically) would lead to limits on water withdrawals from important spring recharge areas if flows drop below pre-determined thresholds. Minimum flows have been established for most springs important to manatees.

A number of projects have been undertaken to restore springs and spring runs. Some spring runs have been dredged to remove human-caused sedimentation, obstacles have been removed, and banks have been stabilised to minimise erosion. To prepare for the eventual loss of industrial warm-water sites, temporary power plant shutdowns have been closely monitored to evaluate manatee response to these disruptions. When certain key power plant discharges were temporarily eliminated during plant modernisation and repowering, interim heating systems were built and operated by the utility for multiple winters to provide warm water to manatees during cold weather (i.e., ambient water temperatures <16.1° C) (e.g., Reynolds and Scolardi 2016). A passive thermal basin has been created near Port of the Islands in southwest Florida—by digging deep pools that tap into warm, saline groundwater—to replace the pending loss of a nearby warm-water site (Edwards et al. 2021). Managers have finalised a long-term plan that outlines strategies for securing a sufficient warm-water habitat network within each of the four manatee management units (Valade et al. 2020); efforts are underway to implement this plan.

Manatee Rescue, Rehabilitation and Release

Thousands of reports of distressed manatees purportedly in need of assistance have been made to the FWC and other resource protection agencies by a concerned public. While most of the manatees do not require assistance, an average of 108 manatees (min-max = 66–159) have been rescued and treated annually over the past decade (2013–2022) (FWC, unpublished data, https://myfwc.com/research/manatee/rescue-mortality-response/statistics/rescue/yearly). A record 159 manatees were rescued in 2021, with many debilitated due to the loss of seagrass on the east coast. Currently, a network of state and local agencies and private organisations (the Manatee Rescue and Rehabilitation Partnership, or MRP)—with oversight from USFWS—evaluates, rescues, and treats these animals (https://www.manateerescue.org/). Reasons for rescue include: cold stress, injuries incurred from boat strikes, injuries from entanglements in crab trap lines and monofilament fishing line, orphaned animals, red tide poisoning, entrapment in culverts and other structures, starvation or malnutrition, and other natural and man-made factors. Programme veterinarians and staff have developed and refined treatments for these animals and have been remarkably successful in their efforts to rehabilitate them. From 1973 through 2019, more than 2000 manatees were rescued (including some that were assisted and released on site); after successful rehabilitation in oceanaria, 676 manatees have been returned to the wild and naïve animals are tracked for up to a year via satellite-linked telemetry (Adimey et al. 2016, FWC, unpublished data).

Media coverage of manatee rescues and releases helps to educate millions of people about manatees, the life-threatening problems that they face, and actions that can be taken to minimise the effect of anthropogenic activities on this species. In addition, many millions of people have had the opportunity to see manatees up close at oceanaria and to participate in manatee education programmes sponsored by several parks. The publicity and outreach inherent in these programmes lead to substantial public support for efforts to protect and recover manatees in Florida.

Public Education, Awareness, and Support

Government agencies, oceanaria, environmental groups, and power utilities have all contributed to manatee public awareness and education efforts over the past several decades. The public has learned about the biology and status of manatees, urgent conservation issues, and the regulations and measures required to assure their protection through the distribution of brochures, posters, videos, press releases, public service announcements, and other media. Outdoor signs have been produced that provide general manatee information and highlight the problems associated with feeding manatees. Several agencies and organisations provide educator’s guides, posters, and colouring and activity books to teachers in Florida and across the United States. Their staff also give many presentations to schools and citizen groups each year and distribute educational materials at festivals and outreach events. Information on manatee viewing opportunities has also been made available to the public.

Many public awareness materials have been developed specifically focusing on boater education. 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. Researchers and managers help to educate law enforcement personnel about manatees and inform them about available outreach materials that can be distributed to user groups. Monofilament fishing line collection sites and clean-up efforts have been established. All such efforts are essential for obtaining public compliance with conservation measures to protect manatees and their habitats.