An Unusual Mortality Event from December 2020 to April 2022, when 1,255 manatees died along Florida’s Atlantic coast, is now officially over, according to the U.S. Fish and Wildlife Service.
UMEs are defined by the Marine Mammal Protection Act as “a stranding that is unexpected; involves a significant die-off of any marine mammal population; and demands immediate response,” according to the National Marine Fisheries Service. The UME closed out by the USFWS earlier this month was triggered by manatee starvation due to a historic loss of seagrass in the Indian River Lagoon and exacerbated by cold winter temperatures, according to the Florida Fish and Wildlife Conservation Commission.
The FWC reports it’s now been two years since researchers documented the last manatee death directly linked to starvation. That was in March 2023. But even though manatees have generally fared better since April 2022, an extensive review of all data available for the UME was still necessary before the USFWS could formally close it out.
“In part, UME’s are designated to understand the cause of each mortality event and the impacts on populations. Sometimes investigations extend several years due to the investigation complexity,” Jonathan Veach, a spokesman for FWC’s Fish and Wildlife Research Institute, wrote in an email. “For example, the 2013 Indian River Lagoon UME was closed 9 years later because the investigation into the cause was extensive and ongoing.”
Additionally, since the vast majority of manatee deaths during this UME happened in wintertime, researchers needed winter data for their analyses.
Researchers first observed improvements in the winter season of 2022-23, “but considering the fragile conditions of habitat and manatee recovery, we waited for winter 23-24 to confirm the positive trend continued,” Veach wrote.
The starvation UME’s long-term implications on the Florida manatee population remain unknown, according to the USFWS.
But Beth Brady, director of science and conservation for the Save the Manatee Club, said one positive sign is that manatees are reproducing again.
There weren’t many live manatee births during the UME, due to a lack of available seagrass forage for female manatees, Brady said. Then, last year, FWC logged a record-high number of perinatal deaths: a category that includes manatee calves, as well as any carcass measuring 5 feet long or less.
Although some of those perinatal deaths were stillborn calves, “we also saw live calf births,” Brady said. “This was also an indication that there is enough forage for these animals to return to reproduction.”.
Brady echoed a view shared by FWC’s marine mammal experts: that the 154 perinatal deaths logged in 2024 may actually be a good sign, as proof of the female manatees' ability to reproduce.
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Still, Brady remains concerned about the potential for future harmful algal blooms, or HABs, which triggered the UME that began in December 2020 by killing off most of the seagrass in the Indian River Lagoon. HABs “bloom” in the water at high nutrient concentrations. Those high levels are caused by things like leaky septic systems, and agricultural and stormwater runoff.
“Have those underlying problems been addressed? That's why this is a rather tenuous situation,” Brady said. “Even though there has been some increase in forage, particularly in the Mosquito Lagoon, we still have a long way to go.”
The Mosquito Lagoon had a particularly good year in 2024, with no HABs to speak of. Of the five basins making up the Indian River Lagoon, Mosquito was the only one found to be in “good” health, according to the Marine Resources Council’s annual lagoon health assessment.
Both the Mosquito and North Indian River Lagoon basins were determined to have “better” seagrass coverage than in 2023, meaning coverage significantly improved from one year to the next. Meanwhile, the Central Indian River Lagoon basin ranked “very poor” for seagrass coverage year-over-year.
“It takes approximately three years for seagrass to grow, and if you don't have the right conditions, it could potentially die off,” Brady said. “So to get back to where we were when we had these really lush seagrass beds in the Indian River Lagoon could take anywhere from 10 to 20 years.”
Seagrass isn’t only crucial for manatees; it also serves as a key indicator for environmental health more broadly, Brady said.
“Seagrass sequesters more carbon than do trees. This is a vitally important habitat that indicates the overall health of our ecosystems,” Brady said. “So when you have problems with those, you are going to see problems for humans as well. It's not just a manatee issue.”
For example, seagrass beds are also critical for healthy fisheries, which humans rely on as a source of scallops, clams and oysters, along with fish, Brady said. “If you don't have nurseries or habitats, how are you going to have good fisheries?”
Brady and other marine experts remain worried about the future of the Florida manatee, which is currently classified as “threatened” under the Endangered Species Act. The USFWS recently decided against upgrading the Florida manatee’s status to the more serious “endangered” designation. Brady finds that decision concerning, as do experts with the Marine Mammal Commission.
“Some of the best available science that we have is showing that forage in many of these areas [is] decreasing,” Brady said.
She said the USFWS relied on outdated criteria, from 2015 or earlier, to determine the Florida manatee’s current status.
“That's 10 years ago. That's not the best available science,” Brady said.
In a letter to the USFWS earlier this month, the MMC urged the agency to list the Florida manatee as “endangered” until it can finish a “finer-scale review” of its status and that of the Antillean manatee, an “endangered” manatee subspecies found near Puerto Rico.
