Wildlife researchers first spotted the trend as the winter weather set in late last year. Florida manatees — the peaceful, lumbering marine mammals iconic to the Sunshine State — were dying in alarmingly high numbers. Many washed up emaciated, indicating they’d starved to death.

It didn’t take long to identify a likely cause. Florida’s community of manatee conservationists has warned for years that water pollution chokes off the sea grass that makes up the bulk of the manatee diet. The problem was now so bad that the underwater pastures in one manatee hot spot were almost completely wiped out.

An update last week from state wildlife officials captured the full magnitude of the devastation: At least 761 Florida manatees — more than 10 percent of the estimated manatee population — have perished so far this year, already surpassing the total manatee deaths recorded in 2020. The current manatee die-off could top 1,000 by year’s end, experts say, exceeding the recent high of 824 deaths in 2018 and threatening to upend the fragile recovery the species has made.

“We’ve never seen anything like this before,” said Jaclyn Lopez, Florida director at the nonprofit Center for Biological Diversity, which works to protect endangered species. “I think it’s fair to call it a crisis. It’s not hyperbole when you see hundreds of manatees dying like this.”

Officials declared the deaths an “unusual mortality event,” which the federal government defines as a significant die-off of any marine mammal that “demands immediate response.” Scientists say pandemic-related reductions on monitoring activity may have prevented researchers from picking up on the problem sooner.

State wildlife commissioners and private groups are considering a suite of potential solutions that range from replanting vegetation to rounding up sickly creatures en masse and rehabilitating them. But the gradual destruction of the manatees’ habitat, combined with the long-term menace of climate change, means there’s no panacea.

Left unchecked, the fallout is all but certain to stretch beyond the manatee population, experts say. Manatees are often referred to as a “sentinel species,” meaning their health serves as an indicator for the welfare of other flora and fauna in the state. Manatee grazing makes sea grass beds more productive, which in turn attracts a greater diversity of organisms to their habitats. If their numbers decline, other plant and animal populations will suffer, too.

“They’re kind of like the gardeners of the aquatic ecosystem,” said Patrick Rose, a biologist and executive director of Florida’s Save the Manatee Club. “And they’re just so defenseless.”

Until recently, the West Indian manatee found on the East Coast represented a ecological success story. They faced extinction in the 1970s, when only a few hundred remained. But decades of intensive conservation efforts have helped them rebound to more than 7,000.

Citing the improving numbers, the Trump administration in 2017 downgraded the species from “endangered” to “threatened” under the Endangered Species Act. The move was decried by conservationists and some Florida officials, who said the Interior Department ignored ongoing threats such as habitat loss and rising injuries from boats. They say this year’s die-off all but confirms their fears that the reclassification was premature.

Water quality degradation emerged as a top concern long before manatee deaths reached their current levels.

Runoff from farming, pesticide sprays, sewage treatment, leaky septic systems and other human sources causes an excess of micronutrients such as nitrogen and phosphorus to build up in the water. Massive algae blooms form. Eventually, they grow so large that they deplete the water of oxygen and block out the sunlight sea grass needs to thrive. What’s left are murky wastelands with little for the manatees to eat.

Year after year has brought dire warnings, but no protective measures have managed to reverse the recent upward trend in manatee deaths.

Experts said there wasn’t an easy explanation for why manatee mortality spiked so dramatically this year. Rather, they said, it appears to be an accumulation of environmental ills, with the diminishing supply of sea grass playing a major role, in addition to such other factors as boat strikes and cold stress.

“I think this is a point on a long, linear trajectory,” said Lopez, of the Center for Biological Diversity.

The situation is particularly dire in the Indian River Lagoon, a 150-mile waterway along Florida’s Atlantic Coast that ranks among the most biodiverse estuaries in the country. Researchers estimate that more than a quarter of the manatee population flocks there during cold months, many drawn in by warm water discharge from a power plant in Titusville, Fla.

Manatees won’t tolerate water that’s lower than 68 degrees, so once they arrive, they tend to stay until the ocean warms again — even if food is in short supply. In previous years, the region was blanketed in tens of thousands of acres of sea grass, making it an ideal place to spend the winter, said Rose, of the Save the Manatee Club. Vast tracts of that vegetation have disappeared since 2011, when protracted algal blooms began to smother the estuary, according to state water regulators.

“Before, they could forage and stay warm in the water from the power plant,” Rose said. “But they kept losing more and more sea grass.”

Central Florida also experienced an unusually cold winter this year. That plus the lack of food created awful conditions for the animals, Rose said. “They came into this winter already malnourished,” he said. “They didn’t have the resources to go out and feed on those warmer days. That resulted in this more massive starvation.”

Researchers might have noticed the advanced state of manatee malnutrition earlier had conservation work not been hampered by the coronavirus pandemic, according to Rose. Wildlife officials and conservation groups spent less time monitoring and tagging the animals, he said.

“A lot of that work didn’t happen,” Rose said. “The fact that it had gotten that bad was not detected.”

But the work is picking back up. The Manatee Rescue and Rehabilitation Partnership — a cooperative that includes Save the Manatee Club, state zoos and wildlife regulators — has helped nurse numerous manatees back to health this year. In May, a male who came in more than 200 pounds underweight was released in central Florida’s Salt Springs.

The Florida Fish and Wildlife Conservation Commission says it has rescued 90 manatees so far this year, nearly topping the number rescued in each of the two previous years. The agency, which conducts the majority of manatee health assessments along with private groups, says it’s working with federal officials to determine what caused the “unusual mortality event” and is exploring possible responses such as habitat rebuilding.

“Environmental conditions in portions of the Indian River Lagoon remain a concern. Preliminary information indicates that a reduction in food availability, sea grass, is the primary factor in this event,” the agency said in a statement late last month. “We will continue with a comprehensive investigation and share information as it becomes available.”

Conservationists are also working out ways to stave off another cold-weather catastrophe in the coming months. Replanting and nutrient filtration projects are underway. Rose and other advocates say they’re also calling on the U.S. Army Corps of Engineers to temporarily stop using sprays to remove plant buildup that restricts boater access so that manatees may have an alternative food source.

Another possibility involves finding a safe facility for large numbers of sick or injured manatees to convalesce. A manatee rescue committee is meeting in the coming weeks to weigh whether an old fish hatchery or other sites could do the trick, housing 100 or more manatees through the winter.

Rounding up the gentle giants would be a major undertaking — trapping them and transporting them poses dangers to both manatees and humans. But all options need to be on the table, Rose said.

“Next winter really has to be quite different. We have to be on top of this,” Rose said. “They’re dependent on us.”

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