Seagrass has been decimated in the 251 kilometre Indian River Lagoon and neighbouring areas.

The aquatic plant thrives in clear, sandy water, but murkier water because of the algae and pollutants has made it harder for seagrass to survive, said Melissa Tucker, director of the Division of Habitat and Species Conservation at the Florida Fish and Wildlife Conservation Commission.

“Our state-wide death count from all sources has been higher than it’s ever been reported before,” Tucker told the House State Affairs Committee.

“This is a starvation issue. There’s not enough seagrasses that are available to the manatees.”

Officials noticed a sharp rise in manatee deaths from December through May, when the sea cows congregate in warm waters.

During that period, 677 manatees died, when typically only 156 die, Tucker said.

While manatee mortality levelled out after May, when the mammals extend their range in summer and fall, the state has recorded 968 manatee deaths in 2021, with more than two months left in the year.

The previous annual high was 830 deaths in 2013, Ms Tucker said.

Big manatee die-offs in past years have been attributed to more transitory events like algae blooms and unusually cold weather, but the seagrass problem could take longer to reverse, Ms Tucker said.

Efforts are being made to replant seagrass and restore clam and oyster beds so the molluscs can help filter the water, she said.

“This is something that we’re going to be trying to manage and improve over the course of years and maybe decades,” she said.

Republican State Representative Thad Altman, who represents Brevard County, where manatees typically thrive, said it will be difficult to regrow the seagrass unless the water gets cleared up.

He said the manatees are now even eating seagrass roots, permanently killing the aquatic plants.

“We literally have a catastrophe on our hands,” Mr Altman said.