Florida’s endangered manatees, already reeling from an unexplained string of deaths in the state’s east coast rivers, have died in record numbers from a toxic red algae bloom that appears each year off the state’s west coast, state officials and wildlife experts say.

The tide has killed 241 of Florida’s roughly 5,000 manatees, according to the state Fish and Wildlife Research Institute, and the toll appears certain to rise.

The number of deaths from the tide far exceeds the previous annual record of 151. Most occurred along the lower west coast of Florida near Fort Myers, where an algae bloom that began last fall was especially severe and long-lasting.

“Southwest Florida is an area where a lot of manatees are during the winter months,” Kevin Baxter, a spokesman for the research institute, said Friday. “It’s a warm-water area. The bloom has persisted there for quite a while.”