Scientists have long known that red tide can be deadly to manatees and suspected it was the same for dolphins, but did not know whether ingesting it could be fatal or how long it could remain lethal.
Study results published Wednesday shed some light – and it doesn’t look good.
Red tide toxins infiltrate the food chain, including sea grasses that manatees munch on and fish that dolphins eat, researchers said. And those toxins can remain in lethal concentrations for weeks, maybe months, after a red tide bloom has dissipated, they said.
The findings are the first evidence that manatees, whose vulnerability to airborne red tide toxins is well-established, can also be poisoned through their food. And it also connects dolphin deaths and red tide for the first time, with the implication that other animals that eat contaminated sea life – such as turtles and sea birds – also could be vulnerable.
Red tides are naturally occurring blooms of plant like algae known as Karenia brevis that can emit a neurotoxin. In high concentrations, red tide can be lethal to fish and marine mammals, make shellfish unfit for human consumption and can cause respiratory irritation in people.
The latest study looked at the spring 2004 deaths of 107 bottlenose dolphins in the Panhandle and a series of manatee deaths in southwest Florida in 2002.
Those dolphins died with stomachs full of undigested fish that were eaten alive, which researchers said was unusual and indicated the dolphins died quickly. Checking stomach contents, researchers found the same toxins in both the dolphins and the fish.
“No one thought a fish could accumulate high concentrations of toxins and still be alive,”
said Leanne Flewelling, a researcher in the Florida Fish and Wildlife Research Institute’s Harmful Algal Blooms group in St. Petersburg and co-author of the study.
In the 2002 manatee deaths, researchers looked into what the mammals had eaten because they began dying well after red tide blooms had dissipated. Researchers discovered red tide toxins can collect in high concentrations in epithytes, microscopic plants that live on seagrass blades.
So far this year, red tide has been blamed for at least 46 manatee deaths in Southwest Florida waters.
Researchers said the latest findings, while revealing, have raised even more questions that will require additional study. Among them: How the toxins get into epithytes, why some contaminated fish die and others survive, and how long – and at what level – red tide toxins can kill.
“We realize we still don’t know too much about the dynamics of red tide blooms,” said N