Hundreds of sharks invade rivers and canals to escape toxic ‘red tide’

Strange footage shows hundreds of sharks swimming in canals in Florida as they flee from deadly coastal algae known locally as the "red tide".

In the clip, which aired on Fox 13 News, several species of shark such as nurse and lemon sharks are swimming with their fins poking through the water.

The uncanny sight, recorded in Buttonwood Harbour on July 26, has perturbed locals who say they have spotted "great big" sharks sometimes appearing beneath the smaller ones.

Janelle Branowner told the news channel: "You could literally walk across the canal on the backs of the sharks in the water.

"We don't have healthy water in the bay right now."

Her concerns were echoed by a shark researcher at OCEARCH, Dr Bob Hueter, who said the sharks were not in the canal through choice but because they were in peril.

Mike Heithaus, a biological sciences professor at Florida International University, told the Guardian: "You just don't normally see sharks piling up like that in these canals, they do go in there but not in the huge numbers that we're seeing reported.

"We don't know what the trigger might be for those sharks going to those areas, but the changes in the chemistry of the water, the oxygen being pulled out of the water, the toxins, combined with the amount of dead fish around, any of those could cause these big concentrations.

"It's not the kind of thing that you would see if it wasn't a big red tide event."

He added that it was time to start taking environmental problems seriously.

Some studies suggest high fertiliser use on farms near the coast may be liked to the Karenia brevis algae, known as red tide.

The scientist said: "We really need to start working very hard in Florida on addressing some of the causes of these blooms, too much nutrient getting into the water, and that can come from lots of different sources, so we really need to be working on all of it."

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