A mysterious outbreak that sickened or killed fish in the lower Florida Keys, including 54 dead endangered sawfish, appears to be largely over, state scientists said Tuesday.
Only one sighting of spinning fish and no thrashing sawfish have been reported since June.
But exactly what caused the startling behavior remains unknown. It’s also not clear whether the spinning fish, which mostly survived, and dying sawfish were sickened by the same thing, said Tom Matthews, a biologist who manages the Florida Fish and Wildlife Research Institute’s Marathon lab.
“Just because it's occurring at the same time and place might not mean it's the same cause,” Matthews told the Florida Keys National Marine Sanctuary water quality committee.
READ MORE: One of the rarest fish on the planet is dying in the Keys. Scientists are struggling to find out why
Spinning fish were first spotted struggling in waters around the Lower Keys in October. The fish included a wide variety of species from tarpon and permit to stingrays and mullet, suggesting whatever was making them sick was not what they ate but the water where they swam. By late January, the first dying sawfish appeared.
A group of scientists, scrambling to keep up with growing reports of sawfish waving their chainsaw-shaped snouts in shallow water, had already formed a working group to look for a cause. They quickly ruled out common suspects, including red tide, low oxygen triggered by high temperatures or chemical spills.
As the weeks dragged on, and reports of distressed sawfish climbed to nearly 190, they focused on ciguatoxins found in tiny algae. Numerous species and elevated levels of the toxin were detected in samples. University of South Alabama researcher Alison Robertson, who specializes in harmful algae, was even able to replicate the illness in fish exposed to Keys water in her lab.
But even that led to more questions, Matthews said.
“The water in the Florida Keys has a soup of these algaes and the toxins associated with them,” he said. “So we have not figured out which one, or if it's a combination of these harmful algaes [that] are the causative agent.”
While ciguatoxins are most known for causing ciguatera, the disease in humans caused by eating fish carrying the toxin, the outbreak did not trigger an increase in human illness, Matthews said. It’s also unknown why fish that can normally tolerate ciguatoxins would fall ill.
The algae are common, but it’s also not clear what caused them to begin harming fish, he said.
“Was there something causative that allowed these harmful, benthic algaes to be more prevalent this winter?” he said. “Certainly last year was that unprecedented marine heatwave.”
The blistering heat wave bleached coral across the Keys and left a swath of other dead marine life: sponges, sea fans and sea grass. Ciguatoxins are opportunistic, Matthews said, so they could have expanded and taken over space where things died. But so far, scientists have not been able to make the link that the soaring temperatures — one gauge near shore in an enclosed bay topped 100 degrees — to the change in ciguatoxins. That’s keeping everyone on alert for signs of another heatwave this summer, he said.
“There is a mix of thoughts among the experts in this field: Are we primed to have it happen again, or that it was a one time thing,” he said. “I hate to say that we will wait and see, but we've got that network of reporting, standing by and watching it.”
Meanwhile, Robertson and Florida Gulf Coast University researcher Mike Parsons are continuing to try to pinpoint the harmful ciguatoxin. They are also trying to determine whether sick fish truly recovered or continue to be impaired.
“They seem to have short-term behavioral responses,” Matthews said. “Sometimes, when the tide changes [and] the flood tide comes in, possibly diluting the water, the animals regain their normal behavior.”
But others also showed continued signs of illness when stressed by either handling or shining lights on them at night. Sawfish researchers are also studying samples from 40 necropsied sawfish to determine why they died and if it was related to the ciguatoxins. They took the unprecedented step of trying to rescue a sick sawfish in March, but ultimately had to euthanize it when the fish's condition worsened.
They also hope to get clues about their recovery from sawfish newly tagged with tracking devices during the event.
“We were in a very good place thinking this population had started to expand until this event,” he said. “We don't know the impact on the population, but we are very worried about it.”
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