What’s Killing Endangered Sawfish in Florida?

Fishing guides in the Florida Keys began reporting unusual sightings to Ross Boucek last fall. Small bait fish, especially at night, would start spinning in tight circles in the water, seemingly in distress.

As the months went by, more reports trickled in to Dr. Boucek, a biologist with the Bonefish & Tarpon Trust, a nonprofit conservation group. Bigger fish — jacks, snook — were swimming in spirals or upside down in the shallow waters of the Atlantic Ocean and Gulf of Mexico. So were stingrays and the occasional shark.

Dr. Boucek called scientists at state agencies and universities. They held meetings, took samples of the water and fish and tried to figure out what might be causing the fish to behave so strangely. A parasite? A sewage spill? Some other contaminant?

Then, in January, the mysterious ailment began afflicting smalltooth sawfish, a type of large, prehistoric-looking ray named for the look of its long snout lined with sharp teeth. The sawfish, which are endangered and reliably found only in southernmost Florida, started dying.

The search for answers became urgent, Dr. Boucek said, “the second an endangered species started dying off at unprecedented rates.”

He now spends much of his time in a wet suit, flippers and a snorkeling mask, collecting samples and recording data from sensors that he deploys along the sea bottom, looking for changes or patterns that might help solve the mystery.

At least 38 sawfish have died so far this year, according to the Florida Fish and Wildlife Commission, which is investigating the deaths. Perhaps only hundreds of breeding sawfish females remain in the wild, said R. Dean Grubbs, a fish ecologist at Florida State University. The fish can grow as long as 18 feet, according to the commission.

A research team led by state scientists has raced to conduct experiments, tag sawfish and sample their blood. Florida lawmakers designated $2 million in emergency funds to help carry out the work.

But scientists have not yet figured out what is going on. They have ruled out a few potential causes, including red tide, the toxic algal bloom that has led to past, massive fish kills.

Some wonder if last summer’s record-breaking sea temperatures, which bleached coral throughout the Keys, may have altered the ecosystem and triggered unusual microalgal growth.

In their best lead to date, they have learned that microalgae naturally present near the sea bottom have produced an elevated level of toxins that acutely affect the neurological systems of fish when they swim into those areas.

That might explain why spinning fish seem to recover when pulled up from the sea bottom (where toxin concentrations are higher) toward the water surface (where concentrations are lower), said Michael Parsons, a marine science professor at Florida Gulf Coast University. Sawfish are sea-bottom dwellers.

Since early April, the National Marine Fisheries Service has been trying to rescue and rehabilitate sawfish spotted in distress, a logistically daunting effort that the agency calls the first of its kind in the United States. The team rescued its first sawfish, an 11-foot male, on April 5, after a member of the public saw it swimming in circles in Cudjoe Bay. It is now recovering at Mote Marine Laboratory & Aquarium in Sarasota, in hopes that it can eventually return to the wild.

More than 150 sick sawfish have been observed since the crisis started. Whirling sawfish have been spotted as far north as Palm Beach County, but scientists have not linked their behavior to that of the fish in the Lower Keys.

Gregg Furstenwerth, who lives in Little Torch Key and has been spotting spinning fish for months, sharing videos on social media, said he glimpsed one struggling sawfish, about 14 feet long, late last month on a beach near Key West.

“My wife started crying,” he said. “I wish it was better. I’m sitting here watching the ecosystem tear itself apart, and I’m powerless to stop it.”

Whatever is happening threatens not only the endangered sawfish and other marine life — some 426 dead fish from more than 50 species have been reported to the state — but also the livelihoods of many in the Lower Keys whose jobs are connected to sport fishing.

Some fishing guides have had clients cancel their trips because they are worried that because people are worried that the fish they catch will not be safe to eat, Dr. Boucek said. The state says that people should not consume any fish that has exhibited abnormal behavior.

One of the microalgae species detected, Gambierdiscus, produces several toxins, including a compound responsible for a common form of fish poisoning in humans called ciguatera. But it typically does not sicken fish.

“It’s been a stressful few months, just trying to piece together a very complicated puzzle,” said Allison Delashmit, executive director of the Lower Keys Guides Association.

More than one thing may be to blame for the sick and dying fish, cautioned Alison Robertson, an associate marine science professor at the University of South Alabama. Fish in the Keys, where a number of toxins have been present for years, could be predisposed to behaving abnormally because of prior exposure.

“We actually think the combined effect of multiple toxins are causing the behavioral effects that we’ve been seeing,” Dr. Robertson said.

To collect fresh data, Dr. Boucek, 39, who lives on a sailboat in Marathon, in the Middle Keys, goes out on a boat every few days to check on sensors.

One recent morning, Capt. Nick LaBadie, a 33-year-old fishing guide, took Dr. Boucek to six sites around Sugarloaf Key, about 15 miles north of Key West, reading GPS coordinates to track sensors identified by floating buoys. The first site, nicknamed Tarpon Belly, was where some of the first spinning fish had been reported, Dr. Boucek said.

“You talk to these guys who are 70 years old, and they’re like, ‘I’ve never seen this,’” he recalled.

He donned his swim gear and dove in, squealing “Woo!” as he hit the chilly water. He cleaned the tip of one sensor and deployed another one. He could see clear to the shallow bottom but still looked out for bull sharks, which, he and Mr. LaBadie agreed, can be “very aggro.”

Back on board, Dr. Boucek recorded his work by hand, in pencil. He added fixative to preserve a water sample and placed it in a cooler.

“Every day, you think you have some kind of pattern, and the next week that pattern’s totally gone,” he said.

He failed to find a sensor at another site where the water was more turbid. But Dr. Boucek noted promising signs, including a nurse shark and a red snapper in the Sugarloaf Marina, where few fish had been seen for a while. Near Tarpon Belly, he spotted “mullet mud,” a dark patch created when feeding fish stir the sea bottom, for the first time this season.

He came upon no sawfish.

Not long after docking the boat and leaving the marina, however, word spread among fishing guides and scientists: A thrashing sawfish had washed up on a beach in Key West. Tourists watched it die.

Kitty Bennett contributed research.