Mystery swirls around dolphin deaths, 105 wash ashore during March

Mystery swirls around dolphin deaths, 105 wash ashore during March

By Kevin Lollar
Gannett News Service

Florida Today

18th April 2004

It's a mystery: The Case of the Dead Panhandle Dolphins.

The suspects: Red tide toxin -- the same poison that killed 149 manatees in Southwest Florida from March 5 to April 27, 1996, and continues to plague area waters -- and a toxin produced by several species of diatom, a type of single-celled alga.

Atlantic bottlenose dolphins began washing up on Panhandle beaches March 11. By the end of the month, the carcass count had reached 105.

A curious fact: The dolphins bore no outward signs of disease or injury. By all appearances, they were healthy animals. Except they were dead.

"Part of the reason biotoxin was suspected is that the dolphins were all healthy looking," said Alex Costidis, a marine mammal biologist at the state's Marine Mammal Pathobiology Laboratory in St. Petersburg. "They had healthy fat storage; there was a pregnant female, and everything looked great. It was a wide range of healthy-looking animals. Whatever killed them, killed them quickly."

Other evidence pointed to biotoxin.

High concentrations of brevetoxin -- which is produced by the red-tide organism Karenia brevis (aka K. brevis) and low levels of domoic acid, produced by the diatom Pseudo-nitzschia -- were found in the stomachs and blood of some of the dead dolphins.

A leading theory is that the dolphins died after eating fish that had accumulated the toxins in their tissues.

But the cause-and-effect relationship is not complete.

Water samples taken off the Panhandle during the die-off showed little or no K. brevis and Pseudo-nitzschia.

In a follow-up study last week, Florida Marine Research Institute scientists Bill Richardson and Matt Garrett spent two days aboard the Florida Fish and Wildlife Conservation Commission patrol vessel JJ Brown sampling water from Carrabelle to Fort Walton Beach. The samples contained neither K. brevis nor Pseudo-nitzschia.

So, brevetoxin or domoic acid or both might be the culprits, but scientists don't know how the dolphins were exposed.

"The basic question up there is the fact that we're finding brevetoxin in the tissues but no K. brevis cells in the water," institute senior research scientist Cynthia Heil said. "We're finding Pseudo-nitzschia at low to moderate levels, but only a trace of domoic acid.

"My first guess is that it's a big ocean. That's one of the reasons I sent Bill up there: It could have been an offshore bloom; the K. brevis could have been trapped at the bottom; or it might have been a quick event that we missed."

Stomach clues

Another clue could come from the dead dolphins' stomach contents.

Early in the die-off, scientists examined the stomachs of eight dead dolphins, five of which were packed with menhaden, said Nelio Barros, manager of Mote Marine Laboratory's Stranding Investigations Program.

"Menhaden have been implicated in other mortality events," Barros said. "They're filter-feeding fish, so they accumulate the toxin. It's the combined effect that does them in. One menhaden won't kill a dolphin. It's like pizza: One slice won't hurt you, but if you're like me and go for two large pizzas, it will catch up with you."

The menhaden were eaten just a few hours before the dolphins died.

"They were not super-fresh, but they were fresh," Barros said. "So the impression we had was that the dolphins gorged on a school of menhaden, and a little while later, for whatever reason, they died.

"As we understand the mechanism, the toxins have to be released from the fish. So what we'd expect is not absolutely fresh, fresh consumed prey. It has to be in there long enough for the digestive process to release the toxin."

Inconclusive tests

This is not, however, positive proof that brevetoxin-laced menhaden killed the dolphins. In fact, scientists don't know how much brevetoxin constitutes a lethal dose for dolphins.

Necropsies or post-mortem examinations at the state's Marine Mammal Pathobiology Laboratory in St. Petersburg hinted inconclusively at brevetoxin as the possible killer.

When red tide toxin kills manatees, the animals show specific symptoms, such as runny blood, lungs congested and heavy with thick, ropy, blood-tinged mucous, and enlarged kidneys and livers. The dead Panhandle dolphins didn't show the same set of symptoms, Costidis said.

"There was some pretty runny blood, which seems to be one of the side-effects of brevetoxin," Costidis said. "But that could be something other than brevetoxin. Beyond that, no, there were no similarities."

Tests on dolphin tissue performed at the Armed Forces Institute of Pathology in Washington, D.C., indicate that neither virus nor disease caused the die-off, although more tests are being done.

While red tide is a familiar phenomenon in the Gulf of Mexico, domoic acid is the new toxin on the block.

Domoic acid

The poison, which has killed sea lions and sea otters in California, can render shellfish toxic to humans. Cases of domoic-acid-related illness, called amnesic shellfish poisoning, have been reported in the Northeast and Northwest but not in the Gulf area.

"We've never really seen domoic acid in the Gulf -- we've seen Pseudo-nitzschia, but not the toxin," said Blair Mase, NOAA-Fisheries Southeast stranding coordinator. "The more we learn, the more perplexing it becomes. Maybe it's a combination of brevetoxin and domoic acid. We're looking very closely at both."

Although scientists found massive concentrations of K. brevis during the 1996 manatee die-off, no dolphins died from red tide poisoning.

Scientists don't know why brevetoxin might have killed 105 dolphins in the Panhandle but no dolphins during the largest manatee die-off ever recorded.

"We're asking the same questions," Mase said. "A theory at one point was that the dolphins in Southwest Florida are more familiar with red tide, and when they see it, they don't feed in the area. So, maybe in the Panhandle it's a naive population that doesn't know about red tide.

"But when we looked at the records, we found out that red tide does occur in the Panhandle often, so maybe that theory doesn't hold true."

Not everybody thinks red tide played a part in the dolphin deaths.

Inconsistent mystery

Aboard the JJ Brown, Lt. Earl Whaley, a 25-year state marine patrol veteran, doubted the connection -- like many Florida natives, Whaley refers to dolphins as porpoise.

"I'm no scientist, but I've seen big patches of red tide out here, and there'd be miles and miles of dead fish, and the porpoise were feeding on them like crazy, and they didn't die," Whaley said. "I think something else is killing these porpoise."

Just one more inconsistency in the mystery.

"That's really confusing," Mase said of Whaley's observation. "Though the event is over, and we're jumping for joy, we still have a lot of work to do. We still have to answer questions like what constitutes the lethal dose of red tide for dolphins, what role domoic acid plays -- maybe it's a combination of the two. And we need to look a lot more at the world of harmful algal blooms."

So, two weeks after the dying stopped, investigators have a pair of likely suspects in the Panhandle dolphin case, but for now at least, the mystery remains unsolved.

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