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Cause of dolphin deaths remains elusive
By Kevin Lollar,

17th June 2004

Circumstantial case built against red tide
Although scientists have built a circumstantial case against red tide toxin, they still have no definitive culprit in the deaths of 107 bottlenose dolphins.

NOAA-Fisheries and a team of state and federal scientists and private research organizations and universities Wednesday issued a preliminary report on the die-off, which occurred between March 10 and April 13 off the Panhandle.

“Brevetoxin is the highest on our list of possibilities,” NOAA-Fisheries marine mammal veterinarian Teri Rowles said. “But we haven’t closed the case.”

Evidence points to brevetoxin, produced by the red tide organism Karenia brevis, or K. brevis. Necropsies (post-mortem examinations) showed high levels of brevetoxin in the stomachs of all the dolphins examined so far and variable levels in their tissues.

The first chamber of most of the dolphins’ stomachs were packed with undigested fish, indicating they died soon after feeding, and fish collected from the area during the die-off tested positive for brevetoxin.

The stomachs of some dolphins were full of menhaden.

“Menhaden target K. brevis, and the menhaden tested extremely high for brevetoxin,” said Jan Landsberg, a red tide expert for the Florida Marine Research Institute in St. Petersburg. “Menhaden could immediately vector that to dolphins.”

Besides brevetoxin, the stomachs, urine and faeces of some dolphins contained low to moderate levels of domoic acid, a toxin produced by the diatom Pseudo-nitzschia delicatissima.

But the levels were much lower than those found in marine mammals killed by domoic acid on the California coast recently. So scientists think the role of domoic acid in the Panhandle die-off is secondary, if it is a factor at all.

Despite the evidence, scientists can’t say for sure that red tide was the cause of the Panhandle die-off, partly because water samples taken during and directly after the event show little or no K. brevis.

Another problem is that, unlike manatees, dolphins that die from brevetoxin don’t show a specific set of symptoms.

“There are two ways to look at how marine mammals are exposed to brevetoxin,” Rowles said. “One is respiration; manatees inhale it, and you see pulmonary lesions, and inflammation of the nares (nostrils).

“We didn’t see that in the dolphins. The primary route seems to be ingestion. But we don’t see a specific pathology that lets us say, ‘Ah ha, this is how brevetoxin affects dolphins.’ ”

This is the second red tide-related dolphin die-off along the Panhandle in five years; during a 1999-2000 die-off, brevetoxin is blamed for killing 120 dolphins.

In the past decade, three red tide-related manatee die-offs have occurred in Southwest Florida, including the largest ever recorded when brevetoxin killed 149 manatees during eight weeks in 1996.

“It’s perplexing for us that brevetoxin has been associated with manatee mortality in Southwest Florida but not dolphin mortality,” Rowles said. “That’s the $64 million question. Why do we see repeated dolphin mortality events associated with dolphins in the Panhandle and not Southwest Florida, which is hit a lot more often?

“There are a lot of things we’re trying to tease out in the course of our investigation over the next few months.”