Scientists baffled over hundreds of fish 'spinning and whirling' dead onto South Florida shores

Scientists are puzzled over what's causing hundreds of fish to wash up dead in South Florida, including an endangered species native to the region.

The fish are "spinning and whirling" onto shore in the Florida Keys, according to the Florida Fish and Wildlife Commission (FWC), which is labeling the incident as an "abnormal fish behavior event."

But there are no signs of a communicable pathogen, the commission published in an update on Wednesday, based on fish necropsy data.

In October, fishers and fishing guides began to notice erratic behavior of fish spinning in circles and upside down, Mike Parsons, professor of marine science at the Water School at Florida Gulf Coast University, told ABC News. The university is working with the FWC, the Florida Department of Environmental Protection and other agencies to determine the source of the unusual behavior.

Dissolved oxygen, salinity, pH and temperature are not suspected to be the cause of the fish behavior or kills, according to the FWC. Red tide toxins have not been detected in water samples either.

The Florida Department of Environmental Protection conducted a lot of analyses looking for different pollutants, pharmaceuticals and other chemicals and nothing showed up, Parsons said.

A different kind of harmful algal bloom or neurotoxin could be at play, Parsons said, adding that the affected fish stopped spinning when they were placed in "clean" water. Researchers are essentially approaching the tests as a "needle in a haystack," he said.

"This is unusual," Parsons said. "We really haven't seen this before -- so trying to figure out what's going on is a bit of a mystery."

The critically endangered smalltooth sawfish are the species among the fish deaths that marine researchers are most concerned with, Adam Brame, NOAA Fisheries' sawfish recovery coordinator, told ABC News. NOAA Fisheries has launched an emergency response effort due to the sawfish deaths and is attempting to rescue sick sawfish -- an complex effort that has never been attempted before, according to the agency.

Additional sawfish tissues are still being processed for analysis, according to the FWC, which sent 52 fish, including and 12 smalltooth sawfish to University of South Alabama for analyses.

While researchers have several theories on what could be causing the unusual fish behavior, it is too soon to speculate, Brame added.

"As it stands currently, it's a mystery that's yet to be solved," he said.

Dozens of carcasses of smalltooth sawfish have been among the piles of dead fish, according to the FWC.

There have been at least 28 confirmed sawfish mortalities and 265 reports to the hotline for fish deaths, according to data from the FWC.

Most of the sawfish mortalities have been in the lower Florida Keys, but wildlife agencies have begun to get reports of sawfish displaying similar symptoms outside of the Keys, including near Everglades National Park, Brame said.

NOAA Fisheries has been getting regular reports of affected sawfish since January, Brame added. Health officials are warning residents to not eat any spinning fish or fish caught in areas where spinning fish were reported, Parsons said.

It is unclear how many smalltooth sawfish are left in the wild, Brame said. They historically ranged from North Carolina to Texas, but they are now only found regularly in South Florida.

The population of this species was decimated as sawfish were captured in fishery bycatches, Brame said. Coastal development has also impacted their habitat, as they rely on shallow steam waters for their nurseries.

"It reduces the amount of habitat that's available for these mothers to come in and concert them," Brame said.

The smalltooth sawfish was the first marine fish to receive protections under the Endangered Species Act in 2003.

Genetic analysis of the species has shown that a limited number of breeding females could be the reason why the smalltooth sawfish population has been unable to recover quickly, Brame said.

Brame stressed the importance of public participation and urged anyone who sees unusual behavior to report it to the hotline at 1-844-4SAWFISH (1-844-472-9347) or Sawfish@myfwc.com.