CHEYENNE, Wyo. - Add another concern for the tanker plane pilots who barnstorm low over treacherous terrain, in vintage aircraft, to bomb fire retardant around raging mountain wildfires: Endangered species.

New U.S. Forest Service rules for the use of fire retardant in dozens of national forests seek to prevent the millions of gallons of fire retardant spread over the landscape every year from poisoning streams and killing off protected plants and fish.

Forest Service officials insist the new rules won't hinder firefighting. The company that operates almost half of the U.S. private fleet of large tanker planes agrees, for the most part.

"It is an increasing workload, there's no doubt about that," said Dan Snyder, president of Missoula, Mont.-based Neptune Aviation Services, which operates eight Lockheed P2V planes.

"It may reduce the speed at which they can affect the fire because they do need to take those few extra minutes to study the charts and plan on how they can put the retardant on the ground and still comply with the rules."

The group that brought about the changes by filing suit says the rules aren't as big of an issue as whether fire retardant even works.

The Forest Service has never proven in the field that fire retardant is effective, said Andy Stahl, executive director of Forest Service Employees for Environmental Ethics in Eugene, Ore.

"Why use it if it's not effective? If it's not effective, I don't care if it's environmentally benign. It's a waste of money and firefighters' lives," Stahl said.

"The case for retardant use is not sufficiently strong to offset the environmental effects."

Rubbish, say Forest Service officials, who cite decades of rigorous laboratory testing and relate the accounts of plenty of ground and aerial firefighters who insist that fire retardant not only works, it works well.

"When enough people in enough places say retardant helps, we have to believe they're not making it up," said Cecilia Johnson, fire chemicals technical specialist at the agency's Missoula Technology and Development Center in Montana.

The lab assesses not just the efficacy but the toxicity of fire retardant, which is blended into water and dumped from airplanes onto fire as a slurry mixture.

"All of the retardants as concentrates are practically nontoxic. They're even less toxic by the time they're diluted," Johnson said.

So who's right? The finer points of the debate get complicated.

Fire retardant doesn't attempt to put out wildfires or even necessarily halt flames in their advance. Consisting primarily of ammonium phosphate - fertilizer, basically - fire retardant is formulated to slow down the combustion of trees, brush and grass.

The idea is to give firefighters time to mount a ground attack. The ground forces clear away flammable material in a wide line around the edges of the fire. They hem in the flames and eventually a soaking rain falls or the fire just burns itself out.

Often, even a fully contained high Rockies wildfire will smolder, sputter and flare for weeks or months, into autumn and the first significant snows.

The U.S. Forest Service spent $19 million on 23 million gallons of retardant last year, which was unusually busy for wildfires.

Documented cases of fish killed by fire retardant are relatively rare. But they've happened.

"We've observed streams for miles be sterilized of all their fish life. Tens of thousands of fish can be killed in one dump," Stahl said.

In 2002, a slurry bomber inadvertently dumped between 1,000 and 2,000 gallons of fire retardant on the Fall River about 25 miles south of Bend, Ore. The retardant immediately killed all of the river's fish, an estimated 21,000 mainly juvenile brown trout, redband trout and mountain whitefish over a six-mile stretch.

The fish population began to recover after a couple years, said Steve Marks, a fish biologist for the Oregon Department of Fish and Wildlife.

"It did not have long-term impacts to the habitat itself. But it did have immediate impacts to the population," Marks said.

Stahl's group filed the first of two lawsuits over the Forest Service's fire retardant policies in the aftermath of the Fall River spill.

Meanwhile, the die-off took Johnson at the fire retardant lab by surprise. The culprit was sodium ferrocyanide, which became toxic to fish under certain environmental conditions.

"We found out the hard way there was a small amount of chemical in some of the products that did have this characteristic," Johnson said.

Sodium ferrocyanide no longer is used in fire retardant. These days, the more likely fish-killer would be ammonia. In 2009, ammonia in fire retardant dropped on wildfires near Santa Barbara, Calif., killed 50 protected steelhead trout in the Santa Ynez River.

Fire retardant manufacturers have cut the ammonium in retardant by half over the last decade without sacrificing effectiveness, according to Johnson.

"It's actually better. It's as good or better," she said.

Meanwhile, very few fish poisonings have been documented. Even Montana U.S. District Judge Donald Molloy, in siding with Forest Service Employees for Environmental Ethics in its second lawsuit over fire retardant, pointed out in his 2010 ruling that only 14 of 128,000 retardant drops over eight years killed protected fish or plants.

Ammonia in watersheds from fire retardant is not a human health risk.

The new Forest Service rules that resulted from the lawsuit require fire retardant drops to come no closer than 300 feet from streams and lakes except when human safety is at risk. Fire retardant also can't be dropped in areas with endangered or threatened plants.

Many endangered Rocky Mountain plants are adapted to thrive in very limited habitats with poor soils. Fire retardant can encourage the growth of invasive weeds that can crowd out native plants that otherwise would have a competitive advantage, said Glen Stein, a Forest Service fire ecologist who led the effort to write the new rules for fire retardant.

Stahl said the Forest Service hasn't proven with field studies that fire retardant helps.

"We throw it on for the air show," Stahl said.

There's a reason why fire retardant isn't tested in the field, said Stein.

"The problem that I have with what Andy wants us to do, is out there in the wild, you have so many variables that are constantly changing. You've got slope, you've got aspect, you've got wind, you've got temperatures. It's hard to know what the result of the retardant is versus some of the other variables," he said.

Anyway, he's seen fire retardant work in the field, such as when a DC10 tanker plane dumped thousands of gallons at a California fire a few years ago.

"Five miles long, I drove that whole road where we dropped it. There were two little spots where it went into the retardant and stopped. And the rest of it just stopped at the retardant. So I know it's effective," Stein said.