A decline in the populations of steelhead trout and other fish species in Malibu Creek has scientists wondering why the previously healthy schools of fish are dying.

Rosi Dagit, senior conservation biologist for the Resource Conservation District of the Santa Monica Mountains, said the death of trout and other fish in the creek below Rindge Dam is puzzling because there are no toxins in the water and nutrient and flow levels are typical for this time of year.

One year ago there was a bumper crop of baby steelhead trout in the creek—more than 3,000, according to Dagit.

“It was the best count we’ve had in years,” Dagit said. “By October 2008, we still had 1,300 (trout) in the creek. To find only 250 left, that’s really a dramatic drop.”

This year’s decline in fish is reminiscent of the massive fish deaths in 2006, Dagit said. Besides the environmentally sensitive species of steelhead trout, heartier populations of catfish, carp, crayfish and largemouth

bass died en masse that year. Since the California Department of Fish and Game and the National Marine Fisheries Department were slow to re

spond to the 2006 event, scientists weren’t able to determine if the dieoffs were the result of environmental toxins.

“We’re starting to see conditions occur again,” Dagit said of the similarities between the 2006 and current die-offs.

This time, Dagit and others at the Resource Conservation District have rallied scientists to discover why the fish are dying.

The volunteer force is made up of what Dagit called the “brain trust” from California’s leading public and private universities, environmental groups, the National Parks Service and other organizations.

Eighteen volunteers collected water and fish samples, hiking out of the Santa Monica Mountains with 26 gallons of water samples on ice in backpacks.

The office of Los Angeles County Supervisor Zev Yaroslavsky paid for air freight to transport the 350 pounds of water samples to the toxicology lab at U.C. Davis.

Dagit, who is preparing a report on the matter, doesn’t believe pollution has caused the fish to die. Rather, a “set of circumstances exist that are causing conditions that make it difficult for critters to live in,” Dagit said.

Lack of oxygen in the water may be one reason so many fish have died.

“It’s really important for fish to have oxygen in the water,” she said. “As temperatures go up, oxygen goes down. If muck is decomposing it will take the oxygen out of the water.”

While such conditions could cause stress to local fish populations, Dagit doesn’t believe it’s the whole explanation. “This is what they’ve lived in for eons,” she said. “Something else is going on.”

Several tests were conducted at the end of July and again in mid-August to gather data that officials hope will show why the fish are dying in such large numbers. Malibu muck, or decomposed algae and other elements, were tested, as was the temperature of the water. The muck, Dagit said in her report, had expanded during both observations.

“There were carcasses of dead crayfish littering the substrate,” Dagit wrote. “Only 188 trout were observed, down from a total of 1,300 counted in October 2008.”

Water flow was tested by the Resource Conservation District. The Tapia Water Reclamation Facility, which is operated by the Las Virgenes Municipal Water District, is required to release water into the creek each year but is prohibited from discharging water from April 15 to Nov. 15. However, if water drops below a certain point, the district may release water even during restricted periods. In Dagit’s report, she said that water flow fell below the required 2.5 cubic feet per second during the week of July 20, triggering the district to release more water.

David Lippman, the water district’ director of facilities and operations, explained that a National Pollution Elimination System permit between the water district and the Regional Water Quality Control Board calls for three consecutive low readings before they can request permission from the board to release water.

“We did that in late July,” Lippman said. “Eventually the creek flows came back up, and we stopped discharging. It has nothing to do with the fish dying.”

The water from Tapia is treated to a very high level so that it is nearly the quality of drinking water, Lippman said. “When we release water, there are no bacteria or chlorine,” he said. After the water is sterilized, chlorine is removed. “We meet all water quality limits before we discharge. The limits are set to be protective of the environment. We believe water going into Tapia is the cleanest source of water going into the creek.”

At the end of the summary report, Dagit said she does not believe the fish died because of typical chemical toxins; rather, the fish could be dying because of biological or environmental conditions. So far the water has tested negative for heavy metals, pesticides, herbicides, ammonia and other toxins.

“This is really important information and tells us that whatever is killing the fish and crayfish is not detectable by the water toxicology tests,” Dagit said.

Dagit and other scientists will compare notes and test results on Sept. 10 at the technical advisory committee meeting in Los Angeles.

“We’re hoping when we do the brainstorming session on Sept. 10 we’ll figure out what our next steps will be,” Dagit said.