TANNER PARK — A dozen dead trout lay on the bottom of Parleys Creek a few yards upstream of the concrete bridge. Beneath the bridge were another six.

The trout, which ranged from fingerlings to about 5 or 6 inches, were a small portion of a fish kill discovered Monday, with the total likely more than 500.

"That's terrible," said Dan Archuleta, Canyon Rim, who was walking his three dogs on the nature trail. "I don't want to let my dogs off this leash."

Along a single stretch of the creek, Brock Richardson of the Salt Lake City chapter of Trout Unlimited, the Stonefly Chapter, counted 453 dead Bonneville cutthroat trout. He hiked the route with Mike Slater, aquatic biologist for the Utah Division of Wildlife Resources.

"Anytime you see more than a couple of dead fish, it's surprising," said Richardson.

Slater said other agencies also were investigating the kill, including the U.S. Fish and Wildlife Service, the Utah Department of Environmental Quality and the Salt Lake Valley Health Department.

Investigators first suspected that sediment, possibly from construction activities, had swept into the stream and the silt had killed the trout. They'd heard reports that silt was in the stream on Sunday, but by Monday afternoon it was as clear as ever.

"And the sediment down in Sugar House did not look bad," Richardson said.

Also, Slater and Richardson hiked upstream beyond the point where construction was taking place and found dead fish there too.

A fish disease is unlikely because the trout died at the same time. A landslide that released silt and the introduction of some chemical were the only other reasons experts could imagine.

At the park, Slater said officers searched diligently for live fish, hoping to get a better idea of what had happened. But they could find none.

"It appears to be a complete kill," he said.

"We did take water samples," Slater noted. They also collected a couple of the fish. Sampling results will not be available for a few days.

Meanwhile, experts checked the acidity of Parleys Creek on Monday and found that it is normal.

"Wow, we're really mystified about what could have happened," said DWR conservation officer Ray Loken, speaking in a telephone interview.

Bonneville cutthroat trout are a native species, surviving in scattered streams after ancient Lake Bonneville declined thousands of years ago. While the species is not listed as threatened or endangered, its population is low enough that the DWR is working hard to re-establish it.