Fisheries biologists, joined by a handful of volunteers, have been spending their nights conducting a trout census in the Green River.

Bruce Schmidt of the Utah Wildlife Resources Division said Friday the annual survey is conducted under a contract with the federal Bureau of Reclamation to determine the impact of changing water releases from the Flaming Gorge Dam on downstream fish populations.The crews began the survey Wednesday, and the work will continue through April 27 on three one-mile stretches of the Green from just below the dam to the Little Hole area on the eastern edge of the Flaming Gorge National Recreation Area.

The captured fish are marked with a fin clip to make sure they are not counted more than once, he said.

One of the division's concerns is "there's no such thing as a natural situation on a river once you have a dam," said Schmidt. "Now we're looking at a host of things in trying to make the river flow as natural as possible."

But the Bureau of Reclamation uses dams such as Flaming Gorge for power generation and to meet utilities' peak demands in the middle of the summer and in the middle of the winter.

"And they even fluctuate releases during 24-hour periods," he said.

Division biologists believe trout evolved with natural flows "keyed into them. In the winter, their metabolic rate goes way down and they try to find a slow flowing section of the river they can survive in."

"Then the bureau releases a lot of water in the middle of the winter to meet heating needs and that makes the trout deal with fast water. What we're seeing is the smaller fish, they just seem to leave" the stretch of the Green River immediately downstream from the dam.

Since those are the trout the division hopes most fishermen would keep, the agency is using the information to adjust stocking rates, he said, and to work with the Bureau of Reclamation on modifying water-release patterns.

The stretch of Green River is considered one of Utah's top trout fisheries, with about 20,000 adult trout per mile of stream. But it is restricted to artificial lures only, and all trout 13 inches to 20 inches in length must be released.

The daily possession limits for other trout are two less than 13 inches and just one 20 inches or longer.