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Grand Canyon fish stabilizing

Published: Thursday, Aug. 31 2006 12:00 a.m. MDT

GRAND CANYON NATIONAL PARK, Ariz. — The population of an endangered fish in the Grand Canyon area of the Colorado River may be stabilizing, according to biologists with the U.S. Geological Survey.

The number of adult humpback chub appears to have stabilized at about 5,000 fish, according to research by federal biologists.

"It means that conditions exist in Grand Canyon that allow adult fish to reach reproductive age," said USGS biologist Matthew Anderson in a statement.

Until recently, the chub population in the canyon was steadily declining, because adult fish were dying at a rate of 15 percent to 20 percent a year, and young fish were not surviving in large enough numbers to replace them.

There also are signs of more juvenile fish in the past few years at the confluence of the Colorado and Little Colorado rivers, where the chub are known to spawn, the research showed. More young fish of three other endangered fish species have also been documented at the location.

The humpback chub population in the Grand Canyon dropped by about two-thirds between 1989 and 2002, from 10,500 to 3,500.

It was listed as an endangered species in 1967. The decline is blamed on Glen Canyon Dam near the Arizona-Utah border, which has lowered the river temperature in the Grand Canyon and eliminated much of the river level fluctuations and sediment deposits. Non-native fish, such as rainbow trout, that survive better in the colder water have added competition, as have other factors.

USGS biologists say the removal of large numbers of trout, drought-induced warming of the river water and a summer flow experiment from the Glen Canyon Dam in 2000 may have helped more fish reach adulthood.