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Endangered Colorado River fish population surges

The humpback chub, a closely watched indicator of the Grand Canyon’s ecological health, has grown steadily in number since 2001 as changing conditions on the Colorado River have created a more hospitable habitat.

The population of the endangered fish grew by 50 percent over the past eight years, the U.S. Geological Survey reported Monday. By the end of last year, there were an estimated 7,650 adult chub, fish at least 4 years old, near the confluence of the Colorado and Little Colorado rivers. That’s up from about 4,000 fish as recently as 2000.

Scientists offered several possible factors for the higher numbers, including drought-related spikes in water temperature, the removal of non-native fish from the river and a series of experimental water releases from Glen Canyon Dam.

Put together, those factors essentially re-created some of the conditions that once supported larger populations of the chub.

“It may be that the synergy, the combined impacts of all of those, is the thing that helps humpback chub survive best,” said Matthew Andersen, a USGS biologist. “We have great confidence in the population trend. We’re still investigating the reasons behind it.”

The chub, found in just six locations on the Colorado River and its tributaries, has become a measure of the Grand Canyon’s overall condition in recent years. The chub’s numbers in the lower Colorado dwindled after the 1963 completion of Glen Canyon Dam shut off the river’s natural flow, altering the habitat.

Finding more fish in the river is encouraging, environmental advocates said Monday, but work remains to ensure the species’ long-term survival.

“This is not a result that should have us sitting back comfortably in our chairs,” said Nikolai Lash, Colorado River program director for the Flagstaff-based Grand Canyon Trust. “It should have us leaning forward, trying to figure out how to take advantage of whatever it was that led to a small improvement.”

A decision is expected in the next few weeks in a case the trust and others filed in U.S. District Court in Phoenix, challenging the government’s management of the river and the chub habitat.

The chub, named for a protruding hump on its back, can grow as long as 20 inches and can live for 30 years or more. It uses its prominent fins to glide through the water and find insects to eat. Over 4 million years, the chub evolved to survive in warm sediment-laden water.

The construction of Glen Canyon Dam to store water and generate electricity changed the fish’s environment on the lower Colorado. The river’s flow was controlled artificially and, because water was released from the lower depths of Lake Powell, its temperature cooled.

As a result, native-fish populations plummeted. Responding to lawsuits from environmental groups, Congress passed legislation in 1992 that ordered federal agencies to manage the dam in ways that would help restore habitat, but until about 2000, fish numbers remained low.

In 2001, the population started to grow, Andersen said. Scientists began looking at three factors:

• A long drought lowered water levels at Lake Powell, which allowed the sun to reach deeper into the lake and warm the water.

• Non-native fish have been removed from parts of the river where the chub live. Non-native fish compete for food and eat young chub. From 2003 to 2006, the non-native rainbow trout population near the Little Colorado confluence dropped 80 percent.

• The U.S. Bureau of Reclamation has conducted a series of experimental test releases from Glen Canyon Dam. Andersen said it’s possible some of those tests have helped improve conditions.

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