Colorado River’s humpback chub downlisted from endangered to threatened

Humpback chub

The humpback chub, one of four endangered fish in the Colorado River and its tributaries, is now a “threatened” species instead of “endangered.” 

The status takes effect on Nov. 17. 

“We are reclassifying the humpback chub from endangered to threatened (i.e. ‘downlisting’) because we have determined that the species is no longer in danger of extinction throughout all or a significant portion of its range,” said Matt Hogan, acting regional director for the U.S. Fish and Wildlife Service.

According to the federal Endangered Species Act, a species is endangered when "there is a danger of its extinction throughout all or significant portions" of its range. A “threatened” species is likely to become endangered in the “foreseeable future.” 

After the USFWS conducted a “thorough review of the best available scientific and commercial information available,” it determined that since only one of six original populations has increased, the fish was no longer endangered throughout its range. Of the original six populations that existed in 2002, one has since died out.

Along with the humpback chub, the Colorado pikeminnow, bonytail chub and razorback sucker were originally listed as endangered due to habitat loss from the construction of several dams. Climate change has now made the situation worse. Only the humpback chub has been downlisted.

The humpback chub, having evolved in warm, muddy waters, normally grows to a maximum size of about 20 inches and 2.5 pounds.

In the Grand Canyon, the chub’s growth is stunted by the cold water flowing out of Glen Canyon Dam and is only doing well in the warmer waters of the Little Colorado River. 

The humpback chub is part of the minnow family, and by minnow standards it is considered a big fish, dwarfed by the Colorado pikeminnow. In normal conditions, a humpback chub can survive more than 30 years in the wild.

The humpback chub was first listed as endangered in 1967. Its numbers started dropping shortly after the construction of three dams on the Colorado River drainage:  Glen Canyon, Hoover and Flaming Gorge.  

Originally, six populations were identified, limited to the Colorado River and three of its tributaries: the Green and Yampa rivers in Colorado and Utah, and the Little Colorado River in Arizona. 

According to 2002 data from the USFWS, 2,000 to 3,000 adults were estimated to exist in the Black Rocks and Westwater canyons in the Colorado River near the Colorado/Utah border. An estimated several hundred to more than 1,000 adults existed in Desolation/Gray Canyon in the Green River. 

Populations in Yampa and Cataract canyons were small, each consisting of up to a few hundred adults. The Yampa population has since died out, according to the Center for Biological Diversity, leaving five populations today.  

The largest population of humpback chub is found in the Grand Canyon in the Little Colorado River and its confluence with the main stem Colorado River.

In 2009, the U.S. Geological Survey announced that this population in the Little Colorado River had increased between 2001 to 2008 to between 6,000 and 10,000 adults, the only population to show an increase. 

Recently, there was a proposal to build two dams in the Little Colorado River canyon, but these projects have not moved forward, in part because of the chub’s presence. Instead, proposals to pump out of springs in the Little Colorado drainage remain, making the future of the chub uncertain.  

The chub’s mortal enemy is the smallmouth bass, a non-native species that thrives in the rivers above Lake Powell. So far, smallmouth bass have not established themselves in the Colorado River below Glen Canyon Dam. With reservoir levels dropping, the river in the Grand Canyon will likely become warmer and make it more habitable for the smallmouth bass.  

Instead, the brown trout threatens the humpback chub. Brown trout are non-native and are increasing in number and constitute around 25% of current trout catches in the Lees Ferry area.

The rest are rainbow trout, considered to be a less severe a threat to the chub. Glen Canyon NRA continues to pay a reward to anglers who catch and kill brown trout, but so far the policy has not suppressed their numbers.

Other options to help the humpback chub, discussed by a technical working group advising the government on the management of Glen Canyon Dam, is to build hydropower turbines in the dam's bypass tubes to allow colder water from the depths of Lake Powell to flow down the Grand Canyon when needed. 

However, this would reduce the capacity of the bypass tubes when they are needed as they were in 1983 when Lake Powell rose higher than the design height of 3,700 feet.

With only one population out of the five remaining populations of humpback chub on the rise, some question the wisdom of downlisting. 

In a press release dated Jan. 21, 2020, the Center for Biological Diversity said that “in delisting the chub, the (US Fish and Wildlife Service) only looked 40 years in the future, despite the fact that climate models extend to 2100 and paint a grim picture of water availability in the Colorado River.” 

Alicyn Gitlin, the Sierra Club’s Grand Canyon program manager, said, “This is a political decision, not an ecological decision.” 

The threatened status still bars people from killing it and keeps the weight of the Endangered Species Act behind is conservation. Should an angler accidentally catch a humpback chub, it is expected that the fish be carefully removed from the hook and put back in the river.

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