GRAND JUNCTION, Colo. (AP) — Just as it approaches recovery, the endangered Colorado pikeminnow faces a new threat, a predator that eagerly scarfs down young pikeminnow, taking a jagged bite out of the species' overall numbers.

Unfamiliar predation on the pikeminnow comes from a species of fish that's native to the flatwaters of Canada and the northern tier of the United States. Walleye is a prized food fish, but its voracious appetite for pikeminnow is proving to be a setback to expectations that the pikeminnow could be removed from the endangered species list.

"Walleye have gone gangbusters in Lake Powell," said Tom Chart, a fisheries biologist who heads the Upper Colorado River Endangered Species Recovery Program for the U.S. Fish and Wildlife Service.

It's likely that walleye found their way up the Colorado from Lake Powell, Chart said. It's also possible that the hungry newcomers could have swum down from Rifle Gap and Elkhead reservoirs in Colorado and from two reservoirs in Utah, Red Fleet and Starvation, both near Vernal, said Henry Maddux, director of Utah's species recovery programs.

On a barely encouraging note, "We haven't seen walleye reproduce in the rivers," Chart said. "Yet."

Walleye nonetheless have taken up residence in waters where they don't belong, feasting on the young pikeminnow that hatched in the critical 15-mile reach of the Colorado River through the Grand Valley and washed downriver into the slower waters where for eons they have grown to become the apex predators on the river.

It's not just connecting the dots between fewer numbers of pikeminnow and the presence of walleye, Chart told The Daily Sentinel .

Fish biologists checking the stomach contents of walleye caught in the Black Rocks stretch of the river have found their guts stuffed with young pikeminnow.

Pikeminnow once were so numerous and large that when they moved upstream in large schools, they were dubbed "white salmon" by residents of western Colorado and eastern Utah, many of whom considered them a staple in hard times.

Once they reach a length of about 18 inches, pikeminnow can fend for themselves, but they're slow to mature and can't reproduce for as many as eight years. They can live for about 40 years and grow to 6 feet in length.

Walleye, however, appear to be interrupting the normal pattern, and the numbers of pikeminnow have taken a dip. After reaching a high of as many as 800 adults in the mainstem of the Colorado as recently as 2013, they're down to an estimated 400 or 450.

The Fish and Wildlife Service estimated in 2013 there might be as many as 2,000 adult pikeminnow in the Green River, down from nearly 4,000 in 2008.

That's enough of a stall, especially when combined with a similar drop in the Green River, that the Fish and Wildlife Service is tamping down expectations that it might downlist or delist the pikeminnow, Chart said.

"I think the service needs to see some stabilization and the population turning positive before they consider a change in status," Chart said.

Biologists are working on an assessment of the species' long-term prospects for survival, beginning with a population estimate, Chart said.

But the pikeminnow still appears to be positioned for recovery, eventually.

Biologists identified more than 1,300 young-of-year pikeminnow in 2015 in Colorado River backwaters, the highest catch in 30 years, the officials said.

Young-of-year are small fish, hatched from eggs spawned in the current year.

"We have had one really good year," Maddux said. "It helps us buy time."

"I think success is on the horizon for the recovery of the endangered fish," said Eric Kuhn, outgoing chief of the Colorado River Water Conservation District, who noted that control of non-native species such as the walleye is necessary for that.

"Just when it looked like we were out of the woods with these fish, the invasive, predatory non-natives came into the mix and started causing trouble," said Patrick McCarthy, deputy director of the Colorado River Program for The Nature Conservancy.

"Everybody knew that it wasn't going to be straightforward," McCarthy said. "This is a little bit of a wrench in the works."

Already efforts are underway in Colorado and Utah to put pressure on the walleye and other non-native predators. Colorado is working to make sure walleye are prevented from reaching the rivers by putting screens in flatwaters even as small as Highline Lake in Mesa County. Utah officials have introduced sterile walleye into the river to inhibit the possibility that the pointy-toothed predator establishes itself, Maddux said.

Sterile walleye also have been introduced into Rifle Gap Reservoir, even though there's no evidence that the walleye in that lake have left it for the Colorado River below.

The change in the prospect of delisting the pikeminnow comes as officials from three states, the U.S. Bureau of Reclamation and the Upper Colorado River Endangered Species Recovery Program changed the way they operate dams on the Colorado and Gunnison rivers to mimic the kinds of flows that allowed the pikeminnow to survive for at least 2 million years.

"From a habitat perspective, we've really been good," Chart said.

"If we could get rid of the non-natives, these fish would be doing tremendously," Maddux added.

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Information from: The Daily Sentinel, http://www.gjsentinel.com