Rising numbers of invasive brown trout and green sunfish in the Colorado River downstream of Glen Canyon Dam have spurred the National Park Service to propose a range of new actions to control those and other non-native aquatic species in Glen Canyon and Grand Canyon.
But a few of the ideas, including long-term electrofishing and the introduction of more non-native fish to the river, are drawing concern from fishing and environmental organizations as well as the Arizona Game and Fish Department.
The Park Service is gathering public comments on its initial slate of ideas until Friday.
The agency stated that it is looking for “additional tools” to prevent, control, minimize and eradicate invasives that threaten native species in the Colorado River as well as the blue ribbon rainbow trout fishery at Lees Ferry.
Among the proposed invasive species control options is a still-experimental tactic involving the introduction of non-native male fish with YY chromosomes, which produce only male offspring. As the fish continue to reproduce, that gender imbalance ends up crashing the population of whatever invasive species managers want to reduce, said Chris Cantrell, chief of fisheries with Arizona Game and Fish.
Fish barriers, pumps and piping in backwater areas where invasive sunfish are spawning and the application of fish-specific chemicals are other ideas the Park Service included in the proposal.
It’s the agency’s suggestion to use long-term intensive and repeated electrofishing that drew opposition from the Arizona Game and Fish Department as well as Trout Unlimited and Fly Fishers International. The process involves sending an electric current into the water to stun invasive fish so they can be scooped up and removed.
The prospect of weeks or months of electrofishing around Lees Ferry would negatively impact people’s perceptions of the fishery to the point where they just won’t come, said John Hamill, a volunteer for Trout Unlimited who serves in the Glen Canyon Dam Adaptive Management Work Group.
That would have “a lot of collateral damage to the local community up there,” Hamill said.
Electrofishing at a rate that would reduce brown trout populations also would negatively impact the local rainbow trout population, Cantrell said. The technique has a low catch probability and isn’t an efficient or effective use of resources, especially in a big river like the Colorado, Cantrell and Hamill said.
A 2017 white paper on the brown trout also noted that several tribes including the Zuni Tribe have expressed concerns with the physical removal of non-native aquatic life. State wildlife managers would advocate other alternatives like stocking more rainbow trout that compete with the brown trout, Cantrell said.
But the idea of introducing more exotic species to manage existing exotic species?
“I feel like history has told us that rarely works," said Alicyn Gitlin, with the Sierra Club. Gitlin said she instead supports the idea, also included in the Park Service proposal, of introducing more native species like Colorado pikeminnow to prey on and compete with non-natives in the river.
What isn’t in the range of options that she would like to see, Gitlin said, is the modification of water releases from Glen Canyon Dam to better mimic the river’s pre-dam spring and summer flooding regime instead of maintaining steadier flows with high-flow releases in the fall.
The 2017 brown trout white paper supports that idea, saying extremes in discharge, with both floods and drought, often inhibit the trout’s recruitment, “even to the point of population collapses.”
Brown trout numbers, after holding steady for more than a decade, exploded between 2012 and 2016, with the number of fish caught per minute in the Lees Ferry reach of the river increasing from .01 to .06 over that span. Brown trout prey on juvenile humpback chub, which are native to the Colorado, and also prey on and compete with rainbow trout.
Sunfish, a predatory invasive, also have continued to breed in a backwater of the Colorado River in the two years since thousands of fish were first discovered there. Fish managers tried two chemical treatments to wipe out the fish, but they just keep getting back into that area, said Scott Vanderkooi, chief of the U.S. Geological Survey’s Grand Canyon Monitoring and Research Center.
The sunfish are up in Lake Powell and scientists think with lake levels dropping, the warmer surface waters where the green sunfish live get closer to the dam’s turbines, so some fish are slipping through, Vanderkooi said.
Scientists are less sure about the reason brown trout numbers are increasing so dramatically. The white paper lays out seven possible hypotheses with the most likely being the start of regular fall high flow releases from Glen Canyon Dam that “cue the migration of ripe brown trout into Glen Canyon” and cleanse spawning grounds.
It’s most likely a combination of reasons though, especially in a large ecosystem like the Grand Canyon, Vanderkooi said.
“We're trying to understand what’s happening and why now, given that we’ve had brown trout in the system for a long time," he said.
Rob Billerbeck with the National Park Service stressed that this is just the beginning of the agency’s planning process.
“There is a range of alternatives we are looking at. We are really open at this point and have tried to be very inclusive,” Billerbeck said. “As we hear back from the public and conduct our analysis then we will likely weed through those actions and may weed out some of them. We also might get a few new ideas.”