Biologists Poison Fossil Creek Bass

Crews hope weeks-long effort has stopped bass from invading premier refuge for native fish

Biologists last week poisoned non-native smallmouth bass that sumounted an unmaintained fish barrier and threatened to wipe out restorted native fish populations in the spring fed stream.

Photo by Pete Aleshire. |

Biologists last week poisoned non-native smallmouth bass that sumounted an unmaintained fish barrier and threatened to wipe out restorted native fish populations in the spring fed stream.


Crews last week used chemicals to kill smallmouth bass in a 2.5-mile-long stretch of Fossil Creek, invaders that had already spawned and so threatened to wipe out one of the premier refuges for endangered native fish in the Southwest.

The six-man crew spent weeks netting perhaps 100-300 native fish to move upstream, before using a chemical to kill all the fish remaining in the final 2.5 miles of the stream before it merges with the Verde River.

The bass had moved into the stream sometime after floods in 2010 piled up rocks and sand below a permanent fish barrier, creating a fish ladder for the resourceful and voracious bass.

“We’re cautiously optimistic that we got them all,” said team leader Roger Scott. “We’ll gain confidence with every survey from here on out.”

Scott estimated the cost of the effort to “renovate” the stream at $200,000. However, without the removal, the bass would gobble up most of the young, predatory headwater and roundtail chub within a couple of years, not to mention the desert suckers, Sonoran suckers, razorback suckers, longfin dace, spikedace, loachminnows and Gila topminnows.

“A dozen bass don’t remain a dozen bass for very long,” said Scott. “They become thousands of bass. It’s an arms race and the smallmouth bass always win. They’re not native and the chubs just didn’t evolve with that kind of an ambush predator.”

The spring-fed Fossil Creek has offered a refuge for native fish driven to the brink of extinction by water diversions and non-native species in almost every other river system in the Southwest. After Arizona Public Service agreed to dismantle a hydroelectric power plant and return the spring water to the streambed about six years ago, the Arizona Department of Game and Fish and the U.S. Fish and Wildlife Service poisoned out the non-native fish and built a fish barrier to keep bass, catfish, sunfish and other non-natives from getting back into the stream from the Verde River.

The chubs and other fish have since thrived in the crystal-clear, travertine-rich stream, despite the discovery of the resurrected stream by some 90,000 summer visitors last year.


Photo courtesy U.S. Fish and Wildlife Service

Biologists last week used a fish-killing chemical to rid a 2.5-mile-long stretch of Fossil Creek of invading smallmouth bass, which have threatened one of the premier habitats for native fish.

But last year, a researcher noticed bass in the lower reaches of the stream, above the laboriously constructed fish barrier.

A quick survey revealed many bass just above the permanent barrier.

Game and Fish put up a temporary fish barrier about a mile above the breached permanent barrier, although U.S. Forest Service regulations barring the use of machinery in wilderness areas delayed the construction of that temporary barrier.

Earlier this year, another survey discovered about eight bass above the temporary barrier, raising the possibility that the voracious predators had swum upstream out of control.

Scott said his team spent weeks using nets and electroshocking to remove the bass seen above the temporary barrier. They determined that none of the bass they removed had yet spawned. They spent hundreds of additional man-hours hiking and snorkeling upstream to look for any bass that had escaped. Scott said simply looking down into the pools remains the best way to spot the bass, which are clearly visible in water that remains so clear you can see to the bottom of 20-foot deep pools.

“I feel fortunate that we detected them, contained them and caught them when we did,” said Scott. Renovating the stream from the temporary barrier another four miles upstream to the popular waterfall near the site of the decommissioned power station would have proven far more difficult, given the number of deep pools and vegetation, said Scott.

“It would have proven phenomenally more difficult and expensive,” he commented.

Killing all the bass in the bottom reach of the stream proved arduous enough.

The crew first put out about 100 fish traps, baiting with food used in hatcheries. The chub swim into the nets, but then can’t find their way out. For some reason, the nets catch a lot more chubs than bass.

The crews then pulled the chubs and other native fish out of the nets, plopped them into buckets outfitted with battery-operated bubblers and hauled them out the 1.5 miles back to the road in backpacks. The buckets could carry about 10 grown fish or about 20 small fish. In that manner, the crew moved as many as 300 native fish upstream above the bridge below the power plant site.

Crews set up a dispenser to inject Rotenone into the water, a chemical that essentially smothers any gill-breathing creature. Laboratory tests have shown that the chemical can cause Parkinson’s-like symptoms in laboratory animals. However, the concentrations used to kill the fish in a stream remain far below those levels. The chemical breaks down quickly into harmless byproducts. In addition, the crews released a second chemical that neutralized any lingering traces once it had killed all the fish in that 2.5-mile stretch of stream.

Scott said the game and fish crew collected many large bass killed by the chemical treatment. The bass spawned this spring, so the survey revealed many schools of recently hatched bass as well, which means the bass had already made a good start on taking over the lower section of the stream.

“We know we found groups of 40 or 50 small bass there in a couple of locations. When you find a few, it means there are many,” said Scott.

Scott says he’s confident the treatment will have eliminated the bass from the lower reaches of the stream, but said managers will have to increase surveillance and maintain the existing fish barriers.


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