Smallmouth bass have invaded the lower reaches of Fossil Creek, threatening the premier stream for endangered native fish in Arizona.
The voracious, non-native predators first invaded the creek a year ago after a flood piled up debris on the downstream side of a fish barrier intended to keep bass from moving up the creek from the Verde River.
Arizona Game and Fish biologists thought they’d dealt with that invasion by building a temporary barrier several miles upstream and repairing the permanent structure.
But now they’ve discovered at least nine additional bass several miles above even that temporary barrier, said Arizona Game and Fish Fisheries Branch Manager Kirk Young.
“It was a disappointment to find bass above the temporary barrier, but it was a possibility we knew was there — either way. It doesn’t change our chances of success at containment — it just increases the area we’re going to have to deal with. We’ll weigh our options: but we are going to respond. We will conserve the non-natives and the fishery in Fossil Creek.”
The discovery poses the first serious threat to what has become a native fish refuge since 2005, when Arizona Public Service agreed to return water to the creek bed that for 100 years had been diverted into a flume to generate electricity.
Small numbers of endangered native fish like Sonoran and desert suckers, roundtail and headwater chub, speckled and longfin dace and others already lived in the creek, but so did voracious non-native predators like bass, catfish and sunfish.
Biologists used electro-shocking to capture and remove as many native fish as possible, while eliminating the non-natives. Then they used a fish poison to get rid of rest of the non-native fish they hadn’t managed to capture. Finally, they returned the native fish to the stream after APS put back the water.
The Forest Service, in cooperation with the U.S. Fish and Wildlife Service, and the Arizona Game and Fish Department also built a permanent fish barrier several miles upstream from the confluence of Fossil Creek and the Verde River, where non-native fish like bass and catfish dominate.
In the six years since, the populations of the native fish have exploded, creating one of the best streams for native fish in the whole state. Only a portion of the Little Colorado River below a spring in the Grand Canyon and the spring-fed Arivaipa Creek in southeastern Arizona can compare.
The number of chubs in the creek grew from just a handful to more than 20,000, surprising biologists at the productivity of the crystal clear waters of the travertine laden stream.
Better yet, biologists monitoring the stream saw no sign of returning non-native fish and even the normally voracious crawfish remained rare.
Populations of the trout-like native chubs grew so quickly that Game and Fish even introduced a unique catch and release fishery.
However, last year stream managers discovered that a flood had piled debris up on the downstream side of the fish barrier, located in a federal wilderness area. The biologists discovered small bass in several pools just upstream from the breached fish barrier.
Game and Fish built a new, temporary fish barrier several miles upstream to keep the bass from spreading, while seeking permission from the Forest Service to repair the permanent barrier. It took nearly a year to obtain that permission, in part because the barrier was built in a wilderness area and the Forest Service wouldn’t allow the use of vehicles or machinery to repair it.
Crews hiked in and finally completed repairs on the permanent barrier several weeks ago, said Young.
Soon thereafter, biologists from Northern Arizona University with a contract to monitor native fish populations in the creek discovered a large bass in a pool several miles above the temporary barrier — not far downstream from the bridge over the creek on the Fossil Creek Road.
“The Fish and Wildlife Service, Game and Fish, and NAU went up there and we did find some smallmouth bass above the temporary barrier. The bass weren’t very cooperative, in terms of warm temperature ideal collection,” said Young.
Biologists with snorkels and nets flippered about trying to catch the elusive bass in the crystal clear water. “We visually saw up to nine fish up there and pulled out six of them. We’re planning to go back in until we get all of them.”
Young said biologists found no evidence that any of the invading bass had reproduced, but still don’t know how they got so far upstream from the breached fish barrier.
“These most recent fish look a little suspicious to me. They’re a different size than the ones we saw coming over the barrier. We need to go back in and survey above and below the pool. If we find fish, then it means they must have gotten past the temporary fish barrier before we got it up. If all we find is this group of fish, it’ll point to something else.”
Biologists have fretted for years about the possibility people might deliberately break the law by dropping non-native sport fish in the creek above the barrier.
Those surveys will determine how serious a problem the bass pose. Biologists already assumed they would have to systematically remove bass in the two-mile stretch between the permanent and temporary barrier. Now, they may have to deal with an infestation for one or two miles above the temporary barrier.
One way or another: The bass must go to protect a unique refuge for a host of endangered species driven nearly to extinction in almost every other river in Arizona.
“We don’t think the stream can accommodate the bass,” said Young. “The Forest Service, Fish and Wildlife, Game and Fish, Bureau of Reclamation — we’re all working together to manage this. This is an important fishery — important water.”