Prior to the construction of Glen Canyon Dam, the warm, muddy flow of the Colorado River was home to eight native fish. Forty years after the dam’s opening, the waters released from the depths of Lake Powell are cold and clear, one example of a change that impacted fish.
Fish can be collected in this weir, located on Bright Angel Creek below the foot bridge. (Photo courtesy of Jeffrey Cross/National Park Service)
Four of the eight native species are gone from the Grand Canyon stretch of the Colorado River and a fifth is being seriously threatened. In an effort to learn more about the decline of native fishes in the Grand Canyon, the National Park Service began a three-month study a few weeks ago.
"The river has gone from warm, highly variable, highly silty pre-dam conditions to the constant flows and clear water we have today," said Jeffrey Cross, Grand Canyon Science Center director. "And different non-native species have been introduced — trout, catfish, carp, sunfish, bass."
The non-native brown trout’s impact on the endangered humpback chub has become a focus of the NPS study.
"We’re looking for things the park can do to help the humpback chub," Cross said. "One thing is to look at controlling non-native species, particularly brown trout."
The project contractor, SWCA out of Flagstaff, is overseeing a weir that has been set up in Bright Angel Creek.
"Fish come up the creek and it’s like a fence on each side," Cross said. "They follow the fence and swim into the trap."
A team of fish biologists led by SWCA’s Bill Leibfried pop open a lid on the weir to retrieve the fish. Various types of information is collected, including the fish’s sex, stomach contents and whether or not it has been tagged by the Grand Canyon Monitoring and Research Center, which is doing a population study.
The predatory brown trout have been known to feed on the humpback chub. In previous studies, Cross said around 10 percent of brown trout were found to have humpback chub in their stomachs.
On the other hand, around one-tenth of 1 percent of rainbow trout had humpback chub in their stomachs. Although not a predator such as the brown trout, the rainbows do compete with humpback chub for food. Both feed on floating insects on the water.
Bright Angel Creek is where brown trout spawn, therefore that’s where the weir was located. Rainbow trout spawn in the Lee’s Ferry region at Marble Canyon.
Humpback chub stay in an area around the confluence with the Little Colorado River. During monsoon season, humpback chub spawn up the Little Colorado River. Later in the year, they head down to the Colorado River, where predators are usually waiting.
"The young fish come out of the Little Colorado River and when they come into the (Colorado) river, they don’t stand a chance," Cross said.
Brown trout, a golden brown fish widely stocked in North American waters, originated in Europe. The rainbow trout is native to the mountain streams and rivers of the Pacific coast. Humpback chub, with its small head and snout, have called the Colorado River home for a few million years.
Cross estimates the population of the brown trout at around 50,000 and the rainbow trout between 500,000 and 1 million.
Only 1,000-2,000 humpback chub remain in the river, Cross estimated. A recent study had the chub population at 1,100.
Besides the information to be gained by studying brown trout, the project will also test the feasibility of using a weir to trap fish. Cross said it seems to be an effective method, especially when compared to other options such as dragging nets or using poison.
The study, funded by the Bureau of Reclamation, began Nov. 18 and is expected to last into mid-January. In those three months, fish biologists hope to figure out what triggers brown trout to head up Bright Angel Creek to spawn.
After the study concludes, Cross said a decision will then be made on how to manage brown trout with one goal being the eventual recovery of the humpback chub.