State and federal fish biologists have surveyed the lower Dolores River to gauge native fish populations.
Crews from the Bureau of Land Management and Colorado Parks and Wildlife took advantage of water releases from McPhee Reservoir from March into July to raft the lower Dolores and conduct population surveys for the three main native fish below the dam – the roundtail chub, flannelmouth sucker and bluehead sucker. The Nature Conservancy assisted with river ecology studies.
Wildlife officials also arranged for a dam release of 4,000 cubic feet per second to flush the lower Dolores for a few days in mid-May. And officials reported gains in their effort to reduce numbers of smallmouth bass, a troublesome non-native species.
One survey focused on Slick Rock Canyon, a 32-mile stretch of the Dolores River from Big Gypsum Valley Bridge to Bedrock. It was last surveyed by raft in 2007.
During the surveys, biologists place an electrically charged pole in the water to stun fish, record their type and size and fit native fish with a Passive Integrated Transponder tag to track their movements.
About 400 native fish were captured and tagged, according to BLM fish biologist Russ Japuntich.
Other species captured were the native speckled dace and non-native white sucker.
Native fish in the remote Slick Rock Canyon had not been surveyed recently, said Jim White, an aquatic biologist with Colorado Parks and Wildlife.
“Of the fish we caught (in Slick Rock Canyon), 94 percent were native, so that is good, but on the other hand, they were not very abundant,” he said.
Crews caught an occasional 25- to 30-inch flannelmouth, which can live 30 years. The larger roundtail chubs caught were 18 to 20 inches.
Biologists also documented the first known non-native white sucker in Slick Rock Canyon, White said. The white sucker is a problem because it can hybridize with native suckers.
Smallmouth bass present a risk to native fish because they prey on them and compete for food. As a case in point, one bass removed barfed up a bluehead sucker, White said.
In mid-July, Parks and Wildlife targeted the non-native bass by scheduling a three-day, 4,000 cfs release at a time when males were guarding the nest.
On a 14-mile stretch from below Snaggletooth Rapid to Slick Rock Canyon, biologists removed 600 smallmouth bass during the electro-fishing process and threw them to shore “for the raccoons.”
White said it was encouraging that very few smallmouth bass were found in Slick Rock Canyon, and he speculated that they couldn’t handle the amount of sediment that is dumped there by Disappointment Creek.
The lower Dolores receives significant flows only when McPhee Reservoir receives more water than it can hold. This year, above-average snowpack led to a significant boating season for the lower canyon, and the high flows had numerous ecological benefits.
“We could see that it cleaned up the cobble beds and helped form new channels and slow-velocity backwaters that are a nursery habitat for smaller fish,” White said.
Flannelmouth and bluehead suckers spawn in late spring as water levels increase, and roundtail chubs spawn as flows decrease in early summer.
Increased water flows flush fine sediment from rocks and clean the river bed during spring spawns. After the embryos develop, larvae emerge and begin to drift downstream until they arrive in calm pools and backwaters.
“These slow-water habitats are important to the fish larvae because chironomid larvae also utilize these areas and are an important food source,” Japuntich said. “This habitat is also advantageous because water temps are fairly warm, which increases growth rates in the fish.”
Bluehead suckers occupy swift riffle and run currents. Flannelmouth suckers occupy run and pool habitat, and chubs occupy pool and backwater habitat.
Near the mouth of Coyote Wash, biologists found a large number fish that had been born in the past year, hinting at the importance of the tributary.