A better understanding of the needs of native fish on the Lower Dolores river below McPhee dam is key to their survival, two fish biologists told local water boards recently.
Jim White and David Graf of Colorado Parks and Wildlife briefed board members of the Dolores Water Conservancy District and Montezuma Valley Irrigation District on current surveys for the flannelmouth sucker, bluehead sucker and roundtail chub.
Both DWCD and MVIC depend on McPhee reservoir to store water and deliver it for customers and shareholders. But below the dam, fish populations struggle in low water conditions.
As part of an ongoing collaborative process to accommodate all users of the river, the native fish are getting some attention with an aim toward establishing where they are and what can be done to help them.
White, an aquatic biologist, and Graf, a water specialist, gave an update on their activities monitoring fish populations.
The first 11 miles from McPhee dam is managed for the cold-water trout fishery. The spillway is monitored for bass and unwanted sucker species that may have slipped through from the reservoir. Bass can out-compete trout for food and habitat.
From Bradfield Bridge to the Dove Creek pump station, the lower Dolores flows for 19 miles through the Ponderosa Gorge, a pristine wilderness canyon only accessible by boat.
“Intensive surveys have been done on that section in 1993, 2005 and 2007. But it requires at least 400 cfs to run the boat crews through there with the electro-shock equipment,” White said. The technique places electrically charged rods into the water to momentarily stun fish so they can be identified, examined, tagged and counted.
During lower flows, the river below Bradfield Bridge can warm to the point it is not favorable to trout but may become more acceptable for native fish.
Below the Dove Creek pump station, the summer season can leave only deep pools on the river and electroshock techniques are less feasible, White said. Instead, biologists use seines, or nets, to herd fish for counting and tagging surveys.
Hard numbers are difficult to pin down. Instead, CPW uses statistical populations estimates based on its surveys, and rough estimates are inferred.
“We look to see if numbers go up or down over the years. If one year there are 700 and the next 200, we know something happened,” White explained.
For example, after the Narraguinnep fire, sediment flushed into the systems and had a negative affect on trout, but native fish weren’t affected as much because they have evolved to handle turbidity and murky water.
Antenna arrays that track fish movements are set up at two locations on the lower Dolores — one just downstream from the Dove Creek Pump station and the other near James Ranch. Fish handled in surveys are tagged by injecting a small glass tube with a copper wire inside (a PIT tag) into their abdomen. Stretched across the river are antennae lines that ping off the imbedded tag when the fish swims underneath. The information is relayed to a satellite and downloaded by biologists.
Radio telemetry is also used, but less often because it is expensive and labor intense. The technique uses tags that can be tracked in real time.
Biologists are learning what the native fish need. Roundtail chubs are more particular about what habitat they want to be in. They are more site specific and prefer complex pools with cover like logs and bank overhangs.
“They are not aimless swimmers; they will swim four miles downstream, and return to the exact same spot,” White said. “We don’t know as much for the flannel and bluehead suckers.”
The board wondered if cost-shares could be arranged to install additional antennae arrays to better estimate the native fish population. The units cost around $90,000.
An additional array at Bedrock would give biologists a clearer picture of how native fish handle a high salinity gradient in that area. Does the salty water act as a fish barrier, forcing native fish to back up into the San Miguel River, or is it not a problem for them?
Native fish have become a focus as of late. Working to preserve a healthy habitat and population helps avoid species being listed under the Endangered Species Act, which prompts federal intervention and tougher conservation regulations.
“We want to avoid the (Gunnison) sage-grouse situation and get in a better position to make progress on sustaining native populations,” said Mike Preston, DWCD general manager.
The U.S. Fish and Wildlife Service is recommending that the sage grouse be listed as endangered around Dove Creek because of rapidly declining populations there.
More study is also needed to determine ideal conditions for native fish larvae. Knowing where they go would help to adjust flow management to improve their chances.
Bluehead suckers have been showing up at the Dove Creek pump station region. Additional surveys are needed to determine how far upstream from there is good habitat for that species.
White explained that crayfish are common on the lower Dolores, but they are an invasive species that compete with native fish.
“People think all they do is eat scum off the bottom, but before they resort to that they first eat the microinvertebrates depended on by native fish,” he said. “They also crowd fish out of their hiding places.”
Native fish are not equipped to contend with non-native sport fish like trout and bass sharing the water. Trout will build nests and lay eggs in a protected depression to avoid predation. But during the spawn, native fish have not evolved to build nests. Instead they broadcast their eggs into the current where they are conveniently preyed on by crayfish.
Tweaking the way the rafting spill is managed, such as ramping it up earlier to mimic more natural spring flows, is also a key component to native fish recovery.
“Temperature and hydrology prior to a spill can have a deleterious effect for a spawn,” added Graf. “Making adjustments will give fish a better opportunity to have a successful spawn.”
The native fish spawn is timed for a natural hydrograph such as occurred before the dam was built. Eggs are laid after the spring snowmelt when the water begins to warm in late May and June.
But to accommodate better weather conditions for boaters, the whitewater spill from the dam has delayed until late May, allowing the river’s low flows leftover from winter to heat up.
The unnatural early warming triggers a premature spawn, and biologists think that the eggs and young fry are wiped out by the sudden rush of cold water during the whitewater release.
White disagreed with the notion that native fish are adapted to persistent low water conditions and can survive in deep pools.
“They can hang on for a few dry years, but they need flowing water for long-term survival,” he said. Improved flows create important riffle and channel habitat for spawning and feeding, and perennial flows wet the sand, triggering insect population growth for fish food.
“The question becomes what is the carrying capacity for native fish for the Dolores River? Are we close to that, and can we do better?” White said.