The Colorado River is now officially “positive” for invasive zebra mussels in the latest failure of containment for the voracious species, after three new samples came up with larvae July 3, from between Glenwood Springs and Silt.
The main stem Colorado River discoveries piled on top of a confirmed “large number” of adult zebra mussels in a private body of water in western Eagle County, and two more positive larvae tests, at Highline Lake and Mack Mesa Lake, both near the Utah border, Colorado Parks and Wildlife officials said Wednesday. Sampling was redoubled throughout June after tests found a single zebra mussel larvae, or veliger, in the Colorado River from a June 9 collection.
It’s the second year in a row veligers are being discovered in the West’s key river channel through Colorado, and now CPW officials are also dealing with a full-blown adult zebra mussel invasion in the privately owned Eagle County water. A year ago, researchers called the first discovery of larvae in the main stem river “devastating.”
Wildlife officials are trying to stay positive this time around, though zebra mussels have eaten their way through other states with a multiplication rate of a million eggs per female mussel per year, and 700,000 adult mussels massing in one square meter. They can eat up all the food sources needed by native wildlife, and clog and erode river and lake infrastructure.
“Not great news,” acknowledged regional CPW spokesperson Rachael Gonzales. “But from the very beginning, it was never great news. But these little steps are crucial in helping us figure out, OK, where do we go next? What do we do next?”
For now, “next” means even more sampling up and down the river, while considering options for how to eradicate the adult mussels in the private Eagle County water body, Gonzales said. The state continues to emphasize boater and floater education, with boat inspections and cleanouts a key to preventing the spread of mussels that can survive relatively long periods out of the water.
Highline Lake, now positive again for the larvae, was only recently refilled after a long drainage and eradication period to combat the state’s first discovery of an adult mussel population in 2022. New samples were taken at the refilled lake, at a patrol dock and an inlet, on June 10, with veligers later confirmed in each sample.
“While we had maintained hope that our eradication efforts at Highline Lake would be successful, we have always known this was a lofty goal,” CPW invasive species program manager Robert Walters said. “This is the primary reason we have continued the implementation of the containment watercraft inspection and decontamination program” there.
At Mack Mesa Lake fishing pier in Highline Lake State Park, more samples of plankton were taken June 10. Suspected zebra mussel veligers were confirmed in lab testing. More samples were taken July 8 that contain suspected veligers, meaning Mack Mesa is now considered “positive” for the mussels.
The Colorado River channel samples were taken June 16 between Glenwood Springs and Silt, with three zebra mussel veligers confirmed July 3.
Adult zebra mussels lining a plastic tube. (Colorado Parks and Wildlife, Contributed)Expanded sampling this summer has not found any veligers in the Eagle or Roaring Fork rivers, and no adult zebra mussels in the Colorado River, CPW said.
The state is considering options for Highline Lake and Mack Mesa, but does not have plans to drain them for treatment, Gonzales said.
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