NEDERLAND — Over the weekend, a Nederland local drew attention to a strange phenomenon occurring at Barker Reservoir, where a large number of dead fish were found rotting on the shores.

The sight of dead fish washing ashore is not too common to the area, though some experts noted that the heat and humidity of the 2026 summer season is not too common either, and may be playing a factor in the mystery. 

Peter Kornicker expressed his concern to the community after having witnessed, and personally disposed of, up to 20 dead fish—on several different occasions—at the reservoir where he walks his dogs. Roughly the same amount of dead fish have washed ashore periodically over the last two weeks, accordingt to Kornicker.

The community immediately began speculating, echoing Kornicker’s concerns surrounding the potential hazards of the dead fish to both their dogs and to the water supply. Among the most popular of the theories being tossed around online was the heat and lack of oxygen causing increased stress on the environment, and the influx of amateur anglers using inappropriate equipment.

Many hypothesized those coming to the reservoir to fish were using barbed hooks and were killing the fish when catching and releasing them —this theory was backed by the evidence that the majority of the dead fish found were White Sucker fish, which those fishing for sustenance will always catch and release. 

On Monday, July 20, the reservoir was packed with anglers, and the shore sported some dead fish, though not as many as had been recently reported. A guide with Rocky Mountain Anglers—a Boulder-based fishing club that takes fly fishing amateurs on tours across Colorado—was showing the members of his group the ropes before stopping to speak with The Mountain-Ear about the phenomenon.

He noted a member of his team had reported over 50 dead fish on the reservoir beach just a day prior, which almost prompted the guide to cancel his Monday-morning tour. He surmised a combination of the heat and of anglers using spin casters and other such gear for recreating was what was most likely causing the issue. 

The guide reiterated the importance of teaching conservation in addition to technique when leading a group through Colorado’s waters. Many amateurs who utilize Barker Reservoir do so opportunistically, he said, as the County stocks the fish, and are not necessarily gentle in their methods. 

Reports of dead fish on the shores of Barker Reservoir have raised concern within the community about the hazards to their dogs and to the water quality.

Photo by Christopher Kelley

When water temperatures rise above 68 degrees Fahrenheit, oxygen levels and water depths drop, putting severe stress on cold-water fish species and increasing their buildup of lactic acid. Such stress can cause mass death, especially when anglers do not adjust their gear or methods to compensate. Barbless hooks are recommended for such scenarios, and it is suggested to keep the fish in the water when unhooking them for release.  

Ben Swigle, Aquatic Biologist with Colorado Parks and Wildlife, also spoke with The Mountain-Ear about the mystery, and provided some additional context. 

“The lake was recently stocked with several thousand Rainbow Trout, and those went out quite nice,” Swigle said. “Suckers likely were making a spawning run, and the stress of spawning resulted in a small number of fish passing. Should you find 20 or more fish that include multiple species, I would then have heightened concern.” 

Though photos from Kornicker show up to 20 dead fish, the majority of them are White Sucker. The number has been significantly dwindling each morning, with just one or two dead fish on the shore by Tuesday morning. 

Regardless, Swigle assured the community there is no cause for concern. He will be monitoring the situation over the course of the week. Nederland Town Manager Jonathan Cain said the Town’s most recent water testing shows that there is no danger to the reservoir’s water.  Nonetheless, the City of Boulder, which owns Barker Reservoir, have been made aware of the concern.

Nederland’s latest Water Quality Report can be viewed here: nederlandco.gov/media/2321 .

As CPW and the City of Boulder monitor the situation, residents are encouraged to be extra aware when walking their dogs at Barker Reservoir, and to keep their pets on leash and away from rotting dead fish. Ingestion of rotting fish can transmit bacteria, toxins, and intestinal parasites to your pet. The contraction of salmonella poisoning or botulism can result in vomiting, diarrhea, dehydration, shock, and even seizures.