Fish die-off at Peanut Lake due to low water levels in 2024-25

“Anoxic conditions” for non-native fish

By Katherine Nettles

It appears there is a significant loss of fish life happening out at Peanut Lake this winter, a phenomenon which local wildlife experts say happens every so often due to low water levels and subsequently low oxygen levels in the lake during the winter months. 

The fish are generally non-native sucker species. Some Nordic skiers using the trails around Peanut Lake this month have observed the dead or dying fish, and concerned citizens have reached out to Colorado Parks and Wildlife (CPW) and the Crested Butte News about the situation. CPW confirmed there is a mass die-off (winter kill) happening in the lake, which also occurred in 2016. 

“Today while Nordic skiing just north of the Gronk, I noticed a raven investigating an unfrozen area next to the trail by Peanut Lake,” wrote Patty Zishka in an email to the CB News last week. “Upon a further look, I saw many adult and thousands of tiny fish that were dead or dying.”

CPW southwest region public information officer John Livingston said they have been hearing from others who have also observed the fish kill at Peanut Lake this year. “This situation is nearly identical to one we had around this same time during the 2015/16 winter,” said Livingston. “Impacted fish species we’ve found are fathead minnows and white suckers (both are non-native to this area).” He noted that CPW does not stock any fish at Peanut Lake.  

“Fish from the Front Range were spread around all over by humans historically before greater attention was placed on native fish. Many of these species have been introduced to waters where they are non-native by humans both intentionally and unintentionally throughout history.

“They also could have come from other connected water systems and eventually made their way into Peanut Lake. White suckers and fathead minnows in non-native waters in Colorado were most likely introduced through bait bucket release and trout stock contamination,” said Livingston. He noted that white sucker and fathead minnows are native elsewhere in Colorado, primarily in East Slope rivers. 

Livingston explained that oxygen levels in the lake are too low for the fish to survive. “When cutting through the ice with an auger, you immediately get a rank smell similar to rotten eggs, indicating a buildup of hydrogen sulfide. That is because the vegetation within the lake is degrading and explains the consumption of oxygen,” he said. “This situation is identical to what we found during the 2015/16 winter fish kill. The vegetation breaking down under ice paired with not enough in-flow of water leads to fish kill in these anoxic conditions.”

 “The lake is shallow,” continued Livingston. “With thick ice, there is not much water (in many areas only 6 to 12 inches). The fish will all push and crowd to the inflow area where there are better oxygen levels. However, the inflow is so small that we expect fish to continue to struggle in that area and for continued fish mortality.”

CPW has investigated the conditions and does not have any larger concerns about the fish kill, said Livingston. The dead fish are not considered dangerous or toxic to other wildlife, and he said the fish populations and the lake’s vegetative ecosystem are expected to recover as the lake’s water inflow increases in the spring. 

“In 2016, we did follow up with our health lab for disease testing to make sure there wasn’t something else also going on and were able to rule that out,” he said.

Zishka shared her understanding of how the fish died due to the anaerobic processes occurring in a shallow, frozen-over lake. “These are sucker species predominantly, as trout don’t usually overwinter in such shallow water,” she said. “It was disturbing nonetheless.”