In the early morning of Aug. 23, Sheldon Lake in City Park was filled with dead fish.

The discovery was made by a keen-eyed reporter from the Coloradoan newspaper, who was shocked to see numerous dead baitfish, carp, bass and sunfish floating on the lake’s surface. Adding to the eerie spectacle, hundreds of mudbugs were observed crawling out of the water onto the ground in a desperate attempt to reach higher oxygen levels.

This massive fish kill was the result of water temperatures rising too high.

“The fish kill that happened in August was due to elevated water temperatures that led to low dissolved oxygen issues impacting the fish in Sheldon Lake,” said Mike Calhoon, Fort Collins director of parks. “These conditions tend to appear in August during the heat of the summer and result in die-offs in lakes along the Front Range.”

Sheldon Lake has a long history of such fish kills, with the most recent previous incident occurring in March 2022. During that incident, the lake suffered from low oxygen levels because the ice covering it prevented sunlight from penetrating the water. The absence of sunlight crippled the lake’s plant life, preventing photosynthesis and, consequently, further lowering oxygen levels.

Sadie Ross Menke, a resident of Fort Collins for 24 years, has seen multiple fish kills in Sheldon Lake.

“(Dead fish) were just floating right near the gazebo in 2018,” Menke said. “I was really confused, and it kind of grossed me out.”

Despite its recurring fish kills, Sheldon Lake has been recognized for its potential as a fishing destination. Colorado Parks & Wildlife stocks the lake with fish to maintain its appeal to anglers. However, the deteriorating water quality and pollution have deterred many Fort Collins residents from indulging in its fishing opportunities.

“There’s a waterpark next door and runoff of pesticides into the lake,” said Aspen Shanke, a Fort Collins resident. “I wouldn’t eat anything from there. Also, the amount of trash in that lake disgusts me.”

Adding to its troubled history, Sheldon Lake became infamous in June 2017 when a woman’s dead body was discovered in its waters. A man named Jeffrey Ethridge later pleaded guilty to the murder and the disposing of the body in the lake.

In light of these recurring challenges, the Fort Collins Parks Department has been working diligently to find lasting solutions to address the persistent fish kills in Sheldon Lake. In 2000, aerators were installed in Sheldon Lake in City Park to improve water quality.

“It seems to have worked over the years since this was the first summertime fish kill that has occurred since we made those improvements,” Calhoon said. “In discussions with the Colorado Parks & Wildlife folks, they indicated there was nothing we could really have done to prevent this one. The conditions were just right, and unfortunately, all we could do was provide the cleanup. We will work with our partners at CPW to restock the lake at the appropriate time to help the fishery recover.”

To avoid fish kills in the future, CPW can take active steps in research, as fish kills will only become more common due to climate issues that lower the oxygen levels in the lake. CPW has installed aerators to combat the fish kills, but there is no information on any other steps taken to improve the water quality. With contaminated runoff making its way into Sheldon Lake, many locals theorize the idea of pesticide poison; however, CPW assured the public that the water was tested for chemical contamination, and none was discovered. An article published by New Mexico State University outlines multiple prevention strategies that Colorado Parks & Wildlife Department could utilize to address fish kills.

Reach Alexander Wilson at science@collegian.com or on Twitter @alexgrey0604.