Sheldon Lake fish kill at City Park blamed on low dissolved oxygen levels caused by heat

Those whose morning habit is to enjoy a walk or run around Sheldon Lake at City Park in Fort Collins instead discovered thousands of dead fish, the vast majority being small baitfish, encircling the shallow water of the lake.

Reports about the kill started coming into the Coloradoan and city of Fort Collins early Wednesday morning.

Mike Calhoon, Fort Collins parks director, said the water was tested for possible chemical contamination but nothing unusual was discovered and ultimately the kill was blamed on low dissolved oxygen levels.

Jason Surface,  Colorado Parks and Wildlife area manager in Fort Collins, said an agency aquatics biologist late Wednesday afternoon confirmed low oxygen was the cause of the fish kill.

Low dissolved oxygen levels can occur in shallow lakes (Sheldon Lake is approximately 13 feet deep) as water temperatures increase during hot spells like Fort Collins recently experienced. Decaying plants and algae in the water also can decrease dissolved oxygen levels, especially when nutrient loading from lawn fertilizers and other contaminants flow into the lake and fuel abnormal high algae growth.

When dissolved oxygen levels become too low, fish and other aquatic organisms start to die.

A Coloradoan reporter walked the perimeter of the 1,230-surface-acre-feet reservoir Wednesday morning and observed many dead shad baitfish along and near the shore as well as some grass carp, bass and sunfish. Colorado Parks and Wildlife stocks the lake with those fish as well as trout.

Hundreds of crawdads were climbing out of the water onto rocks to breathe while other fish packed the water where fresh water was flowing into the northwest corner of the lake.

"Shad are usually the canary in the coal mine," said Calhoon, noting the city posted signs about the kill at the lake Wednesday morning. "If you see them popping up, you know you have something going on. They are especially vulnerable to low dissolved oxygen levels."

Calhoon said the low dissolved oxygen issue cropped up despite aerators placed in the lake years ago during a renovation project that included dredging the lake to deepen it. Those aerators were working properly Wednesday, he said. Aerators help increase dissolved oxygen levels.

It is not uncommon for shallow lakes in the area to suffer fish kills from low dissolved oxygen levels. However, Calhoon said he can't remember a fish kill from such conditions at Sheldon Lake since aerators were installed.

The lake suffered a winter fish kill event in March 2022 due to similar low oxygen levels caused by long periods of ice not allowing enough sunlight into the lake, resulting in plants dying and depleting oxygen levels.

During that fish kill, Calhoon said the lake's aerators were turned off to help form ice on the lake's ice skating rink.

Calhoon said city crews started cleaning up the dead fish late Wednesday morning. He said he hoped to contact someone who could use the dead fish for compost. If he can't find any takers, the fish will be disposed of, he said.

Calhoon said the city had Pleasant Valley and Lake Canal Co., which owns the water, release shares of the city's water into the lake to increase the flow and oxygen to help the remaining fish survive.

"We're hoping by increasing the water flow and with cooler temperatures starting tomorrow that we can start turning the corner on this," Calhoon said Wednesday.

This article originally appeared on Fort Collins Coloradoan: Large numbers of fish die at City Park's Sheldon Lake in Fort Collins