Dead fish in Denver’s Sloan’s Lake killed by heat, lack of oxygen

Extended heat is killing hundreds of fish in one of Denver’s most popular lakes as the city roasts under another long stretch of temperatures nearing 100 degrees.

At least 400 fish died in Sloan’s Lake over the weekend due to the heat, Denver Parks and Recreation spokeswoman Stephanie Figueroa said. Dozens of carp, bluegill and sunfish floated dead along the shore Monday morning as parks and recreation staff worked to remove them.

“This is a direct correlation to the extended weather patterns we now get,” she said. “These 100-degree days really affect the lake ecosystem.”

The amount of oxygen in the water decreases as water temperatures rise, making it difficult for fish to breathe. Warming water simultaneously increases fish’s metabolisms and heightens their need for oxygen.

If water temperatures become too high, fish suffocate.

Sloan’s Lake is shallow — about 3 feet deep on average — and the lack of deeper, cooler water makes it more susceptible to fish kill from warm water, Figueroa said.

Parks and Recreation are adding more water to the lake to lower the temperature, she said.

“We’re doing all that we can, but it’s hot,” Figueroa said.

Temperatures in Denver reached the high 90s over the weekend, several degrees higher than the 30-year average of 90 degrees for those dates, according to National Weather Service data. Temperatures are expected to remain in the high 90s through Friday, according to the weather service.

Temperatures have exceeded the 30-year normal in Denver for 41 of the 58 days since June 1, the data show. Average temperatures in Colorado are expected to rise by up to 5.5 degrees by 2050 as climate change reshapes weather norms here. By 2050, the average year will be as hot as the hottest years Colorado experienced through 2022.

A similar mass die-off of fish in Sloan’s Lake occurred in 2020, when hundreds of fish floated belly-up in the lake.

People visiting the lake should not get in the water or allow their pets to swim because of the blue-green algae, Figueroa said.

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Originally Published: July 29, 2024 at 11:37 a.m.