Low dissolved oxygen leads to partial fish kill at Sweitzer Lake State Park

John Livingston
Southwest Region Public Information Officer
970-759-9590 / [email protected]
@CPW_SW

Sept. 16, 2025

DELTA, Colo. – Colorado Parks and Wildlife has evaluated an ongoing fish-kill event at Sweitzer Lake in Delta County. While CPW will not enact a fishing closure at this time, anglers should expect poor fishing conditions at Sweitzer Lake State Park until conditions improve.

After receiving reports of several dead fish around the lake and at the bottom of the boat ramp, CPW aquatic biologist Eric Gardunio responded and measured low dissolved oxygen levels within the reservoir. This has led to stressed fish populations and several dead fish washing up on shore. 

“The fish behavior coupled with the recent low-water levels within the reservoir made me suspect low dissolved oxygen levels, and sure enough, we measured levels low enough to kill fish,” Gardunio said. “Sweitzer Lake is a shallow, nutrient rich water body, and this coupled with recent reservoir water management likely led to this fish kill.” 

For much of the open-water season, the reservoir was drawn down several feet for necessary spillway construction. This left large clumps of aquatic vegetation exposed on the shallow shorelines. 

During the last month, the reservoir has refilled. This has inundated the dead vegetation that has consumed oxygen as it decomposes, causing the low dissolved oxygen levels observed.  

“This was likely accelerated by the four consecutive days of cloudy weather we had from Sept. 10-13,” Gardunio said. “Cloudy weather causes aquatic vegetation and phytoplankton to stop producing oxygen through photosynthesis and is known to cause fish kills. This likely caused the oxygen levels in the water to further decline.”

The fish kill has impacted all species in the reservoir, including black bullhead catfish, bluegill, common carp, green sunfish and largemouth bass. However, CPW has only observed a partial fish kill at this time with fish still able to survive where there is fresh water feeding into the lake from the inlet.

With improving conditions expected in the coming days and with fresh, oxygenated water flowing regularly into the irrigation reservoir, CPW has opted not to enact a mandatory fishing closure at this time.

“While we don’t have a closure in place, please help us protect this fishery during this period of stress for the fish,” said Sweitzer Lake State Park Manager Scott Rist. “The fishing is likely to be poor for a bit until the conditions improve. We understand this can be frustrating for anglers who were excited to get back out on the lake once it refilled, but protecting the fishery over the next few weeks will help all of us enjoy better fishing days in the future. Hopefully, this is a one-time event.”

Gardunio will continue to sample dissolved oxygen levels over the next few weeks to assess if other management actions need to be taken. CPW will also conduct a survey of the fishery next spring to determine if there were any long-term impacts from the fish kill to inform future management of the fishery.

Anglers are reminded that Sweitzer Lake is a catch-and-release-only lake. Fish in this lake contain selenium and are not edible.