DENVER — If you've been spending time around ponds and lakes in the Denver metro area recently, you may be seeing — and smelling — dead fish.
Jason Clay with Colorado Parks and Wildlife said there could be two factors at play: Heat or an algae bloom.
He said this time of year, when temperatures are regularly soaring into the high 90s, the oxygen is low in the waterways, killing the fish.
"With the heat and water temperatures rising, it causes a big drop-off in oxygen levels in the water and certain fish or species do die as a result," he explained.
The other possibility is an algae bloom, which suck more oxygen out of the water, he said. These blooms are most common in the late summer in still bodies of water.
While most algae is harmless and actually important to the food chain, lakes with high nutrients are more susceptible to blue-green algae, which can produce toxins, according to CPW's website. This can kill fish, but can also kill larger animals like dogs and even humans if too much is ingested.
On Thursday, The Colorado Department of Public Health and Environment said state testing came back positive for toxic algae in Prospect Lake in Colorado Springs, Barr Lake in Adams County, Cherry Creek Reservoir in Arapaho County, and Steamboat Lake in Routt County. CDPHE said it's likely present in other lakes and slow-moving waters, but not in rivers of high mountain lakes.
Denver Parks and Recreation Spokesperson Cyndi Karvaski said these fish kills occur almost every year and in most lakes in Denver. She said the Department of Public Health and Environment is testing the water at Sloan's Lake. She said the heat can cause that shallow lake to hold less dissolved oxygen, which fish require. Algae can also contribute to this, she said.
CPW Aquatic Biologist Paul Winkle said Saturday morning that he had observed an estimated 500-1,000 dead fish at Sloan Lake after the fish kill. He said the dead fish consisted of black crappie, channel catfish, common carp and a few largemouth bass – but the majority were 2-6” black crappie.
“There’s a significant algae bloom currently at Sloans; this, along with water temperatures reaching 79 degrees Fahrenheit have caused dissolved Oxygen (DO) levels to plummet,” Winkle said. “The dead fish showed the classic gaping mouth indicative of lack of oxygen.”
According to a water quality report by Denver Environmental Health in 2015, Sloan's Lake is regularly stocked with a variety of warmwater fish, including walleye, muskie, and channel catfish, as well as trout.
Kristy Richardson, state toxicologist, said if you suspect toxic algae is present in a waterway, do not let kids, pets or livestock near the water.