Hundreds of fish turn up dead in Boulder lake

BOULDER, Colo. -- Hot temperatures are likely to blame for the mysterious deaths of hundreds of fish in a private lake in Boulder.

Neighbors began noticing dead fish floating on the surface of Tantra Lake on Sunday morning.

By Monday afternoon, hundreds of small perch could be seen near the shore along with dozens of carp, many measuring more than 20 inches in length.

“It’s devastating. I’ve been living here about six years or so and I’ve never seen anything like it,” Melissa Miller said.

The lake is on the property of Tantra Lake Apartments, which is managed by Boulder Housing Partners. According to the city of Boulder, the lake is managed and maintained by a contractor hired by Boulder Housing Partners.

Neighbors were upset the homeowners association and complex management did not send a note to residents about the fish kill.

“The smell is horrendous,” Miller said. “There is lots of speculation that somebody did something to the water. It’s just kind of frightening, especially if you have a pet that you’re walking around.”

“I hope the birds aren’t affected by it too because they’re out there swimming in it now,” resident Steven Chase said.

An outside firm began collecting water samples for testing Monday afternoon. The city of Boulder Utilities Department also sent a team of inspectors to check the water.

According to inspectors, the water temperature and pH level were too high to support the fish.

Around 3 p.m. the water temperature was 85 degrees. Crews were unable to estimate what a normal temperature in Tantra Lake should be.

The pH level, which was taken at various points around the lake, measured in the high 8 to low 9 range. According to the field crews, Boulder’s waterways should test in the mid-6 to 7 range.

“From what we can tell, this is a eutrophication event where you get large algal phytoplankton blooms,” Chase said.

Basically, that means an algae bloom, caused by hot temperatures, used up the oxygen in the lake. The fish were essentially suffocated to death in the water.

Fish kills in Colorado are fairly common in the summertime. Boulder sees similar events two to three times a year.

According to the city’s field crews, algae blooms can be spawned by a combination of prolonged high temperatures and runoff from rainstorms if neighbors have recently fertilized lawns.

Boulder had a rainstorm Friday, and neighbors said the apartment and condominium complex put down fertilizer a few days earlier.

“There was nothing in [the] test results to suggest that there was a larger risk to the city’s stormwater system," the city said in a statement.

Additionally, inspectors said they found no evidence of harmful or toxic chemicals in the water.

Water quality and algae testing performed by an outside firm is still being conducted. Those test results will be available Thursday.