Why are fish dying in Fort Collins lake and park ponds?

Large numbers of dead fish that turned up in Sheldon Lake at City Park and ponds at Troutman Park in the last week to two weeks succumbed to winter kill, according to Colorado Parks and Wildlife officials.

Winter kill occurs frequently in Fort Collins and shallow ponds along the Front Range, according to state Assistant Area Wildlife Manager Brandon Muller.

It occurs when shallow ponds experience long periods of ice cover. Plants in the water die because the ice cover does not allow enough sunlight to keep them alive. The plants decompose, causing a depletion of oxygen for the fish.

Mike Calhoon, Fort Collins parks director, said ice was late forming on Sheldon Lake—after Jan. 1 — but it did not break apart until recent wind and warm weather. He said the city informed the state wildlife agency of the dead fish.

“It was weird this year because we had open water on Christmas Day but when it capped, it capped hard,” he said. “We didn’t get much of a break after that and had single-digit temperatures around March 9- 10, so the ice remained on those waters.”

A dead fish lies in the shallows at Troutman Park in Fort Collins on Thursday.  A large number of fish died in the pond and at a Sheldon Lake at City Park in recent weeks.  The fish died from winter kill, which occurs when ice cover does not allow enough sunlight to keep plants alive.  The plants decompose, causing a depletion of oxygen for the fish.

Fort Collins recorded single-digit low temperatures four of five nights March 7-11, according to the Colorado Climate Center.

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The timing of ice breakup greatly fluctuates along the Front Range but generally occurs starting in mid- to late February.

Calhoon said fish killed included grass carp, some up to 10 pounds at Troutman Park, as well as bass and sunfish.

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