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.”
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.
He said the city will work with Colorado Parks and Wildlife to restock the waters.
Calhoon said Sheldon Lake has aerators that help prevent winter kill but that the devices were turned off to help the ice develop for skating. They were turned on when it was determined ice skating was not going to happen, but rapidly freezing temperatures — including minus-7 degrees on Jan. 2 — quickly formed, reducing the aerators’ effectiveness.
Fish kills also have happened in Fort Collins and elsewhere in shallow ponds in summer when warm days and nights occur. That prompts algae to bloom, creating a drop in dissolved oxygen concentration, which in turn kills fish.
Calhoon said the parks department uses water from city park ponds to irrigate grass, trees and flowers. He said it has for years worked on ways to reduce such kills by using aerators, not mowing right up to pond edges and using 50% organic, slow-release fertilizer to help prevent fertilizer runoff into the ponds.
He said Fort Collins Utilities has been working on establishing water quality guidelines for the city to further protect the ponds.
Open-water fishing is allowed on all city park ponds with a valid fishing license.
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