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Fish mortality in lakes and ponds considered natural in the spring

MISSISSAUGA - The cold, harsh temperatures this winter caused extensive ice coverage on local lakes and ponds in the Credit River watershed.

Prolonged cold periods in January and February contributed to thicker ice, which affects oxygen levels in the water and result is higher than normal fish mortality in the spring.

This is a natural phenomenon known as winter die-off or “winterkill”.

“Winterkill happens naturally and is common in shallow lakes and ponds. When ponds and lakes freeze over with thick ice and snow the water becomes separated from air. It reduces the amount of sunlight available to aquatic plants,” said Jon Clayton, Aquatic Biologist for CVC.

“This limits the amount of oxygen available for fish. Without enough oxygen, fish die.”

Ontario experienced one of the coldest Februaries on record this year, which resulted in more ice coverage on our lakes and ponds. Our Great Lakes saw approximately 86 per cent coverage this winter, compared to 61% last year. Once spring temperatures normalize and the ice begins to melt, fish mortalities become obvious.

Fish mortalities are usually reported in places where ice initially melts. The lakes and ponds at Island Lake Conservation Area and Ken Whillans Resource Management Area are susceptible to winterkill events. Please notify Credit Valley Conservation staff if you see results of a large winter die-off.

CVC staff will monitor fish species that experience winterkill and the extent of any die-offs.

If you have any questions or concerns, please contact CVC Aquatic Biologist, Jon Clayton (905) 670-1615 ext. 502 or

April 17, 2015


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