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What Is A Winter Fish Kill & Why Do They Happen? | Ponds & Lakes Q&A


What Is A Winter Fish Kill & Why Do They Happen?I’ve heard the term “winter fish kill.” What is it and why does it happen?

Linda - Stockbridge, MI

A winter fish kill occurs when your pond or lake becomes uninhabitable for aquatic life and a large number of fish pass away. Winter is the most common time for a fish kill because the long, harsh conditions of the season deplete the oxygen content of the water.

Prolonged ice cover on your pond or lake seals off the water from the air and prevents oxygenation of the water. Ice and snow cover also prevent sunlight from reaching your pond plants. Pond plants normally produce extra oxygen for your pond via photosynthesis. Finally, decomposing organic matter in your pond releases toxic gases that can’t escape from under the ice.

Eventually the competition for the limited supply of oxygen in the pond becomes too great. The fish suffocate and a winter fish kill occurs.

The good news is that winter fish kills are preventable. There are many things you can do as a pond and lake owner to maintain proper oxygen levels in your pond. First and foremost, invest in a good Aeration System. Aeration is the process of circulating air through water and it’s critical for overall pond health.  A second option is to add beneficial bacteria products like MuckAway™ or PondClear™ to your pond or lake before the winter season. Beneficial bacteria digest accumulated decomposing organic matter in your pond and convert it to a harmless, odorless gas.

Pond Talk: What fish behavior may indicate low oxygen levels in your pond?

Airmax Aeration Systems

2 Responses

  1. We have about 40 two pound rainbow trout in what was a 30,000 swimming pool. It has a source of fresh creek water supplying it year round. We have continued to feed the trout about twice a week all winter as they eat readily. Is this the thing to do or should we stop feeding them. There is some algae in the pond, not much but I notice they do eat on it. We had hoped they would control the algae. It is hard to use anything for the algae as fresh water pours in and runs out constantly.

    • Algae needs a food source. Could be the (1) the fish poop or (2) the quality of incoming water. Can you increase the flow ? i.e. , flush your pond. (most trout streams i’ve seen run pretty fast). If not, how about
      some type of in-line filter for your source water. Something i’d try would be a small pool big enough to hold 2 or 3 barley straw bales. Let the creek water “filter” through the barley straw and pick up its “anti-algae” benefactors. (honestly, never seen this done on a large scale)
      Good luck

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