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Winter Fish Kill Can Be Prevented

Early spring is often when we are asked about Winter Fish Kills and what causes them. Frequently this question comes from people who have just ventured down to inspect their pond after the ice has finally melted off on a beautiful spring day, only to discover heartbreak. They anticipate a beautiful pond, but what they encounter is dead fish.

These are the same fish that they have stocked and fed and enjoyed all last year, but are now dead, sometimes sunk on the bottom, sometimes floating belly up. This is a frustrating and disappointing moment, but it can be prevented from happening again in the future.

Why Fish Kills Happen

To prevent winter fish kills, the first step is to understand what the real causes are and to dispel a common misconception. The misconception is the idea that the pond “froze to the bottom,” causing the fish to freeze and die. Unless the pond is extremely shallow (like less than three feet), this isn’t what happens. Even in extreme Northern Michigan winters, we rarely see ice more than 40 inches thick, usually it is much less than that. This winter, ice is less than 12 inches in our area, maybe up to 24 inches somewhat farther north.

So, if the pond didn’t freeze to the bottom, what killed the fish? The primary answer is usually a lack of oxygen. Once a body of water freezes over, it can no longer exchange oxygen and other gases with the atmosphere; the ice essentially seals the top of the pond like a big sheet of plastic wrap. This leaves the pond with only the amount of oxygen that was in the water at freeze up to last until the spring thaw. In the meantime, there are lots of biological processes taking place in the pond that use oxygen, such as fish respiration, aerobic bacterial decomposition of weeds, etc. Without oxygen replacement, the pond eventually runs out of available oxygen and fish die.

In addition to the lack of oxygen, other gases can also build up under the ice, again due to various biological processes, and these gases can reach toxic levels. Usually, winter fish kill is caused by a combination of lack of oxygen and the build-up of harmful gases in the water.

How to Avoid Fish Kills

The key to preventing winter fish kill is simply to aerate the pond in winter. Using a properly sized diffused aeration system or circulating deicers will continually introduce oxygen to the pond as well as maintaining some open water. This open water is important to allow the exchange of gases with the atmosphere, allowing oxygen in and preventing the buildup of harmful gas compounds.

Any aeration is better than none, but best results come from aerating the entire water body adequately. This can be accomplished with several different types of equipment including de-icing circulators and diffused (bubbler) aeration systems. Diffused aeration can be AC Electric, DC Solar, or Wind-Powered. There are many options to fit any pond and any situation, whether the pond is on or off the electric grid.

Selecting the right option for aeration may be a little overwhelming, but we are here to help. We will measure your pond, talk to you about your goals and experiences and help you select the system that is right for you. We are available for advice over the phone or you can schedule an onsite pond consultation.

Don’t delay until next fall and then come up with a hurried solution. Getting your aeration solution installed this spring gives you a whole year of aeration benefits. While the subject of this article is winter fish kill, there are also many reasons to aerate a pond for benefits all year around.

Aerating your pond can be simple and we can help you through the entire process from system selection to installation of the equipment. Give us a call or email, or set up a consultation online. We are happy to help!