Winter Fish Kill

pond ice startingHow to prevent a Winter fish kill

It’s never a good day when we walk to the pond only to see the fish gasping for air in the open water. Or worse yet is getting to the pond to see them all floating or already on the shoreline.

Why does a winter fish kill happen? During the summer months the pond does its best to accumulate oxygen by surface area contact, wave action to break surface tension and plants or weeds that produce a small amount of oxygen. All may be well for a while but even summer fish kills can happen.

Let’s stick with winter fish kill and why. Spring, summer and fall help to add oxygen to the pond but only as deep as it can. Below that during the warmer months there is a thermocline created, a barrier of sorts that separate the warm and cold water which could only be a few degrees different in temperature. The warm water having oxygen and the colder water has very little if any oxygen.winter pond cross section

Now visualize a cross section of the pond with an imaginary line called the thermocline separating the warm and cold water. As winter approaches the pond water cools down, plants and fish also slow down their activities. All is good, for now, even when ice forms and covers the pond as sunlight can still penetrate the ice and keep the weeds and plants alive to produce a little oxygen. But then the snow starts to accumulate on the ice and blocks the sunlight letting any weeds or plants to die, sink to the bottom and decompose creating carbon dioxide, ammonia and hydrogen sulfide.

winter pond beginsAs the decaying process continues through winter so does the amount of gasses build and push towards the surface. The fish need to move and stay above these gasses in the water with oxygen. If the pond fills with these gasses the fish will suffocate. That’ why we see them gasping for air at the surface.

How can we help save the fish in this time of need.  An aeration system designed  and calculated for the pond size and volume. During the warmer months the aeration is pushing up and out the bad gasses and allowing beneficial bacteria to the floor of the pond to help consume the muck and nutrients.  Properly sized aeration system should be able to turn over the entire pond volume at least once a day and be able to move water starting at the floor of the pond. Essentially building oxygen from top to bottom, stock piling and saturating for the long winter months.aeration close to zero small pond

But what to do when winter comes?  When the water temps’ hit 55 degrees we want to move one diffuser to half the depth of the pond to keep a hole in the ice allowing gasses to escape, keeping open water for sunlight to keep alive your wanted aquatic plants and water to air oxygen exchange. The diffuser can be placed under your deck, dock or other structure to keep it ice free and from being lifted by the ice.

Do I really need to run the aeration system during winter? No, but only if you have been running the system all summer long building oxygen in the entire volume of the pond. If you have two or more diffusers only leave one operating after it has been placed at half the depth of the pond. (If the pond is 10’ deep, locate the diffuser at 5’ deep) This keeps from super cooling the pond and harming the fish.

Another way to look at it is in recent years we have seen negative 15 degrees weeks of cold winter. Along with ice coming on as early as November and lasting till March. With the ice on the pond for this amount of time toxic gasses are building up with no place to go, which usually ends up suffocating the fish. Some reports over these type of winters came in that folks were going out to ice fish and once the hole was drilled they saw dead fish floating up to the hole.

aerator under farm pond deckOne caution on aerating during winter is to make the commitment to leave it run 24/7. Running the compressor only at night or on and off during a day could be harmful for the compressor. They do generate heat, then with the cold, condensation will accumulate in the compressor when it is off, freezing temps’ will form ice and when the compressor comes on again is when damage could happen. Check out our aeration systems and if you have any questions or need help in designing a system please use our contact page to get a free design.

Other related article is summer fish kill

Proper Sized Aeration


About the Author: Darrell Rhoades is the founder of A one man business, works full time in tool & die. It all started when he built his own pond for the family. Ran into pond issues and started the research with pond suppliers and conferences for pond management. He writes about pond building and pond management and sells pond management supplies, aeration & fountains and Practical hands on experiences at . No physical store, but has items in stock.

Comments on this entry are closed.

Next post:

Previous post: