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What do I need to do to overwinter the fish in my farm pond? – Pond & Lake Q & A


Winter is coming, are your fish ready?

Pond & Lake Q & A

Recently, someone asked me a fantastic question regarding winter aeration and if it can “super-cool” your pond in the winter, possibly causing harm to your fish. I had one of our expert Fisheries Biologists, Justin McLeod, answer this question. Below are some easy solutions that he suggests to ensure a fish-safe winter for those of you in the colder climates. – Jason Blake, The Pond Guy®

Winter aeration (bubblers) can be very important in areas where ponds can freeze over. Along the Northern US border and into Canada, mid-winter temperatures dip well below freezing for prolonged amounts of time. This can put your fish into jeopardy if the pond freezes completely over. To answer your question regarding “Super-Cooling your pond, extremely cold surface temperatures cause ponds to stratify in the opposite way of the summer. Because water is most dense at 39 degrees Fahrenheit, the water beneath the winter thermocline stays around that 39 degree mark, while the water above the thermocline drops down near the 32 degree mark. This is a small difference, but it could mean life or death to a fish.

A “super-cooled” condition is created generally when surface aeration (fountains, High Volume Surface aerators, or really anything pump or pushing water into the atmosphere) is used during winter months. “Super-Cooling” happens when the colder water on the top is circulated to the bottom, leaving no warmer water refuge for the fish. Although it is uncommon to see “Super-Cooling” with sub-surface aeration (bottom bubblers), there have been instances when it has happened. With that said, I would never let this stop me from operating my aerator in the winter – my fish are just too important to me!

Here are some tips to make sure you get the most out of your aeration system and possibly avoid the chance of “Super-Cooling”:

    1. If you have multiple diffuser plates, it is ok to run only 50% of your diffusers. Even though the mixing power of your system is decreased, it will still add oxygen to the pond and allow gases to escape out through the hole it creates in the ice. Note: You only need 10% of your water surface open in the winter for gas exchange.
    2. If your pond is extremely small (1/8 acre or less), you may want to move your plate(s) out from the deepest area into a shallower spot. This will leave room for fish to winter in the deeper water.

POND TALK: What do you do to prepare your lake fish for winter?

18 Responses

  1. I’ll try that product. Thx for the info

  2. With the water going in and out, I dont think the bacteria will stay in the pond?

    • Hi Lillian,

      If you have water flowing in and out of the pond you may want to consider the Muck Away. It is a natural bacteria product but in pellet form so it will sink to the bottom of the pond rather then float at the surface when applied.

  3. We purchased a new home in Lake Tahoe last May. The previous owners turned a swimming pool into a pond and had stocked it with Rainbow trout. There are huge boulders in the pool (pond) and it is fed by a natural creek. The water runs in thru a fountain into the pond which aerates it and provides fresh water. It flows out of the side of the pond and back into the creek. So we cannot treat the pond for algae. The fish keep it fairly clean, however we would like to get rid of some of the muck on the rocks, the sides, and the bottom of the pool. What vacumn would you recommend. I don’t know exactly, but it is probably 25,000 or 30,000 gallons of water??

    • Hi Lil,

      Vacuums are typically used in ponds that are 5,000 gallons or less. For that much water you may actually be better off using a natural bacteria like Muck Away or Muck Defense to eat away at any organic debris. It may not be as quick of a result but a lot less work and offers continual cleaning.

  4. I have a pond that is about 3/4 of an acre and goes up to about 23 feet deep. I have just had it stocked with Bass and bluegill. The company that stocked the pond stocked it with matuer bass that go from about 2 lbs to 3 lbs. They also stocked it with about 50 lbs of bate minnows. The area I am in does not stay below freezing fro long periods at a time. The pond may get a thin layer of ice on it but is gone in no time. Yesterday I went to the pond and at the shallow end there where about 10 bass in about a foot of water. I tried to get them to move by throwing rocks in their area with no luck. I finally went down and pocked them with a stick and they took off. When they took off they seems fine but at first I thought for sure they had died. Do bass go dormant in cold weather and if so why are they in the shallow part of the lake. Is there anything I need to do? I know when it was stocked back on 11/29/2009 the company told me to harvest the first 10 to 12 bass I caught because they felt there was to many bass for the size of the pond. I havent been able to catch any bass yet to take them out? Any issues or are the bass just dormant as we have had a cold wave that has kept us below freezing for the last few weeks? also how do you know if you need areation in your pond?

    • Roby,

      I apologize for a delay in this answer. As the cold seasons set in, a fish’s metabolism will get slower and slower. They will eat less and not swim as much. This is very normal for fish in the winter months. As for a reason why there are towards the surface of the water, this may be because of oxygen levels. If your pond does not have an aerator, there is a separation in the water known as a thermocline. During the Spring, there is warm water with oxygen at the top and cold water with little or not oxygen at the bottom. During the winter, this will flip causing the no oxygen water to mix with the water containing oxygen. When this happens fish are confused as to where to find oxygen. Going to the surface may be their only way to get oxygen from the air to water oxygen transfer. Hopefully this makes sense. Every pond is suggested to have an aerator, especially if you plan to have fish in the pond. With a 3/4 acre pond, 23 feet deep. An AM30 Aeration System would be perfect for that.

  5. I have a working windmill near my 1/2 acre pond.
    Is there a way to convert it for areation during the winter?

    • Bob, You would need to add a diaphragm to your windmill to produce air and an airline with stone(s) to deliver the air into your pond. You can purchase these components individually, but without knowing what type of windmill you have, it may require some retro fitting to adapt these components to your present windmill.

  6. I highly disagree with leaving the aerator on thru the winter as usual. 2 years ago I left one of my 2 diffusers on throughout the winter…it was in about 17 feet of water. Come spring, I had lost at least 150 fish. For 3 years prior, I didn’t have an aerator, and kept a few holes open thru winter with an auger. I never lost a fish. Numerous people told me afterwards that the aerator froze the fish because it kept the entire pond cold…the fish had no warmer water to get to.
    Last year I shut 1 diffuser down completely, and moved the other into 3 feet of water near the edge of the pond. I put it on a timer so that it didn’t run all the time…it just kept the ice open. The animals had a watering hole, and a didn’t lose a single fish.

    • Don, From reading your post, I suspect that when you had your aeration plate at 17 feet deep, this was near the deepest part of your pond. Usually, you want your diffuser at the average depth of your pond, especially for winter time aeration. Fish will go dormant for the winter in the deepest part of the pond, so having the diffuser plate located in that section would cause some disturbance. At average depth, or just a few feet below the surface for the winter, you will avoid this disturbance. Keeping the hole open in the ice for oxygen and gas exchange is the main goal.

  7. Note to All: Super-cooling should not be a reason NOT to aerate. Aeration is an extremely important aspect of a healthy pond as well as healthy fish. Super-cooling happens very rarely and usually happens with surface aerators (which are usually taken out during the winter anyway) and not bottom bubblers. Call us with any questions, we are very happy to help. 810-765-7400.

  8. John,

    During last January you said you lost fish, but you had an aerator in there during that time correct? What model aeration system did you have at this time? Super-cooling is a very rare occurrence, especially with a bottom bubbler. In your case, you mention that there were only a few days in January above freezing. Throughout January, did your bubbler diffuser keep a hole open in the ice? If not, then toxic gases underneath the ice may not have been able to be released, thus causing your fish kill.

    1.) If super-cooling is really what caused the issue, then you would be fine to just move the one diffuser you had to a spot not directly on the bottom of the pond. I would’ve moved yours 3-4 feet above the deepest point. This would allow enough of an area for the fish during the winter.

    2.) But now that you have two diffusers, one in the deepest point and one in 4′ of depth, you would be fine to turn off the diffuser in the deepest point (again only if super-cooling is really an the issue). Having just the 4′ depth diffuser running should be enough to keep a hole open to allow gases to escape. Also, you are still adding oxygen to the pond, just not as much as you were at the deepest point. This should be enough to keep your oxygen levels up throughout the winter.

    3.) Once the ice begins to break.

  9. Last Janurary, here in Michigan, we had a total of three days above freezing. Needless to say I lost a lot of fish. It seems it was caused by super-cooling.

    To avoid this I purchased another difuser to place in the shallows during the winter. Usually I run two diffusers during the summer. The pond is about 1/2 acre with a max depth of 15′ and an average depth around 8′.

    I have a couple questions.

    1. At what point should I stop the aeration in the deep water and switch over to the shallows? It would seem I would want to do this early enough to allow a thermocline to develop in the deep portions.

    2. Will running one bubbler at about 4′ deep on one side of the pond provide sufficent oxyegen and allow for gas release? I plan on shutting down one bubbler completly in the deep water and running the line from the shallow one to the other line and opening the valve to full.

    3. At what point in the spring can I resume my normal two diffuser set-up?

  10. Hi i have a question i wrote to you earlier on heron
    we had our pond for over 15 years and never had a heron till this summer we fenced it up and put net over it. it is just a small pond with waterfall about 300 – 400 gal. do heron go south for winter or do they stay in this cold iowa weather? We have always put fence around pond in fall to keep leaves out and net on top but take the net off when leaves are done falling. I am afraid if we take net off the heron will fly inside?
    thank you….. also can we keep the waterfall going for awhile yet? We have cleaned the bags of lava rocks and took the plants out but our weired weather got hot so we kepted it going instead of turning off. I still have to get inside and cut the water lilles back its still warm where they will grow yet so was waiting on that otherwise the pond is clean. The guy that put our pond in about 6 years ago didn’t like water lillies in the pot he just buried them in big rocks and so I have always just cut them all the way back to the roots and we have done fine. I really appreciate all the help you have been in the past… thank you

    • Susan,

      Herons do fly south for the winter. Now for those who live in the south, plan on seeing herons =). Susan, have you tried using a heron decoy or motion activated scarecrow to scare the heron away?

      As for the waterfall, depending on your weather you can keep running your waterfall for a while. I have some customers that run there waterfall all winter as well (read post here). We are here in Michigan and we are still running ours. You can probably go to November if you’d like. It really depends on when ice begins to form.

      Hopefully this helps!

  11. I wondered about using aeration in my small pond/watregarden. My pond is only 18″ deep.(about 250 gallons). It is in a somewhat protected spot in the yard, a 6 foot fence behind it, and trees surround it on 3 sides. For the past 3 years i have only used a de-icer. The 15 fish who habit there, winter just fine. Last month I won a gift basket full of pond essentials. In it is a one stone aerator. I’m nervous about hooking it up this winter. The pond does freeze over a couple of times through the winter. I live in South Jersey, just outside of Philadelphia.

    • Kay,

      I would hook up the aerator and use it in conjunction with your de-icer. This will ensure a hole is always open in the ice to allow toxic gases to escape. I wouldn’t be the slightest bit nervous about adding an aerator to your small decorative water garden. Hope this helps!

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