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How do fish go dormant? Are they really asleep? – Decorative Water Gardens Q & A

How do fish go dormant? Are they really asleep?

How do fish go dormant? Are they
really asleep?

Ryan – Falston, MD

With the warm days of summer now a distant memory and fall following in its footsteps your Koi are left with a few months of cold weather with nothing to do but relax. Since they don’t have miniature submerged Koi calendars to check what is it that tells your fish it is time to hibernate?

Koi are cold blooded creatures which means their body temperatures and activity levels are directly correlated with the ambient temperature. Koi are active and alert when their environment is warm and will start to slow down as the water temperature decreases. Once the water temperatures start to dip below 46°F your fish tend to stop eating and will retreat to the bottom of the pond. Your fish use the decrease in temperature along with the shortening day lengths as a trigger to prepare for winter. As the water begins to cool your fish will become less active as their bodily functions slow down. Less activity means a slower digestive process, less demand for food. It is this decrease in food digestion that warrants the use of wheat germ based foods like Pond Logic® Spring & Fall Fish Food. These types of food are easier to digest that regular food reducing the risk of leaving undigested food to rot inside a dormant fish which can potentially be fatal.

As the temperatures continue to decline towards 40°F, the blood flow and respiratory rate of the fish will drop to an extremely low rate where their body is hardly functioning. You may hear people say that your Koi are sleeping in the winter and while fish do sleep this goes way beyond the standard drop in bodily functions associated with some much needed shut-eye. This extreme internal slow down ensure survival with even the most limited resources with cases of dormant fish lasting 150 days without food.

The whole over-wintering scenario sounds a little extreme to us but it is truly a natural and normal process for your fish. They do not require much attention in the winter but there are a few things you can do to ensure their winter break is a success. When a layer of ice begins to form over the pond, maintain an opening for gas exchange using your aerator or a de-icer. Also, make sure you feed with a cold weather formulated fish food as water temperatures approach 50° so your fish are able to safely digest it before hibernation begins. You can use a pond thermometer to keep track of the water temperature in your water garden.

Pond Talk: What do you do to help your fish through the winter season?

Get your fish ready for wither with Pond Logic® Spring and Fall Fish Food!

9 Responses

  1. We did are before winter last cleaning of the pond then put a net over. The lilies pots are still in the pond along with a few rocks and the pump. Question is My larger fish 5in have disappeared.. and only see the little ones 1 to 3 in. at night. Where are the larger ones ?

  2. After not feeding my koi during our long Mid-western winter I notice that my fish have actually grown. Is this my imagination or are they getting some nutrients from the microscopic living things like plankton that naturally float in the water? Just curious.

  3. I have a question that im hoping someone can answer!I brought my koi in today,i filled a rubber maid with the pond water and put my koi in it and brought them in the house to get ready to tank them for the winter.One of my koi is still breathing but isnt moving much unless provoked,he lays on his side or belly up up at times.Was the tote to small perhaps?Has he decided to go dorment or is he just gonne die on me?He was 100% befor i took him out of the pond.

    • Hi Michelle,

      Do you have an aerator or filter system running in the tub? The size should not affect the fish much as long as the water is still full of oxygen and being cleaned.

  4. I’m beginning to think that I should turn off my submerged light during the winter as this probably adds heat to the lowest layer of water and may disturb the dormancy of my koi. Am I correct?

    • Susan,

      Generally in the winter months when water temperatures drop below 50 degrees, plants and fish enter a dormant state. Disconnecting your pond lights would allow for the natural process to occur . The wattage of the lights would determine how much heat was being generated0 by the lights.

      Thank you,

  5. When KOI go dormant in the winter months, do they need the oxygen from the waterfall or can it be turned off?

    • John,

      This is a very good question and one that has probably crossed many of our Koi owner’s minds. The Koi do need oxygen during the months that they are dormant, however your waterfall may be turned off. Keeping a hole open on the surface of your pond allows for the toxic gases to escape from decomposing leaves or other organic material and allows for oxygen exchange.

      A de-icer and/or an aeration system will keep a hole open on the surface of your pond. The added benefit of the aeration system is that if will provide more dissolved oxygen into the water.

      I have also provided a link to another Question & Answer from The Pond Guy Blog that discusses de-icers and aeration systems.

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