Koi in Winter
Koi are just Carp... Right?
Image courtesy of http://www.fotosearch.com/photos-images/yorkshire.html Koi have been breed for their beauty, not their ability to survive cold conditions. Is it possible to purchase Japanese brood stock, bring them to Iowa, breed them, and have their offspring be "Hardy Koi" after a single generation, we hardly think so. The Common Carp (Cyprinus carpio) is listed as a Subtropical species with a survival range between 37.4F to 95F.

Koi today are far from being a common carp in as much as a Yorkshire Terrier is in the same Canidae family as the Wolf. How long do you think that Yorkshire Terrier could survive outside in a subzero winter?
Image courtesy of http://www.all-about-wolves.com
How cold for how long?
Bickal Koi Farm is located in Toddville Iowa in climate zone 4b. Average Annual Minimum Temperature Range of
-20F to -25F.

The southern half of Iowa is in zone 5a which can be 10 degrees warmer. Minnesota ranges from 4a to 3a! which can be 10 degrees colder

The water temperatures drop below 50F around mid October and ice forms on the koi pond in early December. The ice doesn't melt off the koi pond until mid March.
Survive or Thrive?
In order for a species to Survive, it must be able to reach sexual maturity and produce offspring. Typical breeding age for Koi is usually 3 years, altho I have had koi that have reached a larger size spawn at age 2. Certainly we want our koi to live longer than 3 years!
Nature does it best!
Lets compare the conditions:
Condition Carp in nature Koi in your pond
Stocking Density Low (thousands of gallons per fish) High (10-100 gallons per fish)
Temperature / Average Depth Small farm pond can average 8 feet deep. Larger lakes can be 30 feet or more. Average depth 2 feet. Even tho your pond may be 6 foot deep in one spot, its the average depth below frost line which is the important factor here.
Water Quality With low stocking rate in lakes, and fresh flowing water in rivers, Carp in natural waters are rarely exposed to high Ammonia levels or pollutants. Do you do water changes in the winter? Your koi are most likely exposed to the same water for at least 3 straight months. By the time spring comes that water is toxic!
Location, Location, Location!
Koi being able to survive in your pond during the winter can really depend on the location of the pond. Ideally the pond should be in FULL sun during the winter months. Blocking the north-west wind will also improve your pond's temperature.
Let it Snow?
Snow is good and bad. A snow cover will insulate your pond preventing heat from escaping, however the snow blocks sunlight from radiating heat to the pond. The lack of sunlight also inhibits algae growth which could cause ammonia levels to rise and ammonia kills koi fast.
My recommendations
If heating your pond isn't an option, than I recommend bringing your koi indoors for the winter. You can enjoy your koi during the winter months and ensure that they remain heathly. Treating a sick koi is alot easier to do inside where its warm instead of outside under the ice in the pond. If you have no other options but to leave your koi outside, please follow these tips:
  • As much sun as possible
  • Keep the snow off
  • Do water changes even during the winter
  • Block the wind
  • Test the water for ammonia and PH
  • Keep your pond as clean as possible going into winter