Protecting Your Pond and Fish During Autumn and Winter

By Dave LaBomascus, Stoney Creek Fisheries, Grant, MI Originally Published in Passion for Water, Fall 2007 Revised Fall 2014

For many parts of the United States, it’s getting to be that time of year again ... leaves fall from trees ... the weather turns cool ... then cold ... then colder! A pond in poor condition after summer will subsequently have a rough time this winter. It’s time to get your pond, fish and aquatic plants ready for winter. Anything you can do to reduce stress prior to and during winter, will pay off appreciably next spring!

Since fish are cold-blooded, their diurnal (daily) routines change in the fall and winter. All biological and chemical reactions start slowing down. Aquatic plant growth slows greatly. Many species of plant die out in winter. Phytoplankton (single-cell algae) continue to produce oxygen during daylight hours as long as some light penetration is enabled through the ice and snow cover. After fall overturn occurs and dissolved oxygen levels are mixed evenly throughout the water column, fish migrate to other areas of the pond. During winter, fish tend to remain near the pond bottom, moving little. Fish activity, including feeding, is greatly reduced. Digestive rates decrease with decreasing temperatures. Depending on species, growth rates decrease or stop completely. It is important to remember that when you feed, you’re “feeding the pond”, not just the fish. Pond owners should feed their fish very little, if at all, during the winter. Over-feeding increases the organic wastes in the pond, which increases the total oxygen demand of the pond ... and this may well contribute to a winter-kill incident. For sport fish ponds, a good idea is to stock fathead minnows in the fall. This natural food source can then be consumed by your fish as dictated by their winter nutritional needs.

As air temperatures cool, so do pond water temperatures. Cold water is heavier than warmer water. At this time of year, cooling surface waters sink to the pond bottom forcing up the hypolimnion, the lower layer produced during summer pond stratification. This phenomenon is known as “fall overturn”. The hypolimnion is void of oxygen and high in waste products (carbon dioxide, ammonia, sulfites, etc.). Pond owners without aeration systems can experience significant problems associated with fall overturn. A correctly designed and correctly placed air diffusion system eliminates the problems associated with all four seasons of the year: fall-overturn, winter-kill, spring-overturn, and summer-kill. (see our article on avoiding winter-kill) Whether a small, ornamental, koi pond or a larger sport fish pond — maintaining an opening in the ice through the use of a pond heater/de-icer or an aeration system helps ensure that your fish will be alive next spring! 

As I mentioned in the opening paragraph, leaves have started to fall. Fallen tree leaves and dead aquatic plants comprise the majority of the organic matter decaying in the winter. Ponds that are surrounded by trees and that have a dense aquatic plant community are likely candidates for winter stress and potentially a winter-kill. If possible, clean your pond in the fall. Small ponds are relatively easy to clean. Remove fallen tree leaves on the pond surface, before they sink to the bottom. Long handled dip nets and “skimming seine nets” work well for this purpose. Larger ponds are certainly more difficult, but whether by physical removal or chemical control, nuisance aquatic plant growth should be eliminated before the fall season. At least one month of warm water temperatures is required to decay dead vegetation. The addition of pond bacteria can accelerate this process before temperatures cool.

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Use these guidelines to protect your pond & fish through the changing seasons

  • Get to know the habits and characteristics of your fish species.
  • Do not overstock your pond. Overstocking leads to over-feeding which increases oxygen demand.
  • Feed minnows to sport fish in the fall. Except for trout, avoid over-feeding commercial pelleted diets during the winter.
  • During cold weather and cold pond water temperatures, avoid handling your fish and returning them to cold water. Handled fish heal slowly in cold water.
  • Use aeration to keep ponds oxygenated and well mixed all year-round (i.e., prevent stratification).
  • Keep your pond clean and free of excess organic matter.
  • Eliminate excess aquatic plant growth before cool temperatures begin in the fall. One should actually initiate aquatic weed control in the spring and maintain this control through summer ... then autumn and winter is much more manageable.
  • Deeper ponds typically experience less stress ... can your pond be deepened?
  • For koi and goldfish ponds, stop fertilizing ornamental plants six to eight weeks before the first frost to prevent excess nutrients from entering the pond system.