Do you have a Pond?  

How do fish survive the winter months?

Have you ever wondered how fish survive in cold winter weather, or where they go when lakes and ponds freeze over? It’s the unique physics of water that makes it possible.

In summer, the water in a lake is typically separated into two distinct zones. Near the surface, it’s relatively warm and oxygenated; but in deeper areas, the water is colder and with less available oxygen.

Because of the differing densities of water at different temperatures, these two zones don’t mix much while the weather stays warm. But as winter approaches, the surface water cools along with the air. Eventually, the cooling water from the surface sinks and the lake’s layers begin to mix freely, a process known as “turning over.”

Almost every other liquid becomes denser as it cools, and is densest of all when it freezes solid. Water, however, is actually densest at 39 degrees F several degrees above its freezing point. This is why ice floats, and why lakes freeze from the surface down rather than from the bottom up. The water under the ice, freshly re-oxygenated from the fall turnover, remains at 39 degrees all winter long, allowing fish and other aquatic life to survive until the spring thaw.

Iced-in fish don’t hibernate, but they do slow way down. Fish are ecotherms. Unlike us warm-blooded creatures, their body temperature changes depending on their environment. Reducing their temperature and activity level means they don’t need as much food or oxygen.

While they wait out the winter, most fish seek out pockets of relatively warm, still water, often near the bottom of the water body they’re in. Trout and salmon prefer cold water and are more active than other fish in the winter, staying closer to the surface.

Fish can’t go entirely without oxygen, though. They still need to move water across their gills in order to stay alive. Despite the fresh influx of oxygen into the water during the fall turnover, this is still the most important limiting factor for fish in the winter. Adding a De-icer or Diffused Bottom Aeration in a pond will help alleviate stress in fish and prevent high kill rates by allowing for open areas where oxygen can enter the water and degassing can occur.

During the coldest months of the year, the photosynthesis of aquatic plants slows down, because the days are shorter and layers of snow on top of ice limit their access to sunlight. At the same time, that surface layer of ice prevents oxygen from the air from mixing into the water. The lack of oxygen can become especially severe at the bottom of a pond or lake, where microbes use up oxygen as they work on decomposing the year’s debris.  I dug a 1.25 acre pond several years ago and use bottom aeration which helps the fish out year round.  

The longer a winter lasts and the shallower a body of water is, the more likely it is that the fish in it will start to die off from a lack of oxygen. Different types of fish have different abilities to tolerate these low-oxygen conditions.

What about the rest of the life in our ponds and streams? Many amphibians burrow into the sediment at the bottom and hibernate. Microscopic life like plankton also often goes into a state of suspended animation until spring arrives.

Next time you look at a frozen river or lake, just remember: It isn’t as dead and still as it appears. Even in the coldest months of winter, our bodies of water are still teeming with life!


Tom Stachler is a licensed real estate broker service the Ann Arbor and surrounding real estate markets of Saline, Dexter, Ypsilanti, Milan and Chelsea Michigan.  Please contact us should you be in need or realty services or looking to sell or purchase residential, commercial or income property.