You have worked hard to maintain your pond or lake and the freshwater fish that live there. The first time the temperature drops, however, you may wonder – what happens to fish during the winter?
What do fish do when the temperature drops? The short answer is: not much.
Fish are poikilothermic, or cold-blooded animals. This means their body temperature adjusts to the temperature around them. It also means the change in degrees triggers changes in their body.
In the case of koi, when the temperature of the water drops below 50 degrees, they enter a state of torpor. Think of it like hibernation: their heart rate slows, as well as their metabolism, and they have a lower body temperature and reduced rate of breathing. Their activity reduces drastically. They swim slowly in the bottom of the pond, where it’s warmer, and conserve their energy.
Other fish may have enzymes systems called isozymes or allozymes that allow them to function at a lower temperature. In arctic regions, several species like cods and herrings utilize antifreeze proteins, which stops their blood from crystalizing in the extreme temperatures.
Fish have another advantage in winter: the properties of water. As the temperature plunges, the density of water increases. This means that ice floats to the top. The fish have a better chance of avoiding freezing since it happens from the top down versus the other direction. Freshwater fish seek thermal refugia, or pockets of warmth at the bottom of a pond.
If you worried about your fish after a cold spell, the best way to protect them is to maintain an opening in the ice. The opening allows gases to escape and an entry point for oxygen can get into the pond. You also might consider upping your predator traps. Many fish in a state of torpor or reduced activity are sitting ducks for predators scrounging outside their traditional haunts during winter.