The often repeated statement that fish hibernate over winter is actually untrue and a bit of a myth.
Technically, fish don’t “hibernate” in the way bears do (which is called torpor). Instead, they enter a state called torpor or brumation.
As fish are an ectothermic (cold-blooded) species, their body temperature and metabolic rate are dictated entirely by the water around them.
When the temperature of your pond drops, everything about your fish will slow down to a very slow crawl as they try to conserve energy.
What Happens Inside the Fish?
As the water cools below 10c, the fish’s body will actually undergo several fascinating shifts:
- Metabolism: One of the main things that happens is that their heart rate will actually slow quite significantly, and as a consequence they will also require very little oxygen.
- Digestion: Alongside the slowing of their heart rate and the requirement for very little oxygen, their digestive system also nearly stops. This is why feeding them in winter is actually quite dangerous as the food will sit in their gut, undigested, and can eventually rot or cause them to have bacterial infections.
- Immune System: As well as the other changes, their immune response also effectively “goes to sleep,” this makes them much more vulnerable to parasites in the early spring before the water warms back up and their immune system is back to full speed.
Where Do They Go?
In your natural pond, the densest (and therefore “warmest”) water will always sink to the bottom. During the depths of winter this water usually hovers around 4c.
- The Bottom Hug: Fish will huddle at the deepest part of the pond.
- Minimal Movement: They stay nearly motionless, though they aren’t “asleep.” If a predator or a bright light disturbs them, they can still move, albeit very sluggishly.
The “Danger Zone”: Gas Exchange
The biggest threat to “hibernating” fish isn’t actually the cold—it’s suffocation.
If your pond freezes over completely, toxic gases such as carbon dioxide and hydrogen sulfide produced by decaying organic matter become trapped. Meanwhile, the oxygen levels drop.
Crucial Tip: Never smash a hole in the ice with a hammer. The shockwaves can actually kill or severely stress the fish in their dormant state. Instead, use a pond de-icer or a small heater to keep a small “breathing hole” open.
When Do They “Wake Up”?
As the water temperature rises consistently above 10c in the spring, you’ll notice them moving into shallower, sunnier spots. This is the time to start introduced Wheatgerm food—but only once they are actively looking for it.
