Originally Posted by rlitman
Naturally occurring ponds have way less fish per gallon of water. That leaves much more available dissolved oxygen. A frozen over man made backyard pond is like stuffing an elevator full of people, and duct taping every seam shut. Eventually they'll run out of air.
This analogy may have some merit if the pond already has a stocking density too high for the pond's capacity. I hear of just as many reports of Koi (and Goldfish) surviving the winter in an iced-over pond as I do of Koi (and Goldfish) mortality in ponds that provided venting in the ice.
Koi and Goldfish are known to survive in anoxic (low oxygen) conditions for extended periods under otherwise normal conditions. Cold water will retain considerably more Oxygen than warm water and fish metabolism is extremely low at low temps. Koi and Goldfish require very little Oxygen to survive in cold (near freezing) water. Koi because they are a warm water specie and can adapt to low Oxygen levels and Goldfish because they have the ability to store Oxygen in their muscle tissue when necessary. This has been proven in several scientific studies.
I would need to see more than anecdotal evidence that venting or aeration would be required in an iced over pond with the appropriate fish load.