Sepp Holzer Type Fish Ponds

After watching the video of Sepp's farm, amazing. I have a question. Hoping someone can help. I'm not new to small backyard ponds, living in the Midwest, where ice completely covers the pond, and without a heater to keep a hole in the ice, the fish will die. My question, is how does Sepp keep a hole in his fish ponds? It seems to me, since he's a permculture guy, he would use something that does not require an NRG source, even though he generates his own electricity.

Any thoughts?

PS - I wish there was a dvd/book on how to construct ponds like Sepp has. With detailed instructions.

i don't think that the fish necessarily  need a hole all winter...  where i grew up in northeastern ohio z5 we had a pond behind my house that always had fish in it... until last year ... in the spring my father found that all the fish were floating ( we made fish emulsion) but what i found out was that the fish will die if the ice doesn't melt early enough... they will stay below it for as long as they can.... the cold water actually holds more oxygen... but if the ice stays later than usual there can be serious oxygen depletion and the fish will die... like i said this usually isn't a problem... but it was for the first time that I can remember last year... from what I can tell this also becomes less of a problem the deeper the pond is (ours is only 6 ft deep in the middle).... btw we have recently constructed another one with a bulldozer.... we just dug the hole and ran the dozer over it a bit to compact the clay.. we have a lot of clay here though.

While I do not consider myself an expert on ponds (not even close), I did put in a pond last year and added four goldfish I bought for about $.50 apiece.  I remembered hearing that ponds had to be at least 2-1/2 feet deep if fish were going to survive the winter (I live in Nova Scotia) so I dug part of it that deep.  Long story short, the fish survived the winter, even though the pond was thoroughly iced over and there wasn't anything remotely resembling a hole in it.  I believe that if a body of water is deep enough, fish will survive months under ice.

Sepp's ponds mostly have rocks in them.
The rocks protrude above the water surface or are just below it.
The idea being that the rocks will hold heat from the sun
shining on them as well as reflection off the water onto the rocks.
This to prevent the ice forming.
I suspect that using darker colored rocks will give warmer rocks.

Make it really deep and they should do ok (so long as they are the right kind of fish for the area).

its the logs and rocks in the ponds that let the pond breathe. im pretty sure that its in one of his videos, might have been the movie that came out recently.

Sepp also had ponds at different elevations and as the water was piped down it created a lot of turbulence which both oxygenated the lower pond and keep it from completely freezing over.  I remember is also help keep the duck 'island' shelter protected from predators who could walk on the ice, but not swim and it gave the ducks access to water.  I assumed it worked all winter, but it's possible he had to shut it off at some point to keep the pipes from freezing.

Thanks everyone for the advice.