Canadian Winter -30C and lower

My first winter with my pond with fish and so far so good with the fish,
but I think someone forgot to tell the fish thats its winter and they
should just stay at the bottom.

I put a black liner over 80% of the pond and the balance a clear plastic
sheet with support beams to keep the liner about 6" above the water.

Currently there is snow covering the whole pond ( 5ft x 4ft ) and we are
having a cold spell of -25C to -40C with the wind chill.

A few weeks ago I checked the pond for the first time, I have an air
line with a stone at the end that was about 8" deep, Ice had formed over
the whole pond, so I took a kettle of hot water to open a hole, brought
the air line up to 4" below the surface and removed the airstone and
just let the large bubbles from the airline come to the surface, I dont
think the small bubbles were able to keep a hole open with these
tempertures.
I also place a raw piece of wood that sits on an angle above the surface
and sits in the shallow end, I left it this way for a week and since the
weather has been colder since the first time I check, I thought it might
have frozen over again, but it wasnt, there is a good foot opening where
the bubbles are hitting the surface.

I'm sure if I hadnt covered the pond, it would have frozen over and with
regards to the fish, they are still swimming around down there and the
water is below 0C near the surace, I'm curious as to the tempture down
at the bottom.

One thing I'm afraid of is the small fish hiding under rocks in the
shallow end and then the water icing up around them, but from what I can
see, I dont think the ice will form any thicker then a few inces and the
fish will be fine.

Last year I did a dry run without fish and no cover and the ice was over
24inches thick and the airline froze in with the ice.