We live in the interior of BC, Canada - elevation about 3500 feet. It's rare but weather can get as low as -40C. We often get extended periods of -20.

Our pond is roughly 20 X 16 feet with a maximum depth of 5 feet. We calculate it holds about 3300 gallons of water.

Our pond contains Rainbow Trout - we started out with 16 4 inch fish and 8 10 inch fish. Over the summer (our first year with the pond) we lost some fish to birds of prey and this fall we had ended up with 5 of the smaller fish and 4 of the larger. All had grown very considerably.

We pulled the pump and filter in mid October and ran an air stone and floating heater. We had some -10 to -15 nights and the foam from the airstone would freeze and build up ice which the heater could not keep up with. Although during the day, when it warmed, this would melt.

The next forcasted cold snap, we turned off the airstone. The pond froze over although the heater maintained an 18 inch hole. When we turned off the airstone and before the pond iced-up we tested the water. It was about 3 celius and water quality was good.

About 5 days later, we found one of the smaller fish floating. We brought it in to check the stomach contents as we thought maybe we weren't feeding enough and perhaps the fish had starved. We feed pellets but the fish feed deep and we can't always see them to know how much they are eating.

This fish's stomach contained a scud (freshwater shrimp) and lots of chironomid larva - some of which were still alive - so we know he didn't starve. Because there was still live insects in his belly, we know this was a VERY recent death.

We turned the aerator stone back on, fearing they needed oxygen. The warmer weather, combined with the airstone, caused the ice to retreat. Early the next day, we found 2 more of the smaller fish floating on the surface of the deeper end and later that day found 2 more small fish in about 4 inches of water, under the remaining ice. We believe these particular fish had been dead for at least a few days - due to the condition of the bodies and stomach contents.

We have not seen the 4 big fish - they could be sitting somewhere out of sight but we suspect they may have floated up and been snatched by the birds. They would have required more oxygen therefor should have died first - assuming this was an oxygen problem.

Is our assumption wrong that these fish died from a lack of oxygen? And is keeping a hole open, with a heater, adaquate or do we need to run a stone throughout the winter? Why would the pond have run out of oxygen in just 5 days or is there something we're missing?

Can we allow that foam from the airstone to build? We do not want to run the pump and filter because of power outages which occur periodically around here.

It may be worth mentioning that the pond has been going through a cycle of freezing and thawing for a few weeks now and we've noticed that the water is very murky but as mentioned, testing has shown good water quality results - low nitrates and amonia.

Anybody out there who lives in as cold a climate as us who can help us out with some thoughts and advice?

We were going to post a picture of the pond but can't seem to find a way to do that.