http://www.edmontonjournal.com/Alberta+ ... story.htmlThis story in the Edmonton Journal explains it better. Ice pack is part of the equation but it has it also has to do with phosphates and algae using up the available oxygen when the sun can't get through the ice pack as detailed below....
“As more nutrients, typically phosphates from land clearing, agriculture, cows and towns, flow into the lake, it basically fertilizes the growth of algae blooms that create oxygen in the daylight and use oxygen at night — and winter is one long night,” Sullivan said.
“With the cover of ice, light doesn’t get through to the water, the algae uses up the oxygen and the oxygen levels decline throughout so the only oxygen left is near holes of open water where there’s a little bit of oxygen exchange with the open air so the fish cluster around there,” he said.
This is all notwithstanding the current VHS investigation which could be the actual cause of the kill off....