Montana biologists are reporting dead fish in the water by Riverfront Park in south Billings, and they expect more to come as the winter ice melts. Winter fish kills occur when plants in the bottom of ponds freeze, then get covered by snow, according to Montana Fish, Wildlife and Parks. The plants die and are consumed by bacteria, which suck all the oxygen out of the water. Biologists say fish floating on the surface died of anoxia, a reaction to the lack of oxygen in the water.