Why can winter ice cover cause fish deaths in parts of the Ob River?

The seasonal ice cover on the Ob River contributes to fish mortality by causing a depletion of oxygen in the water. This phenomenon occurs during the winter months in the river's reaches between the Tym confluence and the delta.1

While the river experiences significant ice-related activity—such as the formation of ice jams during the spring breakup—the specific winter fish deaths are attributed to the lack of sufficient oxygen levels beneath the prolonged ice cover.231