IDAHO FALLS - The typical Currier & Ives scene of a snow covered pond conjures up peaceful thoughts for humans, but for fish residing in lakes a blanket of snow could have a more ominous meaning. In some cases, the long winter and extended period of snow cover has taken a toll on some of the lake fisheries in the Upper Snake Region. Roberts Gravel Pond and Mud Lake have suffered fairly extensive fish die-offs the past couple of weeks. While unfortunate, such fish kills routinely occur when a lake runs out of oxygen in the winter. Fortunately, oxygen levels in Henrys Lake are holding up well.
"Fish in lakes depend on plants to make oxygen for their survival," said Jim Fredericks, Regional Fish Manager for IDFG in the Upper Snake Region. In the winter, many of the plants die out. Instead of producing oxygen, they decay and actually take oxygen out of the water. How many plants die and use oxygen instead of producing oxygen depends on how much sunlight penetrates through the ice to allow photosynthesis. Sunlight penetration, in turn, depends on how long the ice covers the lake and how much snow covers the ice. This winter, although the mountain snowpack is below average, there has been an unusual amount of snow on the Snake River Plain. The resulting prolonged snow cover led to the decreased oxygen levels in these two lakes.
Henrys Lake, the crown jewel of Idaho's trout lakes does not appear to be heading for a winterkill. In the winter of 1990-91, oxygen levels in much of Henrys Lake dipped below the threshold for trout survival, resulting in a fairly extensive die-off. Since then, biologists have monitored oxygen levels in Henrys Lake through the winter to determine the rate of oxygen depletion. Because biologists anticipated less than favorable conditions this winter, IDFG also earlier began running the aeration system in Henrys Lake to provide additional oxygen.