Can Too Much Snow Be Deadly To Fish?

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. "Aeration systems can ease the problem, but unless we're talking about a very small pond, it's not realistic to think we can aerate a whole lake through the winter, " said Fredericks. Roberts Gravel Pond is a good example of a lake where an aeration system can't keep up with the total demand of the fish. According to Fredericks, "Aerators generally provide a small refuge in a much larger system." The problem is that if oxygen levels get dangerously low in the rest of the lake, fish will concentrate in the aerated area. The oxygen can then very quickly be sucked out of the water in that area. The result can be a fish kill right next to the aerator. "It's unfortunate that we're losing fish in Mud Lake," Fredericks said. Mud Lake has a history of winterkilling every few years, which limits the quality of the fishery. According to Fredericks, "Tiger muskies, and largemouth bass grow slowly. It takes 7-10 years to make truly large fish. Every time we have a winterkill, in essence, we have to start over." The impacts to Roberts Gravel Pond are not so severe, where IDFG periodically uses rotenone to kill stunted perch and non-game fish. "We can rebuild the fishery in Roberts Gravel Pond fairly quickly" Fredericks said. Conditions for fish at Ashton, Island Park, Palisades, and Ririe reservoirs remain favorable thanks to recharging inflows.