Firing Up Aerators at Henrys Lake in Preparation for a Difficult Winter

Ice is a fact of life for nearly half the year at Henrys Lake. During a typical winter the fish in the lake are able to handle the difficult conditions fairly well, but a number of potentially negative factors have prompted fish managers at Idaho Fish and Game to do all they can to mitigate for potential damages. Fortunately Henrys Lake happens to have one of the largest man-made aeration systems in the country already in place to help keep a small portion of the lake ice-free, allowing for much needed oxygen to enter the lake water trapped under the ice. Rather than wait for conditions to turn bad, the aerators are now being turned on. Some of the same factors that make Henrys Lake a world-class fishery able to produce trophy trout can also lead to major problems when a variety of negative factors coincide. Henrys Lake is shallow, normally only averaging about 16 feet, but this year the water level is lower, and higher than normal fish numbers because of natural reproduction mean more fish packed into a smaller space. The lower water also allowed for sunlight to penetrate deeper, resulting in greater than normal growth of fish and decomposing aquatic vegetation. But when the lake freezes over and sunlight cannot penetrate, the vegetation can use up much of the dissolved oxygen in the water. Typically during the winter the ice is about 20 inches thick at Henrys Lake. "This year we are especially concerned due to the lower lake level combined with the high biomass of fish and aquatic weeds, both of which consume oxygen during the winter once the ice and snow cover the lake," said Dan Garren, the Upper Snake regional fisheries manager. While the aerators are an attempt to make a difference, they can only keep about 20 acres of the 6,200 acre lake free of ice. Fortunately nature also provides a little assistance by keeping areas near creek mouths, springs and upwelling areas oxygenated. The cost of the electricity to operate the aerators can run into the tens of thousands of dollars, so the decision to flip the switch is not made lightly. "We decided to be as proactive as we could, we won't be able to save all the fish if things get really tough, but we will be able to provide an additional area of refuge," Garren said. "Fish and Game will be monitoring oxygen levels in the lake throughout the winter, and will evaluate any winterkill in May, once the ice leaves and we can implement our annual population monitoring efforts." To learn more about conditions at Henrys Lake, contact the Upper Snake Region office at 208-525-7290 or the Henrys Lake Hatchery at 208-558-7202.