Idaho Department of Fish and Game responded to public calls regarding thousands of dead carp at Wilson Lake (aka Wilson Lake Reservoir). Fish and Game officials investigated and determined the dead fish (including Bullhead, Common carp, Largescale Suckers, Yellow Perch and Utah Chub) were likely killed under the ice from what is known as a winterkill.
The Department estimated approximately 16,000 Carp, 10,000 Largescale suckers, 4,000 Bullhead, and hundreds of Yellow perch and Utah Chubs died.
“Conditions must have been pretty poor since bullhead were among the dead fish at Wilson Lake,” said Doug Megargle – Regional Fishery Manager in the Magic Valley Region. “The unusually heavy snowpack this winter likely blocked all light penetration.
"It’s also possible hazardous materials washed in during the large spring runoff several weeks back; however, it’s very difficult to determine what that toxic material was when there were no reports of a spill or accident, and it’s been too long between when the fish died and when they were observed," Megargle said.
A winterkill occurs when there is a lack of oxygen in the water under ice. Submerged vegetation and algae require sunlight to produce life-supporting oxygen. During the winter, oxygen production is reduced or eliminated when ice and snow cover block sunlight, which shuts down photosynthesis.
In small and shallow lakes, bacteria feeding on decaying vegetation quickly uses up the available oxygen, leaving none for fish. In many cases, some fish survive because they find a location where enough oxygen exists to sustain life, such as an active inflow, or a place where ice did not form, or they are a fish species that can survive low oxygen levels – like bullhead.
In many cases, a winterkill event goes unnoticed except by those who regularly fish the area. Sometimes only a portion of the fish population die. Sometimes fish die early in the season, sink and don’t wash onto the shoreline.
Fish and Game has not sampled Wilson Lake to determine if there was any survival, but based on the number of dead fish observed, there probably aren’t many that survived.
“We may see more evidence of winterkill this year given the unusually heavy ice and snow cover around Southcentral Idaho,” said Megargle.
Wilson Lake was not actively managed by Fish and Game. Past water management did not provide for predictable winter survival of most game fish. The reservoir is usually drawn down in the winter to protect the dam leaving a small, shallow impoundment. Most fish found in the reservoir are introduced each year through the irrigation system that diverts Snake River water. Fish and Game will evaluate the situation and determine how best to proceed.