A "substantial" kill, which included bass, crappie, and catfish, occurred this past summer on one of the most popular fisheries in north central Iowa. According to the Department of Natural Resources (DNR), the die-off of an algae bloom depleted Crystal Lake's oxygen, causing the kill.
"Boaters reported a nuisance algae bloom over the Fourth of July weekend," said biologist Scott Grummer. "However, anglers were still coming in with nice catches of fish."
But then abnormally cool temperatures and overcast days prompted the rapid death and decomposition of the bloom. "If that algae would have died off over a week or two period, we likely wouldn't have had an oxygen issue," Grummer said.
"The loss is certainly substantial," he added. "However, it is too early to tell how it will impact fishing in the immediate future."
As with many of Iowa's manmade fisheries, this 244-acre impoundment is shallow and algae are common, fed by nutrients in agricultural and urban runoff. Several days of warm temperatures and light winds made this blue-green bloom unusually heavy.
"It just shows how nutrient rich Iowa waters are," the biologist said.
Although Crystal Lake is equipped with an aerator to keep the lake from freezing solid during winter and provide an open-water refuge for fish, the biologist said its use during summer would have minimal impact because oxygen depletion was so abrupt.
While dramatic, the kill is not expected to have a lasting impact on the fishery, Grummer added, and no changes in regulations are planned.
"The setback is the loss of the current fish," he said. "A new population will develop and grow well."
DNR plans to survey the population this fall by netting and electrofishing to determine if supplemental stocking will be needed.
"Management decisions are going to be based on what we have left and how we move forward from that point," Grummer said.