Walleye die-off on Wood Lake in Benson County results from blue-green algae bloom, fisheries manager says
Aug. 19—DEVILS LAKE — A recent summer fish kill on Wood Lake in Benson County south of Devils Lake was caused by a blue-green algae bloom, which depleted oxygen levels and killed hundreds of fish, mainly walleyes, a North Dakota Game and Fish Department fisheries manager said.
The die-off affected walleyes from 8 inches right on up to trophy-size fish of 29 inches, said Randy Hiltner, Northeast District fisheries supervisor for the North Dakota Game and Fish Department in Devils Lake.
"The respiration of blue-green algae, and also the decay process, removes dissolved oxygen from the water," Hiltner said.
Game and Fish investigated the die-off Thursday, Aug. 11, Hiltner said, at which time there were "lots" of yellow perch and other species gulping at the surface, in addition to numerous dead walleyes.
"The water had a very gray color to it, which is indicative of the tail-end of a blue-green algae bloom and the subsequent oxygen sag," Hiltner said.
Hiltner said department staff counted "hundreds" of dead walleyes.
"We didn't see very many dead northerns — there were some, all right — but walleye obviously were taking it the worst," Hiltner said. "There were some dead perch, some dead bluegill, we even saw a dead 38-inch muskie."
According to the North Dakota Department of Environmental Quality, blue-green algae results from hot weather and nutrients that accumulate in the water. Harmful to animals and humans, blue-green algae "can look like grass clippings floating in the water, clumps/puffballs or green cottage cheese," the DEQ said in a news release. "It can also make the water appear like spilled green paint or green pea soup."
Late summer traditionally is the worst time for blue-green algae blooms.
Wood Lake in recent years has suffered minor fish kills, Hiltner said, most commonly right after ice-out when localized areas have low oxygen levels.
"This is the first large-scale (die-off) that I'm aware of" on Wood Lake, he said.
Game and Fish staff checked oxygen levels again this past Monday, Hiltner said, and conditions had recovered to more favorable levels. Water levels in the lake also are normal, he said.
Many of the dead fish had been cleaned up by birds and other critters by Monday, as well, Hiltner said.
"Wood Lake has had blue-green algae blooms before — there's nothing new about it," Hiltner said. "But what led to this magnitude, I guess, is unknown."
The department received numerous reports about the die-off, Hiltner said.
"The lake is surrounded by cabins, and some are year-round residents," he said. "We had lots of walk-in traffic and lots of phone calls. They knew something was wrong, and fish were gulping, behaving strangely in very shallow water, and the water looked terrible.
"We had plenty of notification."
Located in Benson County south of Devils Lake, Wood Lake is 2 miles west and 1 mile south of Tokio, N.D. The 103.6-acre lake has an abundant population of bluegills, mostly under 8 inches, largemouth bass of various lengths, and "fair numbers" of walleyes and northern pike, with some big fish present, according to the North Dakota Game and Fish Department's description of the lake.
Bass, pike and panfish produce naturally in Wood Lake, Hiltner said, while the walleye population is reliant on stocking. Game and Fish since 2015 also has been stocking pure-strain muskies into the lake.
The department will have a better handle on the extent of the recent die-off after it conducts an electrofishing survey in early October, Hiltner said.
"There have been previous kills there, but obviously when you're getting walleyes up to 29 inches or so, they're probably teen-agers. They obviously lived through the other minor kills, but were not so fortunate this go-round," he said. "I'm hoping there's some walleyes left in there, but they certainly seemed to take it the hardest. We saw more dead walleyes than we did the other species for sure."
Anyone who sees a suspected blue-green algae bloom in North Dakota should report it to the state Department of Environmental Quality at (701) 328-5210 or at
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* More info: deq.nd.gov.
