A Thanksgiving fish kill in a Minnesota lake has scientists slicing open dead fish, scouring academic literature and scratching their heads.
“It's definitely still a mystery,” Brad Parsons, regional fisheries manager for the Minnesota Department of Natural Resources, said Monday.
Officials have all but ruled out several possibilities, including chemical spills, sewage spills, weed-killing treatments and foul play. Other possibilities likely to be ruled out this week: disease and infection.
Also ruled out: lack of oxygen in the water.
In fact, one intriguing theory floating around the Minnesota Department of Natural Resources' pathology laboratory was that too much oxygen in the water — yes, too much — might have created a rare bends-like condition that could be responsible.
“I've seen this in fish in hatcheries, but we've never seen it in the wild,” said Ling Shen, supervisor of the DNR's pathology lab.
But the cause might never be known, several scientists said.
The die-off was discovered on Thanksgiving as dead and dying fish became visible under the fresh ice in the shallows of 375-acre Lake Owasso northwest of St. Paul, Minn. At least hundreds of fish had died in the lake, a popular destination for walleye and muskie, both of which are stocked. Largemouth bass, sunfish and crappie also died.
It's unclear whether the kill will have a serious impact on the overall fish population of the lake. A scheduled spring fish population survey might help scientists estimate how many fish died.
Early winter fish kills are rare. So-called “winterkills,” when fish die from a lack of oxygen in the water, happen in early spring, after a layer of snow and ice has shrouded a lake from sunlight — and oxygen-producing photosynthesis — for an entire season.
Low-oxygen kills also can happen in the summer, when water temperatures soar and reduce the amount of oxygen water can hold. Usually, a summertime die-off coincides with a die-off of algae, whose decomposition process soaks up the already-limited supply of oxygen dissolved in the water. Oxygen levels can drop to levels as low as 3 parts per million.
In the fall, as waters cool, the amount of dissolved oxygen rises. When clear ice forms without a layer of snow, as it had on Lake Owasso, sunlight can still penetrate. Underwater plants keep producing oxygen. But the ice prevents oxygen from escaping, and water can become supersaturated with oxygen. Thus, at early ice, lakes often see their highest oxygen levels of the year, perhaps up to 16 parts per million.
On Saturday, DNR workers tested Lake Owasso and measured levels between 10 and 14 parts per million — high but normally healthy for fish.
Shen examined two muskies from Lake Owasso on Monday. Both appeared to have been healthy when they died several days before. One, a 36-inch fish that drew Kimball and Atchison's attention, had an empty stomach. The other, a 48-inch lunker retrieved by lakeshore resident Mike Chapman, held 16 sunfish and one crappie approaching 10 inches.
“That was a happy muskie,” Shen said. “It's really hard to know what killed it.”
As for the possible smoking gun from the supersaturated-oxygen-bends theory — bubbles in the tissues or gills — both fish were too decomposed to tell.
Dave Wright, a limnologist with the DNR, said he's skeptical such a scenario could ever occur in a lake, but he said it couldn't be discounted.
“Either way, what we have here is a very rare situation,” Wright said. “Fish don't normally die this time of year. I've been scratching my head as to what it might be.”