A Thanksgiving fish kill in Ramsey County’s Lake Owasso 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 in St. Paul was that too much oxygen in the water — yes, too much — might have created a rare bends-like condition that could be responsible.
But the cause might never be known, several scientists said.
Best guess now, said Parsons: “some weird chemistry thing that happens right at ice-up.”
The die-off was discovered on Thanksgiving as dead and dying fish became visible under the fresh ice in the shallows of the 375-acre lake straddling the border of Shoreview and Roseville. At least hundreds of fish had died in the lake, a popular destination for walleye and muskie, both of which are stocked.
The dead fish included walleye, muskie, largemouth bass, sunfish and crappie.
The fish were found along the lake’s shoreline, suggesting that the cause wasn’t localized. The last DNR-permitted treatment of an herbicide was in August, long enough for any harmful chemicals to have broken down, officials said.
“I just hate seeing this,” said Gary Kimball of Roseville as he shuffled along the 4 inches of ice Monday, peering down at dead fish below.
Kimball was among a stream of people who had stopped at the public beach parking lot on the lake’s northeast corner to view the fish kill.
Over the years, the lake has become known for producing large northern pike, muskies and walleye.
It’s unclear whether the kill will have a serious impact on the fish population. Parsons said the DNR had no plans to impose special fishing restrictions before a scheduled spring fish population survey is completed. The 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.
Lake Owasso is different.
In the fall, as waters cool, the amount of dissolved oxygen rises. When clear ice forms without a layer of snow, 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.
Yet ice-onset fish kills, while rare, aren’t unprecedented. Last year, Forest Lake experienced one, and several years ago one was reported in Lake Pimusche near Bemidji. Neither lake saw its fishery fundamentally altered by the die-off, but in each case, the mechanism remains unknown.
Ling Shen, supervisor of the DNR’s pathology lab, has a hypothesis: too much oxygen in the fish’s blood and tissues.
So much oxygen, in fact, that it can’t remain there. It bubbles out, forming physical conditions similar to the bends, when dissolved nitrogen in SCUBA divers’ blood “boils” as they ascend too rapidly toward the surface. However, in the case of oxygen in the fish, the situation arises from spiking oxygen levels in the water; the fish needn’t rise rapidly to experience it.
“These bubbles can actually make it harder for fish to absorb the oxygen, and they die from lack of oxygen,” Shen said. “I’ve seen this in fish in hatcheries. But we’ve never seen it in the wild.”
Oxygen levels would likely have to reach 17 to 20 parts per million in the water, and Shen, who has worked at the DNR for more than 20 years, said it’s unclear if that’s possible in a lake. “It’s just a theory at this point,” she said.
Shen examined two muskies from Lake Owasso on Monday. Both appeared to be 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.”
Dave Orrick can be reached at 651-228-5512. Follow him at twitter.com/OutdoorsNow.