Locals living near Pokegama Lake in Pine County, have noticed a strong fishy smell and multitudes of dead fish washing up on shore after ice out last week. The dead fish mainly consist of sheephead (freshwater drum), with some carp and suckers. Reports of fish kill have come from all around the lake. One resident on the east side said they collected over 1,000 pounds of dead fish between four lots. Another east side resident said they hauled out two trailers full, and another east side resident said he counted over 280 dead fish on his lakeshore alone.
As of Tuesday, there have been no reports of fish kills on other lakes, according to Deb Sewell, Minnesota Department of Natural Resources (DNR) assistant area fisheries supervisor.
Sewell said they have no record of previous winter kills on Pokegama; therefore, they do not believe it is directly winterkill-induced. But given the duration of the winter and the fact that the kill is largely focused on one species, they suspect some sort of stress-induced mortality, added Sewell.
“Stress induced mortality” means that when fish are exposed to a stressor (low oxygen, rapidly rising water temperatures, spawning, etc.), they are vulnerable to pathogens (viral, fungal, bacterial) that normally they would be resilient to.
Based on reports of fish coming in to shore still alive, the DNR came to the lake attempting to find a live sample, as that is required to gain an accurate pathology report.
Pokegama and Cross Lakes were the only lakes in the county to be widespread chemically treated for curlyleaf pondweed last summer. When asked if there was a possible connection to the chemical treatment of the lake and the kill, Sewell stated, “The chemicals used are all approved for aquatic use, and break down after several days. In order for an aquatic herbicide to cause a fish kill, fish would have to be acutely exposed to a high concentration of freshly sprayed herbicide, and have no escape route to untreated water. It is impossible that an herbicide application from last spring or summer would have the delayed effect of causing a widespread kill of primarily one species of fish at ice out.”
However, Sewell noted that this year, because of late ice and heavy snow cover on the ice, the curlyleaf pondweed may not have started growing under the ice in late winter like it usually does. The growth of curlyleaf under the ice would provide a late winter oxygen boost to the water under the ice (plants give off oxygen during photosynthesis). If this did not happen this winter, oxygen may have been low enough to cause stress to some species, like sheephead, she added.
Sewell said the DNR has not heard any reports of dead fish on Cross Lake (so far), but Pokegama has a much higher sheephead population than Cross Lake, based on gill net catch rates.
One lakeshore owner proposed the theory that the bottom-feeders may have been chemically targeted, but Sewell stated there is no chemical that would be selective for one species of fish; any chemical would have adverse effects on gamefish as well.
As of now, it is inconclusive what has caused the fish to die off. But Sewell stated, regarding one possible cause, “When I was checking the lake today (Monday, May 8), the dead fish all appeared to be roughly the same size, so there may have been one super abundant year class. When there are that many fish of one species, they compete with each other for food; that may have been an additional stressor (beyond the unusually long winter producing low oxygen levels). The fish kill, while appearing to be an adverse event, may actually benefit the overall fish population of the lake by reducing the overabundant sheephead population.”
She added that this species of fish does appear to be susceptible to a lot of stressors that don’t affect other species of fish. “In Wisconsin, sheepshead kills have been caused by a disease called Viral Hemorrhagic Septicemia (VHS). According to our DNR fish health consultant, this is highly unlikely to be the cause here because there are no nearby bodies of water that are known to have the disease, and VHS testing is required for all fish that are stocked in Minnesota and all bodies of water that are used for rearing baitfish. The disease also causes bulging eyes and red blotches on the fish it affects; none of the fish I saw showed those signs.”
“We will look into the extent of the kill via a netting effort in the coming weeks if additional species, especially those of greater value, begin to wash up,” added Sewell.
There is no health risk to the public from handling or consuming live fish caught from the lake.