Record Heat May be Contributing to Fish Kills in Minnesota Lakes

Record-setting heat may be contributing to fish kills in lakes across the state, according to the Minnesota Department of Natural Resources (DNR).

“Natural summer fish kills are not unusual,” according to Brian Schultz, DNR assistant regional fisheries manager. “In the past several days, however, we’re getting increased reports of dead and dying fish in many lakes from around the state.”

Unusually warm weather has raised water temperatures of many shallow lakes. Schultz has received reports from DNR field staff of surface water temperatures in some lakes reaching 90 degrees, with temps at the bottom only a few degrees cooler where maximum depths are less than 10 feet.

“Those are some high readings, and northern pike are especially vulnerable when the water gets this warm,” Schultz said. “They are a cool water species and just can’t adjust to the high temperatures when sustained for more than a few days.”

Warm water temps can also impact other species such as walleye, yellow perch and bluegills.

Should the high heat continue, there may be die-offs of both northern cisco (tulibee) and lake whitefish in central and northern Minnesota lakes.

Oxygen depletion can be another factor contributing to fish kills of a broader range of fish species. Heavy rains earlier in the summer caused unusually high runoff from fertilized lawns, athletic fields, golf courses and farm fields, starting a chain reaction of high nutrient loads in some lakes.  

The runoff carries nutrients into the lakes, which combined with hot weather, can accelerate the growth of algae. Hot, dry, sunny and calm weather can cause algae growths to suddenly explode, according to Craig Soupir, DNR fisheries habitat specialist.

“Aquatic plants remain relatively stable over time, but algae have the ability to rapidly increase or decrease under various conditions,” Soupir said. 

Algae produces oxygen during the daylight hours, but it uses oxygen at night. This can create drastic daily changes in lake oxygen levels. These daily changes can result in complete saturation of oxygen during peak sunlight and a near complete loss of oxygen during the night.

“All of this can add up to stressful conditions for fish,” Soupir said, “and even summer kill events.”

Fish don’t seem to sense when oxygen depletion occurs and may suffocate in isolated bays, even when another area of the lake contains higher levels of oxygen.

Disease may also be a contributing factor to some fish kills. Schultz explained that when lake temperatures rapidly change, fish can become more susceptible to bacteria and viruses that naturally occur in the water. Columnaris is one of the most common diseases. 

The bacterium is always present in fish populations but seems to affect fish when water temperatures are warming rapidly and fish are undergoing some stress due to spawning. Fish may die or be seen weakly swimming along shores. Species affected are usually sunfish, crappies and bullheads and occasionally, largemouth bass and northern pike.

“It is difficult to pin a summer kill on just one cause,” Schultz said, “and although it is a natural occurrence, it can be disturbing.” 

Fish kills are usually not serious in the long run. Most lakes contain thousands of fish per acre and the fish kills represents a very small percent of that total. 

Some positive effects from partial fish kills is that it creates an open niche in the fish population, allowing the remaining fish species to grow faster with less competition.  

Minnesota lakes are resilient. The DNR has documented these conditions many times over and lake conditions and fish populations do return to managed expectations, either naturally or with the help of stocking if necessary.

If strange behavior is seen in fish, contact the local DNR fisheries office immediately.

“If we can sample fish before they die, we may be able to learn what’s going on in the lake,” Schultz said.  “Once the fish are dead it can be difficult to determine what happened.”