Record-setting heat may be contributing to fish kills in lakes in parts of the state, according to the Minnesota Department of Natural Resources.
But Kevin Peterson, DNR area fisheries supervisor in International Falls, said he’s heard no reports of fish kills in Borderland.
“That’s one of the advantages of living in the Icebox of the Nation,” he joked.
Summer fish kills are a natural process because of the high temperatures and low oxygen levels in the water, and fish populations are normally good at recovering from these types of events, Peterson said.
However, he said it is something local DNR staff will be watching.
He encouraged anyone who sees dead fish or anything that looks unusual on area lakes to contact the DNR Fisheries Office at 286-5220. DNR staff would investigate, if possible, reports of fish kills, he said.
“If warm temperatures continue, it would not surprise me to get some of those reports from some of our small lakes, especially,” said Peterson.
Peterson also noted that he’s heard no reports of excessive algae blooms the area.
“When I’ve been on the lake, it seems quite clear to me right now, which is hard to say why,” he said. “If warm temperatures continue, eventually those same things they’re seeing in southern and central Minnesota could work their way north.”
Unusually warm weather has raised water temperatures of many shallow lakes, according to Brian Schultz, DNR assistant regional fisheries manager. 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.”
Visit http://www.dnr.state. mn.us/areas/fisheries/index.html for a complete list of DNR fisheries offices in the state. People can also call the DNR Information Center at 651-296-6157 or toll-free 888-646-6367.
