Heat killing pike in southern Minnesota
Glenwood, Minn. — More than a week and a half of temperatures typically above 90 and sometimes exceeding 100 degrees is being blamed for a widespread die-off of northern pike, as well as a few other species, across southern Minnesota.
It appeared shallow-water basins were affected most, as surface water temperatures approached, and in some cases passed, the 90-degree mark.
One of the affected bodies of water was the expansive, yet shallow (maximum depth of about 7 feet), Lake Emily in Pope County.
“It was a pretty significant kill (of northern pike),” said Dean Beck, DNR fisheries supervisor in Glenwood, not far from Lake Emily. “There were a few carp, but it was pretty much a single-species type of event.”
Beck said water temperatures in the area were in the mid- to upper 80s.
Given that the heat wave came early in July, prior to the summer’s typically highest-temp days, Beck said it’s possible more fish – beyond just cool-water pike – could succumb to the bathtub-like warm waters. For example, cold-water species like tullibees, found in some lakes such as Rachel and Miltona, could face dangerous conditions.
Other factors could’ve aided in the demise of Emily’s pike, but Beck said no such help was needed, with water temperatures around 85 degrees. “You don’t have to have depleted oxygen for a die-off with cool-water species,” he said, adding that both Lake Christina in Douglas County and the Pomme de Terre River experienced varying levels of pike mortality.
Twice during the first week of July did the nighttime temperature not drop below 80 degrees. That, coupled with calm air, probably didn’t help matters, Beck said. With little “mixing” thanks to a lack of breezes, and aquatic plants “respiring” and using up valuable oxygen at night, the situation simply became worse for fish.
The shallow lakes in the area, Beck said, don’t stratify, thus are unable to “turn over” and cool down; water is sometimes – depending on depth – nearly as warm near the bottom as it is on top.
The Glenwood area wasn’t the only place that proved deadly for northern pike during the past couple weeks. State conservation officers also received reports of fish kills – northern pike, usually – in the Mankato and Albert Lea areas.
On Tuesday, Beck emailed several area fisheries supervisors in southern Minnesota to see if similar reports were being received by their offices, to determine if the die-offs were “widespread around the state or a localized phenomenon.”
What he found out was mostly the former.
Three offices contacted by Beck reported no fish die-offs, including Hinckley, Duluth, and Fergus Falls. However, the Windom area office had received die-off reports regarding five lakes; Waterville had received die-off reports about a similar number of lakes.
Kevin Stauffer, DNR fisheries supervisor in Lake City, reported a “substantial pike kill” of about 200 large adults in a 100-acre reservoir near Rochester. And Jerry Johnson, area supervisor in the East Metro office, responded: “We have received several reports regarding pike kills in some of our small, shallow Fishing in the Neighborhood (FiN) basins.” He said a recent investigation showed surface water temperatures above 90 degrees – and bottom-water temps only a couple degrees cooler.
Officials recently found higher-than expected water temps in the Mississippi River’s Lake Pepin, where surface water temperatures were “solidly in the upper 80s,” according to Brad Parsons, DNR regional fisheries manager in St. Paul.
The surface temperatures on Lake Pepin are pretty astounding,” he said, adding that a more typical reading this time of year for the river-lake is in the upper 70s to lower 80s.
What’s it all mean for anglers? For one thing, fewer pike in some lakes. But DNR officials says it’s also important for fishermen to keep in mind that the fish are stressed.
“It’s more important than ever not to hold fish out of water very long, or if possible, release them (while they’re in the water),” Parsons said.
Beck adds, “Given the stress the cool-water species are under, anglers should know that (the fish) are not likely to be aggressive feeders.”
Beck also asks that anglers report any fish kills they see to the local DNR Fisheries office.
One possibility, though it’s more common in springtime is the disease known as columnaris. It typically kills panfish like sunnies and crappies, according to Parsons.
The disease tends to show up when a rapid warmup occurs in spring, further stressing fish already expending energy during the spawning period.
Recently, it was reported that a few of the dead fish in the Hutchinson area were crappies, suggesting columnaris might be the culprit.
An effect on walleye-rearing ponds?
Prolonged warm water conditions, reduced oxygen levels, and other factors could further stress fish species, and also could influence the level of success in DNR walleye-rearing ponds, those locations from which are drawn fall fingerings for stocking, Beck said.
Typically, not much attention is paid to the rearing ponds during the growing season – they’re usually left to their own devices, and, according to Neil Vanderbosch, DNR Fisheries program consultant, about 30 percent of the ponds fail each year, for one reason or another.
Whether or not the spell of excessive warmth causes any problems in the walleye-rearing ponds will become clear in the fall, but there may be signs prior to then. Beck said a sampling of walleyes must be taken from each pond for viral hemorrhagic septicemia testing.
“If we don’t come up with any (walleyes in nest when drawing that sample), that’s a concern,” he said.
Vanderbosch said it’s probably too early to expect walleye losses in rearing ponds because of the heat.
“Ortonville is one of the hottest places all of the time, and (ponds there) are some of our best producers,” he said.