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Weiss: Unusually warm weather can be harmful to fish population

Trout anglers are getting more and more messages to use a thermometer before taking their first cast. The recent hot weather has caused water temperatures to rise, in some places close to dangerous levels for some fish.

Mark Mayer, in foreground, and Juan Gomez, both of Chatfield, fish for bass June 19 on Willow Creek Reservoir in south Rochester. (John Weiss / sports@postbulletin.com)

In the cool Saturday morning sunshine on June 19, I approached the Willow Creek Reservoir, worried.

For more than a week, ending a week before, temperatures had been an unusual 15 degrees above normal with highs in the 90s. After that, they were a bit above mid-June normals; I feared I would be met at the south-Rochester flood control reservoir with algal blooms and that hot, dead-fish stink.

Much to my surprise, it looked good.

Water was relatively clear. Ah, but what about dissolved oxygen, which goes down with heat and is super critical for fish? I measured about 9 parts per million, maybe three times the minimum needed for that kind of water to support game fish in that reservoir. The temperature was 79 degrees -- warm but not a killer.



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Maybe I was missing something, maybe all the dead fish had blown across the reservoir. As I sampled, Juan Gomez and Mark Mayer, two Chatfield buddies, prepared to kayak looking for big bass. Gomez, who said he has fished the reservoir regularly, said I wasn’t imagining things. “I haven’t had any issues,” he said. He hasn’t seen any big blooms or many dead fish.

“I would say it’s a pretty healthy population (of game fish).” In fact, he said he caught a 21-inch largemouth the week before.

My checks of dissolved oxygen and temperatures at Cascade Lake, and Kalmar and Silver Creek reservoirs, showed much the same thing. The lowest DO was about 5 ppm at Cascade; the others were around 7 ppm. All the water looked decent. Anglers I chatted with generally said they’ve been catching a lot of nice fish, especially bass, and haven’t seen any signs of dead fish or big blooms.

Department of Natural Resources fisheries officials dealing with the reservoirs, the Mississippi River and trout streams, all said that despite the early-summer heat, none was worried. Besides, they said, there’s nothing much they, or anglers, can do, except quit fishing when water temperatures get too high.

How hot is too hot varies with species. Trout, for example, begin to quit feeding around 70 degrees (some experts say quit fishing when it hits 66) while catfish can take much warmer water. Walleye, northern, bass and panfish are in between.


A thermometer is one of the most critical things trout anglers need when fishing in warmer weather. Experts suggest checking the temperature before fishing and avoiding streams where readings are in the upper 60s or warmer. This reading was great for trout fishing. (John Weiss / sports@postbulletin.com)

Brian Beyerl, DNR fisheries specialist who watches Rochester’s reservoirs, said the warm water didn’t bother him. “The species that are in there can tolerate warmer temperatures,” he said. His biggest worry is algal blooms that can decrease the dissolved oxygen. But rain can flush out algae, he said.

The DNR will sample the reservoirs this fall to see if there were any fish kills, he said. Actually, some die offs can help because survivors have more food so they can grow bigger faster, he said. “Those reservoirs some times they need to reset,” he said.

From what he’s seen from past electroshocking of Silver Creek Reservoir, “the size structure (of bass) has gotten to be almost trophy size, especially for Minnesota,” he said. One was 20.5 inches, five were longer than 16.5 inches. northerns are harder to electroshock so they need to be netted to be counted. But at Silver Creek Reservoir, they did see some northerns and “there are some monster pike in there,” Beyerl said.

That reservoir had a major die off of big northerns during a drought more than a decade ago.

One thing that might concern Rochester-area anglers is Cascade Lake, where a company is finishing mining gravel. Once that is done, the DNR will decide what to stock there. The dream has been that it will be stocked with walleye. Cascade, however, is fed by ground water from a geological strata that isn’t overly high in dissolved oxygen, he said. Also, walleye don’t do well in warm water, he said. It’s possible if the warm water continues for years, walleye might be out. Right now, though, it’s too early to tell, he said.

Nick Schlesser, the DNR big-lake specialist in charge of Lake Pepin, said the Mississippi River (Pepin is a natural reservoir of the river) is quite low now but not when compared with 1988; that is the “benchmark” for drought. For example, flows at the Red Wing Lock and Dam were about 10,000 cubic feet per second recently, compared with about 5,000 cfs in 1988. In that year, algal blooms festered in the lake so 1988 “was really famous for the lake stinking,” he said.

Yes, the region got some heavy rains recently but “this rain is not going to do a whole lot to rivers,” he said.


“I don’t know if low water is inherently good or bad for fish,” he said. Warmer water temperatures, however, are worse for walleye and northern, he said. “The saving grace is that when it’s hot, fish don’t bite as much” so fewer are killed by that stress, he said.

Right now, fishing has been quite good, he said. That being said, Schlesser said he wouldn’t mind a cool down.

When it’s hot, the first fish to die are often the bigger ones, he said. And it’s the bigger northern and walleye that often go first. In 2012, the lake had an astounding surface temperature of 92 degrees for a week. “We lost a lot of really big walleye,” he said.

Anglers often find big northerns congregate in the heat near places where cooler-water streams enter the river or lake and target them, he said. That does make the fish more vulnerable to the stress of being caught.

One thing he is certain about in this time -- the lower river is making chances of damaging motors and boats a lot greater. Wing dams and natural points of rock or sand have a lot less water over them, he said. This follows several years when the river was unusually high so boaters might not remember where problems lurked, he said.

For example, a sandbar near his office is now something he has to avoid. But “for the last five to six years, you couldn’t hit it if you tried,” he said.

Vaughn Snook, the DNR’s manager for trout in much of the region, said he’s not all that worried about summer so far. Yes, some springs are running lower or have dried up, but a few are gushing out more cold water into trout streams. Overall, water levels seem to be good from what he and I have seen.

Water temperatures are more critical, he said. When it reaches maybe 68 to 70 degrees, trout usually quit feeding so they are harder to catch. If caught, however, they can die from the stress.

Trout can tolerate a high of 75 degree water for an hour or so, but if all day, “they are probably toast, they are dead, or they are extremely stressed,” he said. Fortunately, many streams have places where cooler water enters from springs so maybe trout can find one of them on the hot days.

Trout anglers are getting more and more messages to use a thermometer before taking their first cast and that’s great, Snook said. An exceptional thing about this region’s trout streams is while some may get too warm, there are always others that are cool because they are fed more directly by springs where water is around 48 degrees. He advises anglers to check temperatures and move on if the readings in the in mid- to upper-60s or warmer.

John Weiss has written and reported about Outdoors topics for the Post Bulletin for more than 40 years. He is the author of the book "Backroads: The Best of the Best by Post-Bulletin Columnist John Weiss"

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