Minnesota can get too hot for proper fishing — and we could be there now

In Minnesota, hot fishing always has meant that walleyes are biting, or perhaps sunnies or crappies, or bass, northern pike or muskies.

Now, as weather vagaries tied to climate change become more apparent — witness this week Twin Cities air temperatures soaring into the 90s, with water temperatures not far behind — hot fishing in Minnesota might describe times when anglers should park their boats and hang up their rods until things cool down.

Otherwise, even some fish that are released by well-meaning anglers will die, succumbing to what's called hooking mortality, or delayed mortality.

How hot is too hot to fish?

It depends, experts say, on the species sought, the type of bait being used, the depth of targeted fish, and whether caught fish are going to be kept or released.

If the goal on a hot day is to catch a few fish to take home to eat, fine.

But if the preference is to release most or all fish that are caught — dooming, probably, some of the fish to their deaths — anglers instead might opt for a day at a beach, and give the state's game fish a break.

"We launched in Gray's Bay of Minnetonka on Monday, and the lake's surface temperature was 80 degrees,'' said Paul Hartman, an avid muskie angler who owns George's Minnesota Muskie Expo, which runs three days each spring in the Twin Cities. "In deeper water, the temperature fell off to 78 degrees. But it's likely water temps will hit 85 throughout much of the southern half of the state by the weekend. That's too hot. It's too hard on fish.''

Muskies in warm water are particularly susceptible to delayed mortality, meaning the process of a fish dying after it is released, seemingly in good shape.

While struggling against an angler's hook, line, rod and reel, temperature-stressed muskies can build up abnormally high levels of lactic acid, possibly leading to death.

Stressed muskies in abnormally warm water also can die of low blood-oxygen levels.

"I was on the St. Croix this week, and the water temperature touched 80 degrees,'' said Josh Stevenson, owner of Blue Ribbon Bait in Oakdale and a noted muskie guide.

"Typically when water temperatures reach 76 degrees, I cut back my trips,'' Stevenson said. "Even at those temperatures, you have to be super ready if a fish is hooked to make sure it is brought in quickly, kept in the water and released within 10 or 15 seconds. This requires two sets of tools in the boat, one in front and one in back, with everything ready to go by people who know what they're doing.''

Walleyes often don't do well in hot weather, either. Just ask anyone who was on Mille Lacs in midsummer in recent years when the Department of Natural Resources (DNR) required, either exclusively or predominantly, catch-and-release fishing for walleyes.

When anglers released these fish, they oftentimes thought their catches were in good shape. But the fishes' appearances were deceiving, and within a day or so many of them died because of a combination of stress and abnormally high water temperatures.

The result was rotting walleye carcasses littering the Mille Lacs surface.

A DNR study of walleye hooking mortality on Mille Lacs found no walleyes died that were caught and released in May of the study period, when water temperatures were less than 68 degrees.

But in July and August, when water temperatures were above 68 degrees, about 12% of released fish died.

Keith Reeves, DNR area fisheries supervisor in Tower who helped lead the study, said fish that were bleeding when caught also were more likely to survive in cooler water.

"In July and August, when fungus and bacteria can grow faster in the warmer water, bleeding fish don't recover as well,'' he said.

Anglers fishing in warm water also should consider the dangers live bait can pose to fish.

Oftentimes, anglers affix leeches, nightcrawlers and minnows to sliding sinker rigs, which allow walleyes or other fish to "mouth'' the bait for a while before eventually swallowing it.

This increases the chance a fish will be hooked in its throat or stomach, which can seal a fish's fate even if released.

Phil Talmage, DNR area fisheries supervisor in International Falls, said water depth also can affect the survival of released walleyes.

"In summer, Rainy Lake walleyes often seek depths of 40 to even 60 feet,'' Talmage said. "We conducted a study to see what happens to those fish when they are caught and released.''

At depths of 30 feet and more, Talmage said, Rainy Lake walleyes were particularly susceptible to delayed mortality, adding the likelihood of delayed mortality increased with each 5 feet of depth beyond that point.

"It's something we told anglers about, so they were aware what happens to released fish in those circumstances,'' Talmage said.

Counterintuitive as it might seem, while the DNR cautions anglers about the dangers warm water can pose to fish, and in some cases has closed fishing during midsummer to protect against warm water-induced hooking mortality, the agency also authorizes more than 300 open-water fishing tournaments each year. Through this weekend, DNR-permitted tournaments are slated throughout much of the state, including in southern Minnesota, for species as varied as catfish, muskie, bass and walleye.

Jon Hansen of the DNR said the agency has placed a number of restrictions on tournament organizers to protect fish, but that it can't predict weather conditions months in advance of issuing a permit.

But if Minnesota summers, and its lakes and rivers, continue to trend warmer, he said, a review of tournament regulations might be necessary.

Meanwhile, some anglers beat the heat by heading north.

Hartman, the muskie angler, moved Wednesday to Mille Lacs, where on Vineland Bay he registered surface temperatures of 77 degrees.

"If it gets to 80, I won't fish,'' he said.