PROTEM, Mo. —  On a scorching-hot day, Fred Richardson dropped his lure into a zone where the fish were much more comfortable than he was.

Sitting in his boat on Bull Shoals Lake, Richardson could feel beads of sweat rolling off his face. It was 6 p.m., and the temperature was in the mid-90s.

But Richardson knew there was a way to beat that heat. Some 30 feet down, the water was much cooler than the surface reading of 91 degrees.

That’s why Richardson was using a heavy jigging spoon, trying to catch fish that were oblivious to the heat wave that had the Ozarks in its clutches.

“The key at this time of the year is fishing the thermocline,” Richardson said, referring to the zone that separates the water with oxygen from the layer without. “The fish will relate to that water. It is cooler, it has oxygen and it has a heavy layer of plankton, so that water is murkier.

“When I used to scuba dive, I found that you can actually see that thermocline and how the fish use it. They’ll hang in that murky water and use it as cover.”

The most productive water, according to fisheries biologist A.J. Pratt, can be found where the thermocline is near the bottom and intersects with good cover, such as brush, timber, dropoffs or humps. But there are places where the thermocline is suspended over much deeper water.

By using his electronics, Richardson often can find that layer on his screen and fish accordingly.

That’s what he did on a recent weekday.

He pulled his boat up to Church Camp flat on the Arkansas side of Bull Shoals and began marking fish near the thermocline. He immediately dropped a three-quarter ounce hammered jigging spoon to the depths and began working it in an erratic motion.

It wasn’t long before he was greeted with a jolting strike, and he set the hook. The fish tugged hard, trying to dive to the depths. But before long, Richardson could peer into the crystal clear water and see what he had on the end of his line — a big Kentucky bass.

Moments after he landed the fish, he put it on a hand-held scale and found that it weighed 2.46 pounds.

“The Kentuckies in here are like little footballs,” Richardson said as he eased his catch back into the water. “Even in the heat of summer, they’re feeding. They’re in good shape.”

But that wasn’t the only fish Richardson caught.

Soon, he was pulling in a keeper walleye. Then several large white bass. Then a largemouth bass. And finally, a few more Kentuckies.

All of this, without another boat in sight.

“A lot of people think you can’t catch fish when it gets this hot,” said Richardson, 70, who has fished Bull Shoals since the early 1980s, when he and his wife, Vera, moved to the Ozarks to run Buck Creek Boat Dock.

“But you can if you’re fishing in the right place. Even in the dead of summer, you can catch fish off this main-lake structure.”

Richardson keys on main-lake points and flats with fairly steep dropoffs. He fishes vertically, using heavy spoons. Instead of merely lifting the spoon and letting it drop, as many fishermen do, Richardson uses an erratic motion to attract the fish.

“When I first got here, an old-timer who had been fishing the lake for years took me out and taught me how to use these spoons,” Richardson said. “He taught me how to make that spoon like a dying shad. He never used line any heavier than 8-pound test. He believed it definitely made a difference in this clear water.”

That lesson definitely paid off. Using a heavy spoon to fish Bull Shoals’ depths, Richardson has caught two stripers weighing 28 pounds and a walleye that was 6 pounds, 7 ounces.

“Just about every type of fish in the lake will hit these spoons,” he said.

They are especially effective in the summer, when the fish retreat to the depths in the clear Ozarks lakes.

But Richardson uses them almost year-round to catch fish. In fact, some of his biggest fish have come in late fall and early winter.

“I’ve caught fish as deep as 55 feet in the winter,” Richardson said. “Once the lake turns over (and there is oxygen throughout the lake), the fishing will really turn on.

“You can catch big white bass like crazy on these flats.”

Even in the summer, there are fish to be caught in Bull Shoals’ depths.

“Too many fishermen are used to casting the banks,” Richardson said. “But in the summer, that’s not where the fish are going to be in these deep, clear Ozarks reservoirs.

“They’re going to be on structure, in deep water — places like points, humps and dropoffs. That’s where the bait is.”

———

FISHING THE THERMOCLINE

WHAT: The thermocline is a zone that separates water that is rich in oxygen from a zone where it is depleted.

DYNAMICS: In deep, clear Ozarks reservoirs such as Bull Shoals, Table Rock, Norfork and Beaver, gamefish often relate to the thermocline because the water is cool, has good oxygen and has a murky layer of plankton, which attracts baitfish and offers gamefish cover.

WHEN: The thermocline usually forms in June and lasts into September and October, according to A.J. Pratt, a fisheries biologist for the Missouri Department of Conservation.

DEPTH: A recent reading by volunteers helping the Missouri Department of Conservation showed that the thermocline at Bull Shoals was 35 to 40 feet deep and the water temperature there was 65 to 68 degrees — quite a contrast with the 87 to 90 degrees at the surface.

... AND BELOW: Just because the thermocline is at a certain depth doesn’t mean fish won’t be found below it. There is diminished oxygen below the thermocline, but species such as stripers and walleyes will still use that cooler water, Pratt said.

FISHING: Fishermen use heavy slab spoons to fish the thermocline and catch a variety of fish, including bass. The best spots are the ones where the thermocline is near a bottom that has cover such as brush, boulders or timber.