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Paul Nelson: Anglers switch to summer patterns on Bemidji area lakes

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Summer is finally here — at least as far as the fishing patterns in the lakes are concerned.

Surface water temperatures in the Bemidji area are now warmer than 70 degrees for the third time this season.

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The first two times the lakes exceeded 70 degrees were short-lived but this time looks like it should last a little longer.

The lakes are starting to “green-up” from the first significant algae bloom of the season. The lakes will continue to add color as long as the warmer water temperatures continue.

Many lakes have developed a thermocline in the deeper portions of the lake. Anglers should be able to notice the thermocline on their electronics as another bottom echo somewhere between 25 and 45 feet deep in most lakes.

The thermocline is the thin, dense portion of the water column that separates the warmer surface water from the cold water closer to the bottom.

The upper layer of water receives oxygen from plant respiration, wave action and springs.

Wave action can turn over the entire water column in the big, round shallow lakes, including Upper Red Lake, Winnibigoshish and Mille Lacs.

Some larger lakes, including Lake of the Woods and Leech Lake, have shallow bays and other large areas of shallow water that will get mixed from the surface to the bottom by the large waves during high winds.

The periodic mixing of the shallow lakes by the wind keeps the entire water column at a similar temperature and also keeps the water well oxygenated during the summer.

Lakes with extensive amounts of deep water will stratify by temperature and establish a thermocline. There is a warm layer of water above the thermocline and a cold layer of water below the thermocline in the deep lakes.

Wind and waves can mix the water in the deep lakes down to about 40 feet, assuming the lakes have enough open water for the waves to grow to sufficient size.

The layer of colder water below the thermocline is not affected by the wind and does not mix with the warmer upper layer of water as long as the thermocline is in place.

Species that prefer colder water will usually stay below the thermocline or move back and forth above and below the thermocline as long as oxygen levels below the thermocline are high enough to support respiration.

Lakes with limited amounts of water below the thermocline may eventually run out of oxygen during a hot summer, while lakes with extensive amounts of water below the thermocline may never run out of oxygen and will be able to support cold water species like trout and salmon.

The thermocline can form a barrier to the fish when the oxygen below the thermocline drops below viable levels.

Each lake is different. Most lakes in the Bemidji area will eventually run low on oxygen in deeper water if the weather stays hot enough for a long enough period of time.

Anglers can expect fish to gradually begin to move toward shallower water as the lakes continue to warm and the algae blooms increase.

Cold water species like tulibees, whitefish and suckers will stay in the cooler portion of the lakes as long as they are able to find suitable oxygen levels below the thermocline.

This year has been cool into mid July so the chance of a significant summer-kill in the lakes has been greatly reduced.

As the summer has progressed some walleye anglers have started to add spinners to their live-bait rigs because the flash helps walleyes target and locate the bait. Bottom bouncers attached to a spinner that is tipped with a nightcrawler or leech also is effective at this time of the year.

The bottom bouncer sinkers are usually 1 to 3 ounces which enable the anglers to run their boats at higher speeds and still be able to keep their baits close to the bottom.

Spinner rigs allow the walleyes to hear the bait coming and the extra speed forces the fish to strike at the bait as it passes.

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Paul Nelson
Paul Nelson writes a weekly fishing column for the Bemidji Pioneer. He runs the Bemidji Area Lakes Guide Service.
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