Lakes in the Bemidji area continue their fall cooldown, with surface water temperatures in the low 60s in most lakes.
The bite continues to pick up for walleyes on some of the large shallow lakes like Winnibig-oshish and the main basin of Leech, while the bite has been tougher on some of the deep lakes like Bemidji and Cass
There is a big difference in the way walleyes in shallow lakes and deep lakes react to cooling temperatures in the fall.
Big shallow lakes like Winnie and Leech will start to develop a thermocline during the summer, but fronts with heavy winds will periodically flip the water in the lake and completely mix it from the surface to the bottom of the lake.
Deep water lakes like Bemidji and Cass are too deep for wind to flip the water, so the lakes stratify during the summer, with the coldest water on the bottom of the lake and the warmest water on the surface.
A thermocline will develop in deep lakes, which is the transition zone between the warm and cold water in the lake. The thermocline separates the two layers of water and is dense enough to be visible on sonar.
The water below the thermocline doesn't mix with water above the thermocline and it can act as a barrier to fish if oxygen levels drop below acceptable levels during the summer.
When air temperatures begin to drop in the fall, the cooling surface water in the lakes will sink and plume through the water column, lowering the temperature of the water below the surface.
As the water above the thermocline becomes closer to the temperature of the water below the thermocline, the thermocline will begin to break down.
Walleyes in deep lakes have been trapped above the thermocline during the heat of summer and a good portion of the walleye population will move into deeper water as the water cools in anticipation of when the thermocline breaks down.
When the thermocline breaks down, the water begins to mix again and the fish are able access the portion of the lake below the thermocline that has been unavailable to them much of the summer.
Walleyes in shallow lakes have been in a different situation during the summer. They actually end up heading toward shore in the fall, instead of heading toward deep water like the walleyes in deep lakes.
Shallow lakes like Winnibigoshish, the main portion of Leech, Upper Red, Mille Lacs and portions of Lake of the Woods have most of their surface area less than 40 feet deep.
The water in the big shallow lakes will start to stratify during periods of calm weather during the summer, but heavy winds (at least 30 mph) will periodically mix the water in the lake and the stratification process will have to begin again.
The coolest water in these lakes will still be on the bottom, but there is no thermocline to act as a barrier to walleyes and other fish. Walleyes in the shallow lakes will use the deepest structure in the lakes during the summer to try and stay closer to their preferred temperature range.
When the shallow lakes begin to cool in the fall, the walleyes will move toward shallow water because there is no longer any temperature advantage in staying deep and there is more food in shallow water.
It all sounds confusing, but the basic point is if anglers want a shallow bite for walleyes, head to one of the big shallow lakes like Winnie or Leech.
The bite has been best along cabbage weed edges or in areas with broken rock on the bottom. A jig and minnow works well, but anglers can also use live bait rigs or troll crankbaits and catch fish.
Anglers fishing in deep lakes like Cass and Bemidji should look for walleyes along steep breaks in deeper water. Key areas can be on the edge of where the hard bottom turns to mud. Vertical jigging and live bait rigs can both be effective for catching walleyes in deep water.
Paul A. Nelson runs the Bemidji Area Lakes Guide Service. He can be contacted by calling 218-759-2235.