Field & Stream Online Editors
Field & Stream Online Editors

Though the holiday weekend will unleash the year’s biggest bass fishing blitz, millions of anglers will have to struggle to catch anything. They’ll blame the crowds, the hot weather, the stress put on the fish, or even their tackle. The problem for most of them, though, is that they’re fishing too deep.

Bass do go deeper as the summer becomes hotter. But in lakes that develop a thermocline, you will not catch fish much below 20 feet on most lakes, no matter how hot it gets.

**A Change in the Water **
Before the thermocline forms, there is sufficient oxygen to support bass from top to bottom. But as the summer sun warms the upper water levels, many lakes develop varying layers of water temperature. Warmer surface water “floats” on top of the colder bottom layer (due to the way warm water molecules interact with cold ones). The line of sudden temperature change between these two layers is the thermocline. This dividing zone may be only 2 feet wide, but the water temperature drops by 8 or 9 degrees within it.

Water temperature and dissolved oxygen levels remain fairly consistent from just above the thermocline to the surface. But as organisms on the lake bottom use up the oxygen beneath the thermocline, the deep portion of the lake can no longer support fish. “Cold, deep water locked in this bottom layer often has no aquatic vegetation to replenish oxygen,” says Fred Snyder, a fisheries biologist with the Ohio Sea Grant. “Fish can’t survive here until the thermocline disappears in the fall when the lake turns over.”

Hence, bass stay up. They have to.

You can easily find the thermocline with a temperature probe (see sidebar) or with a liquid crystal graph. Switch it to manual mode and gradually turn up the sensitivity until you see a definite line of clutter between the surface and the bottom. This is a quick indication of the maximum depth to fish on that lake that day. If the thermocline registers at 18 feet, look for a ledge, a point, a hump, or some other bottom feature that drops off at that depth.

Thermocline Tactics and Lures
When you find structure that correlates to the thermocline, select a lure that matches the depth and water color. For example, if you’ll be fishing a ledge 20 feet or deeper in crystalline water, use 6-pound-test line and 4-inch finesse worms rigged Texas-style or in a drop-shot rig. A watermelon or green gourd spider grub with twin curly tails and a molded skirt, on a 1/2- or 3/4-ounce football-head jig, will also work well at those depths.

If your target structure is less than 20 feet deep in fertile, tinted water, however, your options increase. You can use Carolina-rigged lizards, jig-and-pig combos, and heavy, deep-swimming spinnerbaits that weigh 1 ounce or more.

Conditions are ideal when the thermocline forms less than 16 feet deep. You can use crankbaits then, but you don’t have to resort to the super-deep-diving models that are very tiring to retrieve. Bass will be within the range of typical deep-diving crankbaits. I cast long-billed crankers that dig down 12 to 15 feet, such as Excalibur’s Fat Free Shad BD6F or BD7F, Rapala’s DT 16, and Bagley’s DB3. Use 8-pound-test line to ensure that they get down to where the bass are.