by Cory Schmidt
A story or two beneath the surface lies an almost supernatural veil of water where more life, death, and other astonishing things happen than any of us probably care to imagine.
This disruption in the underwater continuum, the thermocline, represents super-science at work and a vigorous melting pot of living things—in other words, a potential goldmine of angling opportunity. Water, current, algae, temperature, oxygen, fish, and thermodynamics collide in a seemingly implausible, yet natural progression.
In technical terms, the thermocline has been defined as the lake layer where water temperature drops by at least one-half degree for each foot in depth. But consider the hearty stew of fish, phytoplankton, and whatever else might spill out onto the boat floor if you ever towed a seine through this writhing ribbon of life.
Stored forever in my mind’s eye is a scroll of graph paper from a vintage Lowrance X-15 sonar, etched with hundreds, maybe thousands of black boomerangs pinched into a narrow 4-foot layer of the water column. Some time during the mid-1980s, based largely on this striking visual, I became fascinated with the thermocline and the fishing potential it held.
A lot of anglers claim this underwater edge is easy to identify on any sonar unit. Sometimes it is. Especially at night, when plankton ascend into this cool, oxygen-rich environment and baitfish and gamefish converge, you often see a thin layer—usually from 15 to 25 feet beneath the surface—clustered with “stuff.”
But finding the thermocline isn’t always easy, particularly during the day when zooplankton like Daphnia move deeper or otherwise disperse, where sonar won’t show a band of signals. It’s possible for a finely tuned unit to detect water layer gradations by density, but it takes a trained eye. So for accuracy, the only reliable way to know for sure is to drop a temperature gauge. One way is with an Aqua-Vu Micro DVR-DT system that displays camera depth and water temperature as well as real-time video.
Visual confirmation with a camera is helpful to see what can occur as a milky layer, a dense white cloud of zooplankton that often clusters within or just below the thermocline, depending on time of day. The cloud can attract a lot of crappies, and when it touches or hovers just above deep vegetation or other cover, bluegills and perch graze on the zooplankton, as well.
Understanding the Zone
Beginning about early summer, dropping a temperature gauge generally reveals largely constant water temperatures from the surface down to about 15 or 20 feet. Once you notice the temperature dropping rapidly, say from the mid-70s and quickly to the mid-60s, it’s likely you’ve found the upper limit of the thermocline. Continue to lower the gauge and when the temperature stops dropping, typically somewhere in the mid-50s, and levels out again, you’ve found the lower limit of the thermocline. (Note: Temperature ranges vary by latitude, but a substantial change in temperature with depth identifies the thermocline).
Typically, you can see as much as a 10°F drop in temperature within 10 vertical feet or less. Especially later in summer, you might also see the temperature continue to gradually drop beneath the thermocline and into the hypolimnion. Further, as surface temps cool in fall, this heavier cooler water pushes the thermocline deeper until the water column equalizes in temperature and “turns over.”
The dynamics of the thermal stratification process is complex and can be incredibly intricate from lake to lake. That’s because wind and fetch—the distance over which wind travels across surface water—greatly determines thermocline depth. Fetch determines how deeply wind-driven currents can mix the upper layer of the water column, or epilimnion.
So we’re dealing with weather, oxygen, and water, as well as fish. I can’t think of a more mind-boggling set of fishing factors. Yet, other than oxygen—probably the least studied factor of them all—the thermocline is discoverable if you’re carrying an Aqua-Vu or a water temp gauge.
About 10 years ago, In-Fisherman Editor in Chief Doug Stange assigned me an intriguing, though time-consuming, task of studying and documenting daily the thermocline and fall turnover on several Minnesota lakes. Into the field I went, from the first of September that year through the first week of October, armed with a water temperature-sensing Aqua-Vu camera, GPS, sonar, and a few rods and reels.
The results surprised me, dispelling several myths and cementing in my mind that this one remarkable lake zone likely concentrates more food-chain activity than any other—including the surface, lake floor, and any physical edge in between. And yet, other than a few of us who occasionally pull lures through this zone, the thermocline remains the most underfished, overlooked edge in the fishing world. Whether we’re talking about the open-water abyss or the intersection where the ‘cline touches structure, you’ve got a ready-made collection of optimal conditions—temperature, oxygen, and food—waiting to be exploited by predatory fish, panfish included.
Crappie and Sunfish Sanctuaries
The whole thing boils down to food and fish comfort zones. Every species adheres to a range of temperature and oxygen preferences. Water temperature controls the rates of all metabolic processes of cold-blooded aquatic animals, and therefore their requirements for oxygen. Once the thermocline sets up in early summer, you can mostly cross out water below the lower level of the thermocline, which often lacks oxygen in eutrophic and mesotrophic waters. (Note: Some lakes that stratify can have suitable oxygen in the hypolimnion to support fish life. These are often managed as “two-story” fisheries.) This leaves the entire upper 20 to 25 feet for panfish to hide.
“In most mesotrophic and eutrophic lakes, we almost never see enough dissolved oxygen to sustain bluegills, crappies, or perch below the thermocline,” says Mike McInerny, research scientist with the Minnesota Department of Natural Resources, who’s made a career of studying panfish behavior. “Nutrient input below the thermocline robs oxygen, as carbon dioxide is a byproduct of oxidative decomposition. Combine that with a lack of wind mixing and light penetration (needed by algae and aquatic plants for oxygen-producing photosynthesis), and you lose available oxygen very rapidly.”
In studies of different panfish species, researchers have demonstrated that in water 60°F and warmer, bluegills and yellow perch can’t survive where dissolved oxygen drops to 3 parts per million (ppm) or lower. Crappies begin to expire at around 4 ppm. In cold water, all species require much less oxygen but still can’t tolerate much less than about 1.5 ppm, with bluegills having slightly more tolerance.
“The thermocline acts as a sort of trap—both for fish and plankton,” McInerny says. “Dense water meets less dense water. Cool water meets warm water. You can also see a slightly higher level of dissolved oxygen in the thermocline during the day, as photosynthesis occurs via phytoplankton.
“You often end up with a mass assembly of plankton and invertebrates gathered at the deep border of the thermocline. They’re anticipating a move into and perhaps just above the thermocline as light levels fade. On the other side, you’ve often got fish like crappies and occasionally bluegills waiting for the food to arrive.”
Think of it as a sort of “releasing of the hounds” and the type of intense fishing we often experience at twilight. It can happen relative to structure—say a vegetation edge or rocky point—just as naturally as the interactions may occur well out in the open abyss. In more fertile waterbodies, this zone can sizzle with phytoplankton and zooplankton and other invertebrates and tiny fish during all parts of the day, particularly in stained or dirty water. This means you can put lures into these depths and expect to catch something from first light to sunset and perhaps beyond.
Towing Tiny Plugs
Once you’ve established the upper edge of the zone, an effective way to determine what type of fish are there—bluegills, crappies, white bass, or the surprise smallmouth bass—is to put plugs down and tow them until something smacks them. I’ve yet to find a better panfish crank than Rapala’s Ultra Light Shad. If a fish of nearly any species is living there, the Shad scores.
Plug-pulling is a fun, effective discovery process on new water, so long as you’re marking a lot of fish on sonar. But this isn’t a method you want to try just anywhere out there.
You rarely find panfish suspended over water much deeper than 40 feet, though there are exceptions, such as finding smaller bluegills near the surface over the abyss, or suspended crappies in southern reservoirs near channel edges. Ideally, you want to narrow the parameters to smaller basin sections—or lakes within lakes—harboring 20 to 40 feet of water, or a reservoir area such as the mouth of a major creek arm. Good, too, are zones where the thermocline contacts structure, such as a hump with ample feeding shelves in 15 to 25 feet. Panfish goldmines often occur within two casts of the end of a point or a big U-shaped inside turn, starting at the edge and moving out into the adjacent open water.
It’s best to troll small lures like a Rapala Ultra Light Shad around the outer perimeter of structure, zigging in close to the edge and back out into open water. Or, troll across the open waters of a small middepth basin, say the size of four football fields. Crappies can suspend anywhere in these confined basins, but often adhere to depths from 10 to 20 feet down. Big bluegills aren’t as apt to suspend in open water, but might cluster over a certain section of soft-bottom basin hosting Chaoborus (phantom midges), usually with 5 feet of the bottom in 18 to 25 feet; rarely much deeper in summer.
Getting lures down to 15- to 25-foot depths involves additional weight. A Troll Tech Crappie Downrigger by Bullet Weights is designed for a method Wally “Mr. Crappie” Marshall calls “pushing a crankbait.” Marshall rigs up to eight long rods (up to 18 feet in length), and places them in rod holders off the bow like outriggers. He ties 10-pound mono mainline to the weight’s topside eyelet, and connects a 4- to 6-foot leader of 4- to 8-pound fluorocarbon or mono to the swivel on the base of the sinker shaft.
He trolls from 1 to 2 mph, dropping weights directly below the boat until they reach the right depth. Usually he can see the sinkers on his sonar screen. He’s comfortable fishing with bass-sized plugs from Bandit and Strike King. “Those big crappies on Lake Grenada, Mississippi, and Lake Lavon, Texas, suspend in summer,” he says. “But they’re usually in small, confined areas around creek channels or other structure. It’s why trolling Crappie Downriggers is more efficient than straight longline trolling behind the boat. Placed in holders off the bow, the vertical ‘riggers let me turn on a dime and swing back through big schools of fish without tangling lines.”
Pro walleye and crappie fisher Tommy Skarlis recently shared a tip for catching big crappies in 40 to 60 feet of water with Offshore Tackle Tadpole weights and crankbaits on Lake of the Ozarks. He cautions, however, that fishing crappies any deeper than 20 feet eliminates the possibility of catch and release. “Be prepared to harvest a few fish you catch from the depths and then move to shallower water,” he says.
Prior to the thermocline setting up in early summer and again in fall after turnover, fish can freely chase bait, such as shad, to any depth, as anoxic barriers may not exist. And in the upper or lower portions of reservoirs with current, the depths can be constantly infused with oxygen from feeder rivers. In the absence of thermal barriers, scan the water column for the largest concentrations of offshore bait. Skarlis, who super-tunes his Raymarine sonar units, can read fish at 30 to 50 mph, yielding express fish-finding.
Active Panfish Bubbles
Vertical trolling can work in natural lakes, too. Towing Ultra Light Shads and other bite-sized cranks is often the money method for summer panfish of many species, particularly during high-sun periods, and especially in darker, more fertile lakes and flowages. Often, the ceiling of the thermocline or the edge where sunlight penetration ends keys location.
Trolling, however, often simply serves as a means to an end—an efficient way to find an active fish or two before stopping and casting or jigging vertically with other baits. What you frequently discover is an active group (I call them bubbles) of crappies, bluegills, or occasionally, perch, around structure at the depth of the thermocline, particularly at its upper limit, or ceiling.
Some of the best summer spots occur where primary edges or weedflats or rock humps top out at about the same depth as the thermocline. Say the ceiling is at 21 feet, and you find a hump with a deep vegetation edge in 23, which rises to 21, 20, and 19 feet on top, with scattered rock, elodea, and sandgrass. I’d bet you’d find bluegills, perch, or crappies in such an area. Nearly always, vertically jigging a small swimming minnow like a #2 Jigging Rap, Fiskas Red Tail Swimmer, or an Akara Midge, makes good things happen.
Once you find such a spot—having taken the time to identify not just structure, but also where it meets the magical panfish zone—what happens next is mostly inevitable. Pick your favorite lure and let those panfish remind again why you do this.
*In-Fisherman Field Editor Cory Schmidt, Brainerd, Minnesota, has been a keen observer of natural lake limnology for many years.
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