This big pike was holding just below a midsummer thermocline. The fish blasted through the temperature band to hammer a Deep Tail Dancer, in ‘clown flash’ color pattern, trolled across a basin area. Most anglers bypass these fish!
As important as they can be in dictating summertime fish location, thermoclines rarely enter the thoughts of most anglers. But to many fish (and aquatic prey species), temperature zones are the invisible fences of the underwater world.
Find these relatively narrow bands of fast temperature change and you can accurately present lures to fish that hang in and around them.
What is a Thermocline?
Water varies in density depending on temperature. In many systems, summer surface temps are significantly warmer than temps in deeper water. Especially as summer wears on, it’s not a smooth transition from warmer surface water to cooler depths; there is often a ‘quick-change’ zone where temperatures drop rapidly in just a few feet.
That narrow band where water temperature changes quickly is the thermocline.
This is a simple explanation, but if you imagine how layers of water can set up based on density, the dynamic of thermoclines is easier to understand. In essence, the warmer water, because it is less dense, ‘sits on top’ of the denser, cooler water layers. Also, it’s important to know that dissolved oxygen levels can be significantly higher above or in the thermocline than below it.
Thermoclines are most distinct in mid to late summer. As anglers, we don’t have to completely understand the science behind thermocline formation, but it’s important to know that many fish species will choose to hang just above, or just below, a well-defined thermocline. Even though it comes and goes, a distinct thermocline can be an edge that fish relate to.
It’s important to realize that thermoclines do not set up in all lakes, nor do they automatically set up in the same lakes every year. If brisk, sustained winds or other currents cause enough mixing of water layers, that can prevent thermocline formation.
Generally speaking, the deeper the body of water, and/or the more protected it is from wind, the more likely it is that thermoclines will form. Many forces are at work, but the more protected a lake is from big winds, and assuming it has basins of at least 40 feet deep, the more likely that lake will ‘stratify’ or form a thermocline.
Find the ‘Cline
The surest way to determine whether any given body of water has a thermocline is to take a temperature profile with a thermometer that records a set of readings at various depth levels (look in the Cabela’s catalog). But you can also use common sense observation of your depthfinder; if you see lots of fish (even if they’re scattered) at a certain depth, that can be an indicator they are relating to a temperature gradient.
Catching ‘Clining Fish
The surest way to catch fish relating to a distinct thermocline is to troll lures just above and just below the thermocline. You can do this around and in contact with structures that occur at thermocline depth, and in basin waters.
For previous generations this was all but impossible. But thanks to the combination of research into lure diving depths, and innovative deep-diving crankbaits, you can do it consistently.
Rapala, for example, has created the Deep Tail Dancer, a lure that can dive to 30 feet and more when trolled, without the use of any weight. (It’ll reach 30 feet when longline trolled on 10-pound monofilament, and even greater depths on no-stretch lines.)
Down Deep Husky Jerks can get 19 feet down on mono, and mid-20s on superline.
The balsa Shad Rap #9 will swim down to the low 20s on no-stretch line.
Just announced (available in 2006) is the new X-Rap Magnum 30, an extension of the X-Rap family that will give us another distinct profile and swimming action to put down to the thermocline next summer.
Get your hands on a copy of the Rapala Tackle Box Guide, which has casting and trolling depths for all but the newest lures in the Rapala lineup. The research that went into creation of this little reference is solid, and it’s been called “rifle sights for your boat,” by legendary troller Gary Parsons.
Try to run most lures just above the thermocline, unless you are marking most of the fish below it.
This is not a small fish pattern, so don’t expect to load the boat. But when the rod goes, it’s often a memorable battle. Generally speaking, warm- and cool-water species like walleyes, muskies and bass will hang above the thermocline, but are willing to blast through it to feed. Similarly, cold-water species like monster northern pike and lake trout appear to spend most of their time below the thermocline (assuming there is sufficient oxygen down there), but will move into the warmer water to take advantage of feeding opportunities.
It may sound complicated, but the big fish in the thermocline are worth pursuing.
Note: This article was crafted by the Rapala Pro Staff. For more fishing insights, go to www.rapala.com.