It’s getting to be the time of year when the thermocline comes into play. But before we talk about how that affects our fishing we need to consider what the thermocline is, and what it isn’t.
The thermocline is the point at which warm, oxygen rich water is separated from cold, oxygen depleted water. This is caused primarily by changes in the density of the water. Cold water is denser so it drops. Warm water is less dense so it rises. The lower water doesn’t recirculate and so it never gets refreshed.
The thermocline is not something that separates where the fish can live. It’s not at all true that smallmouth can’t live below the thermocline. They do. Divers and anglers agree on that.
Dale Hollow Lake in Tennessee is a deep, clear lake. Lots of divers come here for sport and recreation. They’ve told me for years that if they swim down to the thermocline – usually around 25 feet – and bang two rocks together they’ll see big smallies come up out of really deep water to see what’s going on.
Anglers tell similar stories, although from a different perspective. They regularly catch smallmouth bass when they’re fishing well below the thermocline.
I don’t pretend to know all the biological facts about bass and how they live. However, experience has taught me that fish living above the thermocline are more active and more easily caught. I think their metabolism is higher and that makes them more active. They’ll bite bigger, faster moving lures. They’ll also bite them for longer periods of time during the day or during the night.
It’s the opposite below the thermocline. The water’s cold. The fish aren’t moving as much. They won’t chase baits very far and they don’t seem to want to grab anything that’s very big. That does not mean, however, that they won’t bite or that they can’t be caught. They can. Just fish with something smaller and slow moving like a jig or a tiny Texas rigged plastic.
Finding the thermocline in your lake isn’t as hard as you might think. Modern electronics help a lot. Set your sensitivity high and look for a thin line running horizontally across your graph. It’ll look like fishing line, or at least that’s what it looks like on mine.
You can also find it with a temperature gauge that you can lower down into the water. Drop it down until you find the depth at which the coldness of the water increases faster than it has been increasing. That’s where it’s at.
Now that I’ve said all this I know there are some who will disagree with me, anglers and fisheries biologist alike. They swear that there are no fish below the thermocline, and that even if they are down there they won’t bite. Sorry guys, that’s just not right. I know. I’ve caught them 15 and 20 feet below it.