Anglers often talk about bottom transitions, but beyond the standard mantra of “go fish them,” that’s about all we hear. That’s a bit like saying “structure is good, so go fish it.” Although fundamentally and generally true, such advice doesn’t get you very far, as even relatively featureless lakes are loaded with subtle structural elements. And obviously, all of that real estate doesn’t hold fish all of the time.

The real wisdom when it comes to bottom transitions and structure alike is identifying the most productive ones, and fishing the best specific spots along them. To do this, however, anglers must understand the layout of a given transition area and interpret how—and if—fish relate to it.

Transitions As Structure
A transition, by angling definition, is anychangeinbottomcontent;either subtle or well defined. Fish may relate directly to the transition’s edge, or may hold near the edge on one side or the other.

There are also species-specific transition trends. Smallmouth bass, for example, tend to prefer larger boulders next to soft bottom; walleyes favor rocks of varying size that are adjacent to soft bottom.

Your goal—whatever species you target—is to fish the most productive areas of the transition, not the entire edge. This would seem obvious if we were talking about three-dimensional structural elements such as points, but for some reason, many anglers mistake transition zones as extended buffet lines, with fish set up along their length— anywhere and everywhere.

Not true. Consider a body of water with a mile-long breakline. Are fish set up at random all along it? Most of the time, no, they’re positioned at the best spots along that giant structure, places that are laid out in such a way that they funnel prey and provide ambush points, as well as offer quick access to both shallow and deep water.

Few anglers realize the same is generally true of bottom transitions—the shape of the hard-bottom zone is critical.

And just like three-dimensional structure, fish will relate to points, funnels, inside turns and places where cover intersects the transition.

Tracing Transitions
The first step in exploiting this phenomenon is properly interpreting what a sonar unit reveals. As most anglers already know, sonar displays soft and hard bottoms differently because hard substrate reflects more sound, resulting in a stronger echo and a more intense color (or darker shade of gray in monochrome units) on the display. Color palettes varybymanufacturer,but across the makes and models, the harder the bottom, the wider the band depicting it on the screen; narrower bands indicate softer substrates.

Extremely hard bottoms are also accompanied by a double echo—a second band displayed below the true bottom. That’s because the initial sonar ping bounces back from the bottom with such intensity that it rebounds off the boat hull, heads back to the bottom, and returns to the transducer. The double echo is shown deeper than the first because of the additional time it takes to bounce back-and-forth.

But not all transitions are so easy to see; some are extremely subtle. Some, for example, are indicated by just a slightly thicker/thinner band depicting bottom.

As I mentioned earlier, however, the transition itself isn’t nearly as important as its shape. With that said, it’s wise to map out a target transition area by slowly running your boat along the shift in bottom hardness as you closely monitor the sonar display. Meanwhile, record the track to your GPS. Once you’ve circumnavigated the area, you’ll be left with an easy-to-view shape, which will let you home in on any points, corners and other spots likely to hold fish.

Powerful New Tool
But there’s an easier, more accurate way of doing this , one that’s forever changed the way I think of—and fish— transitions.

For the past several years, anglers have come to rely upon detailed electronic lake maps they can view alongside sonar data on their display screens. The newest high-definition cartography from companies like LakeMaster and Navionics provide even more detail and accuracy.

For all their strengths, such maps obviously don’t show bottom hardness. However, now accessible technology has emerged to fill that void and provide substrate hardness maps that are just as accurate and easy to use.

Back in the February 2009 issue of North American Fisherman, I discussed the emerging DrDepth program, which let anglers create their own contour maps of the waters they fished (“Map Quest,”February 2009).

Recently, the company has applied the same technology to a new affordable program thatlets anglers map bottom hardness in the same way, DrDepth BT.

As with the original programming, the angler need only make several passes of the target area, and the program automatically creates a combination contour/hardness map, with colors indicating differentlevels of hardness.

Although these can currently only be viewed via home computer, the information can be converted into transition outlines that you can save on an SD card and use on your GPS.

New Technologies, Discoveries
It wasn’t until I began using this program that I started to give a degree of serious thought to the shape of transitions, because I’d always assumed that most were laid out similar to nearby structural elements.

For example, in the case of an isolated hump surrounded by deep water, I’d originally presumed that the top of the hump and at least part of its sloping sides were hard bottom, while the base and surrounding deep water was soft.

But I was dead wrong.

After creating a few bottom-content maps with the new program, I soon found many instances where the hump itself was half-hard, half-soft, with hard bottom extending down the break and well into nearby deep water. It quickly became clear why I’d marked concentrations of fish in specific spots off the primary structure.

That’s a lesson that applies whether you map transitions with your sonar and GPS track, or an advanced program like DrDepth BT. Understand how hard substrate zones are shaped, and you’ll be one huge step ahead in finding the spot-on- the-spot, wherever you fish, and whatever species you target.
 

See Subtle Bottom Transitions
Some bottom hardness transitions are extremely subtle and difficult to see on conventional sonar. Many, for example, are indicated by just a slightly thicker/thinner band depicting bottom. As seen here, the soft bottom lies on a flat next to a drop-off; the edge of the drop-off is harder. The only difference is a slightly thicker band of red below the yellow. In this case, fish were holding only on the soft side of the transition.