Published in the August 2011 Issue August 2011

During the warmest days of the fishing season, water temperatures reflect the conditions in the atmosphere above. Of course, that's the case year-round, but at no time do water temperatures affect fish behavior more than when they peak late each summer.

Actually, that should read "reduce fish behavior," for many of our favorite cold-blooded gamefish species shut down to a degree when their fluid surroundings assume bathtub temps. Warmer water holds less of the life-giving oxygen that fish need, and by staying somewhat lethargic the fish require less food for fuel-read that food-and the oxygen it requires to pursue same.

The fish also often go deep this time of year to find the coolest water possible. But the fish can only plummet to a certain point before hitting the wall called the "thermocline," a depth below which temperatures dramatically plummet and, oddly enough, oxygen is suddenly scarce and most fish cannot survive. During August, the thermocline may set up as shallow as six feet or as deep as 60. Each water body is different and the line between oxygen rich and barren may fluctuate by the day, hour or area of the lake. And it is a lake or reservoir phenomenon, although deep, slow-moving rivers have been known to display a thermocline.

Relegated to remaining above the thermocline, this time of year fish tend to stack-up and suspend at particular depths where they find just the right combination of temperature, oxygen and food. These comfort zones can be extremely specific, and added to the lethargic ways of many of the gamefish, which are not inclined to move very far out of their way to investigate-let alone eat-an offering, it can make it difficult for an angler to induce a strike.

This is when controlled-depth fishing devices come into their own as invaluable tools for boat anglers. While there are many fishing accessories that can be used to control the depth of a presentation (see my August 2010 column on Diving Planes), none perform the act as precisely as downriggers.

In its simplest form, a downrigger consists of an arm or "boom" with a pulley at one end and the other fitted to a spool of cable that can be threaded through the pulley and cranked to raise and lower a weight into the water from where it is mounted on the back or side of a slow-moving boat.

That weight, usually made of lead weighing from a pound to 12 or more, is fitted with a release clip that is snapped onto the fishing line, which the weight pulls down to a specific depth and holds, as it trails the bait or lure behind. One or more additional lines (called "stackers") can be held with clip releases set on the cable above the weight or "ball" to present baits at various depths. The near-vertical presentation is dictated by the weight of the ball and the forward speed of the boat, the heavier and slower the more vertical the weight hangs.

What a downrigger allows the angler to do is present baits at a precise depth-and keep them there. When a fish finder-or trusted fishing buddy-tells you that the trout are suspended at 23 feet, with a downrigger you can drop your baits to run at exactly 23 feet, putting them into the feeding range of the fish.

That precision is key this time of year when many of our most popular gamefish species suspend at the coolest oxygen-rich water they can find, which is often just over the thermocline. By noting on your fish finder the depth level of the thermocline, which most modern sonars can discern and display as a visible line or shadow, you can drop your baits to troll just over that level-and into the maws of the fish that lurk there.

Downriggers come from small, portable manual models that actually fit into an oarlock, to sophisticated powered rigs capable of trailing devices that measure speed and water temperature at the ball, even video cameras that transmit live images to display screens monitored by anglers on the boat above. In between are simple manual downrigger models that can be temporarily fitted to mounts attached to the transom, gunwale or rails of deck and pontoon boats and used only when conditions dictate-which across much of North America just might be right now!

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