Knowing the water you plan to fish, plus knowledge of the feeding and habitat characteristics of the fish species you're after are key ingredients for productive angling. The reason is simple an entire lake or reservoir can quickly and efficiently be prospected by trolling, and concentrations of fish can be pinpointed with minium effort. The troll and lure are let out behind your moving boat, with the amount of line varying depending upon the size lure or troll you select and how deep you wish to fish. The forward speed of your boat will dictate just how fast or slow the troll or lure will run and also will control its depth. Once the troll and lure are in the water and working properly, you must then find the fish.
Most lakes stratify into three layers during late spring, staying that way until late fall.
The middle layer of water, the thermocline, contains both a large amount of dissolved oxygen and forage fish. To be most effective, you should troll close to or in the thermocline. This will be from 15 to 50 feet down in most lakes. All fish relate one way or another to structure, shelter or cover. They use it for protection from predators, to escape from direct sunlight, for feeding and, in some cases, for spawning.