Trolling is a method of fishing where one or more fishing lines, baited with lures or bait fish, are drawn through the water behind a moving boat. Trolling is used to catch pelagic fish such as mackerel and kingfish.
Trolling can be contrasted with trawling, where a net (trawl) is drawn through the water instead of lines. Trolling is used both for recreational and commercial fishing whereas trawling is used mainly for commercial fishing.
Trolling from a moving boat is a technique of big-game fishing and is used when fishing from boats to catch large open-water species such as tuna and marlin. Trolling is also a freshwater angling technique.
Trolling involves moving a boat quite slowly through the water; this may be accomplished by use of a special trolling motor.
From the Boy's Own Book of Outdoor Sports (early 1900s):
One of the branches of angling which is generally practiced at mid-water or thereabouts, and includes spinning with a live, a dead, or an artificial bait, with a small fish generally, or its representative. When neither fly fishing nor bottom fishing can be practiced, in consequence of certain forbidding circumstances of water and season, trolling can be resorted to as an excellent substitute. The fish most commonly taken by any sort of trolling in our rivers are pike, perch and trout. Trolling is divided into three parts, viz: sinking and roving, trolling with gauge and snap-hooks and spinning. Sinking and roving is practiced with a live bait; a minnow or a loach for the common trout or perch; bleak, gudgeon, dace or roach for pike or large trout. The best general bait for all sorts of trolling is the gudgeon. The rod used should be a long bottom one, with a good winch, and prepared plaited silk trolling line. For foot-line, about a yard and a half of the best gut. The link to which the hook is tied, should be of fine gimp, if pike are sought fore; but gut, or three-twisted hairs, will do for trout and perch. The baits must be strong and lively, and placed on the hooks with as little injury to them as possible. Allow the bait to swim, here and there, generally at mid-water, but in deep places, deeper, drawing it up gently to the surface now and then, letting it sink again and guiding it to the best looking spots of the locality. Snap-baits are mostly used at seasons when pike do not feed with sufficient voracity to pouch their baits promptly. Their merit lies in allowing the troller to strike quickly, before the fastidious fish suspecting something wrong, has time to eject the bait from his mouth. The rod used must be short and stiff; that known as the punt barbel rod being the best. Snap-baits are two-fold - one, which does not spring when you stroke the fish, and the other which does.
The first-named consists of three hooks - two large ones, tied back to back, with their barbs pointing different ways and one smaller hook tied on at the top of the shanks of the others, and pointing straight out from them. The spring-snap is generally used with dead bait; it requires deep insertion in the bait to allow the spring to act, which it will not do without some considerable resistance. Spinning is a dashing, killing method of angling, and the practice of it requires considerable muscular exertion. The best spinning rod is made of a single piece of East India mottled cane, fourteen or sixteen feet long, well ringed, with a screw winch, requiring no winch fittings. With a rod of this description, salmon and large trout can be trolled for in the deepest and widest waters. In narrow streams, the angler can spin with a very small portion of line out and almost avoid casting, the length of the rod allowing the bait to be dropped noiselessly wherever it is wished, and to spin it accordingly. The baits used in spinning should be of the most brilliant colors; the brightest minnows, gudgeons, you can procure. The hooks used in spinning should be of the bright steel color of the wire, no changed to the ordinary blue line of hooks; and they should be whipped on with light-colored silk, waxed with white wax. Artificial spinning baits are sold at the various tackle stores. They all kill fish more or less successfully; but the majority of them are inferior to the natural bait. A small sail boat, or skiff is used, with an attendant to manage the boat as you direct. You can sue the live bait, or an artificial bait, as most convenient. Some sportsmen are very fortunate with the artificial bait. A stiff rod and reel, with the same tackle as before described, and no sinker - is all that is requisite. The boat should move gently, and let your line drag far in the rear. With artificial bait the fish is hooked almost instantly. If you use live bait, be exceedingly careful in determining when the fish has gorged it. You should give him several minutes after he has seized it, for this purpose. On seeing the bait, the pickerel will generally run off with it, and will then stop to gorge it, but does not always do so. The sign that he has swallowed it, is a peculiar slackening of the line, which experienced anglers can easily understand. But if he has gorged the bait, he will soon start off a second time, and sometimes will stop and start off the third time. In these cases, you should never be in a hurry. When you are convinced that he has taken down the bait, draw a tight line, and strike for your fish. If he is large, you should play with him until he is quite exhausted, or you may lose him in the attempt to land. The difficulty of taking a pickerel from the hook may be obviated in a measure by gagging. For this purpose some anglers provide themselves with prepared sticks of various lengths. If the hook is completely swallowed, as is frequently the case, open the stomach in the middle, cut away the hook, and unslipping the knot that holds the gimp, draw it out that way rather than through the mouth.