Take away the downrigger, the diver, the lead core and the boards, and what is left is a lure on a line behind a boat. That is flatlining, and it catches an enormous number of fish.
It is worth learning properly rather than treating as the thing you do before you can afford the hardware, because there are conditions where nothing with a piece of metal in the water will beat it.
What the words mean
Flatlining is trolling a lure on an unweighted line straight off the back of the boat. No board, no weight, no diver. Depth comes from the lure, the line and the speed and from nothing else.
Longlining is the same idea with the emphasis on distance: a lot of line out, the lure a long way back. In freshwater crappie fishing it is a named technique with its own following, and it means pulling small jigs on long lines behind a boat moving barely faster than a drift.
In saltwater the term flat line has a slightly narrower sense. On a boat with outriggers, the flat lines are the ones fished straight off the transom rather than up an outrigger, and they are usually the closest baits in the pattern.
One thing to be clear about before going further, because the word collides. A commercial longline is a completely different object: a heavy mainline carrying many baited hooks on droppers, set out and left to fish unattended. That is regulated gear with its own rules, permits and restrictions, and it is not what recreational anglers mean by longlining. If you are reading about longline regulations and they are talking about hook counts in the hundreds and soak times, you are in the wrong document. Everything below is rod and reel.
Walleye Crankbait Trolling Setup (Small Boat)Where the depth comes from
With nothing on the line, four things decide how deep your lure runs.
The lure itself is the biggest one. A deep-diving crankbait has a bill angled to drive it down and it will reach a depth on its own that a spoon never will. A shallow stickbait stays in the top few feet no matter what you do.
Line out is the second, and this is the one you actually control minute to minute. A diving crankbait gets deeper as you let more line out, up to a point, and then it levels off. It does not keep going down forever. Somewhere past that point you are only adding distance, not depth.
Line diameter is third and it matters more than most people believe. Thin line has less drag, so it lets a crankbait dive noticeably deeper than thick line at the same length out. Switching from heavy monofilament to thin braid can change a lure's running depth by a meaningful amount without you changing anything else, which is a nice trick and also a good way to be confused about why yesterday's setting stopped working.
Speed is fourth. Most diving lures have a speed at which they run deepest and get shallower on either side of it, and past a certain point they simply blow out and start skating.
The manufacturers of serious trolling crankbaits publish dive curves, and there is published trolling data covering many popular lures on specified line at specified speeds. That is the number worth having. Nothing we could print here would be more useful than the curve for the lure actually tied to your line.
Why anyone chooses it
Three real reasons, and none of them is that you cannot afford a downrigger.
Quiet. There is nothing in the water but the line and the lure. On clear water, in bright light, over fish that get trolled at every weekend, that is a genuine edge and it is the same argument that makes lead core worth owning.
The wake. A lot of predatory fish investigate a boat's wash rather than fleeing it. Bait gets churned up, the white water hides an ambush, and a lure worked into that zone is in a place fish are already looking. This is why offshore boats deliberately run flat lines in the prop wash rather than avoiding it. It is also why a very long line, a hundred and fifty feet back into flat water behind the disturbance, catches the fish that scattered and then settled.
Simplicity. You can change everything in twenty seconds. Different lure, different length, different speed. When you do not yet know what the fish want, the rig you can adjust fastest is the rig that finds out first. Boards and riggers are for exploiting a pattern. A flat line is for discovering one.
Longlining for crappie, which is its own world
The crappie version deserves separate treatment because it looks nothing like salmon trolling.
The boat moves slowly, in the range where a trolling motor rather than a kicker is doing the work. The baits are small jigs, often an eighth of an ounce or lighter. And the lines are long, well past a hundred feet, with several rods fanned out around the boat in holders.
The depth control is entirely jig weight, line length and speed, and the whole technique is an exercise in finding the combination that puts a very light bait at the depth crappie are suspended. Speed up and everything rises. Slow down and everything falls. It is the most direct relationship between throttle and depth in any kind of fishing, and people who are good at it steer with the depth in mind the way a rigger fisherman turns a crank.
It is deadly on crappie suspended over open water in spring and fall, which is the situation where they are hardest to find by any other method. Black crappie and white crappie both come to it.
Brad Chappell Shares Crappie Long-Lining TechniquesThe S-turn, which is a free experiment
Because a flat line has nothing damping it, it responds to the boat immediately, and that makes the humble S-turn more informative here than anywhere else in trolling.
Steer a long lazy S instead of a straight line. On the inside of the turn, those lures slow down and drop. On the outside, they speed up and rise. Every pass is running your lures through a small range of speeds and depths at once, and if the fish have a preference, they will tell you which side of the boat they took it on.
The trick is to notice. If three fish in a row come on the inside rods, the fish want it slower and deeper, so slow the boat down. If they all come on the outside, speed up. That is a real adjustment derived from real evidence, and it costs nothing except steering in a curve instead of a straight line.
This works with every trolling method to some degree, but flat lines respond fastest and most obviously because there is no ball, board or diver averaging out the effect.
The saltwater version
Offshore, flat lines are one part of a layered pattern rather than the whole thing, and the way they are positioned is deliberate.
A boat trolling offshore typically runs some baits up on outriggers, spread wide and high, and some straight off the transom. Those transom baits are the flat lines and they are usually the shortest in the spread, deliberately placed in and just behind the prop wash.
The reasoning is the opposite of the freshwater instinct to get away from the boat. The wash is white water, it is turbulent, it obscures the outline of what is in it, and it looks a great deal like a school of bait being harassed. Predators come to look. A bait placed there is the first thing they see when they arrive, and offshore crews argue about exactly how many wake lengths back to put it with the seriousness other people reserve for politics.
The general principle carries into freshwater more than most people use it. If your lake gets heavily trolled, running one rod very short, right up in the disturbance, is a cheap experiment on a fish population that has learned to associate boats with something.
Snap weights, the bridge
Somewhere between a flat line and a proper weighted system sits the snap weight, and it is worth knowing because it extends flatlining without abandoning it.
A snap weight is a small weight on a clip that you attach to the line partway out. The common arrangement is to let out a set length of line, clip the weight on, then let out more line. That puts the weight in the middle of the system rather than at the lure, so the lure still has an unweighted section in front of it and keeps its action.
The reason to use one is that it is removable. You can go from a flat line to a weighted line and back in a minute without retying anything, which is the flexibility that makes flat lines useful in the first place. The reason not to is that you are back to reeling hardware past the rod tip on every fish.
The related tool is the keel sinker or rubber core sinker, which does the same job further up the line and adds a little resistance to line twist, which is worth having if you are pulling anything that rotates. Neither of these is elegant. Both of them are the difference between fishing the depth you found and fishing the depth your lure happened to reach, and that is usually the difference that matters.
What it catches
Almost everything that gets trolled.
Walleye on crankbaits over a basin is the classic freshwater flatline. Northern pike along weed edges take a long-lined stickbait as well as anything. Smallmouth bass will chase a crankbait pulled over a flat.
Inshore and offshore, Spanish mackerel and bluefish are close to a flat line specialty, because they are fast, shallow and actively hunting the disturbance rather than avoiding it. Striped bass in shallow bays get long-lined with plugs at night.
The gear
Less than any other trolling method, which is most of the appeal.
A line counter reel is still the one thing worth buying. Line out is your only depth control, so a rig where you do not know the number is a rig you cannot repeat. Everything else here is negotiable and this is not.
A moderate-action rod, seven to nine feet, with enough give to keep a treble hook from tearing out on a strike. Flat lines have no cushioning device anywhere in the system, so the rod is doing all of it.
Diving crankbaits and stickbaits across a range of dive depths, because lure choice is your primary depth control. Buying five lures that all run to twelve feet is buying one lure five times.
Thin main line, and consistency about which one you use. Diameter changes your depth, so switching line without recalibrating means your numbers from last season are wrong.
Snap weights and a few clip-on weights, for the days when the fish are just below what the lure will reach on its own.
Loop knots or quality snaps, because a lure with a knot cinched tight to the eye has less freedom to work than one on a loop, and the difference is visible in the water.
And rod holders spread across the transom, because even without boards, the way you keep four flat lines apart is by fishing them at four different lengths from four different positions.
Rules
The same standing point applies as to every other trolling article: rods per angler is set by the state, sometimes varies by water and season, and every flat line counts as a rod. Longlining for crappie in particular often involves more rods than people fish anywhere else, so it is one of the more common ways to end up over a limit without meaning to. Confirm the number for the water you are on.
The other thing worth checking is unattended lines. Some states have rules about how far an angler may be from a fishing line, or require lines to be under immediate control. A spread of very long rods in holders is fine everywhere we are aware of, but it is worth knowing that the rule exists in some form in most places.
For the wider picture on what varies between states and why, see fishing access laws by state, and for getting legally onto the water in the first place, how to find public fishing access.
The case for starting here
If you are new to trolling, do this before you buy anything else.
A flat line teaches you the relationships that every other method depends on. You will learn what speed does, what line out does, what a fouled lure feels like, and what your boat sounds like to a fish, and you will learn all of it without a piece of hardware masking the feedback. When you do put a downrigger on the transom, you will know what you are asking it to do.
And on the days when the fish will not tolerate anything in the water, you will already own the answer.
