Trolling Speed, Lure Depth, and How to Know Where Your Bait Is

There are two questions a trolling boat has to answer every minute it is running: how fast is the bait moving, and how deep is it. Everything else — color, spoon size, leader length — is a distant third. Boats that answer those two questions precisely outfish boats that guess, and the gap is not close.

Speed over ground is not speed through water

Your GPS tells you how fast the hull is crossing the earth. It says nothing about how fast water is passing your spoon 70 feet down. On the Great Lakes those numbers separate constantly. A seiche, a wind-driven surface current, or a thermocline shear can have the top 30 feet moving one direction at half a knot while the deep water sits still.

The practical symptom is familiar: you troll north at 2.4 and the riggers fire, you turn south at 2.4 and everything goes dead. Nothing about your baits changed. The water changed. Turning back into the current bumped actual lure speed to 3.0, and the kings quit.

Three ways to close the gap:

1. A paddlewheel transducer on the transom reads speed through the surface water. Cheap, useful, and blind below about 10 feet.
2. A downrigger probe on the cannonball transmits speed and temperature at the ball. This is the only measurement that is actually true at fishing depth, and it changes how you fish within one trip.
3. Turn discipline. In the absence of a probe, note your GPS speed on the productive heading and adjust the throttle on the reciprocal heading until the rods load the same way. Watch how hard the divers pull. Rod tip load is a crude speed probe you already own.

Starting numbers you can trust

• King salmon, summer: 2.4 to 2.8 mph GPS. Spoons pull best in that band.
• King salmon, cold spring water: 1.8 to 2.2 with flasher and fly.
• Coho and steelhead: 2.6 to 3.2. They like it moving.
• Lake trout on the bottom: 1.8 to 2.4, and slower on Superior than anywhere else.
• Erie walleye on crankbaits: 1.4 to 2.2, with 1.6 to 1.8 doing most of the damage.
• Erie walleye on crawler harnesses: 0.9 to 1.5. Slow enough that boat control in wind becomes the hard part.

Those are start points, not answers. Change speed in tenths, fish the new number for 20 minutes, and let the rods vote.

Knowing your depth: four systems, four kinds of error

Downriggers are the only trolling system where you set a number and get that number. A 10-pound cannonball at 2.5 mph blows back some — usually 10 to 20 percent at 80 feet — so a rigger reading 80 is fishing closer to 70. Heavier balls reduce blowback. A probe eliminates the guessing entirely.

Divers move a bait out to the side and down at the same time. Depth is a function of the diver setting, the line diameter, and line out. Braid dives deeper than mono at the same counter reading because it has less drag. Manufacturer dive charts get you close; the honest way to calibrate is to run a diver over a known depth in shallow water until it ticks bottom, then note the counter reading. Do that once and your charts become yours.

Lead core and copper put the bait down by weight of the line itself. Lead core sinks roughly five feet per color at 2.0 mph. Copper is heavier and more consistent — 45-pound copper runs about 22 feet down per 100 feet out at typical walleye and salmon speeds. Both lift when you speed up and dive when you slow, which means a turn changes depth on every rod in the spread at once. The inside rods on a turn drop; the outside rods rise. Fish often bite on exactly that change.

Flat lines and crankbaits dive by lip design. Precision Trolling-style dive curves exist for most Great Lakes crankbaits and they work, but only if you know your actual line diameter and your line counter is honest.

Line counters lie, and they lie predictably

A line counter measures spool revolutions, not feet. As the spool empties, each revolution pays out less line, so a counter that reads accurately at 50 feet reads long at 250. Different line diameters change the number again. Two identical reels spooled by two different people will not agree.

Fix it once a season: mark a known distance on a dock or a lawn — 100 feet is plenty — pull line to your mark, and note what the counter says. If it says 112 at a true 100, you now have a correction factor and you can stop wondering why your buddy’s 150-foot setting outfishes yours.

Then keep your spread honest by spooling all the reels on a given rod group with the same line, filled to the same level, at the same time.

Snap weights and inline weights: depth without a rigger

An 8-ounce inline weight ahead of a crawler harness on Erie is not a compromise, it is the standard. The 50/50 method — 50 feet of line, add weight, 50 more feet — is repeatable, and repeatable is the whole point. Torpedo-style inline weights fish cleaner in weeds than snap weights and cost you a rod-holder fight when a big fish comes in.

Whatever you use, write it down. The boats that consistently produce are not smarter, they just keep a notebook: date, speed, depth, weight, bait, and what it caught.

Building a spread that answers the question fast

Start wide and vertical. Two riggers at two depths, two divers spread outside them, one lead core or copper running the top. Different colors on every rod. You are not trying to catch fish in the first 30 minutes — you are trying to eliminate 80 percent of the water column and 80 percent of the color chart.

When two rods at the same depth fire, move a third to that depth. When two rods with the same spoon fire, tie that spoon on a fourth rod. Collapse the spread onto the pattern and stop experimenting. The willingness to abandon a pretty, balanced spread the moment the fish tell you something specific is what separates a good day from a long one.