Fish Finders for Trolling and Downrigger Fishing: What to Look For

For trolling and downrigger fishing, look for a fish finder that shows boat speed and water temperature beside depth, holds a steady bottom line at your speed, and lets you build a screen layout of big readouts with sonar and a chart. Marking waypoints and recording your track matter more here than they do for casting.

Trolling is a moving-search technique. You are covering water at a steady pace, watching depth change under the boat, and trying to keep lures at a target depth. That changes which features earn their place. This guide is about the electronics, not tactics or fish behavior, and it names no models or prices.

01Speed readout comes first

A trolling spread depends on speed, because speed affects how deep a lure runs and how it behaves. So the first feature to check is a speed readout you can read at a glance.

• GPS speed. Units with built-in GPS can show speed over the ground. That is helpful, but current and wind can make speed over the ground differ from speed through the water, so some anglers also want a water-speed reading.
• Speed sensors. Some transducers and accessories add a speed sensor or a temperature sensor. Check the listing for what is included, and the transducer maker's information for compatibility with your display.
• Big digits. Look for a screen layout that lets you show speed in large numbers while keeping sonar visible.

If your display does not have GPS, check whether a speed sensor is supported before you buy, and read the manual's data page options.

02Depth tracking and a stable bottom line

When you troll, the bottom line moves up and down the screen as you cross contours. A good display keeps it readable without constant adjustment.

• Auto range and bottom lock. These keep the bottom in view as depth changes. Know how to switch them on and off, because a fixed range can be easier to judge depth by.
• Zoom. A split zoom beside the main view helps you see the lower part of the water column.
• Clean returns at your speed. Trolling is slower than cruising, but a poorly placed transducer can still lose the bottom or fill with noise. If yours does, see our guide to losing the bottom at speed.
• Charts with contours. A chart lets you follow a depth contour instead of reacting to it. Our guide to reading contour lines shows how to use them.

03Reading depth for a downrigger spread

Downrigger fishing adds a second job: knowing what depth your gear is at compared with the marks on the screen.

1. Use the cursor to read the depth of a mark. Most units let you move a cursor onto a mark and show its depth. Compare that with your downrigger counter and adjust.
2. Check temperature. A temperature readout shows the water temperature at the transducer, and some setups add other sensors. See our guide to reading the summer thermocline for what a layer can look like on sonar.
3. Don't rely on the screen alone for gear depth. Your downrigger's own depth reading or cable counter is a more direct measure of where the weight is. Use the sonar to find the layer and the marks, and check the manual for how your unit displays targets.
4. Allow for the delay. A mark you see on screen is already behind you. A short pass over it again, or a waypoint, helps you return.

Our walleye fish finder guide covers more of the sonar side of trolling for a particular species.

Running repeat passes

Downrigger fishing often means running the same line several times at different depths. Record or mark the start and end of a good line, and run it again at the same speed, so that what you change is the depth. Keeping speed steady also keeps the screen easy to compare from pass to pass. Write down the speed and depth you settle on so that you can return to it.

04Screen layout and controls

Because you are watching a screen from a rod holder or helm, readability matters more than extra features.

• A split page. Sonar on one side, chart on the other, and a data bar for speed, depth and temperature. Make sure the unit lets you save the layout as a preset.
• Screen size. Bigger helps at a distance. Our screen size guide goes through the trade-offs.
• Controls. Touchscreens are quick to use, while buttons work with wet or gloved hands in chop. Our touchscreen vs button guide compares them.
• Brightness and glare. A trolling boat spends hours in one position, so check the screen in your lighting before you buy.

05Marking waypoints and recording your track

A trolling pass is easy to lose. A waypoint on a strong mark, a depth change or a good fish lets you loop back on the same line.

• Waypoints. Learn the one-touch shortcut for marking your position, and test it before the trip. See our guide to using GPS waypoints.
• Track recording. Many units record where you have been, so you can repeat a productive line or avoid retracing it.
• Sonar logs. Recording sonar lets you review a pass at home, and it uses card space, so check the capacity first.

Before the trip, check how much space the card has, and back up your waypoints to a card or phone when you get home.

06Power and mounting for a long day

A trolling day can run for hours with the screen on and the sounder pinging the whole time. Work out what the display draws from its manual, and compare it with your battery. Our battery and power requirements guide explains how to do the sums, and your battery maker's instructions decide charging.

Mounting matters too. Trolling happens at modest speeds, so many anglers find a transom or trolling-motor transducer works well, but position, angle and noise still decide how clean the picture is. Keep the cable clear of downrigger cables, rods and lines so nothing tangles with it, and think about where the display sits relative to the rod holders so you can see it without turning around.

A quick feature checklist

• Speed readout that suits your method, either GPS or water speed.
• Temperature readout, and a sensor that your transducer supports.
• Cursor depth readout and a stable bottom line.
• Split screen with large data digits, saved as a preset.
• One-touch waypoint and track recording.
• A transducer that suits your boat and reads cleanly at your speed.