A fish finder, or sounder, shows you what is happening under the boat. Offshore, it helps you find bait, spot gamefish, read structure, and track water temperature. Used well, it turns blind trolling into targeted trolling.
Basic settings for offshore
- Frequency: lower frequencies reach deeper water; higher frequencies show more detail in shallower water. Many modern units use CHIRP, which sweeps a range of frequencies for clearer images.
- Range: set the depth range so you can see the full water column or the depth where fish are likely holding.
- Gain: adjust so you see bait and fish without too much clutter.
Reading bait
Bait schools show as clouds or balls. Dense, tight clouds often mean bait under pressure. Scattered specks may be plankton or loose bait. Marking bait is a good reason to slow down and work the area.
Reading gamefish
Larger fish often appear as arches or individual marks around or below bait. Tuna frequently show as strong marks moving through the water column. When you see marks under bait, troll back through and consider adding a deep line.
Bottom and structure
Watch for ledges, humps, and drop-offs. Mark them on the plotter, along with any bites, so you can return. See trolling ledges and canyons.
Water temperature
Most fish finders show surface temperature. Watch for jumps that mark temperature breaks. See trolling temperature breaks.
Speed matters
At trolling speed, sounders still work well, but image quality can drop in rough water or at high speed. A well-mounted transducer reduces interference.
Turning readings into bites
- Mark bait and fish on the plotter as you see them.
- Circle back through the marked area with the spread.
- If fish are deep, add a planer or downrigger line.
- Note the depth where fish hold for future trips.
Combine with other signs
A sounder works best combined with birds, weed lines, and SST charts. See reading birds offshore and SST charts.
