How to see tuna on the fish finder: marks, bait ball and settings
Finding tuna is, to a large extent, knowing how to read the sounder. But tuna are difficult: they have no swim bladder, which is what returns the strongest echo in other species, and they move fast. On automatic settings they usually go unnoticed. Here are the keys, with our own schematic: not a real screenshot, but what you need to learn to recognise.
Frequency
- 50 kHz: wide cone and long reach; sees more water with less detail. Good for locating bait and layers at depth.
- 200 kHz: narrow cone and fine detail; separates fish from the bait ball down to 60–80 m.
- CHIRP: sweeps a band of frequencies and draws sharper marks. Medium bands work well for tuna; many tuna skippers work around 80–110 kHz when the unit allows it. With a split screen, 50 kHz on one side and medium CHIRP on the other.
Settings that make the difference
- Manual gain, somewhat above what auto would set: tuna return little echo and auto mode cleans them up as if they were noise. Raise it until a little grain appears, then back off one step.
- Manual range, fixed to the depth you are working (for example 0–80 m), so the screen does not rescale every time the bottom changes.
- Fish symbols off (Fish ID on Garmin and Lowrance): they draw little fish where there is noise and hide the real shape of the mark.
- A-Scope or real-time echo bar on: it tells you whether the fish is under the boat right now.
- And most important: do not change settings every day. Only with the same configuration will you learn to recognise a mark.
What a tuna looks like
- A mark thick at the front that tapers at the back, on a diagonal: rising or falling depending on how it crosses the beam. If it swims with the boat, the mark stretches.
- Several streaks at the same depth with the same slope: a school.
- A wide, slow arch is usually a shark or a big fish that moves little; dolphins paint large, very fast streaks and come up to breathe.
The bait ball and the thermocline
The bait ball is a dense cloud, often with a hole in the middle when predators attack from below. The thermocline shows as a faint, continuous line at mid-water: tuna often work just above it, where the water is warmer and food is. If you see bait and thick marks below, it is time to put the spread in the water.
Settings on Garmin and Lowrance
- Garmin: Sonar → Menu → Gain (Manual), Range (Manual), Fish Symbols (Off), A-Scope (On); on CHIRP units, “Medium” frequency for tuna.
- Lowrance: Sensitivity (Manual), Range (Manual), Fish ID (Off), Colorline to taste to separate bait from big fish; on CHIRP, “Medium”.
Names vary between models: your unit's manual has the last word.
How much you see: angle, frequency and power
The transducer does not look at a point: it looks at a cone. The wider it opens, the more water you see and the less detail you get. The usual numbers:
| Transducer | Frequency | Angle | Indicative reach |
|---|---|---|---|
| 50/200 kHz, 600 W (sea) | 50 kHz | ~29° | up to ~350 m |
| 200 kHz | ~12° | ||
| 83/200 kHz, 120 W (fresh water, inshore) | 83 kHz | ~52° | ~120 m |
| 200 kHz | ~22° | ||
| Low CHIRP | 28–60 kHz (or 42–65) | wide | reaches deepest: 400 m and more |
| Medium CHIRP | 80–135 kHz | medium (6–11° on a 1 kW) | the favourite for tuna and deep jigging |
| High CHIRP | 150–250 kHz | narrow (25° on a 1 kW TM185HW) | ~150 m, maximum detail |
Cone diameter = 2 × depth × tan(angle/2). Over 50 m of water a 12° cone covers about 10 m across; 29°, 26 m; 52°, 49 m. A tuna 15 m off the boat does not exist for a narrow cone: that is why the split screen (wide on one side, narrow on the other) is the skippers' standard.
600 W or 1 kW? Power does not give you “more fish”: it gives usable echo deeper and with the boat under way, with less bottom loss when accelerating. Under 100 m a well-installed 600 W is plenty; to read 150 to 400 m, or hold bottom at 20 knots, the kilowatt shows. What loses readings most is not power but a badly placed transducer: bubbles, strakes, prop wash or a through-hull at the wrong deadrise.
Sensitivity and palette
- Sensitivity (Lowrance) or gain (Simrad) in manual, between +2 and +10 over auto. Jigging skippers' rule: over 100–150 m of water, better to saturate with noise the layers you will not fish than lose a mark glued to the bottom.
- Noise rejection and surface clarity: off unless other boats interfere; they also erase shallow fish.
- Palettes: with bottom fill (palette 13 on Lowrance) to see fish tight to the bottom; without fill (1–12) to judge hardness. Hard bottom = thick, intense line, often with a second echo; soft bottom = thin line. Palette contrast between 70 and 78%.
- Thermocline: two thin horizontal lines across the whole screen.
Where the fish is inside the cone
A fish entering at the edge of the cone returns little echo (cold colours) and, if it crosses the centre, climbs to the hot colours: the mark changes colour from outside in. With the A-Scope (real-time echo) you see whether it is under the boat right now. Full arch: the fish crossed the whole cone. Half arch: it came in and out one side, or the boat changed course. And mind the blind angles: behind a rock or a drop-off the cone cannot see; run the area from another direction.
Ping speed and scroll speed
Ping speed: how many pulses per second. Raising it helps with interference from other boats and with following the lure vertically; lowering it gives a little more detail at depth. Scroll speed: how fast the screen runs. Trolling, normal; vertical, fast, to see the reaction to the lure instantly. Neither changes what is below: only how it is drawn.
Other predators and bait
- Snapper, grouper and dentex: thick, clear arches tight to the bottom or the rock; best seen with bottom fill.
- Horse mackerel, mackerel and sardine (bait): dense, compact clouds, sometimes with a hole in the middle; sea bass and bonito work the edges.
- Swordfish and bigeye: thick, very deep marks (100–400 m), low CHIRP and kilowatt territory.
- Shark: wide, slow arch. Dolphins: big, fast streaks in a group, coming up to breathe.
The skippers' trick
Every time you land a fish, take a screenshot of the sounder. In a few trips you will have your own collection of confirmed marks, for your boat and your unit. It is worth more than any photo on the internet.
Sources
- Woodward, C. How to identify fish on sonar. Sport Fishing Magazine. Article
- Simrad Yachting. Fish identification with Simrad sonar technology. Article
- Garmin and Lowrance user manuals for the exact names of each setting on your unit.
- ONNautic (marine electronics, El Puerto de Santa María), sounder tutorials (Spanish): frequencies and angles, fixed vs CHIRP frequencies, sensitivity, hard or soft bottom by palette, fish position in the cone, blind angles, scroll and ping speed, transducer installation and TM185HW transducer.
- Airmar, CHIRP bands and reach by model (TM185M, R599): Sport Fishing Magazine.
