Fish on the fish finder: how to recognise tuna, bait, snapper, sharks and dolphins

Published on 24/09/2026 · Aerovik

The sounder does not show fish: it shows echoes. Learning to read them is what separates a day of searching from a day of fishing. Here is the essential, condensed from Lowrance's guide, Garmin's catalogue, ONNautic's tutorials and what skippers share on forums, with our own schematic instead of other people's screens.

Why a fish is an arch

The transducer emits a cone. When a fish enters at the edge it is farther from the transducer than when it passes the centre, and on leaving it moves away again: the sounder draws that distance as an arch. Hence the three basic shapes Lowrance describes: full arch (the fish crossed the whole cone with the boat moving), half arch (it came in and out one side, or the boat turned) and line (the fish stayed still under a stopped boat, or swam along with it). A tuna following the lure behind the boat draws no arches: it draws a long streak.

Thickness and colour

Thickness = relative size, compared with neighbouring marks at the same depth and scale. A big fish on the edge of the cone marks thin; a small one through the centre, thick.
Colour = echo strength. Cold (blue) at the edge of the cone, hot (yellow, red) in the centre or with a big fish. A mark shifting from cold to hot is a fish approaching the vertical of the boat.
Tuna have no swim bladder: they return less echo than a snapper half their weight. That is why they vanish on auto gain.

Schematic of eight fish marks on a fish finder — The eight marks, schematised. Colours follow the usual palette: blue weak, red strong.

The eight marks

Single tuna: thick at the front, tapering behind, on a diagonal (rising or falling depending on how it crosses the beam). Swimming with the boat, it stretches.
School of tuna: several streaks at the same depth and slope, often above the thermocline.
Bait ball: dense, compact cloud, often with a hole in the middle when attacked from below; predators appear as thick marks just underneath or at the edges.
Snapper, grouper, dentex: thick, clear arch tight to the bottom or the rock; easier to pick out with a bottom-fill palette.
Shark: wide, slow, almost horizontal arch; big but unhurried.
Dolphins: large, very fast streaks in a group, climbing: they surface to breathe. If you see them there is food, and sometimes tuna below.
Swordfish and bigeye: thick, very deep marks, 100 to 400 m; low CHIRP and kilowatt territory.
Horse mackerel, mackerel, sardine: long band at mid-water, no hole, moving little; sea bass and bonito work the edges.

What changes on DownScan and ClearVü

Scanning sonar (DownScan, ClearVü, SideScan, SideVü) transmits at 455 or 800 kHz with a very thin fan-shaped beam: it draws structure almost like a photo, but fish are no longer arches, they are dots or light blobs, and bait shows as grainy clouds. 800 kHz gives more sharpness in shallow water; 455 kHz, more reach. Useful down to 60–150 m depending on the unit, so for tuna it serves to see bait and structure, not to work at 300 m. Lowrance FishReveal and Garmin's combined view overlay the CHIRP arches on that image: the easiest way to learn, with both views at once.

How to train your eye

Every fish you land, take a screenshot and note species, depth and frequency. In ten trips you will have your own catalogue.
Compare with screens identified by others: forums such as fishing.net.nz (kingfish, bait and snapper on Garmin) or BD Outdoors (bluefin, yellowtail, kelp) have real screens with the species confirmed. Look, do not copy: their value is in matching what you see on your own boat.
Change one setting at a time and do not change the configuration every day: the eye learns by repetition.

The concrete settings are in how to see tuna on the fish finder and menu-by-menu settings for Garmin, Lowrance and Simrad; the hardware, in transducers for tuna.