Sonar Decoded: Read Your Fish Finder Better

Your fish finder is showing you a real-time scroll of sound waves bouncing off everything in the water column below the boat. What most anglers miss is that the picture is not a photograph — it is a graph of time versus depth, and the further left on the screen, the further back in time you were. The boat was over that piece of structure minutes ago, not right now. Understanding that one thing changes how you interpret almost everything else on the display.

Sonar works by timing an echo. The transducer fires a pulse of sound downward. That pulse hits objects — fish, structure, the bottom — and some of the energy bounces back. The unit measures how long the return takes and converts that to depth. Denser, harder objects reflect more energy. Softer, less dense objects absorb more and return a weaker signal. That's the physics that lets you read bottom type, fish size, and bait density from the same display.

Getting your gain setting right — the make-or-break adjustment

Gain is the most important setting and the most commonly misadjusted. Turn it too high and you're looking at noise — every bubble, particle, and plankton cloud clutters the screen and masks the fish returns you're trying to find. Too low and you're missing fish that are showing up below the threshold.

The double-echo method gives you a reliable starting point. Increase gain until you can just see a faint second bottom echo at roughly twice your actual depth. That faint secondary line confirms your unit is capturing the full return signal. According to Australian fishing instructor Ryan Moody, who has run fishing sounder courses and holds 30 years of daily on-water experience from shallow estuaries to 400 m offshore water, the transducer and its settings interact — but gain calibration against the double echo is a consistent baseline regardless of which unit you're running.

Auto-gain modes on modern units are decent for general motoring, but drop into manual when you're fishing a specific spot. The auto mode optimises for an average of conditions across the last few seconds of travel; manual lets you dial in for exactly what's under you right now.

Reading the bottom — the display tells you what's down there

Bottom hardness is visible if you know what to look for. A thin, sharp bottom line with hot colors (reds and yellows on a colour display) means hard ground — rock, reef, shell beds, coral. The sound returns fast and clean from a dense substrate. A thick, fuzzy bottom line means soft ground — sand, mud, or weed. This is the part that trips up most beginners: because mud is partially penetrated by the sonar pulse before it bounces back, it produces a fat, smeared return that looks substantial. Hard reef does the opposite — it returns a thin, bright, definitive line.

On colour units, follow the reds. A bright red or yellow inner band with orange fading outward on the bottom trace is the clearest indicator of hard substrate. When you see that thin red line switch to a broad, orange-brown smear, you've crossed from reef onto sand or rubble. Mark the transition — fish often hold right on the edge between the two.

"A thick bottom line does not mean solid ground — in soft mud, the sonar partially penetrates before returning, creating a wide false echo that beginners routinely mistake for rock."

Fish arches — what they tell you and what they don't

The arch shape comes from the cone of the sonar beam. As your boat moves over a stationary fish, the leading edge of the cone hits the fish first, the centre of the cone passes over next (producing the strongest return — the peak of the arch), and then the trailing edge of the cone exits. The result is an arch shape drawn on screen as the boat moves. A fish that holds perfectly still while you pass over it will produce a clean, full arch. A fish that moves with the boat produces a partial arch or a straight line.

Arch length tells you how long the fish was in the beam — not how big it is. A small fish sitting motionless directly under the transducer will produce a long arch. A large fish that crosses the beam at speed produces a short arch. The actual indicator of fish size is the thickness of the arch in the vertical dimension — a wider, stronger, brighter arch return indicates a larger swim bladder, which means a larger fish. This is one of the most common misreadings on a fish finder, and correcting it immediately improves how you assess what's below.

Baitfish look different from target fish. Bait schools show up as clouds, smears, or dense blobs — the individual fish are too close together and moving too fast for the unit to resolve them as separate arches. A concentrated bait cloud near the bottom with cleaner arches sitting above or just below it is one of the most reliable signs of active feeding. The target fish sit at the edge of the bait, not through the middle of it.

Thermoclines — that horizontal line is telling you something

A thermocline appears as a faint horizontal band running across the water column. It marks where a warmer surface layer meets cooler water below — a density difference that partially reflects the sonar signal. To see it clearly, increase your gain above normal. The line typically shows up around mid-water column in offshore environments from late summer through autumn.

Fish concentrate at thermoclines for two reasons: oxygen and bait. The mixing zone supports zooplankton, which attracts small baitfish, which attract predators. Tuna, mahi-mahi, and marlin commonly hold in the 5–15 m band above a thermocline. Snapper and other demersal species are less affected, but in very warm surface conditions they can push deeper to sit just above the cool layer. If you're seeing fish arches at a consistent depth and can't explain why they're holding there rather than near the bottom, check whether that depth correlates with the thermocline band on your display.

2D sonar vs CHIRP vs DownScan vs SideScan — choosing the right view

Standard 2D sonar is the traditional scrolling display — fast, works in all depths, shows fish arches well, and gives you the clearest picture of individual fish. It is the right tool for most fishing situations, particularly offshore and in deeper water.

CHIRP sonar sweeps a continuous range of frequencies — from low to high — rather than pinging a single frequency. Ryan said Moody's guidance on sounder technology, CHIRP provides better target separation because the low end of the sweep gives depth penetration while the high end gives resolution, combining both in the one pulse. The result is individual fish visible as distinct arches where traditional 2D shows a blurry mass. CHIRP in the 260 kHz range handles deeper water well; in the 455 kHz range it is well suited to water under 50 m (164 ft).

DownScan (Down Imaging) produces an almost photographic representation of the bottom directly below the boat. It is excellent for reading structure — identifying rocky bottom versus weed patches, seeing submerged timber or reef formations, and locating fish that are sitting tight to structure. It is less useful for individual fish identification at depth.

SideScan (Side Imaging) fires beams out to each side of the boat, covering a wide swathe of bottom — up to 40–50 m (130–165 ft) each side depending on depth. It is the prospecting tool: cover ground quickly, identify structure away from your current track, then go back and fish it. For estuary fishing, SideScan is useful for finding oyster leases, snag piles, and drop-offs without having to drive directly over them. Run it at under 6 knots (11 km/h) to give the processor time to render accurate returns.

For most Australian fishing scenarios — estuary work in under 10 m (33 ft) for bream, flathead, and whiting; offshore bottom fishing at 20–80 m (65–260 ft) for snapper and pearl perch; or surface trolling for pelagics — a CHIRP-capable unit with a quality transducer covers all bases. The transducer selection matters more than the screen size.

Chart plotter integration — marking what works

Every time you find productive structure, drop a waypoint. This is one of the most under-used features on combo units. A sonar pass over a reef edge that shows fish arches is useless the next time out if you can't find it again. Mark the waypoint the moment you see the return — not when you've drifted 50 m past it. Label it with enough information to jog your memory: depth, species suspected, date, and tide state. Tide state matters because a snapper or bream bite over a structure can vary significantly between high and low water. Checking the Seabreeze tide chart for your session and comparing it against past waypoint notes is how you start building a picture of when and why spots fire.

Build a waypoint library, not a collection. Three waypoints you fish systematically are worth more than 300 random marks. Revisit them in different conditions — different tides, different seasons, different times of day — and note what produces. Over time, the sonar picture and the waypoint data together tell you more than either would alone.

Once you've got your gain dialled and you understand what the bottom trace and fish arches are actually telling you, the next step is learning to read moving vs stationary targets — that pattern is what separates fish from structure, and it's worth a dedicated session just working it through on known ground.

Common questions

My gain is on auto and I see almost no fish — what's happening?

Auto gain often runs conservatively to minimise false echoes. In auto mode you can miss fish that a manually set unit would pick up, particularly in shallow, busy estuaries. Drop into manual mode, apply the double-echo method to set your baseline, and increase gain slowly until the screen makes sense for your conditions.

What frequency should I use for estuary fishing under 5 m depth?

High frequency — 200 kHz or above — gives you the resolution you need in shallow water to distinguish individual fish from structure at close range. The low-frequency advantage (depth penetration) is irrelevant when you're fishing in 3–5 m (10–16 ft) of water. If your unit offers CHIRP and you're in shallow estuaries, the higher CHIRP range (around 455 kHz) is the pick.

I can see baitfish clouds but no arches above them — why aren't the predators showing?

Two possibilities: the predators are mixed through the bait (making them hard to resolve separately), or they've already moved. Bait clouds without active feeding predators nearby often mean the school has already been worked over. Look for bait sitting tight to the bottom or against structure — inactive bait holding on the bottom, rather than suspended mid-water, more often has target fish somewhere close. Check the edges and bottom of the cloud.

Is DownScan or SideScan necessary for basic boat fishing?

Neither is necessary; both are useful. If you're fishing known ground repeatedly, a standard CHIRP unit without imaging modes is fine. If you want to find new ground, discover structure in unfamiliar water, or prospect estuaries efficiently, SideScan pays for itself quickly. DownScan is the easier first upgrade — it adds structure clarity without the need to understand sideways imaging geometry.