How to choose a fish finder: five decisions, in order

Decision 1: what you fish from

This eliminates most of the shelf before you look at a single specification, and almost nobody starts here.

A bank or a dock. You need a castable sonar ball, not a mounted unit. Our castable against mounted page covers the honest limits of those.

A kayak or a canoe. Small screen, low current draw, and a transducer mounting problem. Our kayak sonar page and canoe page cover both.

A jon boat or aluminum boat. Transom mounting works, but rivet lines and strakes cause bubbles across the transducer face. Our aluminum boat page covers placement.

A bass boat or a bigger hull. You have power, space and a console. This is the only category where a large unit makes sense.

Decision 2: screen size, which costs battery

Screen size is the most visible specification and the one people overbuy.

Two things stand out. Most makers do not publish a current draw at all, which matters on a small craft running a 7 Ah battery. And brightness in nits is published by Lowrance and not by the others — a specification you appreciate at noon in July.

The practical rule: 4 to 5 in on a kayak or small boat, 7 in if you have a console and a real battery, 9 in if you run split screens all day.

Decision 3: do you actually need imaging?

Traditional sonar draws what is under the cone. Imaging draws a photograph-like picture of the bottom, either straight down or out to the sides.

Traditional CHIRP alone is enough on small water, for depth, bottom hardness and fish marks. Our pond and small lake page makes that case.

Down Imaging is worth it if you fish structure and need to know whether a breakline is rock, gravel or mud, or want to separate a fish sitting hard on the bottom. The HELIX 5 DI publishes it at 455 kHz.

Side Imaging is worth it on large flats where you are searching. The Eagle range publishes up to 600 ft per side, which covers a 1,200 ft swath in one pass. On a small lake it is money spent on coverage you do not need.

Our sonar types page explains what each channel is physically doing.

Decision 4: mapping, or make your own

Pre-loaded charts mean you arrive with contours. The Eagle range publishes C-MAP inland coverage of 17,000+ US lakes. If you fish named water, this is instant value.

Self-mapping builds contours as you drive. Garmin publishes Quickdraw Contours at 1 ft resolution, which is finer than most commercial charts, and it works on lakes nobody has ever charted.

Waypoints only is the entry level: no chart, but you can mark a spot and come back. The STRIKER 4 publishes GPS with a waypoint map, and on a pond that is most of the value.

Our maps and charts page covers the formats, and our walleye sonar page argues mapping matters more than imaging for structure species.

Decision 5: the transducer, which people forget

The transducer is the part in the water and it decides what the screen can show. A great unit on a badly mounted transducer reads worse than a cheap one mounted properly.

Frequency sets the cone. Higher frequency means a narrower cone and more detail. The PiranhaMAX publishes 28 degrees at 200 kHz and 16 degrees at 455 kHz. In 10 ft of water that is a 5 ft circle against a 2 ft 10 in one.

Mounting decides whether it works at all. It has to sit in clean water with no bubbles crossing the face. Behind a rivet line, behind a strake, or in the wash of a trolling motor prop are all places it will not read.

Our installation page covers the placement, and it is worth reading before you buy rather than after.

Specifications you can ignore on inland water

Depth rating. A unit rated to 1,200 ft and one rated to 600 perform identically in 20 ft of water. You are paying for a number you will never approach.

Transmit power, mostly. Power buys penetration through deep water and thermoclines. In the shallows it largely buys surface clutter you tune out.

Color count. A 256-color TFT and a 16-bit display both draw a bottom line and an arch.

Power is part of the purchase

A fish finder needs a battery, and on a kayak or a small boat that battery is a real cost and a real weight.

A 7 Ah sealed lead-acid battery runs a small unit for a long day. The Eagle 9’s published 640 mA draw over ten hours is 6.4 Ah, which is most of one.

Do not share it with a trolling motor. The voltage sag when the motor loads up shows as noise on the sonar. Our marine batteries page covers sizing.