A fish finder shows you the underwater world in real time. Most anglers who buy one glance at it occasionally and never learn what they're actually looking at. Once you understand how to read the display, it changes where you fish, how long you stay, and how you present your lure. This is worth spending 20 minutes on.
How It Works
A fish finder sends sonar pulses from a transducer (mounted on your hull or transom) down through the water. Those pulses bounce off the bottom and anything in between, and the unit translates the return signals into the display you see. The speed of the return tells the unit how deep something is. The strength of the signal tells it how dense (hard or soft) the object is. Everything on the screen is historical data scrolling from right to left as you move.
Reading the Bottom
The bottom line runs across your display and its thickness tells you about composition. A thin, well-defined line means a hard bottom (rock, gravel, compacted sand). A thick, fuzzy line means soft bottom (mud, silt, weeds). This matters because walleye and bass relate differently to hard versus soft structure. A sharp drop-off where a hard bottom transitions to soft is often where you'll find fish holding on the edge.
The depth reading (shown in the corner of most units) updates continuously. When you see depth change rapidly, you're passing over structure. Slow down or mark that waypoint and come back to it.
Fish Arches
Fish appear as arches on a standard 2D sonar display. A complete arch means a fish swam through the full cone angle of your transducer while you were moving. Half arches are more common and just mean the fish was at the edge of the cone or you moved past quickly. The height of the arch indicates the fish's depth; the width reflects how long the fish stayed in the sonar cone.
Bigger arches don't always mean bigger fish. A small fish close to the transducer can arch as prominently as a large fish deeper down. The depth at which arches appear, combined with the proximity to structure, tells you more than arch size alone.
Fish sitting tight to the bottom often show as marks on or just above the bottom line rather than clear arches. This is typical walleye behaviour. If you see the bottom line getting "lumpy" or showing bumps, look more carefully, those might be fish.
Vegetation and Structure
Weeds appear as vertical streaks rising from the bottom. Dense weed beds show as a thick forest of returns above the bottom line. The top edge of the weeds is the productive zone for bass and pike. If you see weed growth ending at 12 feet and fish arches just above that line, set up and fish at 10 to 11 feet.
Submerged points, humps, and rock piles show as the bottom line rising toward you on screen before dropping away. These are fish magnets. Mark them as waypoints on your GPS and return to them in different conditions and seasons.
The Thermocline
In summer, lakes stratify into temperature layers. The thermocline is the boundary between the warm upper layer and the cold, oxygen-depleted lower layer. On a fish finder, it shows as a faint horizontal band running across the display. Fish won't typically go below it in summer because oxygen levels are too low. If you see the thermocline at 22 feet, you can stop fishing deeper than that.
For more on how water temperature shapes fish location, see our guide on How Water Temperature Affects Where Fish Are.
Sensitivity and Zoom
Most beginners run their fish finder on auto settings. That works, but learning manual sensitivity control gives you more information. Turn sensitivity up and you'll see more detail (including false returns from suspended particles). Turn it down and you filter out noise but might miss suspended fish. The right balance depends on water clarity and what you're looking for.
The zoom function lets you expand the bottom portion of the display. If you're fishing in 40 feet and all your action is in the bottom 10 feet, zoom into 30 to 40 feet and get a clear, expanded view of that zone. It's one of the most useful features on modern units.
Recommended Units for Quebec Freshwater
For a first fish finder, the Garmin Striker 4 ($130 to $150) is hard to beat. It has GPS, a clear display, and handles everything on Quebec lakes and rivers. The Lowrance HOOK Reveal series ($200 to $350) offers better chart integration. For kayak anglers, the Garmin Striker Cast ($130) clips to the gunwale or casts out on a float and connects to your phone via Bluetooth, no installation required.
20+ years fishing Quebec's freshwater systems. Kayak angler, catch-and-release advocate, and founder of Sub Urban Anglers.
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