Why Fish “Arches” Happen (And Why They Don’t Always Look Like Arches)
An arch isn’t a picture of a fish. It’s a timing effect created when a fish passes through a sonar cone and the return strength changes as it moves closer to the center of the beam.
- Big arches: often strong targets passing near the center of the cone.
- Dots/streaks: fish on the edge of the cone, fish moving fast, or your boat moving fast.
- No arches at all: down imaging/clear views often show fish as bright dots, not classic arches.
The 3 variables that change arch shape
- Boat speed: faster movement stretches targets into lines.
- Cone angle/frequency: wider cones “hold” fish longer; narrow cones can show tighter marks.
- Sensitivity + noise rejection: too high creates clutter; too low hides fish.
How Bait Schools Look (2D, Down Imaging, and Side Imaging)
Bait is one of the most useful things to identify because predators often stack around it. The look changes by sonar type:
- 2D/CHIRP: bait often appears as a “cloud” or fuzzy mass, sometimes with stronger marks around the edges.
- Down Imaging: bait can look like a tight cluster of specks or a textured blob.
- Side Imaging: bait schools can show as light patches with shadows (especially when suspended).
Tip: when you see bait, slow down and cross it from a second angle. If it’s real bait, it tends to show consistently across passes. Random clutter usually doesn’t.
Thermoclines: The “Invisible Layer” That Fish Love
A thermocline is a layer where water temperature changes rapidly with depth. In many lakes during warm seasons, fish often relate to it because oxygen and temperature conditions change above and below that layer.
On a fish finder, the thermocline often shows as a faint, consistent band at a similar depth across open water—especially on 2D/CHIRP. It’s not always visible (lake conditions vary), but when you see it, it’s a powerful clue.
How to confirm you’re seeing a thermocline (not noise)
- It stays at a consistent depth as you move around open water.
- It looks like a thin band, not random speckles.
- Reducing sensitivity slightly doesn’t make it disappear completely.
Settings That Make Everything Easier to Read
- Start with auto, then tune: get a baseline before you change 10 settings at once.
- Adjust sensitivity until clutter calms down: you want fish and structure, not constant snow.
- Set correct depth range: too deep a range compresses everything.
- Use a reasonable scroll speed: match it to your boat speed so targets don’t smear.
If you want a full beginner walkthrough, start here: How to Read a Fish Finder Screen. For choosing sonar types (CHIRP vs side/down vs live), this guide helps: Fish Finder Sonar Types Explained.
Next step: If you’re ready to upgrade (or buy your first unit) and want picks that match fishing style and budget, jump to our updated guide: Best Fish Finders (2026).
