A modern fish finder uses sonar (sound navigation and ranging) technology to send sound waves downward into the water. When these sound waves hit something—like a fish, a bait ball, or the lake bottom—they bounce back to the transducer and are translated into an image on the screen.

Most units used on charter boats are dual-frequency or CHIRP (Compressed High-Intensity Radiated Pulse) sonar. These provide better clarity and depth detail than older single-frequency models.

Here are the key elements of a fish finder screen to understand:

• Water Column: The vertical portion of the screen shows depth from surface to bottom. Fish and bait will appear somewhere in this column.
• Bottom: This is the flat (or sometimes uneven) line at the base of the screen. Its color and thickness can tell us about bottom hardness.
• Objects in the Water: Fish, bait balls, and debris show up in the water column, and their shapes give clues about what they are.

Interpreting Fish Arches

Fish arches are the most iconic and sought-after symbols on a sonar screen. But why arches?

When a fish passes through the cone of sonar waves emitted by the transducer, it first enters the edge of the beam, then the center (which is closer and stronger), then exits. This creates a curved, arch-shaped return on the screen.

How to recognize fish arches:

• Medium to large fish—like king salmon—often create well-defined, rounded arches.
• Smaller fish—such as alewives or smaller coho—might appear as partial arches or dots, depending on their size and movement.
• Multiple arches in a cluster can suggest a school of salmon actively feeding.

Pro Tip:

When you see solid, thick arches near a bait ball or around the thermocline (more on that shortly), it’s a strong indicator of active fish. These are great zones to set your trolling spreads.

Identifying Bait Balls

Bait balls are dense schools of small forage fish—mostly alewives in Lake Michigan—that salmon feed on. On your fish finder, bait balls usually look like large clouds or blobs suspended in the water column. They may be tightly packed (a good sign of predator pressure) or more spread out.

What to look for:

• Color intensity: On color screens, dense bait will show up as red or orange centers with yellow or green edges.
• Shape: A round, tight ball suggests baitfish are trying to avoid predation. A streaky, elongated shape might indicate they are swimming freely or being scattered by predators like salmon or lake trout.

What it means for trolling:

• Troll near or around bait. Salmon are usually not far behind the buffet.
• If fish arches are interspersed with bait clouds, keep lures running just above or through the bait ball to mimic a fleeing alewife.

Finding and Understanding the Thermocline

The thermocline is a critical temperature barrier that forms in summer when the top layer of Lake Michigan warms while the bottom stays cold. Salmon love this feature because it marks the boundary where cold, oxygen-rich water meets warmer surface layers. Alewives often school near it, and salmon patrol it for easy feeding.

How to spot it:

• On sonar, the thermocline appears as a faint horizontal line between 30 to 60 feet down (typically).
• It may show a consistent band or a slightly fuzzy stripe that maintains depth across the screen.

Tactical adjustment:

• Run your baits just above or at the thermocline. This puts your presentation in the temperature “comfort zone” for salmon.
• Use downriggers or weighted lines (copper lines, copper, dipsy divers) to dial in this precise depth.

Pro Tip:

If fish are hanging just below the thermocline, try dropping your lure a few feet lower and slow the boat slightly to allow the presentation to sink just into their zone.

Bottom Contours and Structure

Even in Lake Michigan’s seemingly vast and featureless expanse, bottom structure matters. Submerged ridges, shelves, and drop-offs can concentrate fish and bait.

On your fish finder:

• Flat bottoms show as a steady line.
• Rises and drops appear as sudden inclines or declines in the bottom line.
• Hard bottom shows as a thick, bright line; soft bottom appears thinner or fuzzy.

Why it matters:

• Salmon often hang near ledges or shoals where currents concentrate bait.
• Bottom contours can help predict thermal layers and current breaks.
• During early spring and fall, bottom-oriented fish may include lake trout or salmon hugging cooler depths.

Using GPS and Chartplotter Data

Your fish finder isn’t just a sonar—it’s also a GPS tool. The chartplotter overlay shows boat position, trolling tracks, waypoints, and contours.

Here’s how your captain uses it:

• Mark productive spots. When salmon hit, GPS pins are dropped to return to the same area.
• Plot trolling paths. Zig-zagging through known fishy waters helps cover different depths and angles.
• Current and drift tracking. GPS lets captains counter drift or wind by maintaining a straight line or S-pattern through target water.

What to watch for as a customer:

• If you see the captain make repeated passes over one area, it likely means that sonar is showing consistent marks—either fish, bait, or thermocline zones.
• When you hear the click of a waypoint, it’s often your sign that the boat just passed over fish or caught one.

Adjusting Your Trolling Spread Based on the Screen

Now that you know what to look for—arches, bait balls, thermoclines, and structure—how does your captain change the fishing strategy?

1. Adjusting Depths

• If bait or fish are shallow (20-40 feet), expect more use of copper lines lines or smaller diver setups.
• If they’re deep (60-100 feet), downriggers and weighted copper lines come into play.

2. Changing Speeds

• If fish are tight to bait, speeding up may trigger aggression.
• If fish are scattered or hugging the thermocline, slower trolling allows lures to stay in the strike zone longer.

3. Choosing Lures

• Flashy spoons or meat rigs may be deployed when fish are near bait clouds.
• If fish are suspended alone, a more subtle presentation (like a fly or plug) can entice reluctant biters.

4. Direction and Angle

• Trolling across the current or at an angle to structure often creates a more enticing “flutter” in lures.
• “S-turns” can vary lure depth and speed, helping dial in the exact presentation that draws hits.

Common Screen Scenarios You’ll Encounter

Let’s put it all together. Here are a few scenarios and how to interpret them:

Scenario 1: Thermocline with Bait and Arches Above

Interpretation: Perfect salmon conditions. Fish are above the thermocline and feeding on bait.
Tactic: Set lines just above the thermocline and troll at a moderate pace.

Scenario 2: Big Cloud at 45 Feet with No Arches

Interpretation: Large bait school, but predators may not be nearby.
Tactic: Try trolling through it; if no hits after a pass or two, move to new water.

Scenario 3: Arches at 80-100 Feet with No Bait

Interpretation: Staging or neutral fish, maybe waiting to feed.
Tactic: Use meat rigs or glow spoons, slow the troll, and run deep lines tight to the marks.

Scenario 4: No Thermocline, Scattered Dots 30–60 Feet

Interpretation: Mixed bag—fish may be scattered after a front.
Tactic: Stagger depths using copper lines, divers, and downriggers to cover the whole column.

Getting Involved on the Water

As a customer on a Saugatuck charter, you’re not just along for the ride. Understanding what’s happening on the fish finder screen lets you participate more fully:

• Ask the captain or first mate questions about what you’re seeing.
• Watch how sonar marks turn into bites—it’s often just a matter of seconds or minutes.
• Keep an eye out for patterns: How does the action line up with what’s on screen?

The more you engage, the more you’ll understand the fishing process—and it’s a great way to pick up tips if you plan to fish on your own someday.

Final Thoughts: Reading the Water Like a Pro

Lake Michigan is a vast and sometimes mysterious body of water. But with a quality fish finder, a sharp eye, and smart trolling strategies, it becomes a canvas of information that can lead to exciting catches. From identifying thermoclines and bait balls to marking fish arches and adjusting presentations, the sonar screen is your window to the underwater world.

Next time you’re on a charter out of Saugatuck, take a glance at the screens at the helm. What may look like a bunch of lines and blobs at first quickly transforms into a detailed map of where the action is. Trust your captain, trust the electronics—and keep your rod ready. When the sonar lights up, it’s only a matter of time before the drag does too.

Ready to put this knowledge into action? Book your Lake Michigan salmon charter today and learn firsthand how we use fish finder data to stay on the bite!