Here are six actionable tips for using a sounder (fish finder) and GPS together to locate fish effectively, with a focus on bait identification, structure reading, and current interaction:
1. Identify Bait with Your Sounder
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Use your sounder to locate dense, cloud-like shapes in the water column—these are often baitfish.
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Bait typically appears as tightly packed clusters or amorphous blobs, often mid-water or just off the bottom.
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Predators are usually nearby; look for larger arches or streaks around the bait.
2. Mark and Return to Productive Spots Using GPS Waypoints (Commonly Called ‘Marks’)
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When you find bait or fish-holding structure (e.g., a ledge or reef), mark it on your GPS.
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Save multiple waypoints in promising areas to create a “milk run” of productive spots to revisit.
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Use GPS trails to replicate good drifts for dropping divers.
*take note of prevalent current on the day you mark these spots because if the current is running in a different direction or there is no current at all, these spots/drift lines will have 0 relevance.
3. Scan for Obvious Structures Like Drop-Offs, Ledges, and Caves
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Use your sounder to detect sharp changes in bottom contour—these often attract bait and predators.
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Look for overhangs, caves, or sudden depth changes. These appear as steep vertical lines or shadows on the display.
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Fish tend to hold near the edges of structure where they can ambush prey.
4. Understand How Current Interacts with Structure
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Fish face into the current, positioning themselves where food is carried to them.
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Use GPS to identify up-current sides of structure—these are often more productive than the down-current sides.
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Target eddies and backflows near points, humps, or reefs where predators wait for bait to pass.
5. Layer Information to Pinpoint Fish Zones
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Combine GPS, sounder data, and chart overlays (like depth contours and current flow).
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If bait is hugging a drop-off and current is pushing over that structure, predators will likely be on the pressure side.
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Look for convergence zones where structure, bait, and current intersect—these are prime drop zones. Keep in mind that if you are dropping divers in to target these zones, you will need to drop them off further up current to give them time to load their spearguns, relax, breathe up and prepare.
*low revs for the win: doing a few passes over the pressure points to assess conditions/fish life on the sounder is fine however do it with low revs to spook less fish and give your divers the best chance on a good drift
6. Adjust Depth and Frequency Settings Based on Conditions
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Use higher frequency (e.g., 200 kHz) for shallow, detailed bottom imaging; lower frequency (e.g., 50 kHz) is for deeper water (generally line fishing).
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Adjust gain/sensitivity to distinguish between bait, fish, and bottom clutter.
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In deeper water or faster current, slow down your boat to allow better returns and more accurate reading of structure.
To wrap up, mastering your sounder and GPS is crucial for successful spearfishing. By understanding how to identify bait, read underwater structures, and interpret current interactions, you can significantly enhance your fishing efficiency. This is often why people with a line fishing background make great spearos – they already know how to find fish and understand basic things about where to find them.
For those looking to deepen their knowledge, several resources can guide you:
Books:
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The Ultimate Fish Finder Guide by Martin Jacobs and Jens Biegemeier offers over 135 illustrations to help you interpret sonar images and locate fish hotspots. The Fishfinders+1Amazon+1
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AFN GPS Fishing Marks Australia Wide by Jim Harmon is a valuable resource for Australian anglers, offering over 3,000 GPS fishing marks across the country. Booktopia
Online Courses:
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Sounder Skills 2 by Ryan Moody Fishing is a self-study program that teaches you how to effectively use your fish finder to locate and catch more fish. Ryan Moody Fishing+1Ryan Moody Fishing+1
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Fish and Depth Finders Made Easy by New Wave Fishing Academy covers various fish finder brands and provides downloadable content for repeated learning. New Wave Fishing+2New Wave Fishing+2New Wave Fishing+2