Do not fish only where your sonar shows marks. A sonar beam is a cone that reads near zero at the surface and about 120 feet wide at 120 feet deep, so deep water shows everything while shallow, high fish stay invisible. Predators stack under shad balls, but many target trout and kokanee ride 5 to 20 feet down where the boat pushes them off the beam. Use sonar for temperature, depth, and speed as much as for fish. If others are catching or you suspect fish, stay and show your bait, even at 50 feet deep with no marks on the screen.
Why do I see fish deep on sonar but they bite shallow?
Because your beam misses shallow fish. The sonar cone is very narrow near the transducer, so unless a fish sits directly under the boat, which it usually will not since the boat spooks it aside, you never mark it. Deep fish fall inside the wide part of the cone and dominate the screen, but target trout at 5 to 20 feet still see your gear.
How does a sonar cone angle work?
Sonar fires in a beam shaped cone that widens with depth. As a general rule the beam is basically zero at the surface and about a 120 foot circle at 120 feet deep. That is why deep water picks up nearly everything in that wide circle while high, shallow fish slip past the narrow top of the cone unseen.
What are the big balls I see on my fish finder?
Those are massive schools of threadfin shad, often 20 to 30 feet tall, which is very typical seasonally. The fish holding below them are predators: bass, trout, catfish, and salmon. Expect a heavy line of fish somewhere in the 50 to 100 foot range stacked near that bait through much of the year.
Why is my sonar screen clear above and cloudy below?
That change usually marks a water temperature difference, a band the fish find comfortable. Something about that layer, whether oxygen level, temperature, or light penetration, holds them there. Reading where the screen goes from clean to cloudy tells you the depth zone the fish prefer on that stretch of the lake.
Should I leave if I am not marking fish?
No. You do not always have to see a fish to catch it. Cal Kellogg caught a kokanee about 50 feet deep at Bucks Lake without ever marking it. If you suspect fish are there or you see other anglers hooking up, stay put, show them your bait, and try to establish a pattern.
What should I use my sonar for besides finding fish?
Plenty. Use it to read water temperature, monitor your trolling depth so you do not tangle a downrigger or drag bottom, and check your speed. Watch the screen closely in unfamiliar water where a rock pinnacle could rise up. Seeing fish is a bonus, not the whole job of the unit.
How do I target suspended fish I cannot see?
Send your gear to the depth near the bait line and near the marks you do have, but keep offerings in the zone where active fish feed. Many trout and kokanee ride 5 to 20 feet down where they can see a lure, even when the clearest marks on your sonar sit far deeper.
Watch the technique
Reading sonar for trolling FAQ
No. Cal caught a kokanee about 50 feet deep at Bucks Lake without ever seeing a mark. A quiet screen does not mean an empty area, so do not abandon water where you expect or see fish being caught.
The narrow top of the cone misses high fish and the boat spooks any sitting directly beneath it. Target trout at 5 to 20 feet stay invisible on the screen yet still rise to see and eat your gear.
Predators stacking under schools of threadfin shad in a comfortable band of temperature, oxygen, and light. That deep line shows up clearly because it sits inside the wide part of the sonar cone.
Threadfin shad schools, frequently 20 to 30 feet tall. They are a reliable seasonal sight and a strong clue to where predators are feeding.
Not necessarily. Send gear down near the bait, but keep working offerings through the 5 to 20 foot zone where active trout and kokanee often feed, even when the strongest marks are deeper.
Temperature, depth, and speed. Knowing your trolling depth keeps a downrigger from tangling or dragging bottom, and watching the bottom protects you in unfamiliar water.
Roughly as wide as it is deep. The beam is near zero at the surface and grows to about a 120 foot circle at 120 feet, which is why deep water reads so many more fish.
No. Stay and show your bait. Others catching fish is proof there are fish present, so work the area and try to trigger a bite rather than running off looking for marks.
