Identify Fish on Sonar

“Those are white seabass,” said Capt. Yet skippers such as Wisch and others have learned how to ID species they see on fish finders. On dense surfaces, it bounces well. “After years of comparing fish marks to what’s biting, I’ve learned to distinguish striped bass from bluefish returns,” says Pitasi. “Bluefish show as shorter, rounder marks.” Pitasi fishes with the Raymarine CP470 CHIRP sounder with an Airmar B175H transducer aboard his 205 Triton 26-foot center console, targeting striped bass and bluefish along the rips and reefs off Rhode Island. “Most of the time, we’re after striped bass,” he explains. “So it is helpful to know how to distinguish schools of stripers from bluefish, which often feed in the same waters.” Humminbird’s Helix G2 or G2N (networking) unit with GPS now accesses the Humminbird basemap for coastal and inland waters. “Squid are soft-bodied, with no bones or swim bladder, and barely mark on a fish finder.” On many color fish finders, all you see is blue fuzz when a school of squid passes under the boat, as opposed to the distinctive red blob from a school of sardines. Coleman’s boats are also equipped with Raymarine CP470 CHIRP sonar systems with Airmar B175M transducers. “We basically mark two species when fishing offshore — schools of tuna (albacore) and groups of blue sharks, but we are interested only in tuna,” he says.
Identify Fish on Sonar
Striped bass tend to make more elongated marks on a fish finder than do bluefish.

“Those are white seabass,” said Capt. Mark Wisch, peering at the fish-finder display in the dark of night as we bobbed in 120 feet of water on the backside of Santa Catalina Island off Southern California.

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“Get your bait close to bottom,” he said. I promptly dropped a live squid on a lead-head into the depths. Within seconds, I was hooked up. A series of violent head shakes were all I needed to assume that Wisch was correct.

Five minutes later, the captain sunk the gaff in a silvery 45-pound white seabass, a glory fish that locals call a tanker.
Sophisticated fish finders and sonar systems from brands such as Furuno, Garmin, Humminbird, Lowrance, Raymarine and Simrad make it pretty easy to distinguish fish from other underwater elements such as wrecks or the seabed.

However, there’s one thing these advanced echo-sounding devices cannot do: identify a particular species of fish or any other sea creature. Yet skippers such as Wisch and others have learned how to ID species they see on fish finders. How do they do it?

Identify Fish on Sonar
These longer returns show striped bass on the edge of a reef.

Keys to Learning
One of the steps in recognizing species is developing an understanding of how echo sounders work. To simplify things, think in terms of bouncing a tennis ball. On dense surfaces, it bounces well. On softer surfaces, it does not.

Similarly, fish-finder signals bounce strongly off certain parts of a fish, including the skeleton and air bladders, Wisch explains. So it’s also important to understand the anatomy and habits of different fish. This can lead you to identify a particular species you mark.

“For example, croakers like white seabass have heavy bone structures and large swim bladders,” says Wisch, “so the croakers generate relatively large marks on a fish finder.

“White seabass also move slowly, and so on a stationary boat, they tend to stay inside the sonar beam angle longer,” he adds. This results in elongated returns, sometimes called “worms,” on the fish-finder display. On the other hand, California yellowtail, a species of amberjack that…

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