Hint: Different kinds of fish have different shaped & sized swim bladders, which is why sound reflects in a different manner from each species.

Are you still wondering?

Modern spearos who are familiar with the sonar have the ability not only to locate fish, but also to identify the different species.

How does it work?

By studying the return echoes imprinted on the screen, the modern spearo can now identify the particular “signature echo” for specific species of fish.

(I am using SIMRAD NSS12 EVO3 S with AIRMAR transducers).