Fish schools on sonar

[jmax]

What kinds of fish cause headaches for sonar operators? The picture for 'biologic fish school' in the database looks like Bluefin Tuna, which makes sense, but what else might register as a significant sonar contact?

[SeaQueen]

Anything with a swim bladder has a high active target strength. Really big schools of baitfish can appear as giant amoebas on an active sonar screen.

On the passive end, lots of fish and shellfish make surprisingly loud noises. I once saw a video of a tiny little shrimp that uses its claws to make a popping noise through supercavitation. Other fish, like various sorts of croakers and snappers make loud noises. It's actually pretty common for many forms of life to make all kinds of noises. They use noises for mating, threatening, and other forms of communication. Sometimes echolocation.

Urick's Principles of Underwater Sound has a whole section devoted to the target strengths of fish and includes a formula to approximate it based on the size of the individual fish. That would be useful for fish finding sonars. There's also a whole section on biological sources of ambient noise. He shows examples of the sound spectrographs of toadfish, porpoises, drumfish, croakers, shrimp and sea robins. He also shows how it varies seasonally.

ORIGINAL: jmax

What kinds of fish cause headaches for sonar operators? The picture for 'biologic fish school' in the database looks like Bluefin Tuna, which makes sense, but what else might register as a significant sonar contact?

[jmax]

Hah, yes I've heard of snapping shrimp being a particular nuisance, but I assume things like that are factored into the acoustic clutter of shallow littorals generally?

Anyway, I suppose that answers my question, I was wondering whether or not I should include things like Atlantic cod and herring and other fish that form schools and spawning shoals. I'm building a scenario around the Gulf of Saint Lawrence and wanted to make the biologic side as realistic as possible—distribution, migration patterns, and whatnot. Good to know, thanks!

[SeaQueen]

It depends. There's the "ambient" noise level in the sonar equation which reflects the level of general background noise and is considered to be directionless. For theoretical purposes you can think of it generally representing distant sound sources. It's the level of a sort of constant "fuzz." More nearby there's directional sources of sound which have identifiable narrow band signatures. That's actually not entirely true, there is some directionality to ambient noise but for our purposes it's easy to think about it that way.

Biological sources of sound in the ocean aren't limited to just the littorals, they can be found everywhere. In fact, there's a lot of mysterious noises in the sea that have been recorded and not yet identified. Many of them are suspected to be biological in origin.

ORIGINAL: jmax

Hah, yes I've heard of snapping shrimp being a particular nuisance, but I assume things like that are factored into the acoustic clutter of shallow littorals generally?

Anyway, I suppose that answers my question, I was wondering whether or not I should include things like Atlantic cod and herring and other fish that form schools and spawning shoals. I'm building a scenario around the Gulf of Saint Lawrence and wanted to make the biologic side as realistic as possible—distribution, migration patterns, and whatnot. Good to know, thanks!

[jmax]

Maybe I could add a hundred tiny explosions? [:D]

Many of the mysterious sounds are suspected to be ice-related, too. That's what they now figure the Bloop was.