Dolphins may rely on a nonlinear analysis method to "see" through clouds of bubbles that they create to trap fish, according to scientists at the University of Southampton in the UK. The researchers have devised a nonlinear sonar processing scheme that enables them to identify targets through clouds of bubbles, opening up the possibility that dolphins may employ similar tactics when they hunt by creating bubble nets that disorient their prey.

Unlike solid objects such as the sea floor or fish - which reflect acoustic signals linearly, as straight-forward echoes - bubbles reflect nonlinearly. This means that acoustic echoes from bubbles contain harmonic frequencies in addition to the fundamental frequencies of the outgoing signal - producing a "clutter" that is incredibly confusing to sonar. And to make matters worse, bubbles are extremely efficient at reflecting sound.

"Bubbles are the most powerful naturally occurring acoustic objects in water," explains team member Timothy Leighton. "When you send sound at them, they'll emit sound rather like any instrument would - if you shout at a guitar, the strings will rattle back at you."

Given this, Leighton was taken aback when he saw a wildlife documentary showing dolphins purposely blowing nets of bubbles to catch fish. "There should be no way the dolphins' sonar can get through these bubbles - it's like they're making fog while hunting and blinding their own sonar," he says. "Unless there's something spectacularly good about their sonar that we haven't discovered yet."

Considered alongside manmade sonar devices, a dolphin's sonar equipment appears fairly mediocre in terms of the frequencies it can cover and the power it can generate. Dolphins' agility and speed in the water lends them the advantage of being able to send sonar from a number of different directions and thereby see in 3D, but, even so, the bubble problem remains.

"They've got these huge brains though, and they've been living in the oceans for 10 million years where there's always bubble clutter. Why shouldn't they have developed a way of navigating it?" asks Leighton.

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