Sonar is a method for detecting and locating things submerged in water by sending out sound waves, then listening to the echoes that result when the sound waves bounce off an object. The process is also called echolocation.
Dolphins don’t see very well and can’t hear underwater. However, they are equipped with the equivalent of a sophisticated high tech echolocation capability, which they use to locate prey. Their standard means of communication with one another is the transmission of clicking sounds, which they can also release in a focused beam in order to sweep their surroundings to locate objects and create a mental picture of their environment.
To some extent, the dolphin’s echolocation sensory system is a mystery, but it is known that they can both locate and examine objects several hundred yards away. They focus a beam of clicking sounds and when echoes return they can tell where the object is; whether it’s moving, how fast, and in what direction; how big it is; and in some cases their sound waves can penetrate deeply enough to tell them something about the prey’s internal structure. They create the beam of clicks using lips located in their nasal passages and echoes through their lower jaws.
Sound travels about 4.3 times faster in water than in air, yet water is much denser. With a stream of rapid, short clicks, dolphins can tell the difference between the way sound is impeded by water, and the way it is impeded by the matter in the skeletons and swim bladders of fish. Often they can even even locate prey hiding under sediment.
Occasionally, dolphins will sneak up on sea lions in groups and attack. More often they eat fish and squid, generally hunting in cooperating groups. As they approach potential prey, they decrease the amplitude of their clicks, but increase the frequency. The change in output both maintains each individual animal’s stream of incoming information and communicates their position to their fellow hunters.
Dolphins will often herd a large quantity of fish into a pack by surrounding and circling them ever more tightly. When the mass of fish is sufficiently dense, the predators begin feeding one at a time while the others continue to keep the fish sufficiently packed.
Other times, a group of dolphins will chase a school of fish to shallow waters where the latter become trapped between their pursuers and the shore. And again, once the school is packed tightly enough against the shore, the dolphins feed one at a time.