We communicate using language. Dolphins and whales communicate using pings of sound at high frequencies in order to create sound waves that provide them with spatial information when they are in the sea. They use this process to navigate in the ocean, and find prey.
It was once believed that fish could not hear the high frequencies that dolphins and whales emitted. Scientists have discovered that herring and other fish can hear sounds as high as 180,000 Hertz, nine times as high as the highest frequency a human being can hear. No wonder other fish have survived so long!
Even fishermen who have a lot of experience with the sea know that they should attach ultrasonic beepers to their nets. The ultrasound annoys dolphins who swim away from the nets. Other devices protect divers from sharks by surrounding divers with ultrasonic waves.