A fish detector is a device that uses sonar technology to detect the presence of fish in a body of water. Sonar stands for Sound Navigation and Ranging, and it works by sending out sound waves and measuring the time it takes for them to bounce back. This information is then used to determine the location and size of objects in the water, such as fish. Fish detectors are commonly used by fishermen to locate schools of fish, as well as by marine biologists to study fish populations.