Underwater detection apparatus and underwater detection method

An underwater detection apparatus for detection of mobile targets, such as schools or shoals of fish, as well as targets such as trawling equipment, nets and otter boards, which have zero velocity relative to a fishing boat that tows them. The apparatus includes a transmitter and a receiver, which receives reflected transmission waves and generates a reception signal. A Doppler shift module calculates the frequency shift of the reflected wave relative to the transmission wave. A first signal processor retrieves an echo signal from the reception signal based on the frequency shift amount. A second signal processor retrieves an echo signal independently of the doppler shift. An image signal generation module (14) that creates a target echo image of static and mobile targets based on doppler corrected and non-corrected signals respectively.