Abstract: A depthsounder for selectively measuring either bottom depth or the depth of any fish located beneath a vessel. When operating in a depth mode, the depthsounder automatically adjusts the gain of its receiver to minimize interference and provide a filtered display of bottom depth. When operating in a fish finding mode, the depthsounder likewise automatically adjusts the gain of its receiver to minimize interference in its detection of fish. In addition, the depthsounder suitably processes any return pulses to distinguish those produced by fish from those produced by other objects in the water. When fish are detected, the depthsounder provides a visual display of their depth and an aural alarm whose repetition rate is proportional to the number and size of the fish.