pulses directed from the surface or from a submarine vertically down to measure the distance to the bottom by means of sound waves. Echo sounding can also refer to hydroacoustic "echo sounders" defined as active sound in water (sonar) used to study fish. Hydroacoustic assessments have traditionally employed mobile surveys from boats to evaluate fish biomass and spatial distributions. Conversely, fixed-location techniques use stationary transducers to monitor passing fish. History After the disa… (More on Echo-sounding)