Echo Sounder is a type of SONAR (originally an acronym for SOund Navigation And Ranging) used to determine the depth of water by transmitting sound pulses into water. The time interval between emission and return of a pulse is recorded, which is used to determine the depth of water along with the speed of sound in water at the time.
After the disaster of the Titanic in 1912, the German physicist Alexander Behm conducted some research to find a way to detect icebergs. He discovered the technique of echo sounding which turned out to be inefficient in spotting icebergs, but a great tool to measure the depth of the sea. Behm had his invention patented in 1913. While the first serious attempts to quantify fish biomass were conducted in the 1960’s, major advances in equipment and techniques took place at hydropower dams in the 1980’s. Some evaluations monitored fish passage 24 hours a day for over a year, producing estimates of fish entrainment rates, fish sizes, and spatial and temporal distributions.