Midwater Fish Surveys at the AFSC Alaska Fisheries Science Center Quarterly Report : January, Febuary, March, 1997 [feature article]

Echosounders, devices which transmit sound waves and "listen" for returning echoes, have been used for fish detection since the 1930s. The use of echo integrators, which sum returning echoes, was first proposed for fish detection by Dragesund and Olsen in 1965, with scientific applications beginning in about 1970. Early applications were subject to considerable error due to the unsophisticated nature of the equipment as well as the unknown acoustic scattering characteristics of the fish being surveyed. The first digital echo integrator, developed in the early 1970s, greatly improved the accuracy of echo integration systems, though the assumption of linearity between echo integrator output and fish density was still under discussion.