Decades of progress in active fisheries acoustics: Many challenges remain

The technology in this field has evolved significantly over the past 6 + decades to one involving operational scientific echosounders (single-, split-, and multi-beam; broadband) used across a multitude of types of platforms. In parallel with hardware development has been significant development of various methods of data processing, scattering models, and associated interpretation, yielding new understanding of behavior of fish and zooplankton. Overall, this field is very active and productive as well as one that is fertile for new growth and understanding. Trends of development, key milestones, and current challenges are briefly reviewed.