Twenty-five years with Van Holliday in the development of high-frequency technology and analysis algorithms to measure zooplankton distributions

Initial studies using high-frequency acoustics at four individual frequencies (0.5-3.0 MHz) were begun in the 1970s to measure acoustical scattering from zooplankton. Acoustical measurements were made at sea by profiling vertically in the water column. Zooplankton were collected, identified and measured, and target strength measurements were made on individual zooplankton in the laboratory. Concurrently, various acoustical scattering models were analyzed to enable the calculation of the size-frequency distribution of zooplankton from the acoustical data. A multi-frequency acoustic profiling system (MAPS) was then developed (21 different acoustical frequencies). This system was used to measure oceanic structure off of southern California, plumes off of central California, Gulf Stream features, and oceanic structure in the Irish Sea. Analyses of these data indicated that 21 frequencies were more than needed. Four to six frequencies were adequate for most studies, and the Tracor Acoustic Profiling System (TAPS) was developed. This system has been used and modified for a wide variety of studies. These studies range from largescale patterns of zooplankton in the Arabian Sea to the measurement of thin layers in many different oceanic systems. The use of these systems now provides us with high-resolution measurements of zooplankton distributions in the sea.