A combination of multi-frequency acoustics and midwater trawl samples are used to estimate the biomass and distribution of mesopelagic and epipelagic (sardine and anchovy) fishes as well as macrozooplankon and identify areas of enhanced productivity and concentration in the southern California Current.

The use of sound to remotely detect aquatic organisms has been used for more than half a century. As a non-invasive technique, the use of sound provides a real-time high resolution technique to study aquatic organisms, both quantitatively and qualitatively, with the capacity to reveal complex dynamics of the local biology that include their response to micro- and meso-scale oceanographic features, such as fronts. The methods and potential to identify species remotely has improved as the technology has evolved, with the addition of correction factors, development of calibration procedures, and studies of the relationship between echo amplitude and size, material properties of organisms and orientation with respect to the acoustic beam. A multi-frequency acoustic system takes advantage of the differential response of targets of different sizes and composition to different wave lengths, thus enabling us to discriminate small organisms (i.e. zooplankton) from large ones (i.e. fish) and fish with air-filled swimbladders from those without .