Use of a two-frequency algorithm to determine size and abundance of plankton in three widely spaced locations

The application of a two-frequency algorithm to the in situ estimation of large zooplankton is explored in an effort to use information from more than one acoustic frequency. To accomplish this, 38 and 120 kHz echo-sounders, found on most fisheries research vessels, were utilized. A two-frequency method using a highpass fluid sphere scattering model was employed to estimate the mean size (length) and the density per m(3) of the dominant acoustic scatterers (mainly euphausiids and shrimps), by selected depth layer. Three locations were surveyed: the Gulf of St Lawrence, the Antarctic, and the Irish Sea. Ground-truth samples were collected from various plankton net systems, the catches being compared to the acoustic estimates. Results are discussed with regard to conditions that apply to the method, the ground-truth performance and the relative complexity of the zooplankton composition at the different study sites. The conclusion is that this method has provided satisfactory results but still has a capacity for refinement. (C) 1996 International Council for the Exploration of the Sea