Comparison of acoustic and net sampling systems to determine patterns in zooplankton distribution

Abstract

An analysis was conducted of the strengths and limitations of acoustic methods to determine biomass and taxonomic distribution patterns over the Oregon continental shelf. Measurements of volume backscatter from two different acoustic instruments were compared with each other and with predicted volume backscatter calculated from a coincident net tow. Spatially and temporally coincident data were collected from a six‐frequency bioacoustic system (Tracor Acoustic Profiling System (TAPS)) and from a 1 m2 Multiple Opening Closing Net and Environmental Sensing System (MOCNESS). The combined net/acoustic tows were conducted over the Oregon continental shelf in August 2001, during both day and night. The TAPS was mounted on the upper frame of the MOCNESS and ensonified a volume of water directly in front of the net mouth. A four‐frequency echosounder (Hydroacoustic Technologies Inc. (HTI)) with downward looking transducers was towed off the port side of the ship during the MOCNESS/TAPS tows. Fine‐scale vertical distributions of zooplankton that were resolved acoustically were not apparent within the integrated depth strata of the individual MOCNESS net samples. The taxonomic and size composition of the zooplankton community had strong effects on volume backscatter (SV), although the magnitude of SV was not linearly related to zooplankton biomass. Predicted SV calculated from scattering models and the contents of the MOCNESS nets agreed best (sometimes within 5 dB) with measured SV from the TAPS at 420 and 700 kHz and measured SV at 420 kHz from the HTI.