Assessing the distribution and abundance of zooplankton: a comparison of acoustic and net-sampling methods with D-BAD MOCNESS

Results are described from the first field study with the D-BAD MOCNESS (Du al-Beam Acoustics Deployed on a Multiple Opening/Closing Net and Environmen tal Sensing System), an instrument designed to collect acoustic data and ne t samples simultaneously from the same portion of the water column. Our pri mary objective was to evaluate the advantages and disadvantages of integrat ing in a single instrument these two very distinctive methods for assessing the distribution and abundance of zooplankton. In this context, we require d a means of comparison that would enable us to groundtruth acoustic remote -sensing data with net sample data. The approach chosen, referred to as the forward-problem approach, compares the acoustic volume backscattering coef ficients observed in situ with those predicted from net sample data and aco ustic scattering models. The results from this study show that the observed acoustic volume backscat tering data are generally consistent with the forward-problem predictions. This consistency is true in terms of both total acoustic volume backscatter ing as well as that portion of the volume backscattering contributed by eac h of the dominant sound scatterer types. The results also provide two examp les of situations in which inconsistencies between the observed and predict ed acoustic volume backscattering can be used to detect potential methodolo gical problems. D-BAD MOCNESS appears to be a useful instrument for groundt ruthing acoustic data; however, due to its slow towing speed, it is not a s uitable instrument for large-scale, acoustic survey work. (C) 1998 Elsevier Science Ltd. All rights reserved.