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

Abstract

Results are described from the first field study with the D-BAD MOCNESS (Dual-Beam Acoustics Deployed on a Multiple Opening/Closing Net and Environmental Sensing System), an instrument designed to collect acoustic data and net samples simultaneously from the same portion of the water column. Our primary objective was to evaluate the advantages and disadvantages of integrating in a single instrument these two very distinctive methods for assessing the distribution and abundance of zooplankton. In this context, we required 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 coefficients observed in situ with those predicted from net sample data and acoustic scattering models. The results from this study show that the observed acoustic volume backscattering data are generally consistent with the forward-problem predictions. This consistency is true in terms of both total acoustic volume backscattering as well as that portion of the volume backscattering contributed by each of the dominant sound scatterer types. The results also provide two examples of situations in which inconsistencies between the observed and predicted acoustic volume backscattering can be used to detect potential methodological problems. D-BAD MOCNESS appears to be a useful instrument for groundtruthing acoustic data; however, due to its slow towing speed, it is not a suitable instrument for large-scale, acoustic survey work.