Acoustic estimates of zooplankton distributions in Prince William Sound, spring 1996

A survey was conducted in Prince William Sound (PWS) in May 1996, to study the volume backscattering of zooplankton using a 420 kHz digital sonar and a multiple opening-closing net environmental sampling system (MOCNESS). Whi le the digital sonar provided continuous fine-scale volume scattering stren gth measurements, the MOCNESS samples provided zooplankton species, size, a nd coarse-scale estimates of density and distribution. This catch informati on was applied to acoustic scattering models to predict zooplankton volume backscattering strength. By least-squares fitting the predicted backscatter values with the measured acoustic backscatter, we determined the reflectio n coefficients for Neocalanus, pteropods, and eupausiids to be 0.0178, 0.15 33, and 0.0341, respectively. By applying these reflection `coefficients an d the measured animal sizes to the same acoustic models, we were able to ca lculate target strengths (TSs) for these tars, which were then applied to t he volume scattering information, yielding estimates of the absolute densit ies of those selected species along continuous transects inside and outside PWS, The comparison of these density values with the density estimates fro m the MOCNESS showed reasonable agreement, although the distribution of aco ustic values was much wider than the MOCNESS due to finer sampling. Since r eflection coefficients vary from species to species, it is inappropriate to borrow this value from another species based on similarity. (C) 2000 Elsev ier Science B.V. All rights reserved.