Acoustic estimaes of zooplankton distributions in prince william sound, spring 1996

摘要

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). While the digital sonar provided continuous fine-scale volume scattering strength measurements, the MOCNESS samples provided zooplankton species,size, and coarse-scale estimates of density and distribution. This catch information 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 reflection coefficients for Neocalanus, pteropods, and eupausiids to be 0.0178, 0.01533, and 0.0341, respectively. By applying these reflection coefficients and the measured animal sizes to the same acoustic models. we were able to calculate target strengths (TSs) for these taxa, which were then applied to the volume scattering information, yielding estimates of the absolute densities of those selected species along continuous transects inside and outside PWS. The comparison of these density values with the density estimates from the MOCNESS showed reasonble agreement,althought the distribution of acoustic values was much wider than the MOCNESS due to finer sampling. Since reflection coefficients vary from species to species, it is inappropriate to borrow this value from another species based on similarity.