Comparison of zooplankton density estimated by acoustic inversion method and net sampling

Abstract

A multi-frequency acoustic zooplankton fish profiler (AZFP, ASL Environmental Sciences) with temperature-salinity and depth sensor was moored at 10 m from the bottom (ca. 30 m) at the mouth of Yamada Bay in Iwate Prefecture, Japan, from 14 January to 16 June 2015 in order to measure volume backscattering strength profiles from the transducer surface to the water surface at four frequencies (125, 200, 455, and 769 kHz). Zooplankton density by size was estimated from the measured volume backscattering strength at four frequencies by the inversion method. Monitoring period was selected to cover the period when juvenile salmon stay in the bay before leaving for oceans. Zooplankton sampling was conducted periodically (once a week or two weeks, day and night) using a Kitahara Net (30 cm dia., 335 µm mesh) and a Norpac Net (45 cm dia., 335 µm mesh) at near the moored position. These observations revealed that large zooplankton, which juvenile salmon prefers, arrived with the Oyashio Current, and the zooplankton density increased from late April to the middle of May. Acoustically estimated variation pattern in zooplankton density over time was similar to that obtained by net sampling. [This work was supported by AFFRC, Japan.]