Simultaneous observations of zooplankton distribution and density using net sampling, a laser‐optical plankton counter, and acoustics

Abstract

Conventional plankton sampling methods are based on net tows, which are often poorly suited to capturing spatiotemporal variability. Various automatic plankton samplers based on optical principles have been developed recently. The laser‐optical plankton counter (LOPC) was developed as the next generation OPC. In addition, acoustic technology has been used extensively to survey zooplankton distribution and abundance. This study compared the density and size spectra determined using net contents, LOPC data, and acoustic data. Surveys were conducted for 3 years around Hokkaido, Japan. The sound‐scattering layer (SSL) was recorded on the echogram of an echosounder. Biological sampling was conducted using a 1‐m2 frame‐type mid‐water trawl (FMT; mesh size 1.5 mm) and a NORPAC net (mesh size 500 microns). An LOPC was mounted inside the mouth of these nets. Net sampling was carried out at dusk, when the SSL moves toward the surface (ca. 10–35 m). Acoustic data were derived using a Simrad EK60 echosounder operating at 38, 120, and 200 kHz simultaneously. A comparison of the use of acoustic data, LOPC data, and net contents to assess zooplankton distribution and density will be presented and discussed. [Work supported by JSPS Grants‐In‐Aid for Scientific Research.]