VOLUMETRIC MULTIBEAM SONAR MEASUREMENTS OF FISH, ZOOPLANKTON, AND TURBULENCE

In late 2002 an evaluation of the mid-water survey capability of a high-resolution multibeam sonar (RESON SeaBat 8125) was performed. This was conducted as part of a larger investigation of zooplankton aggregations in Knight Inlet, B.C., and included simultaneous measurements with a conventional, three-frequency (38, 120, and 200 kHz), calibrated echo-sounder system. This multibeam sonar, adapted from a standard bathymetric system, operated at 455 kHz and provided 240 one-half-degree beams in the across-track plane with a one-degree along-track beam angle. The multibeam sonar was able to provide well-resolved mid-water images of fish schools (likely juvenile herring), crustacean zooplankton (dominantly euphausiids), and near-surface turbulent micro-structure features. Close comparisons with the echo-sounders confirmed the multibeam calibrations, allowing assessment of the multibeam volumetric sampling capabilities.