Convergence of Acoustic, Optical, and Net-Catch Estimates of Euphausiid Abundance: Use of Artificial Light to Reduce Net

Abstract

Euphausiid concentrations in the Scotian Shelf basins were sampled with BIONESS, a multinet sampling system, and quantitatively assessed simultaneously with an in situ optical zooplankton counter (OPC) mounted on BIONESS and with an acoustic backscattering system operating simultaneously at 50, 122, and 200 kHz. Supplementary observations were made with vertically dropped video cameras and video and 35-mm frame cameras mounted on a remotely operated vehicle (ROV). The use of a light source on BIONESS during sampling increased the catch of euphausiids by 10–20 times by reducing active avoidance reactions to the net. Consequently, conventional net sampling has greatly underestimated euphausiid concentrations, a conclusion suggested by previous acoustic measurements. Experimental acoustic target strengths for 28-mm euphausiids averaged −77.5, −73.4, and −68.4 dB at 50, 122, and 200 kHz, respectively, using measured Sv levels and assuming 100% net sampling efficiency. Theoretical scattering models based on randomly oriented cylinders require the euphausiids to be oriented within about 5° of the horizontal to approximate both the experimentally observed target strength amplitudes and their frequency dependence. Acoustic interpretation has been enhanced by incorporation of transducer sensitivity versus temperature corrections and modeling techniques that allow for finite transducer beam widths.