Discrimination of Diaphus theta and Euphausia pacifica using underwater irradiance

Abstract

This study proposes scatterers discrimination method by using underwater irradiance in addition to the usual acoustic method. Mean volume backscattering strengths (MVBS) at 38 and 120 kHz and underwater irradiance were measured at fixed locations in the north-western Pacific from 24 to 27 August, 2008. Two sound scattering layers (SSLs) conducting diel vertical migration (DVM) at different depths were observed and were identified as Diaphus theta and Euphausia pacifica by net samplings, respectively. During DVM, both species followed mostly the constant light level. The mean irradiance at the lower and the upper outlines of SSLs for D. theta were ranged from -81.0 to -57.8 dB re μ mol/m2/nm and that for E. pacifica were ranged from -50.5 to -42.1 dB re μ mol/m2/nm, respectively. The mean and standard deviation of ▵ MVBS corresponding to the scattering layers of D. theta and E. pacifica were -4.7 ± 2.7 dB and 7.7 ± 3.2 dB, respectively. Scatterers discrimination using irradiance parameter in addition to usual ▵ MVBS parameter was found to be more effective to distinguish both species from other scatterers.