Relationship between the Zooplankton Biomass and the Acoustic Volume Backscattering Strength at 38 kHz in the Waters off Northern Taiwan

Abstract

Acoustical and biological samplings of zooplankton in the waters off northern Taiwan were carried out by the ”Ocean Researcher II” in 2008~2009. The volume backscattering strength (Sv) was measured at 38 kHz; meanwhile, a joined Bongo net with different mesh sizes of 200, 330 500, and 1,000 μm was dragged vertically to collect biological organisms.The dominant systematic species groups were copepods which accounted for 77%, 75%, 67%, and 52% of the total number of zooplankton with respective mesh sizes of 200, 330, 500, and 1,000 μm. Linear relationships between the logarithm of the zooplankton density (B; mg/m^3) and Sv (dB) at 38 kHz for the four mesh sizes were found. Using an ANCOVA test to compare the slopes and intercepts of the four equations, the results showed that the intercepts and slopes of the equations for mesh sizes of 200, 330, and 500 μm did not significantly differ, but there were significant differences between the intercepts for the equation for the 1,000-μm mesh.Results suggested that those three equations could be combined into the equation: Sv38 = 9.45 logB200~500 - 94.60 (n = 51, r = 0.76, p ＜ 0.05), while the equation for 1000 μm was Sv38 = 8.56 logB1000 - 87.48 (n = 17, r = 0.74, p ＜ 0.05). The discrepancy between the two intercepts was 7.12 dB. Possible influences for the difference of the two equations are also discussed in this study.