Broadband Characteristics of Zooplankton Sound Scattering Layer in the Kuroshio–Oyashio Confluence Region of the Northwest Pacific Ocean in Summer of 2019

Abstract

Acoustic technology, as an important investigation method for fishery resources, has been widely used in zooplankton surveys. Since the Kuroshio–Oyashio confluence region has an extensive distribution of zooplankton, describing and analyzing the characteristic of the zooplankton sound scattering layer (SSL) in this area is essential for marine ecology research. To understand its spatial–temporal distribution, acoustic data of the Kuroshio–Oyashio confluence region at the Northwest Pacific Ocean, obtained by a Simrad EK80 broadband scientific echosounder in 2019, were used on board the research vessel (RV) Songhang. After noise removal, the volume backscattering strength (SV) was measured to plot the broadband scattering spectrogram of each water layer and to exhibit zooplankton distribution. The results show that the main sound scattering within 0–200 m originate from the zooplankton, and the SV of each layer increases with the rise of the transducer frequency. The magnitude of SV was closely synchronized with the solar altitude angle, which gets smaller when the angle is positive, then larger when the angle is negative. It means that the SSL has a diel vertical migration (DVM) behavior with the variation of solar height. Meanwhile, scattering strength was positively correlated with temperature in the vertical direction and showed a maximum of −54.31 dB at 20–40 m under the influence of the thermocline. The Kuroshio and Oyashio currents had an obvious influence on the scattering strengths in this study, indicating a low value when next to the Oyashio side and a high value on the Kuroshio side. The scattering strength near the warm vortex center was higher than that at the vortex edge. The results of this study could provide references for a long-term study on ecological environment variation and its impacts on zooplankton distribution.

Characterizing the Sound-Scattering Layer and Its Environmental Drivers in the North Equatorial Current of the Central and Western Pacific Ocean

“…Temperature is significantly correlated with the scattering strength (p ≤ 0.001). The temperature range of the NEC warm pool is 15-30 • C, with a large confidence interval and low credibility at 15-18 • C and a nonlinear positive correlation between temperature and scattering strength at 18-30 • C, indicating that planktonic organisms and micronekton tend to live in warmer areas, consistent with previous research results [56]. Therefore, it is speculated that high temperature provides a favorable environment for the growth and reproduction of SSL, shortening the generation time and increasing their reproductive rate and biomass under high-temperature conditions.…”

Impacts of Morphological Characteristics on Target Strength of Chub Mackerel (Scomber japonicus) in the Northwest Pacific Ocean

“…In recent years, using broadband techniques to identify and classify species in echograms has become a hot topic in fishery acoustics (Stanton et al, 2010;Yan et al, 2020;Hasegawa et al, 2021;Xue et al, 2021). At the same time, a growing number of research vessels are equipped with broadband fish finders.…”

Hydroacoustic method and GIS-Dashboard visualization for fish size in Banggai Sea Waters

“…ESP3 is an acoustic data processing application developed by the National Institute of Water and Atmospheric Research (NIWA), New Zealand [16]. This application can be used freely for various acoustic data processing such as studies of gas leaks from underwater faults [17,18], abundance of marine organisms [19][20][21], and other studies within the scope of hydroacoustic [22][23][24]. Fish density calculations were conducted using the Microsoft Excel application based on the results of processing in ESP3.…”