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.
Highlights
The Northwest Pacific Ocean has intricate circulation systems [1], including the Kuroshio current and the Oyashio current
The scattering strength peaks observed at Stations 19, 22, 32, 35, 38, 44, 48, and A1 all correspond to the negative solar altitude angles
(without Stations 24, 30, and 31) shown in Figure 6 indicated that SV was not significantly correlated with longitude (R = 0.147, p = 0.466, p > 0.05), latitude
Summary
The Northwest Pacific Ocean has intricate circulation systems [1], including the Kuroshio current and the Oyashio current. As important secondary producers and feeding objects of economic fishes, zooplankton connect primary producers with tertiary or ultimate producers, and play a key role in the pelagic food web and marine productivity. Eng. 2021, 9, 938 or ultimate producers, and play a key role in the pelagic food web and marine productiv of 12 ity. Zooplankton distribution characteristics are indicators of the mutual transportation and passage of different water systems, and the basis of the transportation and passage of different waterthe systems, and the toindicators distinguish themutual boundaries of different water masses. Promote the conservation, management, and sustainable development of water fisheries in theToNorthwest. Management, sustainable development of fisheries in conducting the Northwest. The distribution and diversity of zooplankton in the Northwest Pacific Ocean have
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