Impact of anticyclonic eddy on fish distribution in the Kuroshio–Oyashio transition area

Abstract

The Kuroshio–Oyashio transition area has water originating from subtropical and subarctic areas and forms productive ecosystems. Mesoscale eddies play important roles in these ecosystems, such as by transporting heat and organisms. Micronektonic mesopelagic fishes play crucial roles in biogeochemical cycles; however, further research is needed to understand the physical, chemical, and biological effects of mesoscale eddies on these fishes. Observation from a ship was conducted using Underway CTD and a quantitative echo sounder in June 2019 to examine the impact of a mesoscale eddy on fish distribution in this area. The observed anticyclonic eddy originated from the Kuroshio extension and had warm and saline water inside it. Acoustic volume backscattering strength (SV) at 38 kHz showed peaks along the density isolines around 75 dbar and 400 dbar, and the comparison of the SV at 120 kHz and 38 kHz suggested that the peaks of the SV were associated with fishes. The distribution of fishes inside the eddy became 13–19 dbar and 27–37 dbar deeper than outside of the eddy in surface and intermediate depth. Mean SV in intermediate depth was higher inside the eddy compared to that outside the eddy, suggesting the presence of denser fish within the eddy.