Abstract
Recent observational and numerical studies have suggested that the decadal modulation of the Kuroshio Extension system, driven by mesoscale variabilities, profoundly affect the basin scale physical and biogeochemical oceanography. However, it remains unclear how these decadal changes affect distribution and abundance of fish species in this region. In this study, 26,964 swordfish catch data obtained by longliners during 2004 and 2010 in the western North Pacific are analyzed with an eddy-resolving ocean reanalysis by using mesoscale dynamic parameters and an eddy detection technique, to clarify the effects of mesoscale eddies and their variabilities on the swordfish catch and distribution. During this period, the Kuroshio Extension underwent two different dynamic phases: stable path state in 2004, 2005 and 2010; and unstable path state during 2006-2009. Based on our analyses, we show here that swordfish are more concentrated in and near the anticyclonic warm-core eddies in the northern site, 36 -45 • N, of the Kuroshio Extension system, especially during the unstable path phase. This is found to be caused by the interannual modulation of mesoscale eddy activities due to more warm-core rings generated from the unstable Kuroshio Extension, making it easier for fishermen to target swordfish in this region.
Highlights
Ocean accommodates variety of marine life from small plankton to large migratory fish species, including swordfish
Interannual changes in the swordfish CPUE are observed in both regions, which depend strongly on the mesoscale dynamic parameters such as the vorticity, the Okubo-Weiss parameter, and the divergence of Q-vector (Figures 4–6)
When the Kuroshio Extension is stable for the years 2004, 2005, and 2010, the eddy kinetic energy shows high values in limited regions along the narrow and stable Kuroshio Extension axis, and the annual swordfish CPUE shows low and high values in the northern and the southern region, respectively
Summary
Ocean accommodates variety of marine life from small plankton to large migratory fish species, including swordfish Within this marine ecosystem, phytoplankton are the most important primary producers of the vital energy source for most of the marine life, forming the foundation of the food web. The enhanced primary and secondary productions near fronts and eddies provide controls over biogeochemical flows and attract large migratory fish species (Braun et al, 2019). In addition to this bottom-up influence through the trophic supply, physical structures in the ocean, such as fronts and eddies, have been known to influence the distributions of marine organisms.
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