Simulating the Oceanic Migration of Silver Japanese Eels

Abstract

The oceanic migration of silver Japanese eels starts from their continental growth habitats in East Asia and ends at the spawning area near the West Mariana Ridge seamount chain. However, the actual migration routes remain unknown. In this study, we examined the possible oceanic migration routes and strategies of silver Japanese eels using a particle tracking method in which virtual eels (v-eels) were programmed to move vertically and horizontally in an ocean circulation model (Japan Coastal Ocean Predictability Experiment 2, JCOPE2). Four horizontal swimming strategies were tested: random heading, true navigation (readjusted heading), orientation toward the spawning area (fixed heading), and swimming against the Kuroshio. We found that all strategies, except random swimming, allowed v-eels swimming at 0.65 m s−1 to reach the spawning area within eight months after their departure from the south coast of Japan (end of the spawning season). The estimated minimum swimming speed required to reach the area spawning within eight months was 0.1 m s−1 for true navigation, 0.12 m s−1 for constant compass heading, and 0.35 m s−1 for swimming against the Kuroshio. The lowest swimming speed estimated from tracked Japanese eels at sea was 0.03 m.s−1, which would not allow them to reach the spawning area within eight months, through any of the tested orientation strategies. Our numerical experiments also showed that ocean circulation significantly affected the migration of Japanese v-eels. A strong Kuroshio could advect v-eels further eastward. In addition, western Pacific ocean currents accelerated the migration of navigating v-eels. The migration duration was shortened in years with a stronger southward flow, contributed by a stronger recirculation south of Japan, an enhanced subtropical gyre, or a higher southward Kuroshio velocity.

Freshwater eels: A symbol of the effects of global change

“…They subsequently penetrate continental waters, turning into pigmented yellow eels, where they colonize a large range of continental habitats from brackish to freshwater (Arai & Chino, ; Daverat et al., ). After a growth phase lasting from 3 to over 30 years, yellow eels metamorphose into silver eels and migrate back to their spawning grounds (Béguer‐Pon, Castonguay, Shan, Benchetrit, & Dodson, ; Chang, Miyazawa, & Béguer‐Pon, ; Righton et al., ). The eels mature en route and presumably die following spawning.…”

Evaluation from otolith Sr stable isotope ratios of possible juvenile growth areas of Japanese eels collected from the West Mariana Ridge spawning area

“…Although Manabe et al (2011) tracked "silver eel" stage A. japonica individuals released near Japan using pop-up tags, the eels neither reached the spawning area nor showed evidence of spawning related behaviors. A modelling simulation study by Chang et al (2016) suggested that ocean circulation significantly affected the spawning migration route and duration of Japanese eels after their departure from the south coast of Japan.…”

Potential impact of ocean circulation on the declining Japanese eel catches

“…The migration process of Japanese eels, however, is not well understood due to limited direct observational evidence on adult migration 23 or larval dispersal 24 . Numerical modelling methods, therefore, have been used to simulate the potential migration paths of Japanese eels, and to evaluate their linkages to ocean-atmosphere changes 12 , 14 , 15 , 25 , 26 .…”