It has been long established through extensive research that the spawning grounds of the Japanese eel (Anguilla japonica)※1 are located on seamounts along the Mariana Ridge. However, the exact routes these adult eels take to migrate from Japan to these distant spawning grounds remain unknown. This study predicts the optimal migration routes that minimize travel time and energy cost by combining optimal navigation theory with ocean forecasting models.
One optimal migration strategy involves utilizing surface currents to minimize time and energy consumption, while the other involves diving deep enough to avoid strong currents and swimming directly toward the spawning grounds. Since real eels do not have precise information about ocean currents, it is likely that they adopt the latter strategy.
As eels do not replenish their energy during migration, it is estimated that traveling at speeds of 0.4 to 0.6 body lengths per second allows them to conserve sufficient energy for reproduction. While deeper migration reduces the impact of ocean currents, it also exposes eels to lower water temperatures, potentially affecting their physiological functions. Therefore, there appears to be an optimal migration depth.
Long-distance migrations are among the most extraordinary feats in the animal kingdom, with species like salmon swimming upstream, migratory birds flying in formation, and herds of herbivores crossing vast plains. Yet, despite their similarly epic journeys, the migration of eels remains largely shrouded in mystery. The Japanese eel (Anguilla japonica), a species widely studied and known to travel thousands of kilometers to its spawning grounds on seamounts along the Mariana Ridge, but the exact paths and strategies they use to reach these distant spawning grounds have long eluded scientists.
A recent study, conducted by a joint research team from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and California State University, Fresno, USA, offers insights into the migration strategies of Japanese eels. The researchers utilized Zermelo’s navigation theory※2 —a method for determining the most efficient travel routes in a flow field—together with JAMSTEC’s ocean prediction model JCOPE2M※3 , to simulate the optimal paths for eels migrating to their spawning grounds. These simulations accounted for critical environmental factors such as ocean currents, water temperature, and migration speed, aiming to identify strategies that minimize both travel time and energy consumption.
The study revealed two primary strategies that Japanese eels might adopt during their long migration. The first, assuming that eels possess precise information about ocean currents, involves navigating surface currents to conserve energy and minimize travel time. However, this approach requires following a more circuitous route, taking advantage of favorable currents while avoiding unfavorable ones. The second strategy, which is considered more likely given the eels' limited access to real-time current information, involves diving to depths below 200 meters. At these depths, ocean currents weaken substantially, allowing the eels to follow a more direct and straightforward path toward their spawning grounds, thereby reducing both the time and energy required for migration.
However, migrating at greater depths presents its own challenges. While currents weaken with depth, water temperatures drop to levels that could negatively affect the physiological functions of eels. The study highlighted that eels swimming at depths around 600–700 meters encounter water temperatures close to 5°C, which may impact their ability to sustain prolonged activity. To balance these competing factors, the researchers estimate that the optimal migration strategy for Japanese eels is to swim at a speed of 0.4 to 0.6 body lengths per second in deeper waters where temperatures remain above 5°C.
This research offers a valuable benchmark for understanding the interplay between migration depth, speed, ocean currents, and water temperature. By providing insights into the strategies that Japanese eels might use to complete their epic journey, the study sheds light on the broader migration behaviors of other long-distance migratory species as well.
This research was supported by JSPS KAKENHI grants JP20K14978 and JP24K07384. The findings were published in Scientific Reports on October 31, 2024 (JST).
Gen Li1, Yasumasa Miyazawa1, Yu-Lin K. Chang1, Ulrike K. Müller2
Japanese Eel (Anguilla japonica)
A species of eel widely distributed across East Asia, including Japan, the Korean Peninsula, and China. It is known that adult Japanese eels use stored fat reserves to travel thousands of kilometers to their spawning grounds, located on seamounts near the Mariana Ridge at approximately 15°N latitude and 140°E longitude, without consuming food during migration.
Zermelo's Solution
A classical optimal control problem proposed in 1931 by German mathematician and logician Ernst Zermelo.
JCOPE2M
An ocean prediction model developed by JAMSTEC's Application Lab, which provides weekly forecasts for the coastal seas of Japan up to two months in advance.
For this study
Gen Li, Researcher, Research Institute for Value-Added-Information Generation (VAiG), Center for Mathematical Science and Advanced Technology (MAT), Applied Mathematical Science Group, JAMSTEC
For press release