[Nagano University/Kyushu University Joint Press Release]: Research Results

On July 8, 2023, an academic paper on the migratory behavior of Japanese eels, which differs between ocean areas, was published in the international journal Animal Biotelemetry, published by Springer Nature. This research was conducted as a collaborative research project between the Nagano University Institute of Freshwater Biology, the Fisheries Research and Education Organization, and Kyushu University as part of the Fisheries Resources Survey and Evaluation Promotion Project (Eel Unit).

Abe et al. Regional differences in oceanic migratory behavior of Japanese silver eel in waters with different vertical temperature gradients. Anim Biotelemetry 11, 27 (2023). https://doi.org/10.1186/s40317-023-00338-x (Abe, Takaaki; Ishmerai Galang; Ayu Daryani; Minamikawa, Shingo; Mochioka, Noritaka; Hakoyama, Hiroshi)

• Key points of the study

1. Satellite tracking surveys of spawning migratory Japanese eels were conducted in three regions of Japan.
2. Eels in all regions showed a tendency to move toward spawning grounds.
3. Migratory behavior differed between ocean regions, and water mass structure was thought to be one of the factors.

• Research Summary A research team consisting of the Nagano University Institute of Freshwater Biology, the Fisheries Research and Education Agency, and Kyushu University conducted a tracking study using pop-up archival tags in three regions of Japan (Tokai Region, Japan Sea Coast, and Tohoku Region) to clarify the migratory behavior of Japanese eels moving to spawning grounds. The results of the tracking study showed that eels from all regions tended to migrate to spawning grounds and that there were regional differences in migratory behavior. It was also thought that the regional differences in migratory behavior were related to water mass structure.

In recent years, the Japanese eel resource has been declining rapidly, which calls for appropriate resource management. However, the underlying resource structure and ocean migration period are not fully understood. Clarifying the spawning migration behavior of eels leaving each region will allow us to better understand the resource structure and migration period of Japanese eels. The results of this research were published in the international scientific journal “Animal Biotelemetry” on July 8, 2023.

• Background and research history Japanese eel are born in the western waters of the Mariana Islands and their larvae are transported by ocean currents such as the North Equatorial Current and the Kuroshio Current to coastal areas of East Asia (Figure 1). After growing for 5 to 15 years in rivers, lakes, and coastal areas, Japanese eel metamorphose into silver eels (Note 1) and begin migrating to spawning grounds thousands of kilometers away. Japanese eel grow in a wide area from the northern Philippines to northern Japan, but it is not clear whether silver eels that leave each growth area will reach their spawning grounds. In recent years, the resource volume of Japanese eel has been declining rapidly, and appropriate resource management is required. As a basis for this, it is important to understand stock structure (whether silver eels leaving each spawning area reach the spawning area) and migration time (how long it takes silver eels leaving each spawning area to reach the spawning area).

Pop-up archival tags (Note 2) are used as a tool to study the migratory behavior of fish in the ocean (Figure 2). Pop-up tags are also used for eel fish both in Japan and overseas, and it has been possible to track the migration of Atlantic species up to more than 2,000 km. It is known that the migratory behavior of European eels differs depending on the region, and that eels from all regions have the ability to move towards the spawning ground (navigational ability), but this is not clear for Japanese eels.

• Research content and results In this study, we conducted a Japanese eel tracking study, which had previously been conducted only in areas facing the Kuroshio Current, in three regions (a total of six sites), including the Sea of Japan side and the Tohoku region, and examined the migratory behavior of silver eels leaving each region.

Pop-up-tagged Japanese eels released in the Tokai region tended to migrate southeast along the Kuroshio Current. On the other hand, individuals released from the Sea of Japan side tended to migrate in the opposite direction of the Tsushima Warm Current, while individuals released in the Tohoku region tended to migrate southward (Figure 3).

Ocean-migrating silver eels are known to swim at shallower depths (200-400 m) during the night and at deeper depths (400-800 m) during the day (diurnal vertical migration). In this study, individuals released in the Tokai region also engaged in diurnal vertical migration (Figure 4). However, eels migrating in the Sea of Japan, although vertically migrating, did not show a diurnal pattern and were found to frequently move between the surface and a depth zone of about 200 m (Figure 4). To account for the differences in swimming behavior between the marine areas, we compared the depth and water temperature profiles of individuals released in the Tokai region and the Sea of Japan. In the Tokai region, water temperature decreased gradually from the surface to 1000 m, whereas in the Sea of Japan, water temperature decreased rapidly with increasing depth, reaching 4°C, the lower limit of eel activity, at a depth of about 200 m (Figure 4). These results suggest that Japanese eels do not perform diurnal vertical migrations in environments where deep water is not available.

• Future issues This study showed that eels reared not only in the Tokai region, but also in the Sea of Japan and the Tokai region, may be able to migrate to spawning grounds. In addition, the migratory behavior of eels differed depending on the sea area, and it was thought that the water mass structure was one of the reasons. For European eels, pop-up tag tracking surveys have been carried out continuously throughout the distribution area and regional differences in migratory behavior and duration, as well as migratory routes, have been clarified. It is expected that the continuation of pop-up tag tracking surveys for Japanese eels will provide basic information that will contribute to resource management.

• Terminology

• (Note 1) Silver eel During the spawning season, eels of the genus Anguilla undergo a morphological change suitable for long-distance migration. They are called silver eels because their body color becomes black and metallic.

• (Note 2) Pop-up archival tag Because radio waves do not travel underwater, data cannot be transmitted. The pop-up tag records data while attached to the living organism, and when it is detached and floats to the surface, it transmits the data to a satellite. The data transmitted to the satellite can be retrieved by a ground base station.

• Acknowledgements This research was supported by a grant from the Fisheries Agency’s Fisheries Resources Survey and Evaluation Promotion Project (Eel Unit).

• Paper information Publication: Animal Biotelemetry Title: Regional differences in oceanic migratory behavior of Japanese silver eel in waters with different vertical temperature gradients Authors: Takaaki K. Abe, Ishmerai Galang, Ayu Daryani, Shingo Minamikawa, Noritaka Mochioka, Hiroshi Hakoyama

DOI: 10.1186/s40317-023-00338-x

• Contact information 1088 Komaki, Ueda City, Nagano Prefecture, 386-0031 TEL: 0268-22-0594 FAX: 0268-22-0544 Hakoyama Hiroshi (Director/Professor, Institute of Freshwater Biology, Nagano University)