Increasing Reach of Japanese Glass Eels Spurred by Shifting Ocean Currents

A team of scientists from JAMSTEC, the University of Tokyo, and other institutions has observed a significant rise in the number of glass eels, the juvenile stage of Japanese eels, appearing in rivers in Hokkaido. Researchers believe that this trend is linked to shifts in ocean currents—most notably, the northward progression of the Kuroshio Current coupled with a diminishing cold flow from the Oyashio Current. These altered conditions help the glass eels, which begin their life near the Mariana Trench, to travel further north along the Japanese coastline.

Originally, glass eels were predominantly found in rivers of regions like Aomori Prefecture, but recent observations have confirmed their presence in Hokkaido’s Iburi region. By employing an ocean current model based on data such as water temperature and salinity, the team compared distributions from 1994-2003 with those from 2014-2023. They discovered that the probability of glass eels reaching Hokkaido’s coastal areas has increased dramatically, by about twenty times, while the strengthened Tsugaru Warm Current appears to deter their arrival on the coast around the Tsugaru Straits.

Experts emphasize the importance of these findings for resource management. Professor Kentaro Morita of the University of Tokyo notes that since glass eels are a valuable fishery commodity, understanding their changing migration patterns is crucial for establishing future catch limits and conservation efforts. Similarly, Kazuki Yokouchi from the Japan Fisheries Research and Education Agency stresses that continued research is essential to determine if the eels arriving in new regions will thrive, ensuring sustainable management of these resources.