Young Japanese eels, which have traditionally been caught at river mouths in the winter and then raised for human consumption, have been reaching the coast around May and June in recent years, possibly because of environmental changes, a research group said Monday.
The group led by Jun Aoyama, an associate professor at the University of Tokyo’s Atmosphere and Ocean Research Institute, found the delay in the peak season for netting the eels, which have been in rapid decline, during a 2009-2011 survey at the Sagami River in Kanagawa Prefecture.
Aoyama said that although the reasons for the delay and whether it is linked to the poor catches remain uncertain, “Young eels are carried by ocean currents for long distances are said to be vulnerable to the impact of global warming. Changes in the marine environment could be the reason.”
Catching young eels traveling upstream on the nights of new moons every month at the river mouth, the group saw the number caught grow to 22 in March 2010 from between one and five from December 2009 to February 2010, and increase again in June that year to 62. They found a similar trend a year later, with peaks in May and June 2011 of 126 and 152.
No peaks in the early summer had been reported in past surveys of this kind across Japan, indicating the group’s findings may differ largely from the conventional seasonal pattern of eel migration from the sea to rivers across East Asia, Aoyama said.
The eels have been confirmed spawning in the Pacific near Guam from May to August. After hatching, the fry are carried north on the Kuroshio current toward Japan mostly from December to January. They then go upstream to reach adulthood.
Young eels that have grown to about 5 cm long are also called glass eels because of their transparent bodies.