First observations of the oceanic migration of Japanese eel, from pop-up archival transmitting tags

Pop-up archival transmitting (PAT) tags were used for the first time to study the spawning migration of Japanese eels Anguilla japonica. Three silver eels (70 to 98 cm) tagged with PAT tags were released in January 2008 and 6 were released in November 2008 at 2 different locations near the Kuroshio Current, along the southern part of Japan. The tags were preprogrammed to ascend 7 to 90 d after release and data on their pop-up positions, eel swimming depths and water temperatures experienced were retrieved from all but one tag via Argos satellites. Most tags ascended in or near the Kuroshio Current up to 1400 km from the eels' release points, and the longest tracking period of 39 d suggested eastward movements of migrating eels. Two eels tracked for about a month or more showed clear diel vertical migrations (DVMs), predominantly swimming between depths of about 100 to 500 m at night and 500 to 800 m in the daytime, and eventually down to 1000 m depth for one eel. All eels over deep water reached similar minimum temperature ranges during the daytime, even if the depth of that range changed, suggesting they were selecting a preferred minimum temperature rather than a particular depth. Calculated straight-line swimming speeds were faster than those determined in previous reports that included larger-sized eels, suggesting swimming tracks along the Kuroshio Current were faster as a result of the strong current. Extensive DVMs may be performed by A. japonica and other anguillid eels not only for predator avoidance, but also possibly to control gonad maturation by swimming at shallower depths at night where water temperatures are warmer and then moving to deeper layers during the daytime where water temperatures are cooler.