Japanese eel Anguilla japonica do not assimilate nutrition during the oceanic spawning migration: Evidence from stable isotope analysis

During 2008 and 2009, a total of 12 adult Japanese eels Anguilla japonica were captured in the southern part of the West Mariana Ridge, the presumed spawning area. We compared the stable carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) between the 'Mariana silvers' (terminal phase) and those of yellow and silver eels caught in rivers, lakes and coastal areas of Japan (initial phase). Profiles of stable isotope signatures between the initial and terminal phases were similar; both characteristically had a wide range for δ¹³C (-24.9 to -12.0% and -20.5 to -11.3% for the initial and terminal phases, respectively) and δ¹⁵N (6.5 to 18.4% and 9.0 to 18.1, respectively). Mesopelagic fishes, including several other anguillid species caught near the West Mariana Ridge, characteristically had a very narrow range of δ¹³C (-16.9 to -15.3%) and a wide but lower range of δ¹⁵N (5.3 to 11.1%) than the Japanese eels. The very similar profiles in stable isotopic signatures between the initial and terminal phase eels, distinct from those of Mariana mesopelagic fishes, indicate that Japanese eels do not assimilate nutrition from the marine environment during long (ca. 6 mo) spawning migration and retain the initial isotopic values of where they ceased feeding.