• Anguilla japonica;
  • New Moon Hypothesis;
  • otolith;
  • Seamount Hypothesis;
  • spawning date


The location and timing of spawning of the Japanese eel Anguilla japonica was studied during two research cruises of the R/V Hakuho Maru and R/V Suruga Maru in the North Equatorial Current of the western North Pacific Ocean during June–September 1998. There were 38 A. japonica leptocephali (10.0–43.2 mm in total length (TL)) collected in three areas: 24 specimens around the Arakane and Pathfinder Seamounts in June (approx. 16°N and 143°E) and five specimens at the southernmost station (13°N) and nine specimens at the northernmost station (17°N) of a transect along 137°E in September. The average total lengths of the leptocephali were significantly different among the three areas, with those around the seamount being smallest, those at the northern station being largest, and none being collected along the easternmost 144°E transect. This and the currents in the region suggested that spawning of A. japonica occurred near some of the seamounts in the West Mariana Ridge. Back calculated spawning dates indicated that most leptocephali were born during the new moon, supporting the hypothesis that A. japonica spawns around the new moon. Analysis of otolith daily rings found a strong correlation between total length and age (r = 0.97), and the average daily growth rate was about 0.5 mm/day.