Immigration Period and Age of Anguilla japonica Glass-eels Entering Rivers in Northern Kyushu, Japan during 1994

The immigration period, age at immigration, and hatching dates of 174 glass-eels Anguilla japonica caught in northern Kyushu during 1994 were examined, based upon continuous sampling and counting of the daily increments of the sagittal otoliths. Immigration of glass-eels to northern Kyushu occurred from February to July, four months later than on the Pacific coast. Examination of otoliths showed that glass-eels had entered into freshwater at the mouth of rivers at the age of 169 ± 15 days (mean ± SD). Hatching dates extended from August 1993 to January 1994 and the mean ages of glass-eels of different hatch months were as follows: August-184, September-166, October-169 and November-168 days. The peak spawning month was September. Otolith microstructure showed three separable phases, i.e., leptocephalus stage, metamorphosing stage and glass-eel in sea water stage. Age at the freshwater mark and duration of the leptocephalus stage of August-hatched eels were significantly older and longer, respectively, than those eels hatched in other months.