Bull. Jpn. Soc. Fish. Oceanogr. 84(1), Page 27–35, 2020   Daily otolith ring formation and the relationship between water temperature and the onset of ring formation of barfin flounder Verasper moseri larvae and juveniles Haruhiko Hinoa*, Taihei Matsuda and Keizo Yoshimura 1 Mariculture Fisheries Research Institute, Hokkaido Research Organization, Muroran, Hokkaido 051–0013, Japan † E-mail: joh-mikimasa@hro.or.jp The present study reared larvae and juveniles of barfin flounder Verasper moseri, observed otolith microstructure and validated daily formation of micro rings on sagittae and lapilli. For both otoliths, the check (distinct ring) was observed near the otolith core and the timing of check formation coincided with mouth opening. In lower water temperature, larval development became slower and the timing of check formation was delayed. Clear rings were formed outside the check, and the relationship between the age and the number of rings indicated that they were formed daily. After the onset of metamorphosis, secondary primordia were formed on the margin of sagittae. In contrast, there was no secondary primordium on lapilli throughout larval and juvenile phases. Thus, we suggest that lapilli are suitable for the otolith microstructure analysis throughout larval and juvenile life. The population of barfin flounder has been enhanced by the mass release of hatchery-produced juveniles that started in 2006. Recently, wild juveniles are found in the shallow water around Hokkaido. This study provides the key to clarifying unknown biology of field-caught juveniles. Key words: flatfish, juveniles, larvae, mouth opening check, otolith daily ring, Verasper moseri