Otolith microstructure was examined and described with regards to the early life history events of black rockfish, Sebastes inermis, collected in a temperate seagrass bed in Matsushima Bay, eastern Japan. The extrusion check was validated upon examining newly extruded reared larvae. Tetracycline treatments and a thermal marking experiment showed that the otolith increments were produced daily. Four zones were identified, from a clear central zone (CZ), which evolved a clear extrusion check to a translucent zone, visible only in large juveniles. Extrusion dates were distributed from late December to early March, with peaks in January and February. Only the CZ and a planktonic zone (PZ) were observed in newly immigrated juveniles. The PZ showed mean values significantly longer in the January cohort (92 days) than the February cohort (76 days), and the increment width varied significantly over time and between cohorts as well. The PZ was delimited by a clear prominent transition check (TCh) accompanied by an abrupt shift in increment width (the post-settlement zone). The TCh, which was formed at 24.5 mm of mean back-calculated total length, seemed to be linked to the settlement of juveniles in Zostera marina belts. The usefulness of thermal marking and those factors that seem to influence the occurrence of the otolith zones are discussed.
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