Daily patterns of settlement and individual growth rates of young-of-the-year of the rockfish Sebastes inermis in a Sargassum bed

Otolith microstructure analysis was used to reveal the temporal patterns of settlement, planktonic duration and size-at-settlement (SAS) of young-of-the-year (YOY) of the rockfish Sebastes inermis in a Sargassum bed in Sendai Bay, northern Japan, during 2000 and 2001. Individual growth rates (IGR) during the planktonic period and after settlement for each settled group were also studied and compared to those of YOY collected in a Zostera marina bed. In both years, YOY settled in distinctive groups at new and full moon phases. SAS was surprisingly invariant (21.4–22.7 mm total length) while planktonic duration varied widely (range: 73–123 days), even within settlement groups and between years (mean = 88 days in 2000; mean = 105 days in 2001). In both years, IGR were remarkably lower during the planktonic period (range: 0.09–0.29 mm/d) than after settlement (range: 0.54–0.73 mm/d), demonstrating that Sargassum and Zostera marina beds are important nursery grounds for S. inermis. In conclusion, the analysis of variation in SAS vs planktonic duration along with the selection pressures on settlement size, were the most important driving factors in the settlement dynamics of this rockfish.