Cohort-specific mortality of black rockfish Sebastes cheni larvae and juveniles in the Seto Inland Sea: implications for conservation of seagrass and macroalgae habitats

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Black rockfish Sebastes cheni is a dominant component of fish fauna in vegetated habitats in temperate waters of east Asia. Larval and juvenile rockfish immigrate to macloalgae (Sargassum spp.) beds in early March at a total length of 20 mm and then inhabit in seagrass (Zostera spp.) beds through summer in the Seto Inland Sea, Japan. We tested the hypothesis that seasonal changes in vegetation of coastal habitats affect mortality and production of the larval and juvenile rockfish and propose the importance of conservation of coastal habitats referring phenology of different types of vegetation (eg. macloalgae and seagrass). Physical and biological surveys were conducted from February to August in macloalgae and seagrass beds in central Seto Inland Sea. Based on the otolith daily ring analysis, rockfish larvae and juveniles were divided into the sama hatch-date cohorts, each covering a 10-day extrusion period. Increase in cohort-specific mortality coefficient with decrease in vegetation and results from mesocosm experiments showed that vegetation contributed as predation refuge for the rockfish larvae and juveniles. The mortality coefficients of fish at 20-50 mm (macloalgae-dependent period: March to May) were higher that those at 50-80 mm (seagrass-dependent period: June to August). The macloalgae grow during winter and seagrass during summer in the temperate waters of Japan. Therefore, co-existence of these two different types of vegetation, macloalgae and seagrass, within a small spatial scale can increase the accumulative survival of young-of-the-year rockfish from post-immigration period (March) through summer in the Seto Inland Sea.