Effect of Stocking Rate on Growth of Pacific Calico Scallop (Argopecten ventricosus) Spat in Land-Based Downweller and Upweller Nursery Systems

To optimize spat stocking rates for Pacific calico scallop (PCS, Argopecten ventricosus) during nursery culture, two consecutive 8-week trials were conducted to evaluate growth in land-based downweller and upweller systems. Four stocking rates (60%, 70%, 80%, and 90% of silo bottom coverage) of spat at 3.83 ± 0.10 mm in shell height were tested in the downweller system, whereas three stocking rates (50%, 70%, and 90% of silo bottom coverage) of spat at 0.56 ± 0.029 mm in shell height were evaluated in the upweller system. In the downweller system, spat stocked at 60% generally exhibited the best growth performance, with significantly higher relative growth rate (RGR, mm/day) in shell height compared with higher stocking rates. In the upweller system, spat stocked at 50% and 70% showed similar growth performance and both outperformed the 90% stocking rate treatment in shell height, dry weight, RGR, specific growth rate (SGR, %/day), and condition index (CI). These results suggest that moderate stocking rates, approximately 60% in downweller systems and 50–70% in upweller systems, provide favorable conditions for PCS spat growth while maintaining practical space-use efficiency under the present culture conditions. The present study demonstrates the feasibility of PCS spat nursery culture in land-based systems and provides practical information for hatchery production and nursery management of this emerging aquaculture species.