The performance of juvenile Greenshell™ mussels (Perna canaliculus) of different sizes during floating upwelling system (FLUPSY) nursery culture and after seeding onto coastal farms

High early spat losses after seeding hinder productivity in global mussel aquaculture. Integrating FLUPSY into nursery stage offers promise for reducing these losses. However, as FLUPSY use in mussel culture is new, key factors such as spat size, density, rearing duration, and husbandry practices need clarification. This study aims to identify the optimal spat size for transfer into FLUPSYs and for seeding onto mussel farms to improve overall success. This study monitored growth, biomass, and mortality of small (1.0 mm) and large (2.6 mm) hatchery-produced spat in a FLUPSY over three months. Subsequently, retention and growth of FLUPSY-raised spat (2.6 mm and 4.4 mm) were observed for eight months after seeding onto a coastal mussel farm. Results showed that large spat outperformed small spat in the FLUPSY, with 59% greater biomass gain, 30% lower mortality, and nearly double the daily growth rate. After seeding onto a mussel farm, loss rates were similar for both sizes (~87–88%), but the large spat demonstrated an 11.8% higher daily growth rate. Over the entire 10.6-month period, including nursery and seeding phases, small spat experienced 4.4% more cumulative loss than large spat. The findings suggest while nursery-rearing of mussel spat to a larger size in a FLUPSY prior to seeding to coastal farms is likely to improve subsequent retention, there is a need to improve the nursery husbandry to better suit smaller spat, as well as improving the methods for seeding singulated nursery-raised spat onto coastal mussel farms to increase their subsequent retention and growth. • Large spat are better suited for FLUPSY nursery culture. • FLUPSY-raised spat size at seeding does not affect their retention. • Integrating FLUPSY nurseries in mussel farming reduces spat losses.