When and where should aquaculture nets be deployed in a nursery site to maximize <i>Cladosiphon okamuranus</i> growth?

Summary Cultivation of Cladosiphon okamuranus (Phaeophyceae, Chordariaceae; known as “Okinawa‐mozuku” in Japan) requires a nursery to promote the initial growth of germlings and juvenile sporophytes. However, in recent years, the quality and the number of nurseries have declined at many aquaculture farms. The characteristics of a good nursery that promotes germling and juvenile sporophyte growth remain poorly understood. In this study, small polyvinyl chloride (PVC) frames with cord seeded with C. okamuranus spores were installed at three sites within a nursery area at Bise Point, Okinawa Island, to investigate when and where Okinawa‐mozuku aquaculture nets should be deployed in the nurseries and to evaluate the optimal conditions for subsequent growth. In one experiment, the PVC frames were deployed at three sites and observed once a month from October 2020 to March 2021. Growth varied among sites, with optimal results observed for frames deployed in deeper sites during December and February. In another experiment, frames were moved among sites; sporophyte growth was slowest on frames that were moved to the shallow site, whereas growth was faster for those transferred to the deeper site. A comparison of the environmental conditions among sites indicated that the shallower sites experienced a greater range of water temperature and strong wave action. Despite the close proximity of the three sites, there were significant variations in growth; therefore, a careful assessment of the environmental conditions of nursery sites along small spatial scales is essential for ensuring stable aquaculture production.