Wild Spat Collection from the Pacific Oyster <i>Crassostrea</i> (<i>Magallana</i>) <i>gigas</i> (Thunberg, 1793) in Osaka Bay, Japan, Using Plastic Bottles

The collection of natural spat is essential for stable oyster farming. In this study, Pacific oyster seedlings were collected from the intertidal and subtidal zones of Osaka Bay, western Japan, for single-seed farming. The collector was made of a harvest net, and numerous 500 mL polyethylene terephthalate bottles for beverages were placed by cutting off both ends. Short- (45 days) and long-term (79 days) trials were conducted in the intertidal zone of Tanigawa Port during the oyster reproductive season of 2021. The number of collected spat in the short- and long-term trials was 3,878 (0.24 spat/cm2) and 2,854 (0.18 spat/cm2), respectively. The number of individuals with shell height of 10 mm or greater was higher in the long-term trial (1,162 spats) than in the short-term trial (274 spats). In August 2022, the plastic bottle collectors were suspended in the subtidal zone. A total of 902 spats (0.057 spat/cm2) and 101 spats (0.006 spat/cm2) were obtained after 17 days using a pre-sunk collector, where bottles were pre-sunk in the intertidal zone for 24 days, and a new collector, where bottles were not pre-sunk in seawater, respectively. These results confirmed the effectiveness of plastic bottle collectors and pre-sunk collectors for collecting Pacific oyster spats in the Seto Inland Sea.