Pacific oyster outplants and benthic surveys on intertidal aquaculture beds (Washington state, USA)

Hypothesis: Pacific oyster (Magallana = Crassostrea gigas) performance and yield differ between ground culture and off-bottom culture methods and depending on the level of co-occurring eelgrass (Zostera marina).
Benthic surveys were carried out on aquaculture beds at 10 sites around Washington state, USA, at tidal elevations of -0.4 to +0.4 m relative to mean lower low water. Beds consisted of ground culture or off-bottom culture (longlines or flip bags). Measurements on each bed included density of eelgrass, size and density of oysters, and sediment grain size. On each bed, Pacific oysters (initially ~25 mm) were outplanted in June and collected for measurements of survival, shell height, and dry meat weight in September (3 months) and March (9 months). Half of the sites were studied in 2020-21, and the remainder in 2021-22.
Survival was better for oysters grown off-bottom in longlines or flip bags relative to ground culture. In ground culture, survival declined in finer sediments. Shell growth did not respond strongly to grow-out method, but condition (dry meat mass per shell height) was better in off-bottom than ground culture. After 3 summer months, no performance traits differed across amounts of co-located eelgrass, and performance also did not consistently improve with eelgrass after 9 months over winter. Relative to ground culture, off-bottom culture produced higher densities of oysters per footprint of area and/or increased yield due to shorter crop cycles.