Mechanisms leading to the decline in Pacific oyster Crassostrea gigas seedlings in Matsushima Bay, Japan

Predicting the larval transport of Pacific oyster Crassostrea gigas during the seedling collection season

“…This would lead to a decrease in the hauling distance, resulting in an underestimation of the larval density. Therefore, the hauling distance has been historically adopted as 2.5 m (Kakehi et al., 2016). Samples were immediately frozen at –18°C.…”

“…Therefore, a low yield of oyster seedlings in the bay can lead to a decrease in oyster production in Japan. Here, seedling collection failed in 2013 due to an unusually low yield (34% of a typical yield; Kakehi et al., 2016). Although the observed density of the hatched larvae (shell height <100 µm) was higher than usual in 2013, the number of subsequent pre‐attachment stage larvae was quite lower than usual.…”

“…Kakehi et al. (2016) found that this decrease resulted from an outflow of oyster larvae from the bay, driven by greater freshwater inputs following increased precipitation during the oyster seedling season. Freshwater derived from the Naruse River, which discharges outside the bay, flowed into the bay through a narrow strait and accelerated the outflow of oyster larvae through other straits (Kakehi, Takagi, Okabe, & Takayanagi, 2017).…”

“…During that year, the conditions were calm and the seedling collection succeeded. We believe the assumption that the calm conditions would persist is not extreme because, other than in 2013, the seedling collections rarely failed due to excessive freshwater input (Kakehi et al., 2016). The third reason we used the boundary conditions from 2015 is that the modeled current driven by these conditions has been confirmed to reproduce the observed residual current well in the bay.…”

Larval occurrence and environmental factors associated with spawning of Pacific oyster Crassostrea gigas in Matsushima Bay, Japan

“…Tanabe (2012) pointed out that seedlings were collected early in the years when the rainy season ended early. Kakehi et al (2016) season. Freshwater from the Naruse River, which discharges outside the bay, flows into the bay through a narrow strait and accelerates the outflow of oyster larvae through other straits (Kakehi et al, 2017).…”