Comparative analysis of two different immersion times in the farming systems for Ostrea edulis and Magallana (Crassostrea) gigas in a brackish basin of Valli di Comacchio (northern Italy)

Emersion/immersion conditions in off-bottom production of oysters is a promising approach to mitigate various specific challenges associated with aquaculture activities. The aim of this study has been to assess the effect of two different immersion times 24 h and 18 h on growth, mortality, biochemical and elemental composition of two species of oyster, reared in brackish basin of Valli di Comacchio (Ferrara, Emilia-Romagna, Italy). Juveniles of Ostrea edulis and Magallana (Crassostrea) gigas, were cultivated with off-bottom methods from January to July 2023. At the end of the trial, the mean individual mass presented the same data in M. gigas for both immersion time, unlike O. edulis. In the same period both oyster species, reared for 24 h in immersion exhibited higher shell length compared to those immersed for 18 h; while the mortality rate was greater for the 18 h immersion only for O. edulis. The daily growth rate of oysters immersed for 24 h showed a value three times higher in May, which can be attributed to an increase of temperature and consequently to food uptake. In summary, the biochemical analyses of the edible part showed significant differences for O. edulis and M. gigas. O. edulis reared for 18 h of immersion, had a higher percentage of saturated fatty acids than oysters reared for 24 h of immersion; probably due to an increase in palmitic one. The elemental composition of the shell showed no significant differences for calcium and manganese, while the other elements analyzed showed important differences between 24 h and 18 h for both species. In conclusion, immersion time O. edulis and M. gigas, had a limited impact on edible part of oysters, while the elemental composition of the shell presented some significantly (p < 0.05) differences. The data obtained suggest that of the use of the two immersion times, based on the growth phase of oyster, could be optimal for feed availability and maximal clearance rate. Moreover, the two oyster species reached the commercial size in 6–7 months with both immersion time, which makes Valli di Comacchio a promising site for oyster farming.