Growth and mortality of oysters (Crassostrea gigas, Thunberg 1793) in Sacca degli Scardovari (Italy)

This study assessed growth and mortality of oysters reared in suspended ropes, and tested the effect of rope emersion time throughout fattening (8 months; October 2016-June 2017). A total of 4,320 triploid oysters were stuck to 36 ropes (320 oysters/rope) and subjected to three emersion systems (12 ropes/system): standard of the farm (F), i.e. variable emersion duration changing according to daily atmospheric conditions; long (L), i.e. 14 emersion hours per day; short (S), i.e. 7 emersion hours per day. Biometric traits were collected at sticking, and after 2, 4, and 8 months. Biometric data were analysed with PROC MIXED (SAS), with emersion system, sampling time and their interactions as fixed effects, and rope as random effect; mortality was analysed with PROC CATMOD. At sticking, oysters exhibited an average weight of 6.04±2.63 g, a length of 39.8±8.38 mm and a width of 23.9±4.39 mm. Oyster length and width were similar from 2 to 4 months (46.1 mm and 33.3 mm on average, respectively), and increased after 8 months (76.0 mm length and 59.6 mm width) (P<0.001). After 8 months, oysters subjected to F and L emersion programs were heavier (68.3 g and 66.3 g vs 56.8 g; P<0.01), longer (78.1 mm and 77.2 mm vs 71.6 mm; P<0.01) and wider (55.6 mm and 55.0 vs 50.8 mm; P<0.10) than those subjected to S emersion. Total mortality reached 44.3% in F, 63.3% in L, and 66.8% in S system (P<0.001). To conclude, under the tested conditions, oysters fattening appeared feasible and promising, but the short fixed emersion system was the least favourable due to higher mortality and lower growth. Acknowledgements: La Perla del Delta; Veneto Region, Reg. (UE) 508 15/05/2014, DGR 213 28/02/2017.