Although substantial advances have been made in the hatchery production of the European flat oyster Ostrea edulis in the past decades, current methods are still insufficient to meet the demands of spats for aquaculture and for restoration projects. One of the factors that can constrain production is nutrition, which account for 30-60% of costs. It has been shown that different ratio of mixed microalgae diet can improve broodstock conditioning and improve growth, survival and settlement rates of larvae. This is why in most hatchery several strains of microalgae are cultivated. Once produced, spat grow-out occurs in micronursery, then nursery or in the wild. In nursery, it is frequent that a single species of diatom such as skelatonema sp. is used as food and added to partially or non-treated local seawater to reduce costs. In areas where spat cannot be placed in nursery using local contaminated water, the first grow-out of spat in biosecured area such as a micronursery should be cost-efficient. We evaluated the effect of single or mixed diets on spat in micronursery.
O. edulis spat produced from a single swarming event in spring 2023 were used for two feeding experiments. Spat were placed in 2L tanks with constant aeration in a flow-through system, supplied with 27.5 ppt 1µm filtered and UV treated seawater at 15 ºC. Five microalgae diets, plus a no-feeding control, were tested: Diet 1: 50% Chaetoceros sp., 25% Pavlova gyrans, 25% Tisochrysis lutea; Diet 2: 50% Rhodomonas salina, 50% Chaetoceros sp.; Diet 3: 50% Chaetoceros sp., 50% T. lutea; Diet 4: 50 % P. gyrans, 50% T. lutea; Diet 5: 100% Chaetoceros sp. After 7 days acclimatization, spat were fed at a constant concentration of 20 cells µl-1 (equivalent T. lutea volume) for total experimental time of 35 days. For experiment 1: 210 spat (10-15 mm) were distributed in 6 treatment tanks for measuring clearance rate and oxygen consumption; for experiment 2: 15g of spat (5-10 mm) were randomly allocated to each treatment in triplicates (18 tanks in total). Weight gain was calculated at the end of the experimental period, and their organic mass was stored at -20 ºC for fatty acid analysis.
Oxygen consumption was similar for spat fed with each of the 5 diets. Clearance rate was significantly higher for spat fed diet 2, whereas the highest weight increment was observed for spat fed diet 2 and 5 (Fig. 1). These results suggest that O. edulis spat show a preference towards R. salina similarly to conditioning adults, when compared to diets including microalgae of smaller cell size (R. salina volume being ~678 µm3, P. gyrans and T. lutea ~217 and 133 µm3, respectively). In contrast to the observations on larval stages, the use of Chaetoceros sp. as single species diet yielded higher growth compared to mixed diets with T. lutea. Conversely, the best results in term of growth were obtained by its combination with R. salina. Fatty acid composition analysis is currently being carried out.
Funding: This study was part of the project LTA BOOST supported financially by Innovation Fund Denmark.