Pacific oyster, Crassostrea (Magallana) gigas, is the most commonly reared shellfish on the United States Pacific coast and a large percentage of the Aquaculture industry is reliant on hatchery production of animals. Pacific oyster rearing techniques use variable microalgal mixtures for different life stages, vary among producers, and over geographic areas. Subsequently, the effects of including Nannochloropsis oculata in algal mixtures used in hatchery production of Pacific oysters is not well understood. This study quantified the effects of N. oculata introduced into algal mixtures containing Tisochrysis lutea and Chaetoceros muelleri on Pacific oyster larvae and spat.
There were two algal ration methods used to identify differences amongst methods; a “grazing” technique where larval “grazing” was quantified and used to calculate the amount of feed each day, and a “non-grazing” feeding method analogous to standard hatchery protocols where animal consumption was not quantified. Two algal treatments were used in this study; one including all three algae species (NOC), and a second without N. oculata (STD). Larvae were reared in static tanks for 24 hours, then standardized to a density of 40 larvae per milliliter and stocked into the Hatfield Ultra Dense Larval System (HUDLS) at the D-stage. HUDLS is a flow-through seawater system that reduces environmental variance between tanks. Larval samples were collected at 24 hours (immediately pre-stocking into HUDLS), six days, and 12 days post-fertilization. Within the non-grazing ration method, inclusion of N. oculata increased growth rate, measured via shell length. No strong effect of ration method was found on larval size. There was no difference in the total number of larvae produced per HUDL between NOC and STD algal diets, however the non-grazing method produced 18,866 more larvae per HUDL, on average, than the grazing feeding method. In the spat experiment no effect on growth was observed between NOC and STD treatments using a non-grazing feeding method.
Our data show that inclusion of N. oculata increases larval growth rate but has no effect on larval survival or growth at the spat stage. This result demonstrates that using N. oculata during larval production can improve a vital parameter for shellfish hatchery success. Future research should investigate the effect of N. oculata across a variety of commercial larval husbandry systems as well as in different feeding methods as our study highlights that different algal diets and feeding methods can influence important shellfish hatchery production parameters.