Effects of salinity, stocking density, and algal density on growth and survival of Iwagaki oyster Crassostrea nippona larvae

To determine the optimal salinity, stocking density, and algal density for hatchery culture of the Iwagaki oyster Crassostrea nippona larvae, three experiments with salinities of 14, 18, 22, 26, 30, and 34 practical salinity unit (PSU); stocking densities of 0.5, 1, 2, 4, 8, and 12 larvae ml(-1); and algal densities of 10, 20, 40, and 100 x 10(3) cells ml(-1) were designed, which included the developmental stages from newly hatched D-larvae to pediveligers. Results showed that larval growth of C. nippona was the fastest at a salinity of 26 PSU, and when salinity was adjusted to a level that was lower or higher than this salinity, survival and growth rate of larvae declined (P < 0.05), resulting both in a decreased mean shell length and a high mortality. Larval growth decreased significantly with increasing stocking density. Larvae reared at 4 larvae ml(-1) had the smallest shell length (198.9 mu m) and lowest survival rate (7.9%), whereas larvae reared at 0.5 larvae ml(-1) had the largest shell length (245 mu m) and highest survival rate (66.3%) on day 13. And the shell length of larvae reared at 0.5 and 1 larvae ml(-1) was significantly (P < 0.05) larger than the values in other treatments, except those reared at 2 larvae ml(-1) (P > 0.05). When feeding the single-algal diet of Isochrysis galbana (clone T-ISO), the shell length of larvae increased markedly as the algal density was increased. Larvae reared at the highest algal density (100 x 10(3) cells ml(-1)) had the largest mean shell length; however, under the conditions of our experiment, there was no significant difference (P > 0.05) in growth and survival rates between the treatments at algal densities of 40 x 10(3) and 100 x 10(3) cells ml(-1). For a large-scale culture, based on the results of this study, a salinity of 26 PSU, stocking density of 0.5-1 larvae ml(-1), and algal density of 40 x 10(3) cells ml(-1) are recommended for an early development of C. nippona.