Effects of stocking density on growth, metabolism, and energy budget of Haliotis discus hannai Ino

In order to examine the effects of stocking density on growth, food intake, energy budget, and expression levels of related genes of the abalone Haliotis discus hannai Ino, specimens (mean ± standard error shell length: 36.25 ± 1.97 mm, weight: 6.88 ± 0.71 g) were cultured at four different stocking densities (600, 900, 1200, and 1500 ind/m2). Four replicates were set for each density, and the experimental period was 60 days. At the end of the experiment, the survival rate, shell length, and weight specific growth rate of abalones in the 600 and 900 ind/m2 groups were significantly higher than in the 1200 and 1500 ind/m2 groups (P < 0.05). The food conversion rate of abalones in the 900 ind/m2 group significantly increased compared with the other groups (P < 0.05). The activity of hexokinase in the 1200 and 1500 ind/m2 groups was significantly higher than that in 600 and 900 ind/m2 groups (P < 0.05). The activity of pyruvate kinase in the 1500 ind/m2 group showed no significant difference compared with the 900 and 1200 ind/m2 groups (P > 0.05), but was significantly higher than that in the 600 ind/m2 group (P < 0.05). In the 1200 and 1500 ind/m2 groups, lactic dehydrogenase activity at Day 60 increased significantly compared with Day 30 (P < 0.05). In the 900 ind/m2 group, the energy accumulated for the growth was significantly higher than that in the 1200, and 1500 ind/m2 groups (P < 0.05), but there was no significant difference compared with the 600 ind/m2 group (P > 0.05). As the stocking density increased, expression levels of Mn-superoxide dismutase, catalase, glutathione-s-transferase, thioredoxin peroxidase, and heat shock protein 70 in the 1500 ind/m2 group at the end of the experiment were significantly higher than in the other groups (P < 0.05). At Day 30, the expression level of heat shock protein 90 in the 1200 ind/m2 group showed no significant difference compared with the 1500 ind/m2 group (P > 0.05), but it was significantly lower than that in the 1500 ind/m2 group at the end of the experiment (P > 0.05). In the 1200 and 1500 ind/m2 groups, the food availability was lower, levels of anaerobic metabolism increased, and more energy was required to resist oxidative damage; therefore, no energy was accumulated for growth. When the stocking density was 900 ind/m2, the survival rate of abalones was the highest with fast growth, the density factor didn't cause oxidative stress to the organism, and more food-derived energy were accumulated and used for the growth of abalones. Considering aquaculture production output and business profitability, it would be more appropriate to control the stocking density at 900 ind/m2 for abalones with a shell length of 3-4 cm.