Effects of stocking density on survival, growth, and food intake of Haliotis discus hannai Ino in recirculating aquaculture systems

To examine the effects of stocking density on growth, food intake, and expression levels of related genes of Haliotis discus hannai Ino in recirculating aquaculture systems, abalones (shell length: 38.42 ± 2.31 mm, weight: 7.04 ± 0.89 g) were cultured at three different stocking densities (high [1500/m2], medium [1000/m2], and low [600/m2], 4 replicates each) for 90 d. At the end of the experiment, the survival rate and specific growth rate of weight of abalones in the high-density group were 86.5% and 0.14 mg d− 1, respectively, which were significantly lower than those in the low- and medium-density groups (P < 0.05). The food intake of abalones in the low-density group was significantly higher than that in the medium- and high-density groups (P < 0.05). At days 60 and 90, the food conversion efficiency of abalones in the medium-density group had declined, but it still was significantly higher than that in the high-density group (P < 0.05). At the end of the experiment, the moisture and lactic acid contents of abalones in the high-density group were significantly higher than those in the low-density group. The protein content in the medium-density group decreased over time and became significantly lower than that in the low-density group at day 90 (P < 0.05). The activities of pepsin and cellulase were significantly higher in the low- and medium-density groups than in the high-density group (P < 0.05). At days 60 and 90, the activity of α-amylase in the low-density group was significantly higher than that in the medium-density group (P < 0.05). At the end of the experiment, the expression levels of the polysaccharide cleavage enzyme genes Hdaly and Hdamyl in the low-density group had increased and were significantly higher than those in the medium- and high-density groups (P < 0.05). The expression level of Hdlam in the high-density group was significantly lower than that in the medium-density group (P < 0.05), whereas the expression level of Hdcel in the medium-density group was significantly lower than that in the low-density group (P < 0.05). At the end of the experiment, the expression levels of catalase and glutathione S-transferase in the high-density group were significantly higher than levels in the medium- and low-density groups (P < 0.05). Overall, although food was readily available in this study, abalones had to first search for food and then spend time feeding and ingesting it. For some individuals, such as those in the corners of the tank or under a stack of other abalones, access to food was restricted. Moreover, extra energy expenditure was required to resist oxidative damage. These were the main reasons for the observed slowdown of abalone growth and the increased mortality rate observed in this study.