Olive flounder (Paralichthys olivaceus) sometimes referred to as Japanese flounder is a highly valued fish in East Asia due to its rapid growth, excellent aquaculture performance, and high market value. This species has been domesticated in Korea since 1980’s and has potential to be a lucrative aquaculture candidate in other countries as well. Recirculating aquaculture systems (RAS) reduce the amount of water and space required to intensively produce seafood products. In landlocked states such as Kentucky where shrimp production in RAS systems has been growing, olive flounder can be reared in the same flat-bottom culture tanks used for shrimp.. Stocking density is one of the major factors affecting animal welfare and system productivity. The level of stress resulting from high stocking density may also affect energy and metabolism, potentially affecting growth rates and suppressing the immune response. This project focused on assessing the effect of stocking density in a RAS on production dynamics and stress response of olive flounder in nursery-level RAS.
In this study, fingerling (~55 gm) olive flounder were stocked at three densities: 2.7, 5.8 and 7.2 kg/m2 (low-density: LD, medium density: MD, and high-density: HD) in 1.2 m2 round fiberglass tanks. Each of these three treatments were replicated in three randomly assigned tanks connected to a common sump and shared filtration system. To maintain clear water, water from the tanks passed through a drum filter with 40 µm screen, foam fractionator, and a moving bed bioreactor (MBBR) aerobic bio filter. Ozone (O3) gas was injected into the fractionator reaction chamber and water then passed through a UV radiation lamp to destroy ozone and further sterilize the water. In order to prevent the accumulation of nitrate, an anaerobic MBBR denitrification chamber was also used. To assess the health of olive flounder, stress indicators including blood glucose, cortisol, growth hormone and Insulin like growth factor (IGF-1) concentrations were analyzed using a Dynex DS2 System (Chantilly, Virginia, USA), an automated ELISA (Enzyme-linked immunosorbent assay) processor.
At the end of the 90 days long experiment, we observed significantly higher average weight in LD (452.7 g) tanks followed by MD (399.3 g), & HD (347.1 g). Survival in all treatments were very high (>95%) with no significant differences. Similarly, significantly higher biomass was found in HD (54.5) followed by MD (40.4) and LD (23.2) kg/m2. In terms of stress response, no significant difference was observed in cortisol, GH and IGF-1 across all treatments. Based on this trial, we can say that with appropriate culture practices, fingerlings of olive flounder can be grown to a final stocking density of ~55 kg/m2 without any negative impact on their health. However, like many animals, there is a tradeoff between density and individual growth rate. Producers must therefore balance production goals and decide what factors are most important. Nonetheless, this species seems well-suited for RAS and the levels of production in this project are encouraging for an industry that must balance productivity with the relatively high cost of equipment