Olive flounder (Paralichthys olivaceus) have a long history of success in flow-through aquaculture production, starting in the 1960s in Japan where the species is commonly referred to as “hirame.” Since that time, many challenges have been resolved such as breeding, first feeding, survival through the metamorphosis stage, and growout to food-size fish. The fish can reach a market size of 0.8 to 1.2 kg in 12 to 18 months, making them a fast-growing animal and market prices are very strong. As a flat fish, they have several key characteristics that make them ideal for aquaculture, including that they lay on the tank bottom most of the time, thereby conserving energy. This fact may also allow the fish to be cultivated in shallow tanks, and because they can lay several fish deep, high rearing densities are possible. The University of Miami (UM) has been cultivating them for nearly a decade in flow-through, partial flow through, and RAS systems. Over the last 14 months, Kentucky State University (KSU) has also developed RAS systems for them.
The opportunities for Olive flounder exist near coastal areas where native flounder fisheries have been decimated. Those opportunities extend well into interior locations where fresh seafood is very hard to obtain, but highly valued by chefs and consumers in all areas. Carefully designed RAS are the key to capturing both of these markets. Another industry that has seen growth in the United States and other regions is shrimp farming in RAS. Interestingly, this flatfish species requires the use of flat-bottom tanks like shrimp do, therefore some overlap may exist where a farm could explore the production of either species using the same production systems.
With regard to RAS design, because flounder lay on the tank bottom most of the time, the fish have a reduced oxygen demand while resting compared with other finfish species that may be swimming constantly. Another noteworthy issue is that the fish will tend to lay on top of air and oxygen diffusers, which can limit the dispersion of oxygen in the tank environment. This issue may be remedied by elevating one end of the diffuser slightly so fish can lie under and around it. The fish grow best at temperatures between 20 and 25°C, and have a high protein requirement of about 50% in their diet. These facts mean that biological filters must be larger than those of more tropical aquatic species that eat lower protein diets. The fish grow well in a variety of tank sizes; KSU was able to achieve biomass levels of 64 kg/m2 with fish an average weight of 429 g in tanks that were only 1.2 m in size. Pilot-scale growout trials at UM in a variety of production systems and tank dimensions have been very successful as well over the years.
Ultimately, this fast-growing, feed-efficient species holds a great deal of opportunity for RAS production. Research at KSU and UM continues into genetic improvements, low salinity culture, growth in various salt mixtures, greenhouse gas capture, culture in IMTA systems, and other innovative areas.