Physical and biological evaluation of co-culture cage systems for grow-out of juvenile abalone, Haliotis discus hannai, with juvenile sea cucumber, Apostichopus japonicus (Selenka), with CFD analysis and indoor seawater tanks

• We investigated the co-culture potential of abalone and sea cucumbers in one cage system.
• The cage structure does not affect the growth rate of the abalone but it does to sea cucumber.
• The growth of the sea cucumber in the UC was 4.15 g higher than that in the TC.

The co-culture potential of juvenile abalone with sea cucumbers presents a simple yet tremendous opportunity to expand existing monoculture techniques. Both products have substantial market demand and if the wastes of the abalone can be used as food for the sea cucumber, then both economic and sustainability goals could be addressed. Designing a simple but effective containment structure suitable for both animals, however, requires both engineering and scientific approaches. The first objective of this study was to investigate the flow field and dissolved oxygen exchange within two potential structure designs using computational fluid dynamics (CFD). The second objective was to conduct a laboratory experiment, with the same containment structures, to determine if the sea cucumbers would use organic matter (food residues and feces) as a food source produced from the abalone and would show positive growth rates. The design of both containment structures included surfaces for both species to reside and move around. One of the structures utilized vertical plates with access holes between sections (called the UC) while the other was a combination of tube and plate sections (called the TC). The procedure included placing both structures in separate seawater flow through tank systems each with juvenile abalone and sea cucumbers for 7 months. The intent was to examine the growth rates of each set of animals under the same, external flow field conditions. During the experiments, only the abalone was fed.The CFD calculations were verified with tank measurements and indicated better dissolved oxygen exchange in the UC structure. Results of the sea cucumber growth experiments showed that the animals in the UC structure grew to 7.50 g from an initial wet weight value of 3.37 g. In the TC structure, however, the results were not as promising with total growth measured at 1.4 g from an initial wet weight of 3.61 g.During the 202 day experiment, specific growth rates of 0.40 and − 0.47% in the UC and TC, were calculated respectively. For the abalone, a statistically significant growth rate difference was not evident between the containment structures. The study also found that the juvenile sea cucumber actually fed on biodeposits from the abalone and that the substantial difference in growth rates between structure types was due to the better flow rate in the UC.