The culture of fish, mussels, sea cucumbers and macroalgae in a modular integrated multi-tropic recirculating aquaculture system (IMTRAS): Performance and waste removal efficiencies

A modular land-based integrated multi-tropic recirculating aquaculture system (IMTRAS) for fish, sea cucumbers, mussels and macroalgae was established to optimize resource utilization, while reducing water requirements and nutrient discharge levels. The first module included California yellowtail (Seriola dorsalis) as the primary “fed” species and Ulva lactuca functioned as biofilters. Effluent from the fish tank was passed through a drum filter for solids removal, and then the filtered seawater entered a sump. Seawater was pumped from the sump into U. lactuca tumble culture tanks and then back to the fish tank. The second module housed Mediterranean mussels (Mytilus galloprovincialis) and warty sea cucumbers (Apostichopus parvimensis) in separate serial units. Backwashes of the drum filter occurred automatically and effluent flowed by gravity to M. galloprovincialis culture tanks, and then to A. parvimensis culture troughs. Two trials of two weeks each were conducted – one at an initial fish stocking density of 5.2 kg m−3 (Trial 1), and another at 20.9 kg m−3 (Trial 2). The seawater flow rates and aeration levels were determined based on earlier pilot studies. Yellowtail grew on average 0.36 and 0.64% d−1 with a food conversion ratio of 1.73 and 1.47, in Trial 1 and Trial 2, respectively. Ulva lactuca showed productivity of 24.39 ± 2.00 and 15.57 ± 2.47 g DW m−2 d−1 with a protein content of 15.13 ± 3.11 and 30.36 ± 1.41% DW in each trial, respectively, which resulted in nitrogen uptake rates of 0.58 ± 0.08 and 0.75 ± 0.12 g m−2 d−1. Mussels and sea cucumbers removed 16.1 to 18.9% and 16.4 to 20.4% of the total solid wastes produced daily, respectively. The condition index of mussels and sea cucumbers showed no difference between the beginning and end of trial. After calculating the nitrogen balance in this IMTRAS, the largest proportion of nitrogen was in the form of dissolved inorganic nitrogen that accumulated inside the system (31.46%), followed by the yield of S. dorsalis (24.64%) and then the harvest production of U. lactuca (14.91%). All other measured nitrogen components were <7 % combined. Nitrogen that was not directly measured (i.e. “black box” nitrogen) including nitrogen accumulated inside plumbing components and lost with the effluent discharge totaled 22.48%. Based on these results, the proper ratio of co-cultured species in this system was estimated as 20.9 kg m−3 of S. dorsalis, six tanks of U. lactuca, three tanks of M. galloprovincialis and six troughs of A. parvimensis with the same volumes and stocking densities reported in this study. Future long-term trials need to be conducted to refine the operation of this IMTRAS.