Nitrogenous compounds such as ammonia and particulate organic nitrogen are the main waste components of marine fish aquaculture effluents. These compounds are also regarded as nitrogen sources for seaweeds and solid waste feeders. In order to efficiently re-use waste containing nutrients and particulate organic matters produced in marine fish aquaculture, we developed a land-based IMTA system to co-culture white seabass (Atractoscion nobilis), warty sea cucumber (Apostichopus parvimensis), and macroalgae D. mollis and U. lactuca. The performance of co-cultured species and waste removal efficiencies of system were evaluated.
In this IMTA system, sand-filtered seawater flowed by gravity through a four tier system of 700L circular tanks. The order of tiering was: 1) A. nobilis, 2) A. parvimensis, 3) U. lactuca, and 4) D. mollis in Phase 1 of the Trial. In Phase 2, the order D. mollis and U. lactuca was reversed. This experiment was conducted from March through to June with three replicate tier groups. The initial stocking densities of A. nobilis, A. parvimensis, D. mollis and U. lactuca were 30 kgWW/m3, 16 ind./tank, 4 kgWW/m2 and 1 kgWW/m2 with densities reset monthly for A. nobilis and weekly for seaweed, respectively. The temperature ranged from 13°C to 21°C. Devaleraea mollis tanks were shaded at 60% to decrease ectocarpus growth. The seawater exchange rate was 63vol./day.
A. nobilis grew 0.63 and 0.73%/day, with an FCR of 1.55 and 1.43 in Phases 1 and 2, respectively. The growth rate of A. parvimensis was 0.077±0.090%/day fed with the waste from A. nobilis tanks, and the apparent digestive ratio and removal efficiency on solid waste was 63.65% and 42.07%. The average total ammonia nitrogen (TAN) of 0.078 mg/L (peaking at 0.20 mg/L) in influents of seaweed tanks resulted in an average productivity of 20.22 gDW/m2/d for U. lactuca and 14.66 gDW/m2/d for D. mollis in Phase 1. In Phase 2, the productivity of U. lactuca increased to 25.90 gDW/m2/d, while D. mollis gradually decreased to negative productivities due to increasing temperatures. The average TAN removal efficiency was 35.88% and 32.51%, and 54.51% and 21.17% by U. lactuca and D. mollis in Phase 1 and Phase 2, respectively. The pH in the seaweed tank effluents was significantly greater than the influents. Based on the nitrogen balance, the largest proportion of nitrogen was the yield of A. nobilis (34.94%), followed by the excretion yield of A. nobilis (17.39%), while seaweed production and leftover waste contributed 9.73% and 9.29% to the total input of nitrogen, respectively. 25.15% of nitrogen was not directly measured (i.e. “black box” nitrogen) including N2 production, trapped in plumbing, assimilation by other autotrophs, and lost with the effluent discharge. Based on these results, 2.36 additional tanks for both U. lactuca and D. mollis (5 tanks total), to assimilate all TAN excreted by A. nobilis, and additional 1.4 times the biomass of A. parvimensis are needed to digest leftover waste. This study showed that A. nobilis can supply nutrient-enriched effluents to co-cultivated A. parvimensis and seaweeds to increase system efficiencies and diversify seafood production. Future trials need to be conducted to refine the operation of this IMTA.