Nitrogenous compounds such as ammonia, nitrate, and dissolved organic nitrogen are the main waste components of marine fish aquaculture effluents. These compounds are also regarded as primary nitrogen sources for seaweeds , and are a limiting factor for the growth of seaweeds. In order to understand how dulse (Devaleraea mollis) and sea lettuce ( Ulva lactuca) affect and are affected by marine fish effluent, we developed a land-based integrated multi-trophic aquaculture (IMTA) system to co-culture D. mollis and U. lactuca with white seabass (Atractoscion nobilis) . A. nobilis has numerous aquaculture characteristics desirable for commercialization and decades of research have brought it to a commercial ready state.
In this IMTA system, the sand-filtered seawater flowed through the A. nobilis culture tanks (700 L each) , and then entered D. mollis and U. lactuca cultivating tanks (700 L each) by gravity . This experiment was conducted from Mar ch through to July in ambient seawater with three replicates. The initial stocking densities of A. nobilis , D. mollis and U. lactuca were 30 kgWW/m3 , 4 kgWW/m2 and 1 kgWW/m2 with densities reset monthly and weekly, respectively. The temperature was 13-21°C during experimental period. Devaleraea mollis tanks were shaded at 60% to decrease ectocarpus growth. The seawater exchange rate was 63vol./day.
Atractoscion nobilis grew 0.88g/ day, with an average weight gain of 68.84g (from 118.76 g to 187.60 g). The average total ammonia nitrogen (TAN) of 0.07 mg/L ( peaking at 0.25 mg/L) from effluents of A. nobilis tanks (no or low TAN detected in ambient seawater) resulted in the productivity of 24.53 gDW/m2/ d for U. lactuca and 13.58 gDW/m2/d for D. mollis with the average specific growth rate (SGR) of 20.95±13.09%/d and 3.16±2.27%/d, respectively. The average TAN removal efficiency was 32.15% by U. lactuca , and 13.19% by D. mollis , and for nitrate, nitrite, and phosphate, the average removal rate was 56.54%, 13.88% and 7.9% by U. lactuca , and 41.53%, 10.83% and 15.06% by D. mollis . The pH in effluents of D. mollis and U. lactuca tanks increased to 7.77-8.32 from values of 7.71-8.08 in effluents from A. nobilis tanks. The results of nutritional quality of the seaweeds (e.g., nitrogen, carbon, lipid) are pending. Our results show that A. nobilis can supply nutrient-enriched effluents to co- cultivated D. mollis and U. lactuca in IMTA systems to increase seaweed growth and productivity . Additionally, D. mollis and U. lactuca can efficiently remove nutrients and increase pH values before effluents are discharged into coastal regions. In future studies, h arvested D. mollis and U. lactuca will be fed to other co-cultured invertebrates (e.g., abalone, sea urchins) in this IMTA system to diversity seafood production and increase system efficiency even further.