The goal of this project was to develop an Integrated Multitrophic Aquaculture (IMTA) system for limu culture using nutrients supplied by fish and shrimp pond effluent. A 3 x 3 experimental design was used to assess the effects of effluent quality on growth and nutrient uptake by three limu species from Hawaii. The experiment consisted of two independent variables (limu species and effluent type), each with three levels for a total of nine treatments, and there were three replicates per treatment. Experimental units consisted of 55-gallon, plastic tanks containing limu with flow-through effluent. Carnivorous fish effluent came from a tank containing giant trevally (Caranx ignobilis), herbivorous fish effluent came from a pond with striped mullet (Mugil cephalus), and shrimp effluent came from a pond with Pacific white shrimp (Litopenaeus vannamei).
Two dependent variables (limu production and nutrient uptake) were monitored in each of the 27 tanks. Limu biomass was measured weekly and nutrient concentrations (TAN, NO3-, PO43-) were measured entering and exiting the tanks twice per week over the 6-week trial. Results indicate that shrimp effluent promoted the highest production for all three limu species, followed by carnivorous fish effluent, then herbivorous fish effluent. Ulva exhibited the highest production in
all three effluent types, followed by Gracilaria, then Gratelupia. Gracilaria production decreased after week 4, in carnivorous and herbivorous fish effluent, and exhibited significant fragmentation. All three limu species were effective at removing TAN from all three effluent types, and all three limu species were effective at removing NO3- and PO43- from shrimp and carnivorous fish effluent. NO3- and PO43- concentrations were not reduced from herbivorous fish effluent under the conditions of this experiment.
Results from this study indicate that limu from Hawaii can be grown in fish and shrimp pond effluent as a potential secondary cash crop while acting as a biofilter to remove nutrients from the effluent stream.