Co-culture of Nile tilapia (Oreochromis niloticus) and red seaweed (Gracilaria tenuistipitata) under different feeding rates: effects on water quality, fish growth and feed efficiency

Nile tilapia, Oreochromis niloticus (4.12 +/- 0.05 g initial weight), and red seaweed, Gracilaria tenuistipitata, were co-cultured in 250-L tanks for 8 weeks. Six feeding treatments were conducted, with the control group (monoculture) being fed a commercial pellet (30% protein, 6% lipid) at 100% feed ration (4% of biomass per day), while fish in the five other groups were co-cultured with red seaweed (1 kg m(-3)) and received pellet at 80%, 60%, 40%, 20%, and 0% of the feeding ration of the control. The results indicated that the concentrations of TAN, NO2-, NO3-, and PO43- in the co-culture tanks were much lower than those in the monoculture. Moreover, the growth rate and productivity of fish in the 80% feeding ration treatment were comparable to those in the control group (p > 0.05). However, fish performance gradually reduced with a decrease in feeding rates from 60% downwards and all were significantly inferior to the control group (p < 0.05). Feed conversion ratio and feed cost were lower at the lower feeding rates. The results also proved that the co-culture of tilapia-red seaweed supplied at 80% feed ration, which could reduce feed cost by 28.9% compared with the control, still sustained normal growth while also maintaining better water quality. The proximate composition of fish fillet (moisture, protein, lipid, and ash) is also discussed.