Low water exchange Gracilariopsis bailiniae Zhang & B.M. Xia culture in intensive milkfish culture effluents: effects of seaweed density on seaweed production and effluent treatment

This study tested the feasibility of a low-cost seaweed biofiltration system for pond-based aquaculture through an indoor-integrated fish-seaweed culture experiment using weekly nutrient supply regime and different seaweed stocking densities. The culture experiment was conducted in glass aquaria that were stocked with Gracilariopsis bailiniae at 3 densities (low = 0.5 kg m^?2, middle = 2 kg m^?2 and high = 3.5 kg m^?2) and provided either with effluents from intensive milkfish (Chanos chanos) culture or with effluent-free seawater (control) as nutrient source. Stocking density was used as a factor in optimizing nutrient availability for growth and nutrient removal under such low water exchange conditions. Our results showed that G. bailiniae cultivated in milkfish effluents had higher growth, biomass and nitrogen yields than those cultivated in effluent-free seawater. Among the different stocking densities tested, highest growth rate (1.03 % day^?1) was obtained in the middle density. Increasing biomass and nitrogen yields were also obtained at this density until the end of the culture period. Poorer growth rates at low and high stocking densities were attributed to light limitation from phytoplankton and self-shading, respectively. Due to seaweed treatment, average outflow concentration of NH₄ ⁺ was reduced from half of its pretreated level. This study showed that a weekly effluent supply at 2 kg m^?2 seaweed stocking density can sustainably support the growth of G. bailiniae as long as the dissolved nutrients are present at high levels.