Ecological engineering in aquaculture:: use of seaweeds for removing nutrients from intensive mariculture

Rapid scale growth of intensive mariculture systems can often lead to adverse impacts on the environment. Intensive fish and shrimp farming, being defined as throughput-based systems, have a continuous or pulse release of nutrients that adds to coastal eutrophication. As an alternative treatment solution, seaweeds can be used to clean the dissolved part of this effluent. Two examples of successfully using seaweeds as biofilters in intensive mariculture systems are discussed in this paper. The first example shows that Gracilaria co-cultivated with salmon in a tank system reached production rates as high as 48.9 kg m(-2) a(-1), and could remove 50% of the dissolved ammonium released by the fish in winter, increasing to 90-95% in spring. In the second example, Gracilaria cultivated on ropes near a 22-t fish cage farm, had up to 40% higher growth rate (specific growth rate of 7% d(-1)) compared to controls. Extrapolation of the results showed that a 1 ha Gracilaria culture gave an annual harvest of 34 t (d. wt), and assimilated 6.5% of the released dissolved nitrogen. This production and assimilation was more than twice that of a Gracilaria monoculture. By integrating seaweeds with fish farming the nutrient assimilating capacity of an area increases. With increased carrying capacity it will be possible to increase salmon cage densities before risking negative environmental effects like eutrophication and toxic algal blooms sometimes associated with the release of dissolved nutrients. The potential for using mangroves and/or seaweeds as filters for wastes from intensive shrimp pond farming is also discussed. It is concluded that such techniques, based on ecological engineering, seems promising for mitigating environmental impacts from intensive mariculture; however, continued research on this type of solution is required.