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

Rapid scale growth of intensive mariculture systems can often lead to adver se impacts on the environment. Intensive fish and shrimp farming, being def ined as throughput-based systems, have a continuous or pulse release of nut rients that adds to coastal eutrophication. As an alternative treatment sol ution, seaweeds can be used to clean the dissolved part of this effluent. T wo examples of successfully using seaweeds as biofilters in intensive maric ulture systems are discussed in this paper. The first example shows that Gr acilaria co-cultivated with salmon in a tank system reached production rate s as high as 48.9 kg m(-2) a(-1), and could remove 50% of the dissolved amm onium released by the fish in winter, increasing to 90-95% in spring. In th e 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)) compa red 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 th e released dissolved nitrogen. This production and assimilation was more th an twice that of a Gracilaria monoculture. By integrating seaweeds with fis h farming the nutrient assimilating capacity of an area increases. With inc reased carrying capacity it will be possible to increase salmon cage densit ies before risking negative environmental effects like eutrophication and t oxic algal blooms sometimes associated with the release of dissolved nutrie nts. The potential for using mangroves and/or seaweeds as filters for waste s from intensive shrimp pond farming is also discussed. It is concluded tha t such techniques, based on ecological engineering, seems promising for mit igating environmental impacts from intensive mariculture; however, continue d research on this type of solution is required.