Promise and pitfalls of locally abundant seaweeds as biofilters for integrated aquaculture

Seaweeds from the genus Caulerpa offer promise for bioremediation in integrated tropical aquaculture in northern Australia, as they are common on shallow reefs adjacent to where aquaculture is developing and their propagation is readily manipulated through fragmentation. Fragments of five varieties of Caulerpa had high growth rates (between 3 and 7% day−1 ) and high nitrogen content (up to 3% dry weight/0.2% fresh weight for Caulerpa taxifolia) in tank-based culture. These attributes combined confirm the promise for Caulerpa in integrated aquaculture, especially as certain species (Caulerpa lentillifera and Caulerpa racemosa) are valuable products. However, this potential was not realised when Caulerpa spp. were cultured in an in situ aquaculture context. Only a limited proportion of fragments (13%, predominantly C. taxifolia) persisted during a 6 week in situ experiment in a flow-through settlement (treatment) pond from an 800 tonne year−1 fish production facility. Mean growth of persisting pond fragments (less than 0.3% day−1 ) wasmuch less than concurrent tank cultures (3– 7% day−1 ). The factor most strongly influencing pond culture was the negative (smothering) impact of blooming filamentous algae (Cladophora and Chaetomorpha spp.). Poor pond growth of Caulerpa was further substantiated in an additional test, determining that persistence and growth (or lack thereof) was independent of initial seeding size of fragments. These results suggest that Caulerpa culture will not be easily integrated into settlement ponds in tropical aquaculture. However, because some species of Caulerpa grew well in tank-based systems (C. racemosa grew at N7% day−1 ) and others are capable of luxury uptake (Caulerpa serrulata and C. taxifolia almost doubled internal nitrogen in nutrient-rich water), Caulerpa species have application in bioremediation of intensive tankbased aquaculture and perhaps treated pond aquaculture effluent