Seaweed Integrated Mariculture : Prospects and Constraints Towards Increased Sustainability

Abstract: Increased global demand for seafood and stagnating catches from capture fisheries contributed to the doubling of aquaculture production taking place the last decade. Expansion within the aquaculture sector is expected to persist, and by many seen as the only mean to meet future demand for seafood. Aquaculture development has however, been associated with different types of adverse environmental impacts. One such impact, originating from the flow-through behavior of modern aquaculture, and with potential negative consequence for coastal waters, is the release of nutrients. The present thesis investigates the possibilities to use seaweeds as biofilters if being integrated with fish farming. Focus is mainly on open-water fish cage cultures (e.g. salmon) and the general objective is to analyze seaweed integration in respect from its ecological and large-scale perspective. From reviewing existing literature on seaweed integration in aquaculture an imbalance between land-based and open-water culture systems becomes apparent. In land-based aquaculture several studies acknowledged the potential of using integrated seaweed as biofilters for dissolved fish waste. Such aquaculture system has also proved successful on a large-scale, both from a technical and economical perspective. However, for open water systems even basic biological, hydrological and technical understandings are lacking. This is unfortunate, as integration may presently be the only viable treatment solution for open water-cage cultures. The red seaweed Gracilaria chilensis, mainly used for agar extraction, showed high biofiltering capacity when being integrated with a salmon cage farm, with increased growth rates and nutrient content. Integrated Gracilaria growth was up to 40% (SGR 7% day-1) higher compared to monocultures and even if agar content decreased, this was more than compensated for by the higher yield and enhanced agar quality. Analysis of stable isotopes (15N/14N and 14C/13C) in seaweed thalli cultivated at different distances (e.g. 10, 30, 60, 300 m) from cages, verifies that the nutrients being incorporated by the integrated seaweed originates from the fish farm effluent. However, the continuous nutrient supply near the cages has limited effect on seaweed growth during seasons when water temperature and light overrides nutreints as limiting factors. Even if the epiphytic abundance were moderate (2-50%) compared to common bottom cultivations, epiphytes and fouling organisms may also constrain the success of open-water integration especially high infestation of bryozoa that hamper thalli growth and mussels causing the thalli to detach due to weight. The development of suitable methods for suspended seaweed cultivation is crucial for possible commercial implementation of integrated systems. Such methods must be easy manageable with respect to stocking, operation and harvesting and also be durable for open-water conditions. Spore-inoculated Gracialria ropes were found to be unsuitable for Chilean open water conditions and trials with seaweed thalli on twisted ropes showed better performance. Limited effects from integration on seaweed growth, due to seasonal variability and its spatial needs, imply that economic incentives for commercial impementation may be weak. However this could be changed if seaweeds with higher market values were cultured or if costs for nutrient abatement were internalized in the production. Seaweed integration may be one option for developing more environmentally friendly culture techniques, but this will only be confirmed from more scientific studies that include large-scale experiments.