New Perspectives in Multi-trophic Aquaculture

Aquaculture, the farming of marine and aquatic organisms, is currently the fastest growing food production sector globally. As capture fisheries have stagnated and the fishing down of marine food webs has been seen to be unsustainable, aquaculture is filling the deficit in providing food to a growing world population. Aquaculture species can be divided into two groups: those that require an external input of a food source, known as ‘fed species’, and those that extract their food and nutrients directly from the marine environment, known as ‘non-fed’ or ‘extractive’ species. Fed species are generally fish and shrimps, while extractive species are predominantly seaweeds and bivalves. While seaweeds and bivalves are good candidates for aquaculture due to their nutritional profiles, compounds and wide array of applications, they are also promising from an environmental perspective. Filter feeding bivalves provide an uptake of particulate nitrogen while seaweeds sequester both dissolved nitrogen and carbon as well as providing an oxygenation effect. Thus, by absorbing carbon and nitrogen, extractive species have a positive environmental benefit as well as providing a valuable aquaculture yield. While the cultivation of other species, such as finfish, has been seen to cause environmental degradation, the cultivation of extractive species can be viewed as truly sustainable if farmed appropriately. Integrated multi-trophic aquaculture (IMTA) is a method of aquaculture that incorporates multiple species into one farm. The aim is to prevent waste nutrients of one species being lost into the marine environment, but rather being captured and used by other species in the system, thus preventing environmental damage while producing an enhanced and more diverse crop. To date, IMTA systems have mostly centred around a fed species around which extractive species are placed. The aim of this thesis is to explore the potential for IMTA system containing only extractive species where no external inputs are necessary. Further, this thesis also explores the role of nutrient enrichment on the response of seaweed to environmental stress, as well as to explore the potential for multi-species cultivations of seaweed. My findings show that by cultivating kelp in association with bivalves, kelp yields increase in terms of both quality and quantity, where kelp grow larger, contain more valuable bioactive compounds and display reduced rates of biofouling. By enhancing growth, the bioremediation capacity of seaweeds is also increased, thus increasing the total assimilated carbon and nitrogen. Further, I also show that the impacts of mussels on kelp are positive even in high nitrogen environments, such as around fish cultivations, resulting in enhancements of bioactive compounds in kelp tissue. Lastly, I report that the cultivation of multiple seaweed species together may be problematic due to nutrient competition and chemical interactions, although nutrient limitation may be mediated by nitrogen enrichment. Overall, this thesis presents new perspectives in multi-trophic aquaculture, where extractive species have benefits beyond bioremediation, and the potential for IMTA systems containing only extractive species is revealed.