Application of seaweeds to integrated multi-trophic aquaculture in southern Australia: identifying and investigating suitable native species

Integrated multi-trophic aquaculture (IMTA) involves strategic co-culture of organisms so that wastes from one species are used to grow another. Seaweeds can be used in IMTA systems to remove and utilise dissolved inorganic nutrients from fish aquaculture, improving environmental performance and providing economic benefits through diversification and increased productivity. IMTA with seaweed could be applied to assist sustainable expansion of fish farming in South Australia (SA), where dissolved nitrogen (N) wastes limit environmental carrying capacity. Seaweed farming is also of interest in Australia to meet increasing demand for seaweed products, of which Australia is a net importer. Several native seaweeds have been identified as potential candidates for aquaculture in SA based on general knowledge of their biology and potential economic value, but specific knowledge of their suitability for cultivation was lacking. I investigated eight candidate seaweeds, comprising four red (Solieria robusta, Gelidium australe, Pterocladia lucida, Plocamium angustum) and four brown (Ecklonia radiata, Cystophora subfarcinata, Sargassum linearifolium, Scytothalia dorycarpa) species, to determine which species were most suitable for farming, with specific emphasis on application to IMTA in SA. I assessed feasibility of cultivation and potential for nutrient remediation of the eight species in two field trials and in laboratory experiments, and applied species distribution modelling (SDM) to identify the most suitable candidate species for aquaculture in the vicinity of current SA fish farms. My research identified the red seaweed Solieria robusta and the brown seaweed Ecklonia radiata as the most suitable species for aquaculture. The red Gelidium australe showed promising growth in a pilot field trial and removed the most N in a 4-week laboratory trial, but S. robusta grew best in laboratory trials and would remove more N over time due to its faster growth. Solieria robusta tolerated a wider temperature range and grew better at higher temperatures than G. australe. SDM results demonstrated that S. robusta has high environmental suitability in aquaculture zones throughout Spencer Gulf, where all SA finfish farming currently occurs, while G. australe was poorly suited to most existing aquaculture zones. Pterocladia lucida and Plocamium angustum had slower growth rates, and SDMs indicated low suitability in aquaculture zones. There was little difference in field performance of the brown seaweeds, apart from Scytothalia dorycarpa, which performed poorly, but Ecklonia radiata was most amenable to hatchery reproduction and cultivation. SDM showed that several aquaculture zones in southern Spencer Gulf had good suitability for E. radiata. Seedstock production methods used for commercially farmed relatives were successfully applied to S. robusta and E. radiata, and I developed protocols that can be employed to upscale production of these seaweeds. Solieria robusta and E. radiata demonstrated the ability to accumulate tissue N, and N uptake rates comparable to other IMTA seaweeds, supporting the suitability of these species for IMTA. Data from my experiments help to inform suitable depths, locations and seasons for cultivation of these seaweeds, and to incorporate N removal by seaweeds into biogeochemical models. These experiments provide the foundation for developing seaweed aquaculture in southern Australia, including IMTA.