Assessing the capacity for sustainable finfish aquaculture in the vicinity of seagrasses
University of Adelaide
Jason E. Tanner
Globally, aquaculture accounts for over 50% of fish production. However, if poorly planned, rapid expansion to meet the ever increasing demand for seafood brings with it an environmental risk associated with eutrophication and organic enrichment of the seabed, adversely affecting marine coastal ecosystems. Approximately 75-85% of the nitrogen discharged from finfish aquaculture is dissolved and dispersed to nearby habitats. A major spatial constraint on aquaculture in nearshore areas around much of Australia is the potential for these dispersed nutrients to negatively affect seagrasses. Seagrasses can be sensitive to increases in nitrogen, which can lead to habitat loss. This loss can have significant environmental and economic impacts with potential losses of ecosystem services including decreases in commercial and recreational fisheries catches, increases in sand instability and erosion, reduced biodiversity, loss of nitrogen assimilation and cycling, and loss of carbon sequestration. In other situations, small increases in nutrients may have a positive effect on seagrasses, and thus it is not clear what the consequences of aquaculture derived nutrients will be. Subsequently, we can’t robustly determine the level of finfish aquaculture that can be sustainably supported by seagrass ecosystems. There is therefore a need to develop a process to determine the likelihood of seagrass growth (or loss) due to aquaculture derived nutrient inputs. This work will develop metrics that can be used in other aquaculture developments and in long-term regional monitoring. Clean Seas Seafood Pty Ltd are developing a new lease for the sea-cage aquaculture of 4500 tonnes of yellowtail kingfish (YTK) in the Fitzgerald Bay region. The nearshore habitats throughout the region are dominated by long-lived Posidonia seagrass. While Fitzgerald Bay was the original focus of YTK aquaculture in SA, it has not been utilised for ~10 years, essentially giving us the potential to study this system prior to the commencement of aquaculture (currently planned for ~ July 2019), as well as while production is increasing, and it thus provides an ideal case study for assessing how to sustainably farm finfish in a seagrass dominated ecosystem.
1. Determine cost-effective approaches to assessing the influence of finfish aquaculture derived nutrients on seagrasses, and using Fitzgerald Bay as a case study, what that influence is.
2. Develop a predictive modelling ability to estimate carrying capacity and allow scenario analysis of future aquaculture development and how it might affect seagrasses, to allow managers to make informed decisions about where to place future developments, and how much to allow existing developments to expand.
3. Use Fitzgerald Bay as a case study to document seagrass condition using a range of metrics both before the commencement of finfish aquaculture, and once production has reached a substantial level.
4. Develop a range of cost-effective indicators for monitoring the effects of aquaculture on adjacent seagrass beds.