Development of high input farming of salmon creates significant challenges and opportunities for ecosystem-based management. Active aquaculture produces organic wastes: feed and faecal matter which become available as an organic matter source for wild communities. Salmon farms can also act as a source of organic contaminants for wild food webs. This is a topic of interest both in terms of the implications for human health and for the marine ecosystems within the farms' ‘zone of impact'.
Here we have used environmental chemistry to track and quantify assimilation of organic waste from aquaculture into key members of the surrounding reef and infauna communities and to measure organic contaminant levels in these species and in salmon feed.
We collected samples from five farm and four reference sites in the Marlborough Sounds, New Zealand. We measured ratios of bulk δ13C and δ15N using isotope ratio mass spectrometry, and fatty acids profiles using gas chromatography flame ionisation detection. These techniques were used to identify biomarkers of organic farm waste. We used accelerated solvent extraction and gas chromatography mass spectrometry to extract and measure concentrations of organic contaminants in samples.
Results/Conclusions
We identified a depleted δ13C signature and the presence of high proportions of oleic acid (C18:1 (n-9))) and linoleic acid (C18:2 (n-6)) in salmon farm waste. We detected these biomarkers in infauna and reef communities within 300m of farm pens. Estimates of organic matter assimilation through Bayesian mixing models along with biomass measures from diver surveys indicated that 3%-75% of organic matter was sourced from aquaculture production equating to 1g-25g of biomass per meter square in soft sediment communities. In rocky reef communities 0%-15% of organic matter was estimated to be sourced from aquaculture production, equating to 0g-45g of biomass per meter square. Significant declines in average community trophic level with increasing proximity to farms were evident for infauna communities, while changes in trophic position for reef communities were inconsistent across sites. We detected elevated levels of persistent organic pollutants in wild fish species around farms. However, concentrations were below limits set by the World Health Organisation.
Results from this study have implications for understanding how marine communities may respond to further development and expansion of aquaculture, for mitigation of negative environmental impacts, and for the development of integrated multi-trophic aquaculture.