Tracking flow and fate of organic matter and contaminants within native food webs surrounding finfish aquaculture

The development of aquaculture to provide a source of protein has the potential to alleviate pressure on wild fish stocks, allowing marine systems to regenerate while providing livelihood to millions of people. Given the rapid expansion of aquaculture anticipated to occur in New Zealand, it is important that development of the industry is managed in a sustainable way so that detrimental impacts are mitigated, and opportunities for humans and native ecosystems are enhanced. Fin-fish aquaculture produces organic and nutrient waste which, in excess, can act to reduce productivity and diversity in communities. However, native marine communities, likely have some capacity to assimilate this waste and utilise it as a resource subsidy. This research firstly aimed to improve the ability of researchers to model resource use in coastal marine food webs associated with aquaculture. Baseline values for stable isotope (SI) and fatty acid (FA) biomarkers in key organic matter source pools available to consumer populations in the Marlborough Sounds were established (Chapter 2). Controlled experiments were then used to estimate parameters that predict the transfer of these signatures to consumer tissue over time (Chapter 2). Secondly, these experimentally determined biomarkers and mixing model parameters were applied to the Marlborough Sounds system to determine the extent at which soft sediment and rocky reef communities assimilate waste from farms, and the consequences this input has on food web structure (Chapters 3 and 4). Finally, after establishing assimilation of waste by reef consumers, organic contaminants were measured in feed and consumer tissues to determine whether imported feed acts as a significant source of these compounds to the New Zealand marine environment (Chapter 5). Stable isotope and fatty acid biomarkers of native marine production and aquaculture production were found to be distinct, establishing these tracers as suitable for use in in-situ systems. Mesocosm experiments found isotopic turnover rates and trophic discrimination of FAs and SIs to differ considerably among consumers and tissue types, underscoring the importance of establishing experimentally determined parameters when modelling resource use and trophic dynamics in marine ecosystems. Application of experimentally determined values to mixing models for the Marlborough Sounds system predicted assimilations of salmon farm waste in all farm associated soft sediment communities, and most farm associated reef consumers sampled. This finding suggests integrated multi-trophic aquaculture (IMTA) may have implications for a diverse range of species in New Zealand. The capacity of communities to uptake and process waste from farms showed high variability both across distance gradients from farms and among farm sites. Assimilation capacity of communities was strongly influenced by trophic structure with the presence of high trophic level consumers indicative of a greater potential to buffer detrimental effects from excess waste deposition. The results presented in Chapters 3 and 4 of this thesis have implications for determining suitable feed inputs to farms, identifying promising species for IMTA development, and informing remedial action to soft sediment communities. Levels of polychlorinated biphenyls, and polybrominated diphenyl ethers measured in both feed and consumer tissues were below limits set by the European Union and, in most consumers, levels were not significantly influenced by the presence of salmon farms. The exception was P. colias, in which PCB levels were elevated around farms potentially as a consequence of P. colias’s trophic link to proximal benthic communities. Identifying P. colias as a candidate for bioaccumulation and/or biomagnification of harmful compounds may guide future monitoring of organic contaminants in species interacting with aquaculture operations. The sustainable development of aquaculture will be essential not only in New Zealand but also globally if we are to ensure a reliable source of protein to the world’s growing population while addressing the impacts of climate change and environmental degradation on natural resource bases. Implications of the present research include, understanding ecosystem structure and function and how this relates to finfish aquaculture, guiding future management and monitoring programmes, and evaluating marine systems and their capacity to recycle organic waste from aquaculture.