Clarifying and optimising the environmental benefits of integrated multi-trophic aquaculture

<p dir="ltr">Integrated Multi-Trophic Aquaculture (IMTA) is the practice of co-culturing species of different trophic levels for environmental, social and economic benefit. It has recently gained popularity in the Global North as an environmentally-sustainable strategy to remediate the negative effects of intensive (fed) aquaculture, but it is not a new system. Worldwide seaweed production far exceeds that of other groups, such that for all major producers IMTA is the norm. Despite this, adoption in the Global North has been slow. In addition to regulatory and infrastructural barriers, uncertainties surrounding risk management and the optimisation of environmental effects have made growers and regulators reluctant to support research into IMTA’s environmental efficacy in situ. However, modelling techniques can circumvent some of these difficulties by testing management strategies with relatively little cost or risk. Additionally, Tasmania has an established and well-monitored aquaculture industry, many species of native macroalgae, and a local community invested in environmental preservation. Building capacity to grow new and existing industries via modelling techniques therefore presents an opportunity to clarify and optimise the environmental effects of macroalgae IMTA. <br>The overall aims of this thesis were to evaluate the potential positive and negative environmental effects of Tasmanian macroalgae IMTA, and provide guidance on how they could be optimised. Since IMTA is not currently employed in Tasmanian waters, these aims sought to identify and develop tools and methods appropriate for further evaluation of the environmental effects of macroalgae IMTA. The main thesis comprises five data chapters that address: 1) previous modelling approaches used to evaluate the environmental effects of IMTA in marine systems; 2) the nitrogen-remediation potential of three species of native macroalgae; 3) flow attenuation from the drag caused by macroalgae grown at realistic densities; 4) system-wide connectivity changes resulting from the addition of farmed sites; and 5) the ecosystem-wide trophic impacts of large-scale macroalgae IMTA. The analyses are based on existing and emerging modelling approaches, all of which have been modified to suit Tasmanian macroalgae species, ecosystems and environmental conditions. The importance of culture scale and its relative influence within the environment is a dominant theme in this study, along with the acknowledgement of macroalgae’s natural role as both primary producer and ecosystem builder. <br>Chapter 2 first reviewed and analysed studies to investigate how modelling approaches have been used to evaluate the environmental effects of IMTA in the scientific literature. The focus on nutrient-related effects was unsurprising, given that nutrient remediation is often a primary reason for IMTA implementation, but has left knowledge gaps specific to macroalgae culture for which existing and emerging techniques could be adapted. These results informed the approach and focus of subsequent chapters, which progressively increased in the scope of environmental effects considered. <br>Chapters 3 and 4 focused on the local environmental effects of culturing native species. While local effects are generally the best-studied aspect of IMTA, the viability of a future Tasmanian industry depends on producing reliable positive environmental outcomes with native species in a geographically-specific context. Chapter 3 parameterised a growth model for three Laminariales species and calibrated it using Tasmanian field data, determining the species’ responses to environmental conditions and the optimum outplanting and harvesting strategy. Macrocystis pyrifera was able to remove the most nitrogen, with a year of culture removing 9.92 g N m−2 . Chapter 4 then explored how maximising nutrient removal would affect local hydrodynamics. A semi-Langrarian particle tracking tool was used to quantify residence time at culture scales of 34.5–1243.3 ha, or 1.9–68.5% of the total domain area. Attenuation within the canopy reached >70% at maximum macroalgae density, but residence time only increased up to 40% and variability was extremely high. The water column depth and naturally high variability at the Storm Bay site mitigated the effect of macroalgae drag on residence time, demonstrating the viability of environmental impact mitigation through site selection.<br>Chapters 5 and 6 focused on the system-wide effects of IMTA. By conceptualising macroalgae culture sites as artificial assemblages of native habitat, Chapter 5 investigated the changes different configurations of culture sites could cause in system connectivity. Potential pathogenic particles of differing viability times were used to characterise farm configurations using a variety of network analysis metrics, with the aim of maximising applicability in this data-poor field. While changes to the system’s properties were possible with targeted site placement, the overall influence of the culture network was less than that of the natural population. Chapter 6 integrated IMTA into an end-to-end ecosystem model to study its effects on ecosystem functionality, diversity, and resilience. Five scenarios of increasing macroalgae IMTA scale were tested, the largest of which aimed to remove the nitrogen inputs of the entire Tasmanian salmon farming industry using a macroalgae:salmon biomass ratio of ∼ 44 : 1. This generally benefited grazers, detritivores and low trophic-level predators at the expense of high trophic-level predators and other primary producers. Effects on nutrient cycling and total system throughput were positive but structural diversity decreased, resulting in a more efficient but less resilient ecosystem in which high trophic-level fished species were particularly impacted. However, a less extreme scenario (a biomass ratio of ∼ 11 : 1) maintained most of the productivity and nutrient cycling benefits while minimising the detrimental effects on high trophic-level species. This suggests the existence of an IMTA ‘sweet spot’, and that complete nutrient remediation should not be the ultimate goal.<br>Chapter 7 synthesised the findings of the previous chapters to evaluate them as a whole. This integrated study has several implications for the management of macroalgae culture and IMTA in Tasmania, and for marine spatial planning and resource management in general. In particular, recommendations are made regarding i) the viability of an IMTA strategy using native species of macroalgae for environmental sustainability in South-East Tasmania, ii) the ability of available modelling techniques and data to adequately assess the environmental effects of IMTA, and iii) opportunities to use the unique context of Tasmanian macroalgae culture and IMTA to inform management of the natural environment.</p>