Introduction
Aquaculture expansion is increasingly required to meet rising demand for aquatic foods, but continued growth depends on reducing environmental pressures, particularly nutrient emissions in sensitive coastal systems (Crona et al., 2023). Integrated multi-trophic aquaculture (IMTA) has been proposed as a strategy to mitigate aquaculture-derived nitrogen (N) and phosphorus (P) emissions by co-cultivating extractive species such as mussels and seaweed alongside fed finfish (Chopin et al., 2012; Buck et al., 2018). However, the biomass scale required to achieve nutrient-balanced production remains poorly quantified, particularly under environmentally constrained Baltic Sea conditions (Sickander and Filgueira, 2022). This study evaluates the environmental feasibility of nutrient-neutral offshore aquaculture using a rainbow trout (Oncorhynchus mykiss) system integrated with blue mussels (Mytilus edulis) and seaweed (Ulva spp.), combining attributional life cycle assessment (LCA) with farm-gate nutrient mass-balance modelling.
Materials and Methods
Primary foreground data were collected from a multi-use offshore aquaculture site in Tagalaht Bay, Estonia, hosting adjacent trout, blue mussel, and seaweed cultivation systems. The assessment followed ISO 14040/14044 and applied a cradle-to-farm-gate system boundary. Nutrient emissions from trout farming were quantified using a farm-gate mass-balance approach based on feed inputs, biomass retention, and residual emissions, consistent with established aquaculture nutrient budgeting methods (Cho and Bureau, 1997). Environmental impacts were assessed in SimaPro using the ReCiPe 2016 Midpoint (H) method. Scenario analysis was used to estimate the mussel and seaweed biomass required to offset trout-derived N and P emissions under different IMTA configurations, and coefficients were further scaled to European aquaculture production levels.
Results
Per ton of harvested trout, offshore production emitted 63.7 kg N and 6.9 kg P to the marine environment, while feed production dominated climate change impacts, contributing approximately 2,880 kg CO₂-eq t⁻¹ trout. Scenario analysis indicated that achieving full nutrient neutrality would require 10.37 t of extractive biomass per ton of trout, comprising 6.76 t of mussels and 3.61 t of seaweed. In contrast, the current pilot IMTA configuration offset only about 0.42 % of N and 0.45 % of P emissions. Scaling these requirements to current European trout production suggests that nutrient-neutral production would require extractive biomass volumes far beyond present seaweed production capacity.
Discussion
The results show that IMTA can reduce eutrophication impacts by converting nutrient losses into harvestable biomass, but nutrient-neutral aquaculture is strongly constrained by the scale at which extractive species must be deployed. Under Baltic Sea conditions, current pilot-scale IMTA systems fall far short of delivering ecologically meaningful nutrient compensation (Buck et al., 2018; Sickander and Filgueira, 2022). At the same time, the analysis reveals a structural divergence between nutrient and climate mitigation pathways: while IMTA can reduce farm-level eutrophication, feed production remains the dominant driver of climate and resource-related impacts, consistent with previous aquaculture LCA studies (Henriksson et al., 2012). Overall, the study provides system-level evidence that nutrient-neutral aquaculture will likely require a dual strategy combining ecologically significant extractive biomass deployment with upstream feed optimization.
Acknowledgements
This work was supported by the European Union’s Horizon Europe research and innovation programme under Grant Agreement No. 101094065 (OLAMUR). The authors gratefully acknowledge BioMar and aquaculture companies for providing the dataset.
References
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