Integrated Multi-Tropic Aquaculture (IMTA) has been promoted as a sustainable and practical alternative to monoculture that could help solve some ecological issues with intensified aquaculture in cage systems (Chopin et al. 2012, Custódio et al. 2020). IMTA involves the co-cultivation of tropically linked species where the lower trophic levels feed on the waste from the upper trophic levels, treating waste as a valuable resource utilised through natural synergistic interactions between the cultured species (Chopin et al. 2012, Kerrigan & Suckling 2018). Thus, the environmental impact is mitigated whilst economic value is added in secondary marketable products and reduced risk through product diversification (Barrington et al. 2009).
Blue mussels (Mytilus edulis) grown in surface waters have often been recommended in IMTA systems to extract particulate waste from finfish production (Cranford et al. 2013). However, it is debated if significant mitigation can be obtained by direct assimilation of fish farm waste (Sanz-Lazaro & Sanchez-Jerez 2017).
In all mitigation applications the local environment and the farming practice of the emitting species must be considered. Thus, a detailed analysis of the system and models of various possibilities must be conducted in order to ensure the best possible ecosystem benefits by the multi-trophic approach.
Salmon farming is the main aquaculture production in the Faroe Islands with most farms located in fjords. Recently the legislation has opened up for farming multiple species in a fjord and thus there is a potential for regional multi-trophic aquaculture.
In this study, the potential for multi-trophic aquaculture with salmon and blue mussels was modelled in a fjord with commercial fish farming. The best mitigation potential by blue mussels was investigated by modelling three spatial arrangements of blue mussels in the fjord with fish farming activity. Two fish farm/blue mussel IMTA conjunctions were investigated and a scenario where the blue mussels and fish farm were not directly connected by the particulate fish farm waste but still located in the same ecosystem.
The production at the commercial fish farm was analyzed to predict the emission of dissolved and particulate nutrients, and the dispersion of particulate waste around the fish farm was modelled. For the two IMTA scenarios the blue mussels were modelled next to the fish farm and below the fish farm and the direct uptake of particulate waste was traced (Fig. 1). In addition to the waste assimilation the nutrient uptake by the blue mussels through assimilation of microalgae was investigated in the three blue mussel farm setups. The hydrographic conditions in the fjord were measured and modelled and a two-year time series of the phytoplankton biomass and nutrient availability, as well as the growth potential of the endemic blue mussels, was investigated.
These data were used to investigate the potential removal of nitrogen by blue mussels and the mitigation potential by blue mussels was investigated in a coastal area management context, in order to predict the best mitigation potential considering the local production and spatial constraints in the fjord.
References
á Norði, G., Lund, I., Andreasen, B., Johannesen, T.T., Taylor, D., Jacobsen, B., Hughes, A. D. (2022) Modelling waste assimilation by blue mussels within the spatial constrains of a commercial fish farm: implications for multitrophic aquaculture. (Manuscript submitted for publication)
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