Integrated multi‐trophic aquaculture (IMTA) seeks to biodiversify fed aquaculture (e.g. finfish or shrimps) with extractive aquaculture, recapturing the inorganic (e.g. seaweeds) and organic (e.g. suspension‐ and deposit‐feeders) nutrients from fed aquaculture for their growth. The combination fed/extractive aquaculture aims to engineer food production systems providing both biomitigative services to the ecosystem and improved economic farm output through the co‐cultivation of complementary species. Major rethinking is needed regarding the definition of an ‘aquaculture farm’ and how it works within an ecosystem. The economic values of the environmental/societal services of extractive species should be recognized and accounted for in the evaluation of the full value of these IMTA components. Seaweeds and invertebrates produced in IMTA systems should be considered as candidates for nutrient/carbon trading credits. While organic loading from aquaculture has been associated with localized benthic impacts, there have also been occurrences of increased biodiversity and abundance of wild species in response to moderate nutrient enrichment and the use of infrastructures as substrates. To develop efficient food production systems, it will be important to understand and use the duality of nutrients (essential when limiting/polluting when in excess) to engineer systems producing them in moderation so that they can be partially recaptured while maintaining their concentrations optimal for healthy and productive ecosystems. Measures of species diversity, colonization rates, abundance, growth and ecosystem functions with respect to nutrient partitioning and recycling, species interactions and control of diseases could represent valid indicators for the development of robust performance metrics.

Knowledge of the quantitative and qualitative properties of salmonid faeces is necessary for aquaculture waste dispersal models, and the design of integrated multi-trophic aquaculture (IMTA) systems. The amount and proximate composition of salmonid faeces can be estimated using a mass-balance, nutritional approach. Indigestible components of salmonid diets have the potential to a¡ect faecal'cohesiveness'or 'stability' . Nutrient content and density of faeces can vary depending on diet and submersion time. Faecal density has a greater in£uence on settling velocity than faecal size. Published settling velocity data on salmonid faeces are highly variable due to di¡erences in ¢sh size, rearing systems, collection time, water density, methodology, the mass fraction tested and diet. Most faecal settling data used in published salmonid waste dispersal models are rudimentary and recent information suggests that such models are highly sensitive to this input. The design of open-water IMTA systems and estimation of nutrient capture and recovery from co-cultured ¢lter feeders is di⁄cult due to limited information on particle size, digestibility, settleable and non-settleable mass fractions of salmonid faeces at cage environments. Implications of faecal properties on the accountability for the e¡ects of aquaculture nutrient loading are discussed.

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In integrated multi‐trophic aquaculture (IMTA), species from different trophic levels are raised in proximity to one another and the co‐products (organic and inorganic wastes) of one cultured species are recycled to serve as nutritional inputs for others. IMTA can reduce the ecological impacts near aquaculture operations, improve social perceptions of aquaculture and provide financial benefits for aquaculture producers via product diversification, faster production cycles and price premiums on IMTA products. We review aspects of IMTA’s economic potential and market acceptance and consider ways to address the current gaps in our understanding. We find that adopting IMTA raises the assimilative capacity of the farm and that IMTA substantively reduces the environmental cost of aquaculture. Moreover, integrating extractive species (e.g. invertebrates and/or seaweeds), with existing fed‐monoculture operations, can increase farm profits. The presence of positive public attitudes towards IMTA, as expressed by a willingness to pay a premium for its products, can further increase the profitability of adopting IMTA. Areas requiring more economic research include the development of comparative bioeconomic models of IMTA and the evaluation of competing production systems and their ability to internalize externalities to demonstrate the true value of IMTA to society. Further exploration of economic incentives, such as instruments needed to foster adoption of IMTA, and investigation of marketing opportunities, such as promoting the eco‐certification of IMTA products, are also needed. Our paper aimed to inform economists and non‐economists alike about the latest developments in IMTA economics, and spur further research on critical topics concerning this important subject.