Integrated Multi-Trophic Aquaculture: Environmental biomitigation and economic diversification of fed aquaculture by extractive aquaculture

Integrated multi-trophic aquaculture (IMTA) seeks to engineer intensive fed aquaculture (e.g. finfish or shrimps) by biodiversifying it with extractive aquaculture of species utilizing the inorganic (e.g. seaweeds) and organic (e.g. suspension- and deposit-feeders) excess nutrients from fed aquaculture for their growth.

The combination fed/extractive aquaculture aims to biodiversify food production systems to provide both biomitigative services to the ecosystem and improved economic farm output through the co-cultivation of complementary species. Through IMTA, some of the food, nutrients and by-products considered “lost” from the fed component are recaptured and converted into harvestable and healthy seafood of commercial value, while biomitigation takes place (partial removal of nutrients and CO₂, supply of oxygen, and beneficial species interactions/controls). Some of the externalities of fed monoculture are internalized, hence increasing the overall sustainability, profitability and resilience of aquaculture farms.

A major rethinking is needed regarding the definition of an “aquaculture farm” (reinterpreting the notion of site-lease areas) and regarding how it works within an ecosystem, in a broader framework of Integrated Coastal Zone Management (ICZM). The economic values of the environmental/societal services of extractive species should be recognized and accounted for in the evaluation of the true value of these IMTA components. This would create economic incentives to encourage aquaculturists to further develop and implement IMTA. Seaweeds and invertebrates produced in IMTA systems should be considered as candidates for nutrient/carbon trading credits within the broader context of ecosystem goods and services.

Our research is also establishing appropriate performance measures regarding environmental mitigation by investigating the responses in wild species (microbial and higher trophic levels) inhabiting the surrounding environment to determine if they can be used as valid indicators of nutrient cycling for aquaculture operations. Measures of diversity, abundance, colonization rates and individual species health (e.g. growth, reproductive output, immune responses) are all potential indicators of how a farm may function with respect to nutrient loading. While organic loading has been associated with benthic impacts (e.g. anoxia and hydrogen sulfide release), there have also been occurrences of moderate enrichment, promoting localized increase in biodiversity and abundance of wild species, as a natural response to changes in nutrient availability and niche space utilization.

Changes in the rates and conditions under which these influences occur have the potential to provide direction for aquaculture management and improved IMTA farm design. Long-term planning/zoning promoting biomitigative solutions, such as IMTA, should become an integral part of coastal regulatory and management frameworks.