Anyone interested in sustainable food systems should be familiar with the First Nations’ “three sisters” farming method, which leverages synergies among maize, beans and squash. It’s often viewed as the archetype of polyculture.
Allow me to introduce a relatively new form of polyculture: the “three cousins” system for cultivating salmon, mussels and kelp. In 2004, a Canadian research project called AquaNet gave it a far less poetic name: IMTA (Integrated Multi Trophic Aquaculture).
Many believe IMTA could resolve the adverse impacts of salmon farming on marine ecosystems. Salmon farming is classified as monoculture because only one species is harvested. Typically monocultures suffer from unsustainable nutrient deficits, however, the issue with salmon farming is that they add nutrients to the ecosystem.
Is it possible to have too much of a good thing? Definitely! For decades, there have been concerns over excess salmon feed contributing to coastal eutrophication. Just Economics estimated this harm caused $29 million in damages to Canadian ecosystems in 2019.
One of the main contributors to early IMTA research, Thierry Chopin, argues that “the solution to nutrification is not dilution but conversion”. By uniting the “three cousins”, the Canadian salmon farming industry would be transformed from a throughput to a circular economy. Once farmers implemented IMTA technology, they would not only be absorbing the negative externality of nutrient waste, they would actually be profiting from it.
I swear it isn’t witchcraft, but something more magical: ecosystem services. Bivalves (such as mussels and scallops) are filter feeders. By placing mussel rafts around the salmon cage, they act as a buffer between the farm and the surrounding ecosystem. The mussels are fed by the excess nutrients from the salmon. A 2012 study found that mussels grown next to salmon cages are meatier than mussels farmed apart from salmon cages. Mussel and salmon farming are both prominent aquaculture sectors. It’s as simple as placing two already-existing aquaculture technologies side-by-side. The addition of kelp to the system provides another filter, via “nutrient scrubbing.” While there is no traditional market for kelp in North America, kelp demand is expected to increase.
The most daunting barrier to commercial IMTA implementation is the operational complexity, however supporters of IMTA push that these transition costs would be repaid with new revenue streams. Fish farmers should think of IMTA as an opportunity to diversify their investment portfolio.
Bottom line: Polyculture has real promise. Unite the three cousins. We’ll all be better for it.
* Please help my Environmental Economics students by commenting on unclear analysis, alternative perspectives, better data sources, or maybe just saying something nice :).