Polyculture and Integrated Multi-Trophic Aquaculture (IMTA)
How polyculture combines species with complementary feeding niches, and how IMTA pairs fed species with extractive species so wastes become inputs, recycling nutrients and cutting impact.
Overview
Polyculture and integrated multi-trophic aquaculture (IMTA) both raise more than one species together to use resources more fully. Polyculture focuses on combining species with complementary feeding niches in the same water; IMTA goes further by deliberately pairing fed species with species that extract and recycle their wastes.
Polyculture
In polyculture, a pond holds one main species plus others with different feeding habits so that all the available natural food is used and water quality is maintained. The classic example is Chinese major carp polyculture, practiced since the Tang Dynasty: silver carp filter zooplankton and phytoplankton, bighead carp take plankton, grass carp eat vegetation and common carp feed on the bottom, occupying complementary niches. FAO cites an experimental stocking ratio of 5 silver carp, 1 bighead carp, 3 grass carp and 1 common carp. The giant freshwater prawn is also often grown in polyculture with carps.
Integrated Multi-Trophic Aquaculture (IMTA)
IMTA combines, in appropriate proportions, fed species with extractive species so that the by-products (wastes) of one become inputs for another. NOAA and FAO describe three functional groups working together across trophic levels.
- Fed species — finfish or shrimp that receive feed and produce waste.
- Organic extractive species — suspension and deposit feeders such as bivalve shellfish and sea cucumbers that consume particulate organic waste.
- Inorganic extractive species — seaweeds and other macroalgae that take up dissolved nutrients.
A widely cited marine example is salmon farmed with mussels and kelp. China has practiced IMTA at scale since after 2000 with combinations of fed fish or shrimp, shellfish, sea urchins, sea cucumbers and seaweeds, and Indonesia uses double-net cage systems holding grouper, shrimp, rabbitfish, sea cucumbers and seaweed in different sections.
Benefits
- Nutrient recycling — uneaten feed and excreted waste feed the extractive species instead of polluting the water.
- Reduced environmental impact — lower nutrient release to surrounding waters.
- Diversified products and income — several marketable crops from one operation.
- Mimics natural ecosystem cycling, supporting a more circular, sustainable production model.
Challenges
IMTA requires the species and their proportions to be matched so the extractive species can actually use the waste produced; site conditions, water flow, harvest timing and markets for each crop all add complexity. NOAA notes IMTA is still an evolving approach in the United States and Canada, although it is well established in China.