How to better recycle nutrients in aquaculture: Indicators and application to three integrated systems

Faced with growing environmental challenges, nutrient circularity (a concept derived from the circular economy) is emerging as an essential lever for reducing losses and dependence on synthetic inputs (nutrients, fertilisers, etc.) in agriculture and aquaculture.

Researchers at Wageningen University and INRAE's SAS joint research unit have set themselves the goal of proposing a set of quantitative indicators to assess the level of nutrient circularity in different aquaculture systems. A review of the literature has made it possible to:

1. Define six key performance criteria for aquaculture systems that depend on the level of nutrient circularity: productivity, efficiency, self-sufficiency, recycling, ecosystem regeneration, diversity and complementarity.

2. Propose 21 simple indicators to quantify these criteria (such as yield, nutrient losses, or the use of co-products in feed).

These indicators were then used to measure the level of nutrient circularity in three experimental integrated aquaculture systems: an aquaponics system, a biofloc system, and a polyculture pond system, each compared to its monoculture equivalent.

• The aquaponics system (tilapia + tomatoes) improves productivity and efficiency compared to the recirculating system without plants, but shows a low capacity for nutrient recycling by plants.

• The biofloc system (tilapia + microorganisms), which uses a microbial community to convert nitrogenous waste into edible biomass for fish, outperforms the clear water system in terms of productivity, efficiency, recycling (60% bioremediation* of nitrogen) and self-sufficiency, although it only produces a single commercial species.

• The polyculture pond system (carp + rohu, a fish in demand in Asia) doubles yields compared to monoculture, with a marked improvement in efficiency and a slight increase in recycling and nutrient self-sufficiency, thanks to the dietary complementarity of the species.

All integrated systems outperformed conventional systems on at least one important criterion. There are therefore different paths to circularity. However, there is often a lack of data to calculate certain indicators, and the authors of this study call for greater transparency in aquaculture practices (feed composition, outputs, energy expenditure, etc.). This study does not recommend a single model, but shows that the principles of circularity must be adapted to each context, taking into account available resources, local constraints and economic opportunities.

This study proposes a simple toolkit for measuring and improving nutrient utilisation rates in aquaculture. The results show that integrated aquaculture is promising, provided that the most complementary species in terms of the food web are selected and that farming techniques are mastered, including close monitoring of farming parameters. The prospects are to extend this assessment to other types of systems, such as integrated multi-trophic aquaculture (IMTA), and to integrate these indicators into sustainability assessment methods, including social, economic and environmental aspects.

* Bioremediation involves using micro-organisms to break down or metabolise organic contaminants present in soil, water, sludge and solids.