Mitigating Nitrous Oxide Production in Aquaculture Facilities

Aquaculture ponds along the south shore of Donghai Island, Guangdong, east of Guitou Village. (Credit: Vmenkov via Wikimedia Commons CC BY-SA 3.0)

As the world turns its focus to refining aquaculture to meet global food demands sustainably, the environmental impact of aquaculture cannot be ignored. In particular, emissions of nitrous oxide and other greenhouse gases from aquaculture facilities can worsen climate change due to their potent ozone depletion potential.

As a result, determining the causes and behaviors of nitrous oxide from aquacultural sources is a key component to improving aquacultural practices to protect the natural environment and still yield high product returns for operators.

Case Study: Nitrous Oxide Origins in Chinese Aquaculture

A 2024 study published in Water Research X examined the origins of nitrous oxide in fish farms by examining the water column and sediment in three different farms in China. One of the farms was for Japanese seabass, the second for giant river prawn, and the last was a whiteleg shrimp farm.

There were two key objectives of the study:

1. Using nitrogen isotope tracer experiments, the study sought to determine the differences in production across the three aquaculture ponds.
2. The primary source of nitrous oxide (water column vs. sediment) will be identified, and the relative contributions of nitrification and denitrification to total emissions will be determined.

Water samples were collected at mid-depth using a YSI EXO2, which measured dissolved oxygen (DO), temperature, pH, salinity, turbidity, chlorophyll a, and total dissolved solids, as well as grab samples for lab analysis. Sediment samples were collected from the uppermost 5 cm of the sediment and also subjected to lab testing.

Lab analysis included the nitrogen isotope tracer incubation experiment, which can directly measure the rate and process of nitrous oxide production.

Results of the study found production was potentially regulated by feeding amount, stocking density, and species behavior. These factors can trigger elevated organic carbon and stimulate nitrification and denitrification, which are likely to impact the resident microbial community, leading to increased emissions.

Production varied between the ponds, with whiteleg shrimp showing the greatest combined sedimentary and water column nitrous oxide production, followed by Japanese seabass and giant river prawn.

Separately, sedimentary production rates were much higher than water column rate in Japanese seabass and whiteleg shrimp ponds, likely because of uneaten feed that sunk to the bottom of the pond and decomposed, producing dissolved inorganic nitrogen and, therefore, nitrous oxide.

In contrast, the giant river prawn site saw greater production in the water column due to the aquaculture facility aerating the sediment and inhibiting the activity of denitrifying bacteria.

Conclusion

Using the results from each pond, the article proposes a few practical solutions for aquaculture managers to implement in hopes of mitigating emissions.

Bottom aeration and plant-prawn coculture can effectively reduce nutrient accumulation and increase oxygen levels, which in turn reduces sedimentary nitrous oxide production and carbon emissions by burying organic carbon.

Overall, a better understanding of nitrous oxide production pathways in aquaculture settings will help improve fish farming practices that are more sustainable and still produce enough fish to meet global nutrition needs.