Last month I was intrigued by — and very much enjoyed — a joint study between the University of Adelaide and the Nature Conservancy, which assessed how GHG emissions from finfish, seaweed and bivalve aquaculture could be mitigated. The study — Climate-Friendly Seafood: The Potential for Emissions Reduction and Carbon Capture in Marine Aquaculture — gives a good insight into the development of climate-friendly aquaculture practices and the issues that we need to prioritise and address. 

When it comes to our food and issues such as climate change and carbon footprint, discussions tend to focus on meat, or corn, or soy. But what about fish and seafood? Can these be climate-friendly food alternatives? Some of us may think so, but both wild-caught and farmed fish generate greenhouse gas (GHG) emissions, with impacts varying widely depending on what the fish eat, how they are caught or farmed, and how they make their way to our plates. We talk a lot about sustainable aquaculture or responsible aquaculture, but these can be quite loose terms that don’t always carry a lot of meaning or specificity. Emissions and climate-friendly seafood are much more specific, which is why this particular study caught my eye. 

Climate-Friendly Seafood

According to the study, aquaculture produces GHG emissions in many ways, for example through feed production or transportation. Finfish aquaculture has the highest GHG footprint, far more than seaweed and bivalve aquaculture. Seaweed has the lowest footprint because production involves limited infrastructure and mechanisation and is closer to shore, therefore lower on farm transport and maintenance emissions. However, on-farm activities such as using electricity and fuel could be classified as emissions-intensive. The study says that attention should be paid to the sources of energy for seaweed cultivation and any change in farm location, such as moving or expanding to offshore environments that could be energy-intensive. 

Bivalve aquaculture doesn’t require feed inputs, so emissions can be minimised. Like seaweed, energy and fuel use on a bivalve farm are the two areas that could potentially contribute to increased emissions, depending on the production system. 

Finfish aquaculture has the highest GHG footprint because of the emissions intensity from feed supply, including wild-caught fishmeal or fish oil, feed processing, transportation of feed to farms and crop agriculture, deforestation or fertiliser use if fishmeal alternatives are used. The fuel used on boats that catch wild-caught fish is also a big factor. Feed is undoubtedly the number one culprit for GHG emissions when it comes to finfish aquaculture, but in order to understand the impact of these emissions, we need to know what kind of feed is used and how efficiently fish convert food into growth. And, finally, feed aside, the location where products are processed can also increase GHG emissions. Shipping something to be processed in another country, or importing a product for sale, raises fuel and energy consumption significantly. The result? Higher emissions. 

Reducing GHG Emissions

One way for aquaculture to reduce GHG emissions is to switch to low-emissions energy sources, biofuels and sustainable building materials. For example, finfish aquaculture could switch from diesel oil to natural gas. This has been shown to reduce GHG nitrous oxide emissions from farmed salmon by 85% and CO₂ emissions by 20%, according to the study. Site selection is also key to limiting GHG emissions. Seagrass and other sensitive habitats should be avoided as much as possible, but if this is difficult, regularly shifting the location of infrastructure within the broader farm area could be a step to take. Farmers could also shift to species that require less feed, or change the composition of feed to reduce eutrophication. 

For bivalve aquaculture, manual harvesting is said to be the least likely to disturb seagrass, while the volume of carbon-rich shell waste could also be taken into account. Rather than discarding bivalve shells in landfills, where they eventually release stored carbon into the environment, upcycling and turning them into calcium carbonate or calcium oxide can provide a cheap, abundant and sustainable resource.

Focusing on climate-friendly farm designs, farm siting and species choices can all help to support reductions in GHG emissions in seaweed aquaculture, while the use of farmed seaweed in future feed or animal feed products could achieve a net reduction in GHG emissions compared to current feed sources, or provide a functional food value such as improving fish health and therefore efficiency in production. There are also opportunities to reduce indirect GHG releases, such as the release of CO₂ associated with calcification. Farming seaweed with bivalves could lead to a net reduction in CO₂ emissions, while cofarming finfish with seaweed or bivalves absorbs excess nutrients, reducing eutrophication. 

Moving Forward

Aquaculture needs to be part of the climate change solution. Sustainable aquaculture or responsible aquaculture are well-established concepts, but perhaps we should also consider low GHG aquaculture. Policy shifts, international cooperation, improved technology and assistance for fish farmers are all necessary changes if aquaculture is to continue improving. Farmers must show that their products and production systems fit into a more environmentally-friendly world, while consumers can continue to signal that they want sustainably-caught, low carbon fish, not just to help fight climate change but also to help preserve the fish that they want to see on their plates. 

As aquaculture increases in prevalence and importance, so too do its climate impacts, but multitrophic aquaculture, such as farming oysters or mussels together with seaweed, can be far more climate friendly and help combat climate change. Not only does this help in terms of ocean acidification, but it also provides a little bit of a carbon sink and sequesters some of the local carbon, tempers the surrounding acidity and produces a better shellfish product. Seaweed creates oxygen pockets that shellfish can thrive in, and also draws out some of the excess carbon in the ocean, which helps mitigate the effects of climate change.

As aquaculture continues to expand, intensify and diversify, we must recognise the relevant environmental and social concerns such as impacts from feed production, water pollution, antimicrobial resistance and competition for land and water, and take steps to address them.