Although aquaculture’s current contribution to GHG emissions from food production is small, there is high likelihood of aquaculture expanding, given the human health benefits, and increasing social preferences for seafood. Therefore, it is critical to identify pathways to advance the growth of climate-friendly practices. Doing so provides an opportunity to avoid further environmental degradation associated with the expansion of food production.
Food production contributes significantly to climate change through both direct and indirect emissions of greenhouse gases (GHG). The GHG emissions per unit of protein produced by aquaculture generally compare favorably with most livestock production and some wild-caught fisheries, but considerable variability exists within each food type.
The lower emissions intensity of aquaculture is mostly attributable to a lack of direct GHG emissions from land use change and more favorable feed conversion ratios. Responsible development of aquaculture is a key strategy to meet growing food demand and nutritional needs and to achieve food security within planetary boundaries.
Responsible development of aquaculture is a key strategy to meet growing food demand and nutritional needs and to achieve food security within planetary boundaries.
Aquatic algae cultivation is dominated by the production of seaweeds in shallow to moderately deep coastal waters and, rarely, in offshore marine environments. As non-fed organisms that can be readily grown in a range of conditions and locations, seaweed mariculture often has fewer environmental impacts than other types of plant or animal food production.
Similar to seaweed cultivation, bivalve farming tends to have fewer environmental impacts than many other forms of food production and may provide positive ecological functions relevant to the health and resilience of marine environments.
As non-fed organisms that can be readily grown in a range of conditions and locations, seaweed mariculture often has fewer environmental impacts than other types of plant or animal food production, similar to seaweed cultivation, bivalve farming tends to have fewer environmental impacts than many other forms of food production and may provide positive ecological functions relevant to the health and resilience of marine environments.
Fed finfish production via mariculture is not yet a major contributor to total global aquaculture production, but the sector has comparatively large negative impacts on the marine environment and significant potential for future global expansion.
“Responsible development of aquaculture is a key strategy to meet growing food demand and nutritional needs and to achieve food security within planetary boundaries.”
In the present article, opportunities are explored for these three mariculture sectors to support climate change mitigation through climate-friendly design and operational practices that can lead to either avoided emissions (reducing the quantity of GHGs emitted) or enhanced carbon sequestration.
Mariculture’s greenhouse gas emissions footprint
GHG emissions from mariculture occur via many pathways, including upstream on-farm, and downstream emissions. Previous studies indicate that up or downstream activities contribute a considerable proportion of GHG emissions in mariculture, often more than on-farm operations, particularly when feed production is included as an upstream process.
With post-farm transport emissions excluded, the most emissionsintensive aspects of seaweed mariculture are usually on-farm activities, particularly electricity and fuel use, although there is variability across the studies in the activities included as a part of on-farm production.
Although seaweed mariculture may represent a comparably low emissions production opportunity, attention should be given to sources of energy for cultivation, especially given efforts to move or expand seaweed cultivation to potentially energy intensive, offshore environments.
Bivalve mariculture does not require feed inputs, which minimizes the associated land-based emissions from agricultural products. Consequently, bivalve mariculture is increasingly discussed as a sustainable, climate- friendly source of nutrientrich protein production for human consumption. Importantly, bivalve shell formation is a net source of CO2.
“The larger GHG footprint of fed finfish is commonly attributed to the emissions intensity of feed supply. Emissions from feed supply include crop agriculture and associated landuse change, wild-caught fish meal or oil, as well as feed processing and transport to farms.”
The expansion and intensification of fed finfish mariculture have considerably increased the acumulative nutrient load and subsequent eutrophication in coastal marine environments. Increased nitrogen and particulates in the water can lead to the loss or degradation of seagrass habitats below and adjacent to the farms.
This can result in GHG emissions through release of stored blue carbon in the plants and sediments below them and can reduce the capacity for future blue carbon sequestration.
Opportunities for climate-friendly mariculture
Unless low-emissions alternatives to fossil fuels can be readily adopted, the potential socioecological benefits of large, offshore mariculture development could be diminished by a need to increase fuel use to enable distant production at scale.
“However, changes in a country’s energy portfolio and the market forces driving the availability and affordability of biofuels are likely to occur at a national or regional level, with single farm operators having little control over these overarching drivers of on-farm GHG emissions.”
Site selection for coastal fed finfish mariculture should exclude areas of seagrass and other sensitive blue carbon habitats where possible, although complete avoidance may not be practical in some regions, due the widespread distribution and seasonal variations in the presence and density of seagrasses.
The method used to harvest mature bivalves has profound impacts on local benthic disturbance, seagrass cover and, therefore, blue carbon burial and storage. Manual harvesting of raised mariculture is the practice least likely to disturb seagrass and buried carbon. Raised culture also avoids the direct competition with seagrass for space and reduces sediment resuspension compared with on-bottom culture.
This both stabilizes sediment to allow seagrass recruitment and enhances or prolongs carbon storage by reducing oxidation of subsurface sediments. Opportunities for avoided emissions and emissions offsets in seaweed mariculture.
Although biomass yields from seaweed mariculture can be very high, often greater than those from terrestrial crops, variability in the marine environment around farm sites affects productivity. This, in turn, an significantly affect production efficiencies and GHG emissions, as well as the potential for negative interactions with the marine environment.
“Emerging markets for climate-friendly, nonfood seaweed bio products provide an opportunity to realize GHG emissions offsets from seaweed mariculture.”
The production of seaweed-based biochar for use as a soil improver can also indirectly support climate change mitigation and offsets through agricultural soil improvement, because it contains recalcitrant carbon that facilitates long-term soil carbon sequestration.
Potential for carbon sequestration through seaweed mariculture. Natural seaweed habitats are typically associated with hard substrates, as opposed to soft sediment and, therefore, have lower inherent potential for direct transfer and sequestration of carbon to the sediment than blue carbon habitats.
“However, naturally growing seaweeds do donate organic carbon in the form of detritus to nearby receiver blue carbon habitats, where the material is trapped and buried in the sediment.”
The transfer of organic carbon to receiver habitats (both deep sea and shallow blue carbon environments) can also occur from seaweed mariculture farms. Carbon from seaweed mariculture may also be moved indirectly into nearby coastal sediments through grazing organisms that consume biomass at the farm and move into neighboring marine ecosystems, although, again, the magnitude of this transfer and its ultimate impact on carbon sequestration are currently unknown.
The intentional farming of seaweed as a means to capture and sequester anthropogenic CO2 could function in a similar way to carbon farming initiatives on land. This approach would rely on a non-harvest mariculture model, where biomass is either retained in situ or allowed to sink to the deep sea where the carbon can be sequestered for long periods of time.
If all the organic carbon from farmed seaweed was sequestered and the seaweed farming operations were carbon neutral; global seaweed mariculture could sequester between 0.05 and 0.29 Gt of CO2e per year.
Is carbon sequestration achievable through bivalve mariculture?
Because bivalve shell formation and respiration are a net source of CO2 from sea to atmosphere, the potential for bivalve monocultures to directly sequester carbon is limited.
Seaweed primary production is usually carbon limited, but when grown close to bivalve mariculture, CO2 released by the bivalves can enhance seaweed photosynthesis. This in turn releases oxygen and improves conditions for bivalve cultivation, with an optimal ratio for carbon capture by seaweeds of 4:1. Of course, the capacity of such cocultures to truly sequester carbon depends on the fate of the harvested bivalve shells.
The influence of bivalve mariculture on blue carbon habitats, whether positive or negative, will be mediated by environmental setting, hydrodynamics and farm design adopting mariculture designs that promote the regulating services and boost seagrass performance is necessary if bivalve mariculture is to indirectly enhance blue carbon sequestration as well as reducing potential for environmental GHG emissions.
Is carbon sequestration achievable through fed finfish mariculture?
There is evidence of sediment accumulation and organic carbon enrichment under fed finfish mariculture net pens. However, some studies suggest that organic carbon accumulated in surface sediments under sea cages is highly labile, with increased carbon turnover rates and returns to baseline levels after fallowing.
This implies that organic carbon enrichment under sea cages is not a feasible mechanism for long-term carbon sequestration.
By linking the provision of food from mariculture to broader environmental benefits, such as GHG abatement, our study can support the development of climate-friendly mariculture practices that generate sustainable ecological, social, and economic outcomes.
Considering the projected global reliance on mariculture for food production into the future and the industry’s persistently high growth rate, sustainable intensification and the broadscale adoption of the Ecosystem Approach to Aquaculture will be critical to mitigating the climate impacts of a scaleup in mariculture production.
This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “CLIMATE-FRIENDLY SEAFOOD: THE POTENTIAL FOR EMISSIONS REDUCTION AND CARBON CAPTURE IN MARINE AQUACULTURE” developed by: ALICE R. JONES – Oxford University, HEIDI K. ALLEWAY – Oxford University, DOMINIC MCAFEE – Oxford University, PATRICK REIS-SANTOS – Oxford University, SETH J. THEUERKAUF Oxford University, AND ROBERT C. JONES – Oxford University.
The original article was published on JANUARY 2022, through OXFORD ACADEMIC – BIOSCIENCE under the use of a creative commons open access license.
The full version can be accessed freely online through this link: https://doi.org/10.1093/biosci/biab126