Estimates of carbon stored in living marine biota are highly uncertain and estimates are wide ranging. As efforts to harness and amplify the carbon cycle in the ocean for CDR grow, some attention has been given to the role marine animals may play in sequestering carbon durably.  This road map will focus on whales and fishes, but for a more comprehensive report on marine biota, see The Environmental Defense Fund’s 2022 report “Natural Climate Solutions in the Open Ocean”.

Whales are thought to play a role in the carbon cycle both directly (living biomass and whale falls) and indirectly (the whale pump, the whale conveyor belt, and bioturbation), however the extent of this role remains uncertain(Meynecke et al., 2023). A 2022 report from The Environmental Defense Fund (Collins et al., 2022)  found there to be fewer than 10 scientific studies from which to draw any broad conclusions in this area and cautioned against the creation of a carbon credit system based on whale carbon sequestration. A 2020 study estimates that restoring baleen whale populations to pre-whaling abundances could sequester 0.032 Gt CO2/year, including carbon sequestered from fertilization via whale feces(Dufort et al., 2020). However, rebuilding whale populations is sure to elicit many other environmental and social benefits, explored further on in this road map.

Mechanisms for CDR

Direct Sequestration

• Living Biomass: Due to their size and longevity, whales can store large amounts of carbon for long periods of time in their bodies as biomass. While whaling indeed resulted in a loss of carbon stored in whales, there is not clear evidence to suggest that whaling was a net source of CO2 to the atmosphere (Heidi et al., 2023).
• Whale Falls: Whale falls, wherein whale carcasses sink to the ocean floor and their carbon-rich biomass is reallocated in various ways, is the “most scientifically comprehensible” of the pathways (direct vs indirect sequestration) (Collins et al., 2022) with a 2010 study estimating that restoration of large baleen whale populations to pre-whaling numbers could sequester 0.0006 Gt CO2e /year.

Indirect Sequestration

• Whale Pump: This is the term often used to describe the role that whales play in cycling nutrients through the ocean via their feces. The whale pump hypothesis sits on two assumptions that have not yet been verified through empirical observations (Collins et al., 2022). The first assumption is around the efficacy of surface water fertilization and subsequent primary production via whale feces. The second assumption concerns the fate of carbon export from primary production.
  • WhaleX Ocean Nourishment synthesizes limiting nutrients (nitrogen, phosphorous, iron, and silica) and disperses them across nutrient-poor regions of the ocean, simulating the dispersion of nutrients via whale feces.
• Whale Conveyor Belt: The whale conveyor belt is based on the idea that as whales migrate yearly, they move nutrients around the ocean, potentially increasing iron availability for phytoplankton growth (Roman et al., 2014).
• Bioturbation: Bioturbation is the process through which whale feeding behaviors stimulate the seabed and resuspend sediments (Meynecke et al., 2023). Conceptually, this is similar to the whale pump, but the pool of nutrients comes from seafloor sediments instead of whale feces (Nelson & Johnson, 1987; Alter et al., 2007).

Rebuilding Whale Populations: Increasing whale populations would likely amplify their role in the carbon cycle, including any sequestration.

• Increasing Krill Abundance:
  • A 2022 article by Savoca suggests krill as a limiting factor to whale population growth. Savoca points to studies that suggest a close relationship between whales and krill wherein whales provide necessary nutrients for krill to thrive. This is further complicated by competition between krill fisheries and whales in the Southern Ocean.

Reducing Ship Strike: A 2001 study found that in some areas, more than one-third of all fin whale and right whale strandings involve ship strike. This can especially impact small populations of whales such as the northern right whale in the western North Atlantic (Laist et al., 2001). Measures that require large, motorized vessels to slow their speeds in particularly vulnerable areas may be beneficial to whale populations.

CDR Potential

Estimated Sequestration Potential:  0.032 Gt CO2e /year, including carbon sequestered from fertilization(Dufort et al., 2021)

Sequestration Durability: 10 – 100 years (Cross et al, 2023)

Challenges

• Difficult to find and track outcomes from whale fertilization events (e.g., feces)
• Tracking the fate of carbon through the water column

Key Knowledge Gaps

Adapted from Pearson et al. 2022

• How effective are whales at creating and maintaining primary production hotspots that lead to enhanced carbon sequestration?
• How bioavailable are whale-derived nutrients?
• What is the carbon flux from cetaceans to the atmosphere?

First-Order Priorities

*Adapted from Pearson et al. 2022

• Investigate primary production hot spots from whales via determination of fecal nutrients to carbon recycling versus export*
• Conduct additional field and laboratory studies to ascertain the bioavailability of krill-derived iron in whale feces*
• Measure the carbon flux from cetaceans to the atmosphere via respiration to understand cetaceans’ net capacity for CO2 removal*

While fishes have not traditionally been considered as important parts of the ocean carbon cycle / biological pump (Sarmiento & Gruber, 2006), there are several recent studies, as highlighted in the 2022 Environmental Defense Fund report “Natural Climate Solutions in the Open Ocean”, that focus on the geochemical contributions of fishes to the global carbon cycle (Wilson et al., 2009; Boyd et al., 2019; Mariani et al., 2020; Bianchi et al., 2021; Saba et al., 2021). Many of the mechanisms by which fish are thought to contribute to carbon cycling are similar to whales (carbon stored in living biomass, bioturbation, carcasses falling to ocean floor, fertilization through feces) but also include contribution by intestinal precipitation of calcium carbonates (Wilson et al., 2009).

There is still uncertainty around the specifics of mesopelagic fish-mediated carbon export, however, the 2022 Environmental Defense Fund report suggests there is enough evidence of these species’ large contributions to global sequestration to warrant “the immediate pursuit of limitations or prohibitions on their harvest”.

Mechanism for CDR

Limitations or prohibitions on harvest may increase the living biomass of fish, inducing higher natural long-term carbon sequestration by increasing carcass deadfall. Previous studies that evaluate the effectiveness of reducing fishing and establishing marine protected areas to address climate change suggest that these efforts support climate adaptation (Gattuso et al., 2018).

CDR Potential

Estimated Sequestration Potential: Unclear, due to large knowledge gaps around biomass estimates of marine fish stocks, uncertainties in direct and indirect carbon export and overall carbon fluxes, few studies observing natural senescence / dead falls of fishes, and a poor understanding of the life cycle emissions of most fish species (Collins et al., 2022).

Sequestration Durability: Unknown.

Challenges

• More research is needed to understand carbon fluxes and life cycle emissions before developing strategies to increase carbon uptake and sequestration.

Key Knowledge Gaps

Adapted from EDF Natural Climate Solutions in the Open Ocean

• Lack of knowledge around the natural senescence or resulting deadfalls of fishes
• What is the contribution of fishes to inorganic carbon export via carbonate excretion?
• What is the total biomass of fishes, the fluxes in these communities, and conversion factors to go from biomass to units of carbon?
• How to discern between active carbon transport from mesopelagic fish versus from zooplankton?
• What is the composition of mesopelagic communities and what are the differences in roles played by small fishes versus other species such as cephalopods?

First-Order Priorities

*Adapted from EDF Natural Climate Solutions in the Open Ocean

• Conduct studies observing the natural senescence or resulting deadfalls of fishes*
• Conduct additional studies to understand the contribution of fishes to inorganic carbon export via carbonate excretion (Salter et al., 2019)*
• Create new models, observing technologies, and data to better constrain the total biomass of fishes, the fluxes in these communities, and conversion factors to go from biomass to units of carbon*
• Create new observational methods and models to better discern between active carbon transport from mesopelagic fish versus from zooplankton*
• Conduct studies to better understand the composition of mesopelagic communities to determine the difference in roles played by small fishes versus other species such as cephalopods*

• Nutrient cycling (Ratnarajah et al., 2014)
• Restoring the role of marine organisms in the carbon cycle (NASEM, 2022)

In contrast to other proposed ocean CDR pathways, there are fewer apparent environmental risks to utilizing animal carbon for carbon dioxide removal. This is in part due to the fact that sequestration pathways largely rely on restoration efforts which typically benefit local and surrounding ecosystems. However, proper restoration techniques that acknowledge the complex nature of ecosystems is critical to ensuring no or limited negative environmental impacts. As with any CDR pathway, incremental scale-up with careful monitoring and verification of impacts will be critical to mitigate risks.

Restoration of marine animal populations may generate many benefits for surrounding communities and economies. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. Many of the environmental benefits relate directly to social benefits, such as fisheries enhancement and coastal protection. (Note that there are significantly more co-benefits in “blue carbon” than any of the other CDR Road Maps.)

• Potential for economic stimulation and job creation from enhanced tourism (e.g., whale watching – of particular value to coastal communities that are dependent on marine-based livelihoods)
• Cultural / intrinsic / recreational value
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Efforts around restoration efforts are often met with social acceptance and public support, especially when compared to other climate solutions seen as “tampering with nature” (Wolske et al., 2019). This may be in part due to the minimal risks (real and perceived) associated with restoration efforts. While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for negative economic impacts to some industries such as oil and gas, fisheries, and mining (NASEM, 2022)
• Stricter fisheries management to increase species abundances can cause impacts on food security, displacement, and economic strain. (for example, Marine Protected Areas can cause varied impacts among communities and social groups and can be met with opposition) (Mascia et al., 2010).