The Biogeochemistry of Natural Climate Solutions Based on Fish, Fisheries, and Marine Mammals: A Review of Current Evidence, Research Needs, and Critical Assessment of Readiness

Several initiatives to conserve, restore or better manage fisheries, fishes, whales, and other marine animals have been proposed as natural climate solutions to sequester carbon from the atmosphere or avoid new emissions. We reviewed the knowledge and uncertainties surrounding carbon fluxes and storage mediated by these organisms to evaluate their suitability to support climate mitigation interventions. Estimates of the carbon stored within fish and marine mammal biomass ranged from 0.1 to 1.9 Pg C for mesopelagic fishes, 0.7–0.6 Pg C for all fishes, 0.0020–0.016 Pg C for great whales, and 0.0065–0.0113 Pg C for all marine mammals, compared to an estimated 1.5–6 Pg C stored in all ocean biota. Mesopelagic fishes, epipelagic fishes and great whales contribute on the order of 1–3 Pg C yr⁻¹, 0.03–0.06 Pg C yr⁻¹, and 0.001–0.004 Pg C yr⁻¹, respectively, to export from the ocean's surface below the euphotic zone, compared to an estimated total marine biological export of 9–10 Pg C yr⁻¹. The combined flux of carbon to the atmosphere from benthic trawling, biomass extraction, and fuel consumption during commercial fishing ranged from 0.05 to 0.25 Pg C yr⁻¹. Substantial uncertainties were associated with nearly all fluxes and reservoirs. The contributions of whales to carbon export and the mobilization of sediment carbon during benthic trawling were least certain, limiting the readiness of associated pathways to provide quantifiable, high-quality carbon credits. Although substantial uncertainties also surrounded mesopelagic fishes, we found that even conservative estimates of these organisms' contribution to ocean carbon export are large enough to justify conservation actions.