Letting “more big fish sink” doesn’t have the touted carbon benefits. Here’s the math.

A new paper in Science Advances, Marianai et al. 2020, makes their position clear right in the title – letting more big [pelagic] fish die naturally and sink to the bottom of the sea will benefit society by sequestering carbon in the deep ocean. It’s a tantalizing idea, but simplistic in its science and missing critical context. In reality, fishing large pelagic species for food has had a trivial effect on the global carbon cycle and is highly carbon-efficient relative to alternatives.

Marianai et al. calculate that fishing of large pelagic species from the global oceans has had a total carbon cost of 187.1 megatons of carbon (Mt C) from 1950 – 2014, which averages to 2.9 Mt C per year. Sounds big, but these are small numbers in a global context. For example, global anthropogenic carbon emissions from fossil fuels and land use change from 1980 through 2009 were, on average, 7766 Mt C per year, over 2,600 times higher than the total carbon cost of the fish (Bruhwiler et al. 2018). A more apples to apples comparison might be between fishing pelagic fish and other protein sources such as beef. Enteric fermentation emissions from beef (which is only part of the total carbon cost) produced in the United States alone have average annual emissions of 121.3 Mt C y^-1 (US EPA 2018). Compare that to total emission of 2.9 Mt C y^-1 from fishing large fish globally. Emission rates that small are just not very meaningful at the global scale.

That’s the carbon sources side. What about comparing the lost carbon sink to other carbon sinks? Fishing large pelagic species has prevented the sequestration of a total of 21.8 Mt C from 1950 through 2014 that otherwise would have occurred through the sinking of fish bodies into the deep sea. That is an average annual rate of 0.34 Mt C y^-1. This is just 1/5,578th of the net oceanic sink (0.0002 %; Bruhwiler et al. 2018) and, as the authors themselves point out, just 0.26% of the carbon sequestered in mangroves and seagrasses globally. Again, trivial at the global scale.

Of course, one could argue that “every little bit helps,” but at what cost? The species in this study are primarily caught for human consumption and protein substitution is inevitable if these fish were to leave the dinner table. The U.S. alone produces on average 11.8 Mt of beef (2000 – 2014, USDA). Thus, conservatively, there is 10.3 Mt C emitted for every Mt of beef produced. Compare that to 2.9 Mt C y^-1 for 5.0 Mt of large fish produced annually, or just 0.6 Mt C per Mt of fish. There is an enormous carbon benefit to eating wild fish over other common meats, especially beef.

All of the above takes the Marianai et al. numbers at face-value. However, their minimal consideration of the food web effects of fishing is simplistic, and excluding these effects likely leads to an overestimate of the foregone blue carbon sink from fishing these predators. This is mostly because of the classic “Eltonian Pyramid,” which describes how only a small fraction (~10%) of the energy at each trophic level is transferred to the next higher trophic level.