Fisheries and the Global Carbon Cycle

In a previous post in this blog, we touched on how bottom trawling was able to remobilise carbon stored in marine sediments and their potential role in exacerbating climate change [1]. I would like for us to consider if there are other ways in which the fishing industry has disturbed the ocean’s carbon cycle. Let us look deeper into how fish and marine animals can play a dynamic role in the sequestration of carbon – and consequently, how fisheries undermine that role.

Fish and Biogeochemical Cycles

Although limited, there have been recent research studies made on the relationship between fish biomass and carbon cycles. A study by Bianchi et al. in 2021 had indicated that fish communities were able to cycle biomass (by consuming primary production fixed by phytoplankton and releasing metabolic and organic waste as illustrated in Figure 1 below) at significant rates in areas of high fish abundance [2]. These areas can be found along the coastlines of continental shelves where productivity is highest [3]. It has been found that such organic waste particles from fish can sequester carbon for up to 600 years, especially at depths below 1000 m when the rate of sinking is at its peak [2,4].

Figure 1: Illustration of biomass cycling via respiration, organic waste particle production and diel vertical migrations (DVM) during preindustrial times [2]

Impact of Fishing Industry on Carbon Sequestration

Now that we are aware of the importance of fish in cycling and sequestering carbon, we need to consider how the fishing industry can negatively impact these engines of the oceanic carbon cycle.

Bianchi et al. also investigated how the fishing industry can significantly alter biogeochemical cycles in the ocean due to reduced fish biomass to cycle oxygen and export carbon [2]. As seen from Figure 2 below, fish biomass has reduced to as much as 20% of preindustrial times, especially in areas with high initial biomass, hence indicating heavy exploitation of productive ecosystems.

Figure 1: (A) Preindustrial biomass of fish, (B) fish biomass during global peak catch, and (C) percentage of biomass left relative to preindustrial biomass [2]

As biomass is reduced greatly from preindustrial times, fish are less able to carry out their roles in cycling carbon by consuming phytoplankton and producing fecal pellets. Carbon sequestration becomes especially hindered when fish species that live near the seabed as the quickest source of sinking fecal pellets are extensively harvested. Additionally, due to the oceans’ biological pumps that transport carbon through the mesopelagic zone and prevents extensive mixing of carbon-rich waters up the water column, the potential exploitation of mesopelagic fish may lead to greater reductions in carbon sequestration [5,6]. Besides fish, the removal of large marine mammals such as whales through whaling (as a subsection of the fishing industry) have also been noted to reduce export and sequestration of carbon in marine sediments [7].

Details are in the caption following the image — Figure 3: Referring to relevant points only, fishing directly impacts carbon cycle by removing (3) pellet-producing species, (4) fish near the seabed, and (7) large fish and marine mammals that should produce dead organic matter [7].

Overall, more extensive research should be carried out moving forward to fully establish how marine animals act as drivers of the ocean’s biogeochemical cycles. This can be achieved by establishing more robust links between geochemical models and marine ecological studies. In doing so, it will be easier to place fishing activities as a disruptive force of such cycles.

References:

[1] Sala E, Mayorga J, Bradley D, Cabral RB, Atwood TB, Auber A, et al. Protecting the Global Ocean for biodiversity, Food and Climate. Nature. 2021 Mar 17;592:397–402.

[2] Bianchi D, Carozza DA, Galbraith ED, Guiet J, DeVries T. Estimating global biomass and biogeochemical cycling of marine fish with and without fishing. Science Advances. 2021 Oct 8;7(41).

[3] Atwood TB, Witt A, Mayorga J, Hammill E, Sala E. Global Patterns in Marine Sediment Carbon Stocks. Frontiers in Marine Science. 2020 Mar 25;7.

[4] Boyd PW, Claustre H, Levy M, Siegel DA, Weber T. Multi-faceted particle pumps drive carbon sequestration in the ocean. Nature. 2019 Apr;568(7752):327–35.

[5] Cavan EL, Laurenceau-Cornec EC, Bressac M, Boyd PW. Exploring the ecology of the mesopelagic biological pump. Progress in Oceanography. 2019 Sep;176:102125.

[6] Davison PC, Checkley DM, Koslow JA, Barlow J. Carbon export mediated by mesopelagic fishes in the northeast Pacific Ocean. Progress in Oceanography. 2013 Sep;116:14–30.

[7] Cavan EL, Hill SL. Commercial fishery disturbance of the global ocean biological carbon sink. Global Change Biology. 2021 Dec 18;28(4):1212–21.

Fisheries and the Global Carbon Cycle

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