Plenty More Fish in the Sea?

A new model of the world’s oceanic life suggests that the impact of fishing on biomass and the biogeochemical roles of fish has been as detrimental as the effects of anthropogenic climate change. Daniele Bianchi, University of California, Los Angeles, USA, and colleagues have estimated the natural global biomass of fish, and demonstrated how it had fallen by almost half by the 1990s. This had deleterious effects on oxygen consumption and natural carbon storage at depth. The amount of fish, or the amount of fecal matter deposited by the fish, can have an impact on carbon storage and thus climate change.

Historical Reduction of Fish Biomass

The researchers suggest that fish have not previously been considered a necessary variable in climate change calculations on a global scale. Indeed, many models consider phytoplankton, zooplankton, and bacteria—the vast, almost invisible masses of oceanic life—as critical to our calculations, but mostly ignore larger organisms.

And yet, the team has shown that the global biomass of fish that have a mass of between 10 g and 100 kg amounted to 3.3 billion tonnes, plus or minus half a gigatonne, prior to human large-scale fishing. This is the mass range that is essentially harvested from the oceans for human use, although much of it is simply waste, and includes invertebrates such as squid which are caught and used analogously to fish although are taxonomically not in that clade. The 10 g threshold excludes the larval stage of fish and other tiny sea creatures.

“By the 1990s, biomass and cycling rates had been reduced by nearly half,” the team writes. “Our results highlight the importance of developing a better mechanistic understanding of how fish alter ocean biogeochemistry.” The global biomass of fish, its change caused by fishing, and the role of these diverse sea creatures on the ocean’s biological pump (the nutrient dynamics and storage and release of carbon) and the chemistry of seawater might, on the mass scale of billions of tonnes, have a significant effect globally.

An Improved Model

Previous models have suggested that the biomass of fish may have ranged from one gigatonne to fifty gigatonnes based on a wide variety of parameters and proxies. This new work offers a more unified approach to modeling mass that combines what the team describes as “diverse constraints to provide more robust estimates of the global biomass and biogeochemical cycling rates of fish.” This data-constrained model of the marine ecosystem allows them to hook a first-order estimate of how fish biomass has fallen historically, as well as to show how biogeochemical cycling rates have changed since the industrialization of fisheries.

“Given the large change in global fish communities that has resulted from industrial fishing and early defaunation, and the emerging effect of additional stressors such as ocean warming, the development of a better understanding of these biogeochemical implications is an urgent undertaking,” the team concludes.

Fish Fecal

The excrement of the fish sinks to the seabed. Since they contain a certain amount of carbon, the researchers see them as carbon stores. The excrement also contains some oxygen, which is consumed by organisms on the seafloor. The researchers’ models show that today the amount of feces deposited by fish is only about half what it used to be. This suggests that only half as much carbon is bound. The rest is likely to end up in the atmosphere and contribute to climate change.

It is vital that we look more closely at the effects of fishing and over-fishing, in particular, as we observe the marked alterations caused by industrial fishing, habitat degradation, global warming, ocean deoxygenation, and acidification, and model their impact on the environment and climate.