A shortage of fish faeces is contributing to shifts in the ocean’s carbon cycle of an equivalent magnitude to that of the impact of climate change on the ocean.
Fish-produced faecal pellets are one of the most efficient natural mechanisms of carbon storage, locking it deep in the ocean for up to 600 years. But the rise of industrial fishing has seen the number of fish in the sea fall, so Daniele Bianchi at the University of California, Los Angeles, and his colleagues decided to investigate how this has affected the flow of faeces.
The team developed a model of the global marine ecosystem that quantifies how the production of fish faeces has changed over time. The model is based on estimates of historical and present-day numbers of fish caught, as well as broader human-driven impacts on marine ecosystems, such as climate change.
The researchers looked at species that industrial fishers try to catch, as well as those they don’t. Their model showed that before industrial fishing began in the early 20th century, the global biomass of species in the first category was about 5 billion tonnes, while the total for fish that aren’t targeted by industrial fishers was nearly double that. Given that the biomass of all the humans on the planet today is an order of magnitude smaller, these numbers are large, says Bianchi.
Almost all of the biomass on Earth is ultimately the product of photosynthesis by plants, so one way to measure an animal’s influence on the ecosystem is to look at how much of this mass, known as global primary production, cycles through it.
The team found that the species that industrial fishers try to catch cycled 2 per cent of this mass before the 1900s, but by the time the number of fish caught industrially peaked in the 1990s, this had halved, as had the rate at which carbon locked up in fish faeces sank into the sea.
These figures suggest that the effect of industrial fishing on the ocean’s carbon cycle is comparable in magnitude to the impact of climate change on the ocean’s carbon, says Bianchi. “We should consider fish as an integral part of the ocean’s biogeochemical cycles.”
Journal reference: Science Advances, DOI: 10.1126/sciadv.abd7554
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