Humans seem to have broken a law of nature
Industrial fishing over the past century seems to have broken a law of nature.
Surprising as it may sound, all life forms in the ocean, from small krill to large tuna, seem to obey a simple mathematical law that relates an organism’s abundance to its body size. For example, although small krills individually weigh only a billionth of the weight of a large tuna, they also tend to be a billion times more numerous in all oceans. The idea, known as Sheldon’s size spectrum theory, was first put forward in the 1970s, but has never been tested for a wide range of marine species and on a global scale. until now. An international research team, including researchers from McGill, has found that not only does the theory seem to have once been verified, but this natural balance has now been drastically altered by widespread industrial fishing.
In a study recently published in Scientists progress, an international team made up of researchers from McGill University, the Max Planck Institute for Mathematics in the Sciences in Germany, the Institut de Ciència i Tecnologia Ambientals in Spain, the Queensland University of Technology in Australia and the Weizmann Institute of Science in Israel discovered that when the oceans were in a more pristine state (before the 20e century and the advent of large-scale industrial fishing), the size spectrum theory seems to have been verified.
“The fact that marine life is evenly distributed across sizes is remarkable,” says Eric Galbraith, lead author of the article and professor in the Department of Earth and Planetary Sciences at McGill. “We don’t understand why it would have to be that way – why couldn’t there be a lot more small things than big things? Or an ideal size that sits in the middle? In this sense, the results show how much we do not understand the ecosystem.
From bacteria to whales – finding a way to measure all marine life
To get a picture of the current numbers in an unprecedented range of species, the researchers used various recent studies to build a large global dataset on marine organisms, including bacteria, phytoplankton, zooplankton, fish and the mammals. Their approach allowed them to differentiate the spatial distribution of 12 large groups of aquatic life across the ocean.
All life forms in the ocean, from small krill to large tuna, seem to obey a simple mathematical law that relates an organism’s abundance to its body size. Credit: Max Planck Institute
“It was difficult to find a way to adequately compare measurements of organisms spanning such a large difference in scale,” recalls Ian Hatton, study lead author and Alexander von Humboldt researcher at the Max Planck Institute. . “While microscopic aquatic organisms could be estimated from over 200,000 water samples collected around the world, large marine animals can swim in entire ocean basins and had to be estimated using entirely different methods. . “
Researchers also used historical reconstructions and models of marine ecosystems to estimate marine biomass in pristine oceans (before 20e century) and compared these data to those of today. They found that, with exceptions to the two extremes – whales and bacteria – there was once a remarkably constant biomass of about 1 gigaton on every order of magnitude of body size. This means that the total amount of life in the oceans between any size and a size ten times the size – for example, 1g to 10g – still totals around 1 billion tonnes, regardless of the starting size. . But industrial fishing has dramatically changed this image.
Human impacts on marine biomass
Unlike an almost constant biomass spectrum in the pristine ocean, the researchers’ examination of the spectrum revealed a major human impact on the distribution of biomass in larger sizes.
While fishing accounts for less than 3 percent of human food consumption, its effects on the biomass spectrum have been devastating. Large fish (i.e. anything larger than 10cm) suffered a total biomass loss of about 2 gigatonnes (a 60% reduction), eclipsing the 0.1 gigatons that fishermen catch each year . Historically, whaling was even more devastating for the larger end of the biomass spectrum, with the larger whales experiencing a 90% loss. Indeed, the authors estimate that the losses caused by industrial fishing and whaling over the last century are far greater than the potential losses of biomass due to climate change scenarios over the next 80 years, even under scenarios. pessimistic programs.
“The biggest surprise, seen from this global perspective, has been the enormous inefficiency of the fishery. When industrial fishing fleets go out and catch fish in the ocean, they do not act like the large predatory fish, seals or birds they compete with, which only consume small amounts of fish populations. in a way that keeps populations stable, ”says Galbraith. “Humans have not just replaced ocean predators, but have completely altered the flow of energy in the marine ecosystem.”
He adds: “The good news is that we can reverse the imbalance we have created by reducing the number of active fishing vessels in the world. Reducing overfishing will also help make fishing more profitable and sustainable – this is a win-win potential, if we can pull ourselves together.
To learn more about this research, read Humans Guilty of Breaking a Basic Law of Ocean Nature.
Reference: “The Global Ocean Size Spectrum from Bacteria to Whales” by Ian A. Hatton, Ryan F. Heneghan, Yinon M. Bar-On and Eric D. Galbraith, November 10, 2021, Scientists progress.
DOI: 10.1126 / sciadv.abh3732