The smaller a marine animal is, the more of it swim through the ocean. And the bigger an animal, the fewer of them there are. At least, until we fished out the oceans.

As surprising as it may sound, all life forms in the ocean, from krill to tuna, seem to obey a simple mathematical law. This states that an organism’s abundance is linked to its body size. Although individual krill are only a billionth the weight of a large tuna, they are usually a billion times more numerous in the ocean. However, this law of nature no longer seems to hold. And that is our fault.

Study

To get an idea of ​​the current numbers of different marine species, researchers researched several studies and compiled a large, global dataset. This consisted of bacteria, phytoplankton, zooplankton, fish and mammals. “It was quite a challenge to find a way to count organisms that span such a huge difference in scale,” recalled researcher Ian Hatton. “While microscopic aquatic organisms can be estimated from more than 200,000 water samples collected worldwide, the numbers of larger marine animals swimming through the oceans must be counted using entirely different methods.” In addition, the researchers used historical reconstructions and models of marine ecosystems to estimate biomass in pristine oceans before the 20th century. They then compared this data with today’s data.

natural law

The researchers find that before the advent of large-scale industrial fishing, marine life was very evenly distributed; it obeyed a law of nature. In fact, the larger an organism was, the less of it swam through the oceans. Basically, an organism’s abundance is linked to its body size.

Image: Ian Hatton et al, Science Advances, 2021)

“This is quite remarkable,” said study leader Eric Galbraith. “We don’t quite understand why this is – why couldn’t there be many more small animals than large ones? Or why is there no ideal size that is in the middle? In that sense, the results show how little we still understand about the ecosystem.”

Exception to the rule
The researchers also discovered two exceptions to the rule. And at both ends of the scale studied. Bacteria appear to be present in greater numbers than the law predicts. And whales to a lesser extent. Why? That’s a complete mystery.

However, studying them becomes quite tricky. The researchers argue that the natural balance has now been disrupted. And by ourselves. Due to the widespread industrial fishing, the natural law in today’s oceans no longer applies. Large fish (longer than 10 cm) have experienced a total loss of biomass of about 2 gigatons; a 60 percent reduction. The intensive whaling industry of the last century was even more devastating, with the largest whales losing as much as 90 percent.

People

Although fishing accounts for less than three percent of human food consumption, the study shows how devastating its effects are on the natural balance in the ocean. “The biggest surprise is the massive inefficiency of the fishery,” notes Galbraith. “When industrial fishing fleets go out, they don’t behave like the large predatory fish, seals or birds they compete with. The latter only catch small amounts and keep the population stable. However, humans have not only replaced oceanic predators, but changed the entire marine ecosystem.”

The study, published in the specialist journal Science Advances, shows the highly destructive influence of humans on the life with which we share our planet. Apparently we even mess up the laws of nature. Still, all hope is not lost. “The good news is that we can reverse the imbalance we’ve created,” Galbraith said. “And by reducing the number of operating fishing boats. Reducing overfishing will also help make fishing more profitable and sustainable – it’s a potential win-win situation if we can join forces.”

With many fish currently facing extinction, researchers are studying some promising alternatives that could potentially end up on our plates in the future. How about jellyfish, for example? Would you eat it?