It sounds like a promising prospect, actually: an ocean where no one has to worry about the threat of a sleek, dark shape rising from the depths with a million teeth and an appetite for limbs.
Seals everywhere would rejoice, no doubt.
But an ocean without sharks is actually a troubling, potentially disastrous prospect in terms of marine ecosystems—and a disturbingly possible one if things keep going at the rate they are.
"This is a pressing concern, and we are in danger of living in a world effectively devoid of sharks," said Stuart Sandin, a marine ecology expert from the Scripps Institute of Oceanography. "Fishermen around the world are devastating shark populations."
It could mean an unfitting end to an animal that has outlived the dinosaurs and plied the oceans for more than 400 million years, surviving near-global extinction events and outliving countless other marine species.
But according to several studies, sharks nearly everywhere are in serious decline due to human activity. A report from the Pew Charitable Trusts this year says global populations are dropping by drastic numbers from the Atlantic to the Mediterranean.
“Declines in population sizes of sharks, as much as 70 to 80 percent, have been reported globally. Some populations, such as the porbeagle sharks in the northwestern Atlantic and spiny dogfish in the northeastern Atlantic, have been reduced by 90 percent or more,” a report called “Sharks: The State of the Science” reads.
While two centuries of overfishing and the constant problem of shark bycatch have been at play in the Mediterranean, a new scourge is the predators’ biggest threat: finning.
According to the Pew report, the Asian appetite for shark fin soup has led to the slaughter of some 73 million sharks annually, many of which are hooked, finned and tossed overboard alive to drown.
With sharks being fished in such huge numbers, the question arises: what would happen if they disappeared? What would the oceans look like in a world without sharks?
The answer, according to experts, is most likely a sickly underwater swamp populated by algae, jellyfish and microbes, where reefs lie in rigor mortis and fish are hard to see in the murky water. Sound awful? It would be.
In pristine reef ecosystems, says Daniel Pauly, shark expert and professor of fisheries at the University of British Columbia, there are tons of sharks and almost no unused by-products that make up the diet of “nuisance” species like jellyfish and algae. Almost everything is efficiently consumed in the food web.
“As we fish the sharks down,” he says, “the food web completely changes in character, and becomes … ‘leaky’ in that biomass that is generated at lower traffic levels—for example, algae—accumulates in the form of detritus that is either not consumed or consumed by animals that are dead ends in the food web.”
In other words, without the sharks to eat the species that prey on the algae-eaters, those algae-eaters will decline, leading to an accumulation of biological refuse that eventually chokes out almost everything else, such as colorful reefs and everyone’s favorite marine animal: the sea dragon.
A great, slimy oceanic bog instead of a thriving reef lorded over by sharks. An improvement?
In other habitats, the loss of sharks could lead to powerful upheavals of other species. Pauly points to a case study from the Eastern Seaboard of the U.S. just three years ago when, after most of the big sharks were fished out, the shellfish took a nosedive. Local scallop and oyster fishermen were devastated.
The culprits were the rays, skates and small shark species that feed on the shellfish. Without the bigger sharks around to compete with and prey on them, the lower species proliferated, and chowed down on the shellfish.
The same study showed that 97 percent of tiger and scalloped hammerhead sharks, and 99 percent of bull, dusky and smooth hammerhead sharks on the East Coast had disappeared.
“The removal of large sharks can negatively impact whole ecosystems by, for example, allowing an increase in the abundance of their prey (fewer sharks eat less prey), or influencing prey species through non-lethal means, by causing behavioral changes to prey habitat use, activity level and diet,” the Pew study reads.
Those ecosystems can be anywhere in the ocean, Pauly says. Though declines in population and health are easy to monitor and study on reefs, the scenario can play out anywhere sharks naturally occur.
“There is no reason to think that it doesn’t happen in the open ocean,” he said.
Sandin said the loss of sharks would come with the loss of predators in general, and the effects of that collapse are hard to divine.
"If sharks are removed, then we certainly see a shift in the predator-prey interactions, namely the way in which prey fish live and die," he says. "However, we will never really be able to know what sharks alone do in the ocean, as the loss of sharks is almost always linked to the loss of other top predators... and without predators in general, the oceans would certainly look different."
So as 'Shark Week' continues on The Discovery Channel, and the public’s fascination with the ocean’s top predators is rekindled, it’s worthwhile to think about whether sharks will be around long enough to give repeat performances—or whether the “obscene consumption,” as Pauly puts it, of shark fins will doom them.
“That is the thing that threatens sharks most, globally,” Pauly said.