An Ocean Without Sharks Is Bad for Everyone

By Laura Dattaro
Published: March 7, 2014

In 2013, sharks killed 10 people around the world, prompting controversial organized kills in places like Western Australia, which has seen six shark-attack deaths in the past four years.

But such shark culls are ineffective and unnecessary: According to the International Shark Attack File (ISAF), humans already kill between 30 and 70 million sharks every year, not including indirect deaths caused by adverse human effects on the environment. And as shark populations decline, their necessity to a healthy ecosystem — and the economies that depend on them — is becoming increasingly clear. The bottom line: Humans need the oceans, and the oceans need sharks.

Sharks are an example of an apex predator — a carnivore with no natural predator of its own. Like other apex predators, sharks most directly affect the creatures right below them, the smaller fish they feed on, which eventually affects creatures all the way down the food chain. Without enough sharks to eat such fish, like tuna, their populations can explode, which can lead to diminished populations of even smaller creatures, many of which feed on plants and algae. Researchers in Australia, for example, found that the decrease in sharks from fishing ultimately led to damage of the already-troubled coral reefs.

The great white shark is listed as a threatened species on the International Union for the Conservation of Nature's Red List, due largely to the large numbers of sharks humans kill every year. (Terry Goss/Wikimedia Commons)

“Because now those fish weren’t around to eat algae, the algae overgrew the reef and smothered the corals,” Neil Hammerschlag, a marine scientist at University of Miami and director of the school’s marine conservation program, told weather.com. “Coral is ... important for the reef and the economically important and ecologically important fish.”

This is not just bad news for the sea creatures, but also the humans who financially depend on — and eat — these creatures. In 2007, researchers studying this effect found that 35 years of large shark decline led to increases in the numbers of rays and smaller sharks. The resulting population growth of the cownose ray, which feeds on the bay scallop, caused the collapse of a scallop fishery that had existed for a century, destroying the livelihoods of the fishermen. Scallops are also known to filter small unwanted debris from the water, Hammerschlag said, providing indirect evidence that a reduction in sharks may contribute to dirtier waters.

Predation is not the only way sharks affect other sea life either. New research, Hammerschlag said, is starting to show that the very presence of sharks can alter the movements, behavior and even reproductive abilities of the fish around them. For a fish, being around a shark is stressful, given that it means constant vigilance to avoid being eaten. This stress can wear on the fish, potentially influencing its ability to bear healthy offspring. The presence of a shark can also drive fish into areas where they wouldn’t normally congregate, causing them to scatter and find food that isn’t necessarily ideal.

All of these effects can be filed under population control, with pressure from sharks keeping the system flowing and preventing it from collapse. With no areas in which shark populations are on the rise, Hammerschlag said, it’s impossible to confirm whether the reverse effect is true, but an analogous situation might provide some proof: the reintroduction of wolves into Yellowstone, which has caused an explosion of biodiversity affecting everything from elk and beavers to aspen trees and insects.

All of which makes the reaction to kill off sharks that have killed people understandable yet a knee-jerk reaction at best. Though rare, shark attacks will continue to happen, particularly as human populations increase and travel to remote areas becomes ever easier, ISAF’s George Burgess said in a release. “Remote destinations are not typically medically equipped to handle a serious shark attack,” he said. “This situation is a key factor in the higher death rate this year.”

Still, Hammerschlag said, it’s clear that sharks are not particularly interested in humans, given that millions of people swim in waters where at least small sharks are never far away. “There would be nothing easier than preying upon 150-pound monkeys swimming around in the ocean, doggy paddling,” he said. “It would be the easiest thing a shark could do and the fact that [attacks] are so relatively rare just shows how uninterested they normally are.”

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