reefshark

For sharks, fish are friends, not (always) food

By Kathleen Ferraro

Sharks strike fear with their reputation as man-eaters. But coral reef sharks are light eaters with no taste for human fare, new research shows.

Reef sharks – species of shark that inhabit coral reefs – eat small prey and only at infrequent intervals, according to scientists at James Cook University’s ARC Centre of Excellence for Coral Reef Studies in Queensland, Australia.

The research suggests that significant reef shark deaths due to human hunts, climate change  and other causes could potentially throw off coral reef food chains, altering coral ecosystems. This insight is especially pertinent considering ongoing coral reef decline in tandem with humans’ killing roughly 100 million sharks per year globally, researchers said.

“The core theme of my research is the ecological importance of sharks to coral reefs. One of the first things to do was look at the diet and see what they’re eating. And through that work we found that they are probably not the apex predator of the ecosystem,” said marine biologist Ashley Frisch, lead researcher. ”The reef sharks seem to be feeding in the same trophic level—the same level in the food chain—as other large fish.”

Frisch, along with a research team, surveyed the contents of reef sharks’ stomachs and body tissue to learn what they eat. They found a variety of small prey on the menu, including smaller fish, sea snakes, crabs and mollusks, Frisch said.

Reef sharks’ diets were similar to that of large reef fish, suggesting that reef sharks do not catch big prey, Frisch said. This is likely due to an efficient metabolism and reliance on opportunistic feeding. Their diet also revealed that reef sharks are not apex predators, or animals at the top of the food chain. Instead, they are mesopredators, or animals in the middle of the food chain, according to Frisch.

This is a far cry from the archetypal image of the bloodthirsty great white shark. Unfortunately, that stereotype remains pervasive. Australian fishermen continue to kill reef sharks en masse, viewing the sharks as competition for tomorrow’s catch, Frisch said. But the sharks aren’t eating the groupers, snappers or other large reef fish the fishermen prize.

And as reef shark numbers dwindle, the potential environmental implications become clear. Because one animal in an ecosystem is connected to every other, a disturbance in reef sharks’ population could have catastrophic effects across the board, according to Frisch.

“If you removed all of the apex predators, the populations of animals at the next level would tend to increase. And because there’s an overabundance of that second level, you get a depletion of the next level down,” he said. “So every second trophic group either explodes or crashes. These changes ripple down from the top level of apex predator right down to the bottom of the food chain, and it can really disrupt the way ecosystems function.”

For the coral reef, that could mean throwing the entire ecosystem out of equilibrium. Changes at the top of the food chain induce fluctuations at the bottom, right down to the microscopic plankton that coral eat. Changes in coral’s food supply would in turn alter the health of the reef system overall.

Aquarists, the marine curators at Maui Ocean Center (MOC) in Hawaii have made similar observations about reef sharks’ mild diets and are on a mission to break down anti-shark stereotypes. The aquarium’s Shark Dive Maui program allows SCUBA-certified visitors to dive with wild sharks, including three species of reef shark, in MOC’s 750,000-gallon Open Ocean Exhibit.

The program aims to expose visitors to the more peaceful, low-risk reality of interacting with sharks respectfully and safely. In fact, “more people are killed every year [due to] selfie sticks,” a naturalist at MOC said she had been told.

“We try to spiritually convey the importance of sharks and the balance they bring to the ocean,” Harry Abrahamsen, curator at MOC, said.

Abrahamsen, who takes care of the sharks in addition to leading Shark Dive Maui dives, echoed Frisch’s insights about reef shark diets. He feeds MOC’s more than 20 resident sharks more than 22 pounds of small prey food several times per day, though an individual shark might eat as little as once a month given their efficient metabolism, Abrahamsen said.

Reef sharks also prey upon sick or injured fish, favoring easy feeding opportunities over more challenging hunts, according to Abrahamsen. Frisch also noticed this feeding pattern.

“Almost all large predators take injured or feeble animals. They take the old, the sick and the young,” Frisch said. “And that’s one of the reasons why, occasionally, humans get bitten. In the water, humans look like a big wounded seal or turtle because we can’t swim {like fish] and we flail around in the water. To a shark, we look like we’re injured animals.”

(Kathleen Ferraro via Piktochart.com)
Data sources: MOC naturalists, Mother Nature Network, Black Friday Death Count (Graphic by Kathleen Ferraro via Piktochart.com)

Unlike other predators, however, sharks typically let go of humans after taking these rare exploratory bites, according to Frisch. Abrahamsen explained that sharks’ keenest sense is taste. Consequently, if they’re unsure about an object in the water they will bite it to learn more, much like humans use their sense of sight to squint and see something better.

And reef sharks’ stomach contents reflected this habit, with a conspicuous lack of any prey larger than a cheeseburger, let alone human flesh.

“We’re finding such small meals, small prey in their stomach,” Frisch said. “Hopefully that will make people feel a bit safer around reef sharks, because they’re just not a threat to humans.”

Photo at top: A black-tip reef shark. (Simon Gingins)