Feeding sharks ‘junk food’ takes a toll on their health

In general, sharks have a reputation as swimming garbage cans that unflinchingly dine on whatever they can fit in their jaws. But in French Polynesia, blacktip reef sharks that frequent places where tourists toss them low-quality scraps are taking a hit to their metabolic and reproductive health, researchers report December 24 in Animal Conservation.

Around the world, snorkeling or diving tourists who want to see sharks in their natural habitat may lure them with food. A particularly popular location for shark feeding is Mo’orea, a small island near Tahiti. On the sand bank shallows, tourists in boats and kayaks congregate to see the sharks and stingrays, tossing them everything from frozen squid to human food scraps.

While at the Island Research Center and Environmental Observatory in Mo’orea, marine behavioral ecologist Johann Mourier was studying the movements, behavior and reproduction of sharks around the island. Since the blacktip reef sharks (Carcharhinus melanopterus) around Mo’orea tend to be homebodies — sometimes spending a decade or more on one reef — Mourier and his colleagues wondered how sharks living at sites frequented by generous tourists might be influenced by all the attention. 

In an earlier study on the sharks’ movements, the researchers found that female sharks from active feeding sites moved around more than those at sites less touched by humans. Female sharks also strayed from the warm, shallow lagoon waters the species prefers when pregnant. The team wondered if the sharks’ behavioral changes were also correlated with certain health markers.

From May 2008 through May 2011, Mourier and his colleagues caught 49 female and 68 male blacktip reef sharks at different locations around the island. Five locations were feeding sites, 12 were nonfeeding. The researchers drew blood from the sharks and analyzed various biochemicals and hormones related to metabolism and reproduction. 

The volume of blood made up of red blood cells — an overall measure of health condition — was lower for both sexes at the feeding sites. 

In feeding site females during the breeding season, “you have a decrease in the glucose in the blood, which means that the food quality was worse at feeding sites,” says Mourier, now at the University of Montpellier in France.

The scraps and leftovers offered by humans are like junk food, Mourier says, and don’t appear to have the same nutritional quality as the sharks’ normal diet. 

Feeding also appears to affect the reproductive hormones of both sexes during the breeding season. Males had higher testosterone levels at feeding sites, possibly due to more aggressive competition between males crowding together over the seafood spatter. Females at nonfeeding sites were universally pregnant and had levels of one form of estrogen three times that of females at feeding sites, where not every captured female was pregnant. 

An unpredictable, low-quality diet at feeding sites may be making it harder for female sharks to invest in the next generation of pups. Further research could determine how feeding impacts reproduction and fitness in the long term and across multiple generations. Notably, blacktip reef sharks are considered “vulnerable” to extinction by the International Union for Conservation of Nature.

“This study represents a significant advancement in our understanding of shark health,” says Natascha Wosnick, a biologist at the Cape Eleuthera Institute in the Bahamas who was not involved with the study. “These results are particularly important given that shark [feeding] practices are often poorly regulated or entirely unregulated, leading to numerous challenges — not only in altering shark behavior but also in compromising their overall welfare.”

Reef sharks inundated with junk food at Mo’orea may be particularly at risk. The sharks stay within small areas, so they can be repeatedly exposed to human feeding, Mourier says. Larger species that roam farther, such as tiger and lemon sharks, may not have the same susceptibility to feeding activities. 

But other shark species may be vulnerable to impacts from human feeding. For example, nurse sharks in the Bahamas that rely on fish scraps may also have key alterations to their behavior and health, Wosnick says. “These sharks exhibit increased daytime swimming activity, likely leading to greater energy expenditure — energy that may not be fully compensated for by the quantity or quality of the food they consume.”

The researchers suggest that regulating the types of foods given to sharks in places with widespread feeding activities — especially around the breeding season — may be necessary to protect the health of the sharks.

“[Shark tourism] is very beneficial for the sharks’ conservation, for us to educate people around that,” Mourier says. “But you also need to better manage the activity.”