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

Many blacktip reef sharks in French Polynesia are commonly fed by tourists. But the low-quality diet is changing the sharks’ behavior and physiology.

Jan 30, 2025 - 23:30
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Feeding sharks ‘junk food’ takes a toll on their health

A food regimen of miserable-quality scraps is altering the behavior and physiology of blacktip reef sharks

A blacktip reef shark swims over a reef

Blacktip reef sharks (Carcharhinus melanopterus) on the island of Mo'orea bag meals scraps from tourists, but it absolutely is doubtlessly no longer great for his or her lengthy-term successfully being.

Johann Mourier

In most cases, sharks maintain a recognition as swimming garbage cans that unflinchingly dine on whatever they can match in their jaws. Nevertheless in French Polynesia, blacktip reef sharks that frequent areas where tourists toss them low-quality scraps are taking successful to their metabolic and reproductive successfully being, researchers relate December 24 in Animal Conservation.

Around the enviornment, snorkeling or diving tourists who are looking for to peek sharks in their pure habitat may trap them with meals. A very neatly-liked location for shark feeding is Mo’orea, a small island arrive Tahiti. On the sand bank shallows, tourists in boats and kayaks congregate to peek the sharks and stingrays, tossing them all the pieces from frozen squid to human meals scraps.

Whereas at the Island Compare Middle and Environmental Observatory in Mo’orea, marine behavioral ecologist Johann Mourier became learning the actions, behavior and copy of sharks around the island. For the reason that blacktip reef sharks (Carcharhinus melanopterus) around Mo’orea are possible to be homebodies — typically spending a decade or more on one reef — Mourier and his colleagues wondered how sharks living at sites frequented by generous tourists shall be influenced by the total consideration.

In an earlier gaze on the sharks’ actions, the researchers chanced on that female sharks from active feeding sites moved around bigger than those at sites much less touched by humans. Female sharks also strayed from the good and comfortable, shallow lagoon waters the species prefers when pregnant. The crew wondered if the sharks’ behavioral modifications had been also correlated with certain successfully being markers.

From May 2008 by scheme of May 2011, Mourier and his colleagues caught 49 female and 68 male blacktip reef sharks at diversified areas around the island. 5 areas had been feeding sites, 12 had been nonfeeding. The researchers drew blood from the sharks and analyzed diversified biochemicals and hormones related to metabolism and copy.

The volume of blood made up of pink blood cells — an overall measure of successfully being situation — became decrease for every and every sexes at the feeding sites.

In feeding situation females at some level of the breeding season, “you maintain a decrease in the glucose in the blood, which manner that the meals quality became worse at feeding sites,” says Mourier, now at the College of Montpellier in France.

The scraps and leftovers supplied by humans are admire junk meals, Mourier says, and don’t appear to maintain the the same nutritional quality because the sharks’ fashioned food regimen.

A couple of boats sit down in shallow water arrive a shoreline
Small boats, kayaks, snorkelers and divers congregate totally on the north aspect of the island of Mo’orea to work at the side of sharks and stingrays. Johann Mourier

Feeding also appears to be like to maintain an impact on the reproductive hormones of each and every sexes at some level of the breeding season. Males had bigger testosterone ranges at feeding sites, almost certainly due to more aggressive competitors between males crowding together over the seafood spatter. Females at nonfeeding sites had been universally pregnant and had ranges of 1 invent of estrogen three instances that of females at feeding sites, where no longer every captured female became pregnant.

An unpredictable, low-quality food regimen at feeding sites may per chance almost certainly be making it more challenging for female sharks to make investments in the subsequent skills of pups. Additional be taught may per chance resolve how feeding impacts copy and fitness in the lengthy urge and throughout multiple generations. Particularly, blacktip reef sharks are blueprint to be “weak” to extinction by the World Union for Conservation of Nature.

“This gaze represents a foremost advancement in our understanding of shark successfully being,” says Natascha Wosnick, a biologist at the Cape Eleuthera Institute in the Bahamas who became no longer involved with the gaze. “These outcomes are critically foremost given that shark [feeding] practices are on the full poorly regulated or fully unregulated, ensuing in various challenges — no longer only in altering shark behavior but additionally in compromising their overall welfare.”

Reef sharks inundated with junk meals at Mo’orea may per chance almost certainly be critically at menace. The sharks cease inner small areas, so that they'll also be repeatedly uncovered to human feeding, Mourier says. Higher species that run farther, similar to tiger and lemon sharks, may no longer maintain the the same susceptibility to feeding actions.

Nevertheless diversified shark species may per chance almost certainly be weak to impacts from human feeding. As an illustration, nurse sharks in the Bahamas that rely on fish scraps would perchance also maintain key alterations to their behavior and successfully being, Wosnick says. “These sharks show cloak elevated sunlight hours swimming enlighten, possible ensuing in elevated energy expenditure — energy that's doubtlessly no longer fully compensated for by the volume or quality of the meals they admire.”

The researchers point out that regulating the types of meals given to sharks in areas with popular feeding actions — critically around the breeding season — may per chance almost certainly be indispensable to present protection to the successfully being of the sharks.

“[Shark tourism] is amazingly helpful for the sharks’ conservation, for us to educate americans around that,” Mourier says. “Nevertheless you furthermore mght should better manage the enlighten.”

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