In the waters off French Polynesia, fishermen lay longlines equipped with multiple hooks in the hopes of catching tuna and swordfish. Tiger sharks, however, are also prone to chomping down on the bait—and many of those that break free are left with hooks stuck in their jaws for years, possibly even a lifetime, according to a new study published in Fisheries Research.
Sharks are a common bycatch animal, meaning that they get swept up in fishing operations targeting other sea creatures. Longlines, which can extend for miles and contain thousands of hooks, are particularly dangerous. Sharks can sometimes break free from these traps by biting through the line, and sometimes they are set free. But “[t]hese interactions frequently result in hooks remaining embedded in sharks,” the study authors write.
It can be difficult to determine how sharks fare after swimming away with these unwanted mementos, because it’s tricky to monitor sharks in their natural habitat. Exceptions to the rule are large sharks that repeatedly return to ecotourism sites—like tiger sharks off the coast of Tahiti, the largest island in French Polynesia.
Carl Meyer, a marine biologist at the University of Hawaiʻi, and three of his colleagues tracked 55 tiger sharks outside Tahiti’s northwest coast between 2011 and 2019, identifying the animals by unique traits like body markings and scars. Thirty-eight percent of the sharks had at least one hook stuck in their body; 20 percent had several, with seven hooks representing the highest number found in a single shark.
The length of time that the hooks remained embedded in the sharks—all were jammed in and around the jaws—ranged from seven days to more than seven years. Carbon steel hooks, which are corrodible, were shed more quickly, with an average minimum retention time of 227 days. None of these hooks stayed stuck to the sharks for more than two and a half years, according to Gizmodo’s George Dvorsky. Stainless steel hooks stayed with the animals for longer, 685 days on average.
Sixty-five percent of the observed hooks were still trailing fishing line. Fortunately, neither the hooks nor the lines seemed to seriously impact the sharks’ wellbeing or their ability to feed. “Overall,” the researchers write, “sharks appeared to be in good condition despite [the] presence of residual hooks and trailing line with only a few hooks apparently causing minor, localized irritation at the point of entry.”
But the study authors also caution that tiger sharks are a particularly robust species—the “Sherman tank of the shark world,” as Meyer tells Dvorsky. Not all sharks are so lucky.
“Internal hooks can cause internal bleeding, while external hooks can interfere with feeding,” Meyer says. “Trailing line can interfere with feeding, wrap around fins leading to necrosis and interfere with swimming.”
The study authors also point out that they only saw one tiger shark with a hook piercing its internal organs, possibly because sharks with these types of injuries are less likely to survive.
It “may be possible” to extrapolate the hook retention and shedding rates documented in this study to other large, warm-water shark species, the researchers say—though they also note that their observations are likely an underestimate. Internal hooks, which get lodged in the throat or stomach, are difficult to spot, for one thing. And some tiger sharks, though devoid of hooks themselves, had scars consistent with previous hooking injuries.
Because interactions between sharks and fisheries appear to be high, and because such interactions have the potential to adversely impact sharks’ health, it is important to take steps to mitigate the risks to these sea creatures, which play a vital role in ocean ecosystems. Switching from stainless steel to corrodible hooks would be a good start.
“[T]he impact on the sharks would be reduced,” Meyer says, “simply because those hooks would fall out much more quickly.”