Great hammerhead sharks are known for their long migrations, but scientists have now discovered that some individuals in the Bahamas instead spend all year snacking without leaving their plentiful and protected home waters. Why would some individuals take off if others are happy to stay put?
The answer to this mystery might be their favorite foods, researchers report March 21 in Frontiers in Marine Science.
Great hammerheads (Sphyrna mokarran) have been recorded making 3,000-kilometer journeys between Bimini, the Bahamas and offshore Virginia. But tracking tags and tissue samples from hammerheads in Andros, Bahamas, reveal that the country, designated as a shark sanctuary in 2011, could be more than just a part-time haven.
“Resource availability is a key driver,” says shark ecologist Tristan Guttridge, founder of the marine nonprofit Saving the Blue, which does most of its research in Andros. “But why don’t all of them stay if food is so abundant?”
Food choices could influence hammerheads’ travel habits, in the same way that you can be surrounded by pizza joints but still drive across town because all you’re craving is a poke bowl.
From 2018 to 2024, 78 hammerheads were observed or captured. Scientists measured and took tissue samples from 22 of them and put satellite tags on seven. Although some sharks made long-distance migrations away from the Bahamas, as is typical for the species, others stayed in Andros year-round.
When Guttridge and his colleagues analyzed where atoms in the sharks’ tissue came from, they found that different sharks prefer different prey. One hammerhead munched on silky sharks for nearly two-thirds of its meals, while others chowed down on mainly barracuda and stingray.
The different habitats of these fish — silky sharks are largely found in the open ocean while stingrays tend to live on the seabed and barracuda can lurk in both environments — could indicate the types of places where the sharks are feeding. “We found evidence of individual variation in their diets, so deciding whether to stay or go might depend on what they feed on,” Guttridge says.
As these critically endangered animals migrate across immense distances, they can travel into waters where they’re at risk of being harmed or killed by industrial fisheries. “The global population of great hammerheads is thought to have reduced by more than 80 percent over the last three generations,” Guttridge says.
Understanding hammerhead diet preferences may help protect the long-distance travelers. “Although some individuals reside year-round in protected waters, others do not, emphasizing the need for international collaboration on conservation efforts for these mobile species,” Guttridge says.
