Why some great hammerhead sharks rarely leave home

Not all great hammerhead sharks are long-distance travelers. In fact, some prefer to stick close to home.

Recent research focusing on sharks near Andros Island in the Bahamas has revealed that while some great hammerheads are known to migrate vast distances, others appear to settle down, likely because the local environment already meets their needs.

This discovery could help improve conservation efforts for this critically endangered species.

The study was led by Tristan Lee Guttridge, director of Saving the Blue, in collaboration with researchers supported by Discovery’s Shark Week.

The team focused their efforts on the Bahamas, especially around Andros Island, an area largely unexplored in terms of great hammerhead activity.

Shark tracking in the Bahamas

“When we started this study there was little to no information about great hammerheads in the central Bahamas and no information about their habitat use at Andros, which has one of the largest fringing reefs in the world and arguably has the most pristine flats habitats in the wider Caribbean,” explained Guttridge. “We had no answers to simple questions like, what do these sharks eat?”

Until now, most shark tracking in the Bahamas focused on areas like Bimini, known for its seasonal shark visitors. However, the role of Andros Island in the sharks’ life cycle remained a mystery.

Scientists wanted to find out if the island served as a year-round habitat, a feeding ground, or even a potential mating site.

Tagging, tracking, and shark science

Between March 2020 and June 2024, the team captured 22 great hammerhead sharks in locations that matched their known habitat preferences.

Using baited block rigs, they carefully caught the sharks, assessed their health, and recorded key data like sex and size.

Muscle biopsies were taken for isotope analysis, and seven sharks were equipped with satellite trackers. Because great hammerheads are sensitive to stress, only the healthiest individuals were tagged.

The researchers didn’t rely on their captures alone. They also included opportunistic sightings, compiling a total of 78 shark encounters between 2018 and 2024. Most of these were large juveniles or adults.

Sightings spiked in January through March, with a second increase in June and July. Two of these sightings suggested recent mating, pointing to the island’s importance as a possible breeding ground.

Some sharks were even recaptured in the exact same areas years later, showing a surprising degree of site fidelity.

A shark’s preferred neighborhood

The sharks didn’t use the island randomly. They favored eastern waters just off the northern and central coast of Andros, consistently returning to a relatively small 400-square-kilometer area.

Other popular spots included edge habitats near steep reef drop-offs, where prey was abundant.

But if the sharks ventured too far south or past the northern tip of the island, they were more likely to keep moving.

Those that left the Bahamas often traveled to the eastern United States, indicating a possible connection to the broader western Atlantic hammerhead population.

Eating habits of hammerhead sharks

Isotope analysis from the biopsies shed light on the sharks’ eating habits. Barracudas and stingrays made up about two-thirds of the diet, with smaller sharks filling in the rest.

However, dietary preferences varied between individuals. One shark, for instance, had a diet consisting almost entirely of silky sharks.

“Resource availability is a key driver of space use, but why don’t all of them stay if food is so abundant? We found evidence of individual variation in their diets, so deciding whether to stay or go might depend on what they feed on,” said Guttridge.

“For other species, why or if migration has been linked to environmental conditions, body size, food availability, competition, and predation.”

“Perhaps, like salmon, there is a genetic component to it? We need to explore this phenomenon further.”

Movement patterns of hammerhead sharks

Guttridge noted that the global population of great hammerheads is thought to have reduced by more than 80% over the last three generations, and genomic analysis revealed low genetic variation and inbreeding.

“Understanding the movement patterns of great hammerheads is important for improving conservation efforts. We find that some great hammerheads show residency in the Bahamas, protecting them year-round from fishing pressures.”

The Bahamas shark sanctuary offers protection to sharks that stay within its borders, but not all sharks remain in these waters. That means local conservation efforts alone aren’t enough to protect the species.

“Our research showcases the importance of Bahamian waters and their protective measures. It also shows that 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,” said Guttridge.

“A key next step is seeing how these hammerheads fit in with the broader north-west Atlantic population.”

Understanding who stays and who goes – and why – may be the key to keeping the species alive.

The full study was published in the journal Frontiers in Marine Science.

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