Scientists reveal why some sharks are ‘homebodies’

By Stephen Beech

Some sharks are "homebodies" who never leave their "perfect" Caribbean hunting ground, reveals new research.

Scientists tracking endangered great hammerheads found that Andros Island, in the Bahamas, is a year-round refuge for some individuals who choose not to migrate

The research team believe some of the sharks prefer to stay at home because their environment provides them with everything they need.

They say their findings, published in the journal Frontiers in Marine Science, could help protect the critically endangered species.

Study lead author Dr. Tristan Guttridge, of conservation group Saving the Blue, said: “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.”

He said that great hammerhead sharks have been recorded making journeys of 3,000 kilometers (1,860 miles).

But they have also been recorded staying in areas that are particularly favorable, including Bimini in the Bahamas, where great hammerheads overwinter but leave in the summer.

But the potential importance of other Bahamian islands to the sharks, their role in local food webs, and whether they stay there long-term was not so well-known.

The research team focused on Andros Island, the largest in the Bahamas.

Dr. Guttridge said: “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.

“We had no answers to simple questions like, what do these sharks eat?”

Using tags, the team captured 22 sharks between March 2020 and June 2024.

They chose sites that reflected great hammerheads’ usual habitat preferences and placed baited block rigs there.

The team measured, sexed, and took small muscle biopsies from each shark, and attached satellite trackers to seven of them.

As the stress of capture affects great hammerheads severely, they assessed the health of each shark before deciding whether to tag them.

Data from captured sharks was combined with records of opportunistic sightings, to provide a total of 78 shark encounters between 2018 and 2024, mainly large juveniles and adults.

Half of the sightings occurred between January and March, although a significant number were spotted between June and July.

Two sightings provided evidence of recent mating, indicating the area could be important for reproduction.

Several sharks were re-sighted in the same locations, and some were captured in the same locations several years apart.

Dr. Guttridge said sharks favored eastern waters just off north and central Andros for long-term use, particularly preferring a comparatively small 400 square kilometer area of water.

But there were also patches of increased use elsewhere around the island, with edge habitats near the reef drop-off, rich in prey species, being particularly popular.

If the sharks swam south, or past the northern tip of the island, they were much more likely to continue in transit.

Sharks that left the Bahamas traveled to the east coast of the US, suggesting they could be part of the west Atlantic population of great hammerheads.

Isotope analysis of the biopsies indicated that barracuda and stingrays made up about two-thirds of the sharks’ diet, while the rest was comprised of smaller sharks.

But different sharks had different preferences. One had a diet that was almost two-thirds silky sharks.

Dr. Guttridge says plentiful prey, combined with access to deep waters that remain cool during the summer wet season, might enable the sharks to reside in the Bahamas year-round, unlike hammerheads near Bimini.

He said: “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.

“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.”

Dr. Guttridge added: “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.

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