Home sweet home: some great hammerhead sharks stick to the perfect neighborhood in the Bahamas instead of migrating

(Press-News.org) New research shows that some great hammerhead sharks are homebodies. Scientists studying great hammerheads around Andros in the Bahamas shark sanctuary have found that while some individuals migrate, others prefer to stay at home — potentially because their environment provides them with everything they need. This information could help protect the critically endangered species. 

“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,” said Dr Tristan Guttridge of Saving the Blue, lead author of the article in Frontiers in Marine Science. “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.” 

Fishing for information 

Great hammerhead sharks have been recorded making journeys of 3,000km. However, 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 is much less well-known. The scientists set out to investigate this, focusing on Andros Island, the largest in the country.  

“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?” 

Using tags and support from Discovery's Shark Week, 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. They measured, sexed, and took small muscle biopsies from each shark, and attached satellite trackers to seven sharks. 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.  

Sharks favored eastern waters just off north and central Andros for long-term use, particularly preferring a comparatively small 400km2 area of water. However, there were also patches of increased use elsewhere around the island: edge habitats near the reef drop-off, rich in prey species, were 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 which left the Bahamas traveled to the east coast of the US, suggesting they could be part of the west Atlantic population of great hammerheads.  

A great hammerhead menu 

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. However, different sharks had different preferences. One individual had a diet that was almost two-thirds silky sharks. Plentiful prey, combined with access to deep waters that remain cool during the summer wet season, might enable these sharks to reside in the Bahamas year-round, unlike hammerheads near Bimini.  

“Resource availability is a key driver of space use, but why don’t all of them stay if food is so abundant?” said Guttridge. “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,” added Guttridge. “Perhaps, like salmon, there is a genetic component to it? We need to explore this phenomenon further.” 

“Our research showcases the importance of Bahamian waters and their protective measures,” concluded Guttridge. “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.” 

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