Articles

Will you stay, or will you go

Bimini Biological Field Station (BBFS)

Words by Lauren De Vos

Cover image

Photo by Michael Scholl

Nomads are nothing new to the islands of Bimini. For adventurous souls with a taste for the outdoors, the islands in this archipelago have been a favourite escape into a quiet corner of The Bahamas that keeps its ‘authentic Caribbean flavour’. Novelists with a restless spirit, bohemian musicians and the philosophically inclined have all crossed the powerful Gulf Stream that separates the mainland coast of the USA from the westernmost islands of The Bahamas. The manifold charms of Bimini prove an irresistible lure to such visitors, some of whom return time and time again. From Ernest Hemingway, who famously penned The Old Man and the Sea here, to Martin Luther King Jr, who retreated to ruminate on his Nobel Prize acceptance speech, and guitar-toting Jimmy Buffett, who wrote Tales from Margaritaville on South Bimini in the 1980s, some of the region’s most famous guests might only have visited once, but those who were hooked loved to return.

As the hurricane season fades and the trade winds keep temperatures balmy, a shark called Gaia makes her way into these familiar waters. Secretive in her ocean home, she travels long distances over a vast sea. Although she is one of the islands’ repeat visitors, she has proved far harder to track than her terrestrial counterparts.

Bimini’s magnetic allure may persuade some iconic landdwellers to keep returning to these mangrove-edged islands, but it is the whereabouts of its ocean inhabitants that have scientists really scratching their heads. The comings and goings of the region’s marine creatures are the subject not of novels but of scientific studies that are probing their lives and behaviours and unveiling their mysteries. Perhaps akin to the spirit of Hemingway’s storytelling, biologists are now writing the biographies of the wilder citizens of Bimini’s waters. Unlike the margarita-sipping, marlin-fishing brigade, Gaia returns annually in winter. Finding out what lures her back each year and exactly where she goes when she leaves will enable scientists to better manage her species’ dwindling populations.

The movement patterns of Gaia and great hammerhead sharks Sphyrna mokarran like her is the subject of a publication by Tristan Guttridge and his co-authors in Frontiers in Marine Science. Tristan was the lab director and senior scientist at the Bimini Biological Field Station at the time that this research was conducted, and if you know anything about his passion for sharks, you’ll understand why he asks the kind of research questions that can uncover important findings to better inform the conservation of these elasmobranchs.

Tristan is equal parts zeal and concern as he explains the need for this kind of research. ‘We’ve never really known much about these sharks,’ he says. ‘There is still so much to learn. We’re just scratching the surface with these first insights.’ For him and his colleagues, the overarching question is ‘how do we best manage and protect great hammerhead sharks?’ Answering it takes time, patience and continual probing into where, when and why these sharks travel. Tristan’s article asks two questions. Firstly, do great hammerheads stay for prolonged periods in, or reliably return to, the coastal regions of The Bahamas and the USA? In other words, do they show site fidelity to these locations? And secondly, how much do they move between The Bahamas and the USA and how far do they travel?

Cover image

Photo by Michael Scholl

Lauren de Vos Project Leader

Tristan Guttridge Director of the Bimini Biological Field Station

Photo by Michael Scholl

Swinging its head from side to side to detect the minute electrical pulses emitted by stingrays as they hunt above the ocean’s sandy floor, the great hammerhead shark is an iconic creature. Named for the cephalofoil that forms the distinctive ‘hammer’ of its head, it ranges widely throughout tropical waters. ‘This species inhabits deep waters, shallow lagoons and coral reefs,’ writes Tristan, adding that it is ‘considered an upper trophic-level consumer’. Being a predator that moves long distances and possibly links different ecosystems would make the great hammerhead key to maintaining the stable functioning of the different regions it visits.

Its distinctive appearance notwithstanding, very little is known about the great hammerhead. Understanding more about how this shark lives is crucial if we are to confirm its effect on the ecosystem. What little we do know suggests that it is vulnerable to the effects of overfishing. It has what scientists call a ‘conservative life history’, tending to grow slowly, live long and have few offspring. This means that its populations are slow to recover after suffering a rapid decline.

It’s difficult to imagine a powerful predator with such a wide range being classified as Endangered on the IUCN Red List, but great hammerheads are caught as target species and by-catch wherever they are found. The result is that their populations are in decline in many places. Concerns about the species have been raised at international level and it is listed on both the Convention on International Trade in Endangered Species (CITES) Appendix II and the Convention on the Conservation of Migratory Species of Wild Animals (CMS) Appendix II. The listings raise the profile of the great hammerhead, making it easier to regulate legal trade in its products and to manage the fishing effort for it. They also emphasise that the discussion of this species’ conservation must be global. Great hammerheads pay no heed to the arbitrary political boundaries that humans have sketched onto maps to divide the ocean into territories. Managing their populations as they roam freely across the boundaries we’ve invented will need, as Tristan writes, ‘cooperation between jurisdictions to ensure great hammerheads receive necessary protection throughout their migrations’.

The great hammerheads of the western North Atlantic range from the balmy Florida Keys northwards to the oyster shores of coastal Virginia and eastwards to The Bahamas. The Bimini Biological Field Station (popularly known as the ‘Shark Lab’) on South Bimini is a hub of marine research in these waters and scientists have been recording great hammerheads here since the early 1990s. In fact, two particularly fascinating events have been reported: one great hammerhead was observed preying on a southern stingray Dasyatis americana and another on an eagle ray Aetobatus narinari. These sightings aside, visits from great hammerhead sharks were historically sporadic and unpredictable. Over the past 10 years, however, the Shark Lab team has been seeing great hammerheads more frequently.

Sharks might be resident in an area, staying there for prolonged periods of time. They might perform return migrations, moving away from a region that they’ve occupied for a period and then returning to it. They might occupy different places in different seasons or travel vast distances. Understanding the hows, whens and whys of shark movements can help scientists to choose the strategy best suited to addressing specific conservation challenges. Is prohibiting fishing in certain areas useful for protecting a highly mobile creature like a shark? If scientists know that sharks spend significant time in an area for a certain period, protecting that area makes a lot more sense. This goes a step further when one understands why an animal moves to a particular location. Is there food there? Is it breeding there? Is there a nursery? It is easy to understand how vital shark movement patterns are when it can be seen how this information underpins smart conservation strategies. Why then is there a distinct lack of these data for the great hammerhead and many species like it?

Tracking great hammerhead sharks is tricky. Tristan writes that they ‘move long distances, live in concealing environments, are logistically difficult to capture and, as upper-trophic predators, are naturally low in abundance’. And because they occur in low numbers, they are hard to find. However, ever-evolving technology provides scientists with new tools and the Shark Lab team took up the challenge with gusto. Tristan and his colleagues used a combination of methods to answer his questions over a period from 2012 to 2016. They were interested in two study sites: Bimini, and Jupiter in Florida. Great hammerheads at both sites were caught and fitted with either acoustic or satellite tags. In Bimini, divers also took underwater photographs of great hammerheads for photo identification. (Any permanent wounds or marks on a shark, as well as unique fin shape, can be used to distinguish an individual in a photograph and ultimately build a visual archive of identified sharks.)

Photo by Michael Scholl

Photo by Brian J. Skerry | National Geographic Image Collection

Acoustic tags emit sounds that are detected by receivers deployed underwater in a range of habitats along the coast. As a shark passes a receiver, its unique code is logged and it is identified as having moved into the region. Researchers can then decipher which habitats the shark spends most time in and how long it stays in an area. With the aid of renowned free-diver William Winram of The Watermen Project, sharks were tagged underwater in Bimini. The sharks tagged in Jupiter were given acoustic tags with a 10-year lifespan so that scientists can track cross-border movements. To investigate where these sharks move in summer and the extent of their migrations, satellite tags donated by Microwave Telemetry Inc. were inserted into five sharks. Satellite tags transmit GPS coordinates and depth, temperature and light-level information every 15 minutes to a satellite constellation that relays the data to the scientists. The tags are scheduled to pop off the shark after 30 days and they float on the ocean surface to be retrieved or are washed ashore.

Mapping the results of all these acoustic and satellite tags revealed the answer to the first question posed in Tristan’s article. Great hammerhead sharks of both sexes do indeed show site fidelity: many returned annually to either Bimini or Jupiter. Males and females tagged at both these sites returned reliably, ‘with some individuals recorded on up to four consecutive winter seasons’, notes Tristan. Not only did these sharks prove faithful to their favoured overwintering sites, but many individuals were actually resident during the season. These sharks followed Hemingway’s habits and hunkered down for up to five months at a time, a tendency scientists call ‘seasonal residency’. And what of Gaia? She’s been sighted at Bimini for five consecutive years. However, results from her external acoustic tag show that she is not simply a lady of leisure languishing in the comfort of Bimini’s waters. Gaia completed two consecutive voyages northward to South Carolina – a round trip of 1,600 kilometres (994 miles).

In answer to the scientists’ second question, Tristan writes that ‘great hammerheads were found to make regular movements across state boundaries, as well as between the US and Bahamas EEZs’. The Exclusive Economic Zone (EEZ) is a country’s designated coastal waters for economic activities. That sharks move between The Bahamas and the USA was evident in the journeys of some individuals between Jupiter and Key Largo in Florida and the Andros, Bimini and Grand Bahama islands in The Bahamas, usually towards the end of winter. The extent of their movements was far-ranging. The team found that great hammerheads travel epic distances – up to 3,030 kilometres (1,883 miles) – in what they dub ‘large-scale return migrations’. Some of these sharks returned from as far afield as Virginia, a return trip of about 3,000 kilometres (1,864 miles). These findings prompt the question why? This would be the next step to investigate.

As far as the sharks’ seasonal residency is concerned, Tristan has a preliminary hunch. ‘At our study sites, there was an absence of fresh mating wounds on female great hammerheads, or swollen claspers in males, which suggests that the main purpose of site fidelity in great hammerheads is not mating, pupping or use of natal sites.’ So they’re not there to mate, perhaps, but to feed? The mangroves of Bimini are nurseries for many other species of sharks and rays that are great hammerhead prey and the Gulf Stream brings warm, nutrient-rich water to the region that is rich in marine life. ‘Both our study sites are productive systems, prompting the hypothesis that great hammerheads return for feeding opportunities,’ writes Tristan. This makes sense if one recalls the earlier Shark Lab sightings of great hammerheads hunting a southern stingray and an eagle ray. What about when they’re moving between The Bahamas and the USA, or travelling long distances while making return migrations? Changing water temperatures and the resultant presence of prey like blacktip reef sharks have been suggested as a possible explanation. Some pregnant females observed on these routes offer the possibility that they may be travelling to give birth.

What does all this mean for great hammerhead sharks? Tristan notes that ‘the predictable, seasonal, return-use of specific locations, areas or migratory routes enhances vulnerability to spatially focused fishing’. That great hammerheads are resident in or return to overwintering sites in the USA and The Bahamas is vital information that can help define what scientists call ‘essential fish habitat’ or, as the National Oceanographic and Atmospheric Administration (NOAA) records: ‘those waters and substrate necessary to fish for spawning, breeding, feeding or growth to maturity’.

Armed with this knowledge, it is possible to look at where and when the majority of fishing activity happens and match it to where great hammerheads are moving. This could inform time-area fishing closures to improve fisheries management or help designate protected waters. If females are indeed pregnant and moving to pupping sites, protecting them as well as the breeding and nursery sites could be important. Knowing whether they are returning to their birthplace to pup would provide vital information about gene flow across the Atlantic. Finally, the evidence that these sharks move across state boundaries underscores the need for geopolitical coordination and cooperation.

In grainy, black and white photographs of famous travellers to The Bahamas, Ernest Hemingway grins roguishly as he poses next to unfathomably large marlin, tuna and mako shark trophies. It’s a strange irony to think that Gaia and the great hammerheads of the region have been ocean voyagers for far longer than Hemingway and his kind. They have roamed these same waters so beloved and pilfered by adventurers, fishers and storytellers without any fanfare or popular records of their travels. It’s perhaps a strange irony then, that the story scientists are beginning to tell about Bimini’s ocean travellers will now be with the aim of saving their populations from overfishing.

Photo by Brian J. Skerry | National Geographic Image Collection