Inside Stories

Dive tourism dynamics

Bimini Biological Field Station (BBFS)

Vital Heim

Words by Vital Heim

Cover image

Photo by Matthew J. Smukall

Large-bodied animals often occupy high positions in the food web and, if they are apex predators, they can shape ecosystems. Yet the populations of many such animals are in severe decline because their habitat is being destroyed by human activities or they are being harvested. In the marine realm, large sharks are especially vulnerable because their growth rate is slow, they mature late and their reproductive rate is low. As a result, many shark populations around the world are shrinking significantly. There is increasing evidence that the loss of an apex predator – the function of some shark species as the predator right at the very top of the food chain – can have a far-reaching impact on an ecosystem.

Diving with sharks is gaining in popularity in many parts of the world and nowhere more so than in The Bahamas, which is bucking the trend of global declines in shark populations. Thanks to conservation measures such as a ban on commercial long-line fishing in 1993 and the declaration of The Bahamas as a shark sanctuary in 2011, the populations of various shark species here are healthy – and contributing to the island nation’s popularity as a diving destination. Of the more than US$100-million generated by the Bahamian dive industry each year, a large proportion comes from shark diving.

Many sharks are by nature both rare and elusive, so food is used to attract these marine predators to locations where ecotourists can experience close encounters with them. As yet, however, little is known about the potential effects of provisioning on both the targeted sharks and other species in the vicinity.

The great hammerhead Sphyrna mokarran is a large shark that inhabits coastal waters and open seas in tropical regions around the globe. Typically solitary, it is rarely seen in the wild, yet every winter numbers of this charismatic apex predator congregate in Bimini, a group of small, mangrove-fringed islands at the western edge of the Great Bahama Bank. Divers from all parts of the world are drawn to this seasonally resident population of great hammerheads – and pour into the local economy more than half of the annual revenue generated by shark diving in Bimini.

Its apparently healthy population around Bimini notwithstanding, the great hammerhead is classified as Endangered on the IUCN Red List of Threatened Species. Previous research conducted by the Bimini Biological Field Station Foundation (BBFSF) found evidence that several individuals return to Bimini each season. Visual identification played an important part in this research and an ID database currently comprising 35 great hammerheads has been set up. This could be done because individuals have certain characteristics that remain consistent over many seasons: distinctive fin shapes, notch patterns on the first dorsal fin and spot patterns on the belly, for example. Newly identified sharks added to the database each year improve our knowledge of the population’s size and structure.

It is important to understand what could impact the behaviour or movements of the great hammerhead, but no work had been done to investigate the effects of feeding and dive tourism on this species. Other studies on the provisioning of sharks indicated that the impacts were site-specific and species-specific, and even that they may have varied from one individual to another. Our research aimed to describe the behaviour of the great hammerheads at the feeding site in Bimini and determine whether the provisioning and the daily dive tourism activities had an impact on how they used the local space. Thanks to our collaboration with local dive operator Neal Watson’s Bimini Scuba Center, we were able to collect data on every commercial dive led by the company.

Cover image

Photo by Matthew J. Smukall

Vital Heim Project Leader

Photo by Sophie Hart

Because they were not able to distinguish between individual sharks, previous provisioning studies could record neither the amount of bait consumed by each shark, nor how often it returned to feed during each session. Our ability to identify individuals gave us an edge, enabling us to record the number of bait pieces – and hence the weight – taken by each animal. We documented how much bait each shark consumed, how long it spent at a provisioning event, how long before it took the first food reward after arriving at the site, and its reaction to both biotic and abiotic factors.

Additionally, we used passive acoustic telemetry to find out how individual great hammerheads moved around Bimini. Of the nine sharks monitored by means of internal acoustic tags, two have been detected within the BBFSF acoustic receiver array but have not been observed at the dive site. Since our study, like other provisioning research, lacks baseline data on the ecology of the species before tourism activities began – in our case in 2012 – these two sharks provided us with a reasonable control group.

To assess to what extent the sharks responded to the provisioning activities, we established two indices. A provisioning index was calculated as the number of times a shark attended a provisioning event divided by the total number of events it could have attended. The feeding index was the product of an individual’s provisioning index multiplied by the average amount of bait it consumed at each event.
During 104 provisioning events held over a period of 94 days we tallied 483 observations of great hammerhead sharks involving 28 individuals. There was remarkable variation in the number of dives the sharks attended. Four females that had been known in Bimini for up to four years before our study were present at more than 50% of the provisioning events. In contrast, recently identified hammerheads attended the events less frequently.

Differences were found in the amount of bait the sharks took each day. Predictably, the length of the provisioning event was a factor, as was the amount of time the sharks were present. It was also determined that individuals were capable of eating a substantial percentage of their body weight. Nemesis, a female well known to us, was reported to have consumed 101 pieces of bait weighing 16 kilograms (35 pounds) during a diving event that lasted nearly four hours. We estimated that she weighed about 200 kilograms (440 pounds), so on this occasion she ate 8% of her body weight.

By looking at the movement patterns of the great hammerheads within the local area, we learnt that they could be affected by the amount of time the sharks spent at provisioning events. The movements of sharks that ate more and spent more time at the provisioning site tended to be more restricted around the site, whereas the core areas of sharks with a lower feeding index were more widely distributed within the environs of Bimini. Our findings suggest that providing food to attract sharks impacts the local movements of individual sharks to a varying degree.

Given what we have learnt about the feeding behaviour and movement patterns of provisioned sharks, our future focus will be on the ecological impacts of provisioning. The effects of predators on the demography of their prey and on interactions within the food web, as described by previous studies, mean that changes in the natural movements of large sharks may have repercussions for animals at lower trophic levels. We will look at the extent to which wildlife tourism activities affect how an ecosystem functions by altering the role of apex predators.

Does the provisioning enable Bimini’s great hammerheads to meet their daily energy demands or do they still hunt local prey species? Are we creating a hotspot for great hammerheads where they would normally not spend much time? Does that affect prey species through changes in normal predator–prey relationships? Will the sharks begin to arrive earlier and leave later in future seasons as a response to the provisioning? Could provisioning affect the total amount of time they spend in Bimini and subsequently their annual regional migrations? These are all questions we will try to answer.

Photo by Sophie Hart

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