Sun, sand and apex predators: taking the plunge with oceanic whitetip sharks
My face is pressed up against the window and my brow is furrowed. For someone about to land in the Bahamas I look surprisingly troubled. I am trying to figure out the size of the swell and the prevailing wind direction from 10,000ft up in the air. For the last week I have been obsessively refreshing the forecast page for Cat Island, hoping that a small weather window will appear.
I’m here to dive with the oceanic whitetip shark – Carcharhinus longimanus, the migratory circumtropical pelagic apex predator. Historically, the oceanic was a highly abundant species, but more recently it has undergone severe population declines. Globally across its range, it is listed by the International Union for the Conservation of Nature (IUCN) red list as vulnerable, and critically endangered in the north-west and western central Atlantic. This reduction in numbers is a result of fishing pressures, most likely related to the global shark fin trade, for which it is targeted and highly prized due to its large pectoral fins, from which the shark gets its name: “longimanus”, meaning “long hands”.
In fiction the oceanic has been immortalised as the shark species most likely to be the one described in Ernest Hemingway’s The Old Man and the Sea. Indeed, in fishing tournaments held today, the oceanics are the first to turn up looking for the opportunity of an easy meal from a game fish caught on the line.
Historically this species is thought to be responsible for causing the most human fatalities. They have been documented as the species which predate on people adrift in the open ocean after shipwrecks or air crashes. Probably the most well-known incident followed the torpedoing of the USS Indianapolis in 1945, when oceanics were thought to be responsible for the deaths of up to 800 sailors. However, it has since been suggested that most sailors died of exposure and the sharks predated on their bodies. Either way, this is characteristic behaviour of an opportunistic open ocean predator.
More recently oceanics have become a huge attraction for divers, particularly in the Bahamas; the shark population here is well known for its bold, unhesitant approach, and is the reason I am here.
Despite being critically endangered in the north-west Atlantic, the oceanics that come to the waters around Cat Island from April through May each year are highly site-specific. That means it’s not the sharks that you have to worry about but the weather! For most experienced divers the wind speed and subsequent swell is not too much of a problem, but due to the nature of the dive – an open ocean drift – too large a swell and the boat crew cannot see you at the surface. I have been exceptionally lucky in the past on shark diving trips, and was hoping my luck wouldn’t run out now.
That evening I chatted to the owners of Epic Diving, who told stories about the oceanics and their endearing counterparts, the pilot fish Naucrates ductor, which left me praying to Poseidon for calm seas as I fell asleep that night. The wind and swell did not drop, but neither did it increase, and I could hardly believe my luck when we left the dock that morning. Almost immediately we spotted a tiger shark, Galeocerdo cuvier, cruising the shallows, and it was all I could do to not ask if everyone would mind if we stopped the boat so I could jump in quickly to take a couple of shots. But with the weather supposed to change by the afternoon I knew I had to stick to my target species.
Upon reaching the dive site the crew placed a couple of dead snapper in a closed plastic crate, and attached a rope with a buoy tied on so the crate would hang at a depth of about 10 metres. The crate was placed in the water and the rope tied temporarily to the stern as we idled along in water just shy of 1000m deep. After around 20 minutes, three oceanics turned up and it was time to kit up and get in. I was like a coiled spring: I’d heard so much about the oceanics’ behaviour: how bold they were, how close they approached, how you should assert dominance; would they prove too much for me?
In less than a minute a large female came on the approach. I fired off a few shots and then observed her behaviour to get an idea of what I could expect: head on approach, close but not uncomfortably so; she turned before I felt the need to react.
During the course of the day’s diving this first female was a good example of most of the sharks. At any one time 12 oceanics were visible, as well as a very bold silky shark, Carcharhinus falciformis. Some were more timid and turned away after reaching a proximity of about two metres, others didn’t turn until they were less than a camera’s depth away, but not once did they make contact with me or my camera housing.
These constant close passes allowed me to have a really good look at these sharks, something which is not possible with many other species. Given the fact that I was about 15 metres or so up-current from the bait crate I had the distinct feeling they were purely checking me out – and not because I smelled like fresh snapper. They were clearly looking at me, and maintained eye contact as they swam past.
Intriguingly, as we drifted over a seamount that had risen up from the depths to provide a bank at around 13 metres or so, the oceanics turned and swam against the current away from this shallow reef, appearing not to want to go over it. Even though they weren’t actively utilising the available water depth, it was as if they at least preferred having the option to do so.
Despite growing conservation concerns, very little was known about the movements and habitat usage of oceanic whitetips in the Atlantic until 2013, when a study was published using data collected by pop-up satellite archival tags (PSATs) attached to 11 mature oceanics near Cat Island. The sharks stayed within 500 km of the tagging site for an average of 30 days before dispersing across 16,422 sqkm of the north-west Atlantic. The sharks spent 99.7% of their time in water depths of less than 200 metres. This is significant because while sharks are resident within the Bahamas exclusive economic zone (EEZ) they are protected from targeted fishing – the commercial trading and longlining of sharks within these waters is illegal. This study highlighted the fact that when oceanics leave these protected between June and October, they become most vulnerable to fishing gear deployed from 0-125 metre depths.
The reasons behind why oceanics return to the Bahamas, and in particular show high site fidelity to the waters around Cat Island, was investigated in a study in 2015. This showed that gatherings consisted of adult individuals, that females were more common and more than half were pregnant. Dietary analysis showed that the oceanics were predating on more large pelagic teleosts (72%) than in the sharks’ long-term diets (47%). This suggests that the availability of these large prey is something that keeps bringing the oceanics back to Cat Island.
It was this level of site fidelity which drew me to the Bahamas in search of this once-abundant top predator, and I’m not the only one. A study in early 2017 established that the Bahamas dive industry is the largest in the world, contributing approximately US$113.8m annually to the Bahamian economy. Elasmobranch tourism generated 99% of the total revenue, and the balance came from film, television and research. The Bahamas is a unique refuge for sharks in the Caribbean, largely due to a ban on longlining in the early 1990’s, followed by the establishment of the Bahamian shark sanctuary in 2011.
As I walked along the shoreline at Cat Island the following day, I thought about how the stewardship shown by the Bahamian government ensured that their waters remained a carefully managed habitat for many shark species, and how there is also a real need for regional Caribbean-wide commitment to the management of highly migratory species. Of course with current estimates indicating that approximately 24% of all chondrichthyan species are threatened with some risk of extinction , this undoubtedly needs implementing on a global scale.
Howey-Jordan, L.A., Brooks, E.J., Abercrombie, D.L., Jordan, L.K., Brooks, A., Williams, S., Gospodarczyk, E. and Chapman, D.D., 2013. Complex movements, philopatry and expanded depth range of a severely threatened pelagic shark, the oceanic whitetip (Carcharhinus longimanus) in the western North Atlantic. PLoS One, 8(2).
Madigan, D.J., Brooks, E.J., Bond, M.E., Gelsleichter, J., Howey, L.A., Abercrombie, D.L., Brooks, A. and Chapman, D.D, 2015. Diet shift and site-fidelity of oceanic whitetip sharks Carcharhinus longimanus along the Great Bahama Bank. Marine Ecology Progress Series, 529, 185-197.
Haas, A.R., Fedler, T. and Brooks, E.J., 2017. The contemporary economic value of elasmobranchs in The Bahamas: Reaping the rewards of 25 years of stewardship and conservation. Biological Conservation, 207, 55-63.
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