My first meeting with Cat Island’s whitetips took place about a mile offshore on a calm, clear spring morning. Edd and Sean had taken one of the institute’s boats, the Cobia, out at dawn and had started chumming. As the rest of us packed up a smaller skiff, the vessel we planned to tag sharks from, they called us over the radio to let us know that five big oceanic whitetips had already found them. I was pretty anxious to get out there; having worked with a wide variety of shark species, I know that sometimes you have only a few minutes to see them before they move on. Now that I have worked with a large number of the Cat Island whitetips, I know that we could have stopped for a cooked breakfast without losing them. These sharks shadow boats for hours on end.

After a half-hour ride, I stood on the bow as we approached the Cobia. The ocean’s surface was glassy and almost immediately I could see the large brown shapes cruising behind the bigger boat. Two sharks broke off and swam directly to our skiff, their round fins breaking the surface as they slowly approached. They were both mature females, the most common life-stage we see at Cat Island, where all the sharks are adults and males are outnumbered by about 5 to 1. In later years, Brenda Anderson and Dr Jim Gelsleichter from the University of North Florida would ultrasound many of the females and show that more than two-thirds of them are carrying young in spring. I had been warned that these sharks were bold and the pair that followed us more than lived up to this reputation, closely circling the skiff and giving us a good look. By the time we docked with the Cobia I was thinking to myself, ‘Who’s studying whom?’

First impressions are usually good impressions. After hundreds of encounters I can unequivocally say that the oceanic whitetip is the boldest of all the shark species I have worked with, which include many other apex predators like white, tiger and bull sharks. Unlike these species, the whitetip spends nearly all its life in open blue water, where prey is patchy and very hard to find. Even in the vicinity of Cat Island and its fringing reef, the local whitetips do not feed on reef fish, as Dr Dan Madison of Stony Brook University has shown by using chemical tracers. The research is ongoing, but it is likely to indicate that the whitetips’ diet consists mainly of other pelagic fish. This could explain the species’ boldness, which may ensure that it takes advantage of a potential feeding opportunity whenever one occurs. Or it could be that whitetips at Cat Island have a higher probability of being hungry than related coastal sharks, which live in a more productive habitat.

That is not to say that whitetips immediately attack potential prey. Far from it; they often circle for hours before feeding. When they arrive, however, they are dogged in their approach and closely circle anything they think might be food. When they ultimately decide to feed, often prompted by a critical mass of several sharks accumulating in the area, they are aggressive and efficient. We have seen sharks 2.5 metres or more in length eating individual pieces of fish smaller than a scrap of sashimi that you might find on your plate.

From the beginning, our primary research question has been: where do these sharks go when (and if) they leave Cat Island? This is interesting from a conservation perspective because they are ‘safe’ while they are in Bahamian waters (commercial shark fishing is not permitted), but if they leave they may encounter commercial fishing gear such as long-lines. We are also interested in the purpose of their migrations: do they move away from Cat Island to mate and give birth, since we never see young sharks or evidence of mating at Cat Island?

The answers to these questions have started to materialise. Having fitted 11 females with PSATs in the first year of the study, we were able to reconstruct their movements and decipher some patterns.

First, about one-third of the tagged sharks stayed in The Bahamas for the whole year. This was a surprise, given the fact that oceanic whitetips are characterised as a highly migratory species. Two-thirds of the tagged sharks, however, lived up to this characterisation by travelling thousands of kilometres from The Bahamas for the summer, when the sea surface temperature through most of the western Atlantic warms to above 26oC. Some of these sharks went north, to an area between North Carolina and Bermuda. The rest headed east to a section north of Puerto Rico, and some of these later continued to the northern area.

All these migratory sharks returned to The Bahamas in the autumn months. We even saw some of them in subsequent years back at Cat Island, a stone’s throw from where they had been tagged. In later years we increased our sample to nearly 90 sharks and, with a few exceptions, found the same three migration patterns. We now suspect that some of these large movements may relate to reproduction.

Regardless of their underlying motivation, whitetips are mobile but clearly spend much of the year in The Bahamas where they are relatively safe. Perhaps this is why we can still find them there, in contrast to so many other regions where the species has been depleted. In any event, this finding helped inspire The Bahamas to argue for stricter international controls on the trade in oceanic whitetips, which were ultimately adopted under the Convention on International Trade in Endangered Species (CITES).

We still have a lot of work to do because oceanic whitetips are full of surprises. Although these sharks normally spend almost 100% of their time in the upper 200 metres of the water column, the PSATs have logged them occasionally diving to below 1,000 metres. The deep dives almost always occur at night and fit a profile whereby the shark descends quickly, spends a few minutes at depth and then ascends slowly.

Why would whitetips venture into cold, dark water and spend hardly any time there? Prey would be rare at those depths. Researchers studying other large marine vertebrates, including other shark species and tuna, have recorded similar deep diving, so it is possible that such dives incur a universal benefit. If we can work out why the whitetips dive like this, perhaps this universal benefit will be revealed.

Remaining mysteries aside, we are in an exciting phase of our research: a portrait of Cat Island’s oceanic whitetips is starting to materialise. These blue-water apex predators can be highly migratory, but their voyages seem to have purpose, with clear way stations that the sharks are able to relocate year after year. This trait may help humankind save this species by enabling countries like The Bahamas to successfully enact protective legislation for them. If other countries follow suit and cooperate on conservation, perhaps the oceanic whitetip will once again exert its rule over the world’s blue waters.