One of the missing pieces in the global jigsaw in understanding whale sharks has been discovered around the isolated island of St Helena in the South Atlantic.
Marine biologists from Georgia Aquarium in Atlanta, USA, had been studying whale sharks since 2005, mostly around the Yucatán Peninsula in Mexico, where up to 400 individuals gather together in the largest whale shark aggregation known to science.
These coastal aggregations, however, tend to be comprised almost entirely of juvenile males, Dr Alistair Dove, Georgia Aquarium’s vice president of research and conservation, told DIVE.
The team wanted to look further afield for ‘the missing parts of the demographic pie,’ said Dr Dove. ‘Where are the really big ones? Where do they mate? Where do they pup? The kind of basic questions that we didn’t have answers for.’
‘That led me to look into the South Atlantic,’ he continued, ‘but there are not that many places where they could go. There’s really only Ascension, St Helena and St Peter and St Paul Rocks in Brazil.’
The small and remote island of St Helena – zoom out to see exactly how remote! (Google Maps)
Following a call to St Helena’s marine research division, resident marine biologist Elizabeth Clingham was invited to Georgia Aquarium for the Third International Whale Shark Conference, hosted by Dr Dove in 2013.
‘Liz shared with me their amazing sightings database,’ said Dr Dove. ‘Ten years of whale shark sightings, and in there were a couple of eyewitness accounts of mating behaviour, which has never been recorded anywhere else in the world.’
There and then, Dr Dove decided to visit St Helena during the annual aggregation of whale sharks from December to March. Although the team did not document any mating behaviour during their two subsequent expeditions, they gathered enough evidence to suggest that St Helena plays a key role in the reproductive cycle of whale sharks.
‘It is one of the only places – if not the only place – in the world where you get equal numbers of adult male and female whale sharks in the same place,’ said Dr Dove. ‘Everywhere else is either male-dominated juvenile aggregations along the coast, or else the giant, pregnant-looking females that we see in the Galápagos where it’s 10:1 female to male ratio.’
A global pattern of whale shark population distribution is emerging, in which research suggests that juveniles tend to stay close to shore, feeding on plankton blooms generated bynutrient-rich upwellings along the continental shelf. In these locations, it’s all about the fish growing as large as possible before heading off into the open ocean.
As to why St Helena attracts the whale sharks for mating, Dr Dove says that he suspects the island ‘is like a beacon in the open ocean, where just finding somebody to mate with is a pretty significant challenge. So when there are places out there, in the middle of all that blue, where you can get together with a mate and fulfil your prime directive, then naturally those are places where they tend to aggregate.’
A tagging project in 2016 showed that the majority of the visiting whale sharks stay around St Helena for at least three to four months, diving to 600m or more on a daily basis before dispersing into the open ocean; but the 50:50 male-to-female ratio is the key to St Helena’s importance in the global story of whale sharks.
Comparing whale shark research to an incomplete jigsaw puzzle in which St Helena is like finding a corner piece, Dr Dove says that this helps to contextualise what’s happening in the rest of the world. Now that researchers know that adults congregate around small islands in the open ocean, they can begin to look at similar locations elsewhere.
‘Isolated though it is,’ says Dr Dove, ‘St Helena has global-level importance for this species, and it is my sincere hope that everybody who lives in St Helena, and everybody who visits, regards the whale sharks of St Helena as a national treasure.’
There are a number of whale shark aggregations that take place aound the world. Here are five of the most noteable:
As many as 400 individuals – mostly immature males – gather each year from June to September around Isla Mujeres and nearby Isla Holbox, the largest aggregation known to science. The sharks gorge on plankton and fish eggs.
Mature females – more than 90 per cent of which appear to be pregnant – gather around the northern islands of Wolf and Darwin between June and November each year, before heading out to unknown destinations in the Pacific.
Red snapper spawning for a ten-day period at some time between April and June brings mostly young male whale sharks in large numbers to Gladden Spit along the Belize Barrier Reef, where they feed on the snappers’ roe.
Upwards of 150 juvenile sharks, some as young as six months and just a few metres in length – again, mostly male – have been spotted feeding in groups on the rich plankton blooms that circulate around the Gulf of Tadjoura in January.
The 1000th individual whale shark to be identified in Philippine waters was recorded in 2016. Regular sightings occur in Southern Leyte, Palawan and Donsol between November and June. Mostly immature males.