In collaboration with the Royal Society for the Protections of Birds (RSPB) and the Tristan da Cunha government, National Geographic Pristine Seas recently conducted an expedition to Tristan da Cunha and its surrounding islands. Our team conducted comprehensive surveys of the health of its largely unknown marine environment, and will produce a documentary film to highlight this unique ecosystem and the people that steward it. Learn more about the expedition.
Text and Photos by Chris Thompson
Blue sharks (Prionace glauca) are one of the great ocean wanderers. These slim, graceful sharks with large eyes and brilliant blue backs are known to make journeys of up to 9,200 km (5,700 mi) with some individuals making multiple trans-Atlantic crossings.
Unfortunately these incredible travelers are at great risk from modern fishing practices. They are the most heavily fished shark in the world with many millions taken annually. They are predominantly caught as bycatch as their meat is little valued. However, their large scythe-shaped fins are highly prized in the fin trade and thus they are often taken for their fins—their carcasses discarded at sea.
While their IUCN status is near threatened, a reduced catch rate of 60-80 percent has been reported in recent years and the available data is considered inadequate to accurately assess global population declines.
The footage collected on our pelagic cameras, however, suggests that the waters of Tristan da Cunha may provide a refuge for these gentle giants. As part of the Pristine Seas science program we have been using remote cameras known as mid-water stereo-BRUVS (baited remote underwater video systems) to non-destructively sample the pelagic wildlife found in the open ocean. These rigs are allowed to drift in offshore waters for two hours per deployment, allowing us to identify, count, and measure the pelagic wildlife occupying these waters including sharks, whales, dolphins, turtles, tunas, and forage fish. These unique camera systems allow a glimpse into one of the most understudied habitats on the planet: the open ocean.
During this expedition by far the most prevalent species we have encountered on the footage is the blue shark. Tristan da Cunha’s waters harbor more blue sharks than we have seen in any of the other locations we have sampled all over the world, and what is most interesting is the composition of the blue shark population here.
We are seeing only large females and very small juveniles, suggesting that the waters of Tristan da Cunha might be a blue shark nursery ground with large females traveling here to give birth; the lack of intense fishing effort seen in other parts of the world providing a sanctuary for the pups to grow in peace before undertaking migrations of their own.
In addition to the sharks seen on camera, the team has also managed to tag three large female blue sharks. This will enable us to learn more about the movements of these animals and see whether they are resident to Tristan waters or if they make the long migrations their kin are known to undertake.
Blue sharks may not be the only sharks using Tristan waters as a nursery ground, a single small Porbeagle shark pup (Lamna nasus), a smaller relative of the great white and mako known predominantly from the population surrounding the U.K., was also recorded on camera. At approximately 60 cm (2 ft), this observation is a first for Tristan waters and suggests it was likely born in these waters.
It may be that the very fact of Tristan being the most remote inhabited island in the world is what provides the protection these mothers and their young need from the fishing fleets which they are so vulnerable to.
If going forward these waters continue to be protected from offshore fishing it could provide a much needed refuge for these sharks and other pelagic species.