Off Remote South Atlantic Archipelago Ocean Species and Habitat Face Threats

Off Remote South Atlantic Archipelago Ocean Species and Habitat Face Threats
Ocean bird
The Pew Charitable Trusts

In the remote reaches of the South Atlantic Ocean, between South Africa and Antarctica, sits Tristan da Cunha, an archipelago comprising four islands: Gough, Inaccessible, Nightingale, and Tristan. Tristan da Cunha is home to 85 percent of the world’s population of northern rockhopper penguins, as well as a globally significant array of seabird colonies, including albatrosses, petrels, and penguins.

Around 250 people live in Tristan da Cunha, and, despite its remote location, the archipelago faces threats from climate change and marine pollution. Great British Oceans, a coalition including The Pew Trusts and other UK nonprofit organizations, and the Pew Bertarelli Ocean Legacy Project are working with those residents and the UK Government to establish a locally managed marine protection regime around Tristan da Cunha, set to be announced later this year.

To learn more about the islands, Johnny Briggs, a senior officer with the Pew Bertarelli Ocean Legacy Project, caught up with Sue Scott, a marine biologist and explorer who has visited Tristan da Cunha on multiple occasions to study its biodiversity. This interview has been edited for clarity and length.

Q: Tell us about your work in Tristan da Cunha’s waters.

Sue Scott

A: I have been lucky enough to visit Tristan 11 times, organising seashore and diving surveys at all four islands. Our main work was to photograph, identify, and catalog the marine life on the seabed, as there had been no comprehensive surveys of that area. I also participated in a British Antarctic Survey visit to sample the marine life in deeper waters, between 100 and 300 metres of depth. I also advised the Tristan government and the islands’ commercial lobster fishing company on the impact on marine life and the response following two shipping accidents. The first accident in 2006 saw an oil rig stranded on Tristan—it was carrying at least 60 species of nonnative marine animals—and the second was a cargo ship that crashed into Nightingale in 2011, spilling 65,000 tons of soya beans over the seabed, smothering benthic marine life. It also spilled 1,500 tons of fuel oil, killing around 3,000 northern rockhopper penguins as well as polluting the lower seashore, a main habitat for young lobsters.

Q: What challenges have you faced when conducting your research?

A: Just getting to Tristan and transporting all the diving and survey equipment is a major challenge, as it takes six days by fishing boat to get there from Cape Town. More than once I’ve been stranded on Tristan due to a lack of ship space for the return leg to South Africa. Weather is a major factor limiting diving and shore surveys. Very little of the island is easily accessible on foot, and boats are needed to access many of the survey sites. Tristan is a very windy place, and there are many days when getting a boat out of the tiny harbour is not possible. Help and advice from islanders and the local fishing company is key, especially to reach Gough, which is 200 miles southeast of Tristan. I feel privileged to have worked in such an amazing place.

Q: What is the Tristan da Cunha marine ecosystem like?

A: Extremely isolated, extremely exposed, very impoverished in terms of numbers of species, as is usually the case for isolated islands, but those species that survive there are often abundant. The waters surrounding the islands are relatively intact, far from the usual sources of man-made pollution on continental coasts—although even here, plastic rubbish inevitably washes ashore.

Q: What are the key habitats and species?

A: On the shores and in the shallow subtidal zone (down to 50 metres or so), forests of giant and pale kelp provide a sheltered and productive habitat for fish, lobsters, and a wide variety of other life. Meadows of smaller seaweeds harbour numerous small animals. Seaweeds provide a key habitat for rock lobster larvae that need to hide from predatory fish. The islands are surrounded by bedrock slopes with the products of marine erosion—boulders, cobbles, and sand—in varying proportions, generally sloping steeply into deep water of over 2,000 metres within a few kilometres of the coastline. We have much less knowledge of the life in deeper water habitats. Abyssal plains cover much of the offshore area. The seamounts arising from these plains are likely to be key areas for a diversity of benthic life. The open waters around the islands are home to a variety of pelagic marine life, with some charismatic species including blue sharks and Shepherd's beaked whales.

Up to 40 percent of Tristan seaweeds are endemic to the archipelago, and we are still working on their identification. The Tristan rock lobster is fascinating and incredibly abundant. Their larvae spend up to a year drifting in the open ocean, then somehow find their way back to Tristan; how they do this is still unknown. Of the larger megafauna, octopus, broad-nosed seven-gilled sharks, fur seals, and penguins are all fun to meet underwater!

Q: How do invasive species affect the islands and marine life?

A: Humans, rats, mice, and domestic animals on Tristan, feral pigs on Inaccessible, and mice on Gough have decimated seabird populations in the past. Nightingale is the only island that has never had introduced mammal species. Here, seabird populations have remained healthy enough to sustain an annual harvest by islanders. Mice on Gough are killing an estimated 600,000 seabirds a year, threatening the iconic Tristan albatross with extinction. Plans are at an advanced stage to attempt to eradicate mice on Gough. Underwater, the silver porgy, a South American fish that eats lobster larvae and was introduced to Tristan with the stranded oil rig in 2006, has reproduced rapidly and is now abundant.

Q: How is climate change affecting Tristan da Cunha?

A: Giant kelp is already at the upper limit of its temperature tolerance at the northern islands, and if the sea temperature increases by even a small amount, that kelp could disappear. As a keystone species, this could have drastic effects on the marine life dependent on it. Climate change may also result in changes in the position of the subtropical convergence, a major frontal system that currently straddles the northern islands and controls water temperature and plankton productivity. As climate change affects the strength and direction of ocean currents, any shift of this convergence could affect lobster larvae, as well as the food of seabirds, including small fish, squid, and crustaceans.

Q: Are you optimistic about the future of Tristan da Cunha's marine wildlife?

A: Yes and no. I'm very encouraged by the go-ahead for the project to eradicate mice from Gough, which would give several endemic bird species a chance to avoid extinction. Lobster fishing is well controlled, and I hope wise decisions will be made on any future diversification of fishing, particularly on the seamounts, which are especially vulnerable to overfishing and damaging fishing methods. Another shipping accident is highly likely, unless new measures are taken to mitigate this risk. In the longer term, I'm pessimistic that human-induced climate change will be tackled by the major countries of the world in time to avert consequences like altered seawater temperatures and changes in ocean currents that could drastically affect Tristan's marine life.