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First quantification of subtidal community structure at Tristan da Cunha Islands in the remote South Atlantic: From kelp forests to the deep sea

Christopher Thompson | Mar 29, 2018

Christopher Thompson

Mar 29, 2018

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Scientists from the University of Western Australia, the National Geographic Pristine Seas Programme and the Royal Society for the Protection of Birds explored the waters of Tristan da Cunha using a combination of sampling techniques.
Photo: Roger Horrocks.

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CITATION

Caselle JE, Hamilton SL, Davis K, Thompson CDH, Turchik A, Jenkinson R, Simpson D, Sala E. 2018. First quantification of subtidal community structure at Tristan da Cunha Islands in the remote South Atlantic: from kelp forests to the deep sea. PLoS ONE, 13(3): e0195167.

HIGHLIGHTS

  • We conducted the first quantitative surveys of nearshore kelp forests, offshore pelagic waters and deep sea habitats using UVC, mid-water stereo-BRUVS and Deep Ocean Dropcams.
  • Kelp forests had low biodiversity and species richness, however the species that were present were in high biomass and abundance.
  • Assemblage structure both in the nearshore and offshore differed significantly between the main island group and Gough Island, located further south across the subtropical convergence zone.
  • Species richness in the deep sea was positively correlated with rocky substrate and saw a distinct differences in fish communities above and below ~750m, matching a break in temperature and salinity noted in concurrent oceanographic sampling.
  • The marine ecosystems sampled at Tristan da Cunha appear to be in a healthy state, however, not without potential threats. The UKs commitment to protect the marine environments of its overseas territories including Tristan da Cunha, is of great value if managed correctly. The results presented will be of use in informing management as well as providing a baseline against which future monitoring efforts can be referenced.

ABSTRACT

Tristan da Cunha Islands, an archipelago of four rocky volcanic islands situated in the South Atlantic Ocean and part of the United Kingdom Overseas Territories (UKOTs), present a rare example of a relatively unimpacted temperate marine ecosystem. We conducted the first quantitative surveys of nearshore kelp forests, offshore pelagic waters and deep sea habitats. Kelp forests had very low biodiversity and species richness, but high biomass and abundance of those species present. Spatial variation in assemblage structure for both nearshore fish and invertebrates/algae was greatest between the three northern islands and the southern island of Gough, where sea temperatures were on average 3-4o colder. Despite a lobster fishery that provides the bulk of the income to the Tristan islands, lobster abundance and biomass are comparable to or greater than many Marine Protected Areas in other parts of the world. Pelagic camera surveys documented a rich biodiversity offshore, including large numbers of juvenile blue sharks, Prionace glauca. Species richness and abundance in the deep sea is positively related to hard rocky substrate and biogenic habitats such as sea pens, crinoids, whip corals, and gorgonians were present at 40% of the deep camera deployments. We observed distinct differences in the deep fish community above and below ~750 m depth. Concurrent oceanographic sampling showed a discontinuity in temperature and salinity at this depth. While currently healthy, Tristan’s marine ecosystem is not without potential threats: shipping traffic leading to wrecks and species introductions, pressure to increase fishing effort beyond sustainable levels and the impacts of climate change all could potentially increase in the coming years. The United Kingdom has committed to protection of marine environments across the UKOTs, including Tristan da Cunha and these results can be used to inform future management decisions as well as provide a baseline against which future monitoring can be based.

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PELAGIC TRISTAN

Bubble plots depicting A. site-level variation in species richness and B. numerical abundance from pelagic BRUV stations sampled across the four Tristan da Cunha Islands. Inset bar plots show mean values for each island ± 1 standard error of the mean Figure: Caselle et al. 2018.

FUNDING & ACKNOWLEDGEMENTS

We would like to thank the National Geographic Society Pristine Seas program for funding the expedition, the Royal Society for the Protections of Birds, for their partnership with the expedition and long standing history and knowledge of the Tristan Da Cunha archipelago and Alex Mitham of Tristan Island Government for originally inviting the expedition to Tristan Da Cunha. We are very appreciative for the leadership and logistical support of P. Rose, J. Hall, A. Schofield, D. McAloney, S. Dews, and D. Meyer and the hard work and dedication of the crew of the SVS Grenville. Thanks to T. Bell for providing maps of kelp canopy. We extend our sincere thanks to the people of Tristan da Cunha who welcomed our team to experience their islands and who provided exceptional expertise on the water and assistance with research logistics. In particular, we would like to thank Tristanians James Glass, Trevor Glass, Ian Lavarello, Rodney Green, George Swain and Julian Repetto for their help in the field and Sean Burns for island hospitality.