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

Kathryn Davis,Scott L Hamilton,Jennifer E Caselle,Enric Sala,Alan Turchik,Christopher D H Thompson,Ryan Jenkinson,Doug Simpson

doi:10.1371/journal.pone.0195167

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.

Highlights

In many places around the globe, the health of our oceans has declined over the past several decades due to a myriad of human-induced stressors, including overfishing, habitat destruction, pollution, invasive species, and climate change [1,2,3,4,5,6]
Tristan da Cunha is a group of islands in the South Atlantic, representing the summits of massive shield volcanoes arising from abyssal depths
We measured properties of the water column with a CTD attached to the deep water drop cameras at all islands (Gough n = 3; Inaccessible n = 1; Nightingale n = 2; Tristan da Cunha n = 5)

Summary

Introduction

In many places around the globe, the health of our oceans has declined over the past several decades due to a myriad of human-induced stressors, including overfishing, habitat destruction, pollution, invasive species, and climate change [1,2,3,4,5,6]. Remote locations provide one of the last opportunities to develop a baseline of the status and health of marine ecosystems where human impacts are minimized [8,9,10]. These quantitative baselines can provide unique opportunities to study future changes in population abundance, community structure, and ecosystem function through time, in addition to current spatial comparisons with locations that have experienced greater anthropogenic impacts. The likely reason for this is a long history of human habitation and exploitation in temperate regions, including the intensive harvesting and bycatch impacts on many fishes, pinnipeds, cetaceans, turtles, and sea birds, from even the most remote islands over the last two centuries [7, 18, 19]

Methods

Results

Conclusion