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An ecological assessment of Ascension Island’s shallow water seamounts 1 An ecological assessment of Ascension Island’s shallow water seamounts 2 ACKNOWLEDGEMENTS This project was funded by grants awarded by the European Union’s voluntary scheme for Biodiversity and Ecosystem Services in Territories of European overseas (BEST; project 1599), the Darwin Initiative (project DPLUS064), National Geographic Pristine Seas and the UK Foreign and Commonwealth Office and involved more than 20 researchers and marine conservationists from 7 institutions. The project team are indebted to the captains and crews of the British Antarctic Survey research vessel the RRS James Clark Ross and the MV Extractor (St Helena) without whose seamanship and tireless support this work would not have been possible. DISCLAIMER: This document has been produced with the financial assistance of the European Union, the Darwin Initiative and the UK Foreign and Commonwealth Office. The contents of this document are the sole responsibility of the authors and can under no circumstances be regarded as reflecting the position of the European Union or the UK Government. CITATION: Weber, S.B., Richardson, A.R., Broderick, A.C., Brown, J., Campanella, F., Godley, B.J., Howard, F., Hussey, N., Meeuwig, J., Rose, P., Shepherd, M., Thompson, C., van der Kooij, J., Witt, M.J. (2018) A baseline ecological of Ascension Island’s shallow water seamounts as candidate marine protected areas. Ascension Island Government Conservation & Fisheries Department. 56 pp. Available at … With funding from An ecological assessment of Ascension Island’s shallow water seamounts 3 TABLE OF CONTENTS SECTION 1. EXECUTIVE SUMMARY……………………………………………………………………………6 SECTION 2. INTRODUCTION………………………………………………………………………………………9 2.1 Seamounts: Ocean Oases………………………………………………………………………….9 2.2 Towards an Evidence Based Ocean Ascension Island Ocean Sanctuary…….10 SECTION 3. THE HABITATS…………………………………………………………………………………………12 3.1 Bathymetry and geomorphology………………………………………………………………13 3.2 Oceanography…………………………………………………………………………………………..14 SECTION 4. THE PELAGIC COMMUNITIES…………………………………………………………………..16 4.1. Hydroacoustic surveys………………………………………………………………………………18 4.2. Pelagic baited remote underwater video systems (BRUVs)……………………….21 4.3. Vessel-based visual surveys………………………………………………………………………26 4.4. Incidental sightings……………………………………………………………………………………29 SECTION 5. AQUATIC TELEMETRY……………………………………………………………………………….30 5.1. Methods……………………………………………………………………………………………………31 5.2. Galapagos and silky sharks…………………………………………………………………………32 5.3. Tuna…………………………………………………………………………………………………………..37 5.4. Other oceanic species………………………………………………………………………………..39 SECTION 6. CONCLUSIONS…………………………………………………………………………………………..41 REFERENCES………………………………………………………………………………………………………………..45 ANNEXES……………………………………………………………………………………………………………………..47 An ecological assessment of Ascension Island’s shallow water seamounts 4 LIST OF FIGURES Figure 1.1 Locations of study seamounts with proposed protection buffers highlighted 8 in red. Figure 2.1 Ascension Island Government’s offshore research charter, the MV Extractor 11 and the RRS James Clark Ross on location at Ascension Island’s offshore seamounts. Figure 3.1 High resolution bathymetry and three-dimensional topography of the Young, 13 Grattan and Harris-Stewart seamounts. Figure 3.2 Locations of Vemco VR2AR receiver assemblies deployed on the summits of 15 the Grattan and Young seamounts. Figure 4.1 Sampling design used for pelagic community surveys of seamounts. 17 Figure 4.2 Nocturnal echograms showing biomass plumes over the Grattan, 18 Young and Harris-Stewart seamounts Figure 4.3 Relationship between distance to seamount summits and total epipelagic (0 – 19 200 m) biomass of three taxonomic groups of marine organisms estimated using hydroacoustic surveys. Figure 4.4 Hotspots of pelagic fish and shark abundance on the Grattan and Young 20 seamounts derived from hydroacoustic survey data. Figure 4.5 Variation in species richness of pelagic fishes and sharks in recorded by mid- 22 water BRUVs deployed at varying distances from the summits of A) the Harris-Stewart Seamount, and B) the Grattan and Young seamounts. Figure 4.6 Probability of occurrence of large-bodied pelagic fish species in mid-water 23 BRUVs deployed at varying distances from the summits of the Harris-Stewart and Southern Seamounts. Figure 4.7 Probability of occurrence of pelagic shark species in mid-water BRUVs 24 deployed at varying distances from the summits of the Harris-Stewart and Southern Seamounts. Figure 4.8 Relationship between abundance (Max N) of six commonly-encountered 25 species in mid-water BRUVs species and distance to the summits of Ascension’s southern seamounts. Figure 4.9 Vessel-based counts of two species of oceanic seabirds, Ascension 28 frigatebirds (Fregata aquila) and sooty terns (Onychoprion fuscatus), and an important prey species (flying fish; Exocoetidae sp.) in relation to distance from the summits of Ascension’s southern seamounts Figure 4.10 Bluntnose sixgill shark filmed using a custom made drop camera at 760 m 29 depth on the Grattan seamount An ecological assessment of Ascension Island’s shallow water seamounts 5 Figure 5.1 Deployment of telemetry tags on marine megafauna at Ascension’s 31 seamounts. Figure 5.2 Detection histories of 29 acoustically-tagged Galapagos and silky sharks on 33 fixed receiver arrays deployed on the summits of the Grattan and Young seamounts. Figure 5.3 Movements of SPOT-tagged silky sharks after leaving Ascension’s southern 34 seamounts. Figure 5.4 Utilisation distributions (UDs) of SPOT-tagged Galapagos and silky sharks 35 during periods of residency on the Grattan and Young seamounts. Figure 5.5 Fine-scale habitat use by acoustically-tagged Galapagos and silky sharks on 36 the Grattan and Young seamounts. Figure 5.6 Probability density plots showing areas of high utilization by yellowfin and 38 bigeye tuna tagged with PSATs at the Young, Grattan and Harris-Stewart seamounts. Figure 5.7 Most probable tracks of 32 pelagic fish and sharks equipped with pop-up 40 satellite archival tags (PSATs) at the Harris-Stewart, Grattan and Young seamounts. LIST OF TABLES Table 3.1 Physical characteristics of the study seamounts. 14 Table 4.1 Abundances of seabird species observed during vessel-based visual surveys 27 of Ascension’s seamount. Table 5.1 Comparison of standard attendance metrics for acoustically-tagged 34 Galapagos and silky sharks on the Southern seamounts. An ecological assessment of Ascension Island’s shallow water seamounts 6 1. EXECUTIVE SUMMARY 1.1 Seamounts are known to provide habitat for a wide range of marine species, often acting as ‘oases’ of life in expanses of comparatively unproductive deep water ‘desert’. However, they remain among the least studied, and least protected, marine ecosystems globally. Recognition of this shortfall has catalysed international efforts to better understand seamount ecology and increase their representation within marine protected area networks. 1.2 Three prominent seamounts are located within the exclusive economic zone (EEZ) of Ascension Island: the Harris-Stewart seamount lies over deep abyssal plain 260 km to the west of Ascension while the Grattan and Young1 seamounts, collectively known as the ‘southern seamounts’, are situated adjacent to the mid-Atlantic ridge 280 – 320 km to the southeast (Figure 1.1). Owing to their extreme isolation, these features have remained virtually unexplored and their significance for biodiversity has been largely inferred from work carried out elsewhere. 1.3 Between January 2017 and September 2018 a major programme of research was undertaken to improve the state of knowledge of Ascension Island’s seamount ecosystems and provide the scientific evidence to inform their inclusion within a large-scale marine protected area (MPA) that is planned for the Territory. The primary objective of the project was to study aggregations of ‘marine megafauna’ - large-bodied marine vertebrates such as sharks, predatory fish, seabirds and dolphins – that often associate with seamounts and are potentially threatened by commercial longline fisheries that operate in Ascension Island’s EEZ. In particular, the study aimed to map the geographic extent of such aggregations as a basis for proposing biologically- relevant MPA boundaries. 1.4 To achieve this, a novel combination of stereo baited remote underwater video systems (BRUVs), hydroacoustic surveys and vessel-based census methods was used to investigate how the abundance and diversity of marine life changes with distance from the seamount summits. Several types of telemetry tags were also deployed to track the movements of individual animals and assess their residency and ranging behaviour, which is important to evaluate the effectiveness of any future MPA. In total, more than 500 hours of underwater video footage were collected and analysed, 540 km of visual survey transects were completed and 73 sharks, tuna and billfish were fitted with telemetry tags. The project also produced the first high- resolution bathymetric maps of Ascension’s seamounts which reveal their varied geomorphology in unprecedented detail. 1.5 As is typical with seamounts, bathymetry had a profound influence on the ecology of the features surveyed. Grattan and Young are shallow seamounts with peaks rising into the sunlit euphotic zone (the top 200 m of the water column) and both supported substantially higher abundance, diversity and biomass of pelagic sharks and fish compared to surrounding deeper