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EBSA overview  |  Delineation |  Summary of description updates  | Revised description

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EBSA overview

Walvis Ridge is a significant seamount chain in the high seas that forms a bridge running east to west from the African continental margin, near northern Namibia, to the southern Mid-Atlantic Ridge. It is a unique geomorphological feature likely to be of special importance to vulnerable sessile macrofauna and demersal fish associated with seamounts. Although bottom fisheries occur on Walvis Ridge, the spatial extent of commercial fishing is limited to a relatively small area. Due to the variation in depths, ranging from slopes to summits and surface waters, it is likely that the area supports a relatively high biological diversity. The feature also supports high diversity of globally threatened seabirds.

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Delineation

Open or collapse the legend as a sidebar by clicking the icon in the top left corner of the map. In the legend you can turn on/off the old/new extents of the EBSA. You can zoom in/out using the mouse or the +/- buttons on the map, and click on the features for more information.

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Summary of updates and revisions to the EBSA description

R1: Substantial edting throughout the description, with 4 new references added.

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Revised EBSA description

NOTE: Read this here, or download the Word document on the right sidebar.

 

General Information

Summary

Walvis Ridge is a significant aseismic hotspot track seamount chain forming a bridge running north-east to south-west from the Namibian continental margin to Tristan da Cunha and Gough Islands along the southern Mid-Atlantic Ridge. It is a unique geomorphological feature likely to be of special importance to vulnerable sessile macrofauna and demersal fish associated with seamounts. Given the complex habitat heterogeneity, including steep slopes, canyons, embayments formed by massive submarine slides, trough-like structures, a graben, abyssal plains, and shallow summits of seamounts and guyots, it is likely that the area supports a relatively higher biological diversity. This has been confirmed by research cruises that have sampled a variety of benthic macrofauna, including fragile species such as corals, and have also recorded a high diversity of globally threatened seabirds. Although bottom fisheries occur on the Walvis Ridge, the spatial extent of commercial fishing is limited to a relatively small area.

 

Introduction of the area

The aseismic Walvis Ridge is a hotspot track seamount chain formed by submarine volcanism, some of which are guyots, that is connected to a continental flood basalt province. The Ridge presents a barrier between North Atlantic Deep Water to the north and Antarctic Bottom Water to the south. The surface oceanographic regime is the South Atlantic Subtropical Gyre bounded by the productive waters of the Benguela Current System and the Subtropical Convergence Zone. The feature as described here is bounded approximately by a 4000‑m depth contour and contains significant areas within the likely vertical extent of near-surface zooplankton migration (1000 m). The area supports high diversity of seabirds, some of which are endemic to the Tristan Group in the southwest. Although biologically significant, data from research cruises are patchy and variable, the area contains several named seamounts, recognized and endorsed by the General Bathymetric Chart of the Oceans (GEBCO) that are likely to support enhanced primary production, abundance and species richness (e.g. Dobrovol’sky, Ewing, Filippov, Valdivia Bank, Wüst, Radostnaya, Schedraya, Smejnaya and Zubov).

 

Description of the location

EBSA Region

South-Eastern Atlantic

 

Description of location

Walvis Ridge extends obliquely (NE-SW) from the northern Namibian shelf (18°S) to the Tristan da Cunha island group at the Mid-Atlantic Ridge (38°S). Although the focus area is predominantly located outside of national jurisdiction, it does extend into the Namibian exclusive economic zone. The EBSA boundary links tightly to the important benthic features comprising Walvis Ridge (produced by combining GEBCO data with that from www.bluehabitats.org: see Harris et al., 2014, and data from Holness et al., 2014). Those features that are continuous with the Ridge, as well as isolated hills that are in close proximity are included. The component within Namibia’s jurisdiction includes areas with a high selection frequency in the regional spatial prioritization to meet biodiversity targets efficiently, as well as key features that form part of the Ridge. Given the global rarity of the connection between the hotspot track and continental flood basalt province, it is imperative that the full extent of this feature is encompassed within the EBSA.

 

Area Details

Feature description of the area

This is both a benthic and water column feature; essentially, it’s a chain of seamounts that individually and collectively constitute an ecologically and biologically significant deep-sea feature. Seamounts and seamount chains are recognized as such by, for example, the Census of Marine Life project CenSeam (http://censeam.niwa.co.nz). Walvis Ridge also includes a number of deep-sea features in addition to the seamounts and guyots, such as steep canyons, embayments formed by massive submarine slides, trough-like structures, a graben, abyssal plains, and a fossil cold-water-coral reef mound community (GEOMAR 2014). Based on these physical features, the focus area can be divided into a northern section (extending SW from the Namibian shelf, with a steep NW scarp, ridge-type seamounts, and guyots with rift arms), a central section (comprising a SW narrow and a NE broader section, with the former incised by two trough-like features, and the latter including Valdivia Bank and a prominent graben of 200 km x 20 km) and a southern section (that bifurcates into two NE-SW-trending ridge-like arms each about 450 km long, one heading towards Tristan da Cunha and the other towards Gough, with a seamount chain lying approximately parallel between the ridges; GEOMAR 2014).

The high habitat heterogeneity supports moderately diverse biological communities, including benthic macrofauna such as brachiopods, sponges, octocorals, deep-water hexacorals, gastropods, bivalves, polychaetes, bryozoans, cirriped crustaceans, basket stars, ascidians, isopods and amphipods (GEOMAR 2014). Productivity seems to increase from SW to NE along Walvis Ridge, with sediment organic carbon and the abundance and diversity of phytoplankton communities increasing towards the Namibian shelf, likely reflecting patterns of nutrient transport and upwelling in the north-flowing Benguela Current that are more intense closer to the African continent (GEOMAR 2014).

Research has been carried out several times at Walvis Ridge in various oceanographic cruises. These include: Russian Federation cruises; Spanish-Namibian surveys (see summary of knowledge in Perez et al., 2012); US Walvis Ridge cruise MV1203 Expedition (March 2012; http://earthref.oth/ERESE/Projects/mv1203) and the GEOMAR cruise SO233 WALVIS II (GEOMAR, 2014). These cruises have collected both physical and biological data/samples.

 

Feature conditions and future outlook of the proposed area

Walvis Ridge is primarily recognized as a geological feature but the biota in the area could be vulnerable to fishing (e.g., orange roughy, alfonsino, southern boarfish, deepwater crabs, fragile sessile benthic megafauna; SEAFO report in FAO Statistical Area 47 and a portion of 34). The fisheries beyond national jurisdiction are managed by SEAFO, which has introduced area management, catch quotas, and a suite of bottom-fishing regulations. Fisheries within the Namibian EEZ, however, are managed by Namibia’s Ministry of Fisheries and Marine Resources. Concentrations of ferromanganese nodules on the deepest adjacent areas (adjacent Cape Abyssal Plain; Perez et al., 2012) have been observed, thus in future, seabed mining may also be a consideration and would be subject to management by the International Seabed Authority. Oil exploration has already taken place within the EBSA, namely Welwitschia-1 well, which was drilled in 2014 at 20°11’9.79”S, 11°19’3.27”E. Although it was found to be dry, future drilling activities in the general area, including inside the extended EBSA, are likely. The Namibian sections of the EBSA are largely in good condition, though some impacted areas exist on the far eastern edge (Holness et al., 2014). No evidence exists to suggest that the high seas portion of the EBSA is impacted to any large degree, it is therefore reasonable to assume that it is also in good condition.

 

References

BirdLife International. 2009. Designing networks of marine protected areas: exploring the linkages between Important Bird Areas and ecologically or biologically significant marine areas. Cambridge, UK: BirdLife International. www.cbd.int/doc/meetings/mar/ewbcsima- 01/other/ewbcsima-01-birdlife-02-en.pdf.

BirdLife International. 2010. Marine Important Bird Areas toolkit: standardised techniques for identifying priority sites for the conservation of seabirds at-sea. BirdLife International, Cambridge UK. Version 1.1: May 2010. www.birdlife.org/eu/pdfs/Marine_IBA_Toolkit_2010.pdf.

Census of Marine Life project CenSeam http://censeam.niwa.co.nz, http://seamounts.sdsc.edu.

Clark, M.R., Vinichenko, V.I., Gordon, J.D.M, Beck-Bulat, G.Z., Kukharev, N.N., Kakora, A.F. 2007. Large scale distant water trawl fisheries on seamounts. Pp. 361-412 in Seamounts: Ecology, Fisheries and Conservation. Fish and Aquatic Resources Series 12, T.J. Pitcher, T. Morato, P.J.B. Hart, M.R. Clark, N. Haggan and R.S. Santos, eds, Blackwell Publishing, Oxford.

Durán Muñoz, P., Sayago-Gil, M., Murillo, F.J., Del Río, J.L., López-Abellán, L.J., Sacau, M., Serralde, R. 2012. Actions taken by fishing nations towards identification and protection of vulnerable marine ecosystems in the high seas: The Spanish case (Atlantic Ocean). Marine Policy, 36: 536–543.

FAO FIRMS (Fishery Resources Monitoring System) firms.fao.org.

GEBCO (General Bathymetric Chart of the Oceans) Available at http://www.gebco.net/data_and_poducts/gridded_bathymetry_data/.

GEOMAR, 2014. RV SONNE Fahrtbericht / Cruise Report SO233 WALVIS II: Cape Town, South Africa - Walvis Bay, Namibia: 14.05-21.06.2014. Hoernle, K., Werner, R., Lüter, C (eds). Helmholtz-Zentrum für Ozeanforschung Kiel, Germany: Nr. 22 (N. Ser.), 153 pp.

Harris, L.R., Nel, R., Oosthuizen, H., Meyer, M., Kotze, D., Anders, D., McCue, S., Bachoo, S. Managing conflicts between economic activities and threatened migratory marine species towards creating a multi-objective blue economy. Conservation Biology, in review.

Harris, P.T., Macmillan-Lawler, M., Rupp, J., Baker, E.K. 2014. Geomorphology of the oceans. Marine Geology, 352: 4-24.

Holness, S., Kirkman, S., Samaai, T., Wolf, T., Sink, K., Majiedt, P., Nsiangango, S., Kainge, P., Kilongo, K., Kathena, J., Harris, L., Lagabrielle, E., Kirchner, C., Chalmers, R., Lombard, M. 2014. Spatial Biodiversity Assessment and Spatial Management, including Marine Protected Areas. Final report for the Benguela Current Commission project BEH 09-01.

Jacobs, C.L., Bett, B.J. 2010. Preparation of a bathymetric map and GIS of the South Atlantic Ocean: a review of available biologically relevant South Atlantic Seamount data for the SEAFO Scientific Committee. National Oceanographic Centre Southampton, Research and consultancy Report No. 71 (unpublished manuscript).

Perez, J.A.A, dos Santos Alves, E., Clark, M.R., Bergstad, O.A., Gebruk, A., Azevedo Cardoso, I., Rogacheva, A. 2012. Patterns of life on the southern Mid-Atlantic Ridge: Compiling what is known and addressing future research. Oceanography, 25: 16-31.

Reid, T., Ronconi, R., Cuthbert, R., Ryan, P.G. 2014. The summer foraging ranges of adult spectacled petrels Procellaria conspicillata. Antarctic Science, 26: 23-32.

Rogers, A.D., Gianni, M. 2010. The implementation of UNGA Resolutions 61/105 and 64/72 in the Management of Deep-Sea Fisheries on the High Seas. Report prepared for the Deep Sea Conservation Coalition, International Programme on the State of the Ocean, London UK. 97 pp.

Sanchez, P., Alvarez, J.A. 1988. Scaeurgus unicirrhus (Orbigny, 1840) (Cephalopoda Octopodidae): First record from the South-east Atlantic. South African Journal of Marine Science, 7: 69-74.

Zibrowius, H., Gili, J.M. 1990. Deep-water Scleractinia (Cnidaria Anthozoa) from Namibia, South Africa and Walvis Ridge, southeastern Atlantic. Scientia Marina, 54: 19-46.

 

Other relevant website address or attached documents

Table 1: Summary of ecosystem types and threat status for Walvis Ridge. Data from Holness et al. (2014). Note that most of the area was not evaluated as it falls outside the Namibian EEZ.

Ecosystem Threat Status

Ecosystem Type

Area (km2)

Area (%)

Vulnerable

Central Namib Shelf Edge

18,113

2.4

 

Cunene Shelf Edge

6,458

0.9

Vulnerable Total

 

24,571

3.3

Least Threatened

Cunene Abyss

5,920

0.8

 

Cunene Lower Slope

8,664

1.2

 

Cunene Seamount

3,818

0.5

 

Cunene Upper Slope

2,298

0.3

 

Namib Abyss

383

0.1

 

Namib Lower Slope

16,573

2.2

 

Namib Seamount

2,290

0.3

 

Namib Upper Slope

4,931

0.7

Least Threatened Total

 

44,877

6.1

Not Evaluated

High Seas

671,542

90.6

Not Evaluated Total

 

671,542

90.6

Grand Total

 

740,990

100.0

 

Assessment of the area against CBD EBSA criteria

C1: Uniqueness or rarity High

Justification

As the only extensive seamount chain off of the Mid-Atlantic Ridge in the Southeast Atlantic, the Walvis Ridge is a unique geomorphological feature. It is also one of the few hotspot tracks on earth that connects to continental flood basalt.

 

C2: Special importance for life-history stages of species High

Justification

Seamount chains may facilitate connectivity between individual seamounts over extensive distances. The varied topography and geomorphology support demersal fish resources (based on demersal fisheries records in locations shallower than 2000 m). The varied bathymetry dictates distribution area and provides significant habitat for bentho-pelagic species (e.g., hotspots for orange roughy) and is likely to do so also for epipelagics (Clark et al., 2007, Rogers and Gianni, 2010). These seamounts are significant habitats for cold-water corals and sponges (Zibrowius and Gili, 1990; GEOMAR 2014). Thus, the Ridge is of special importance for sessile macrofauna and for demersal fish associated with seamounts (FAO FIRMS species distribution maps) (http://firms.fao.org). The feature is also an important post-breeding area for Spectacled Petrel (Procellaria conspicillata), an Atlantic Ocean endemic species breeding only on an island within the Tristan group and recorded as foraging along the Walvis Ridge (Reid et al., 2014). It includes parts of foraging areas of globally threatened seabirds, including Tristan Albatross (Diomedea dabbenena), Wandering Albatross (Diomedea exulans) and Atlantic Yellow-nosed Albatross (www.seabirdtracking.org). The series of seamounts provides a potential stepping stone feature for organisms from coast to mid ocean (e.g., dispersion of the benthic octopod (Scaeurgus unicirrhus; Sanchez and Alvarez, 1988).

 

C3: Importance for threatened, endangered or declining species and/or habitats Medium

Justification

Bluefin and big-eye tuna occur in the area (e.g., FishBase), and orange roughy hotspots are known (SEAFO information). The far south EEZ of Tristan da Cunha is a foraging area for albatross, penguins, shearwaters and petrels (www.seabirdtracking.org). Historic whale capture data in the mid-ocean portion of the feature (Right Whale and Sperm Whales) indicate former concentrations (maps derived from OBIS presented at workshop), and an opportunistic survey within the area in 2009 as part of the South Atlantic MAR-ECO project recorded 23 sightings of cetaceans (Perez et al., 2012). Critically endangered leatherbacks nesting in South Africa also pass over Walvis Ridge during their post-nesting migrations to the feeding grounds near St Helena Island (Harris et al. in review).

 

C4: Vulnerability, fragility, sensitivity, or slow recovery High

Justification

Habitat-forming sessile megafauna are fragile and vulnerable to bottom contact fishing gears and slow to recover from damage. Habitat prediction models (maps available at workshop) and observational data (Durán Muñoz et al., 2012, GEOMAR 2014, Perez et al., 2012 and papers cited therein) indicate presence of cold-water corals and sponges, and other delicate fauna such as basket- and feather stars. Based upon empirical evidence (e.g. observations from Spanish/Namibian cruises on the Valdivia Bank, and along the whole ridge; GEOMAR 2014) the seamounts and deep-sea features along the Walvis Ridge have sensitive habitats, biotopes and species, justifying a medium (bordering on high) criterion ranking.

 

C5: Biological productivity Medium

Justification

Productivity appears to increase from SW to NE along Walvis Ridge, as seen in the sediment organic carbon load and abundance and diversity of phytoplankton that both increase closer to the Namibian shelf (GEOMAR 2014).  Several seamounts also extend into the photic zone and may have enhanced primary production. Significant areas are within the likely vertical range of epipelagic zooplankton migration (Jacobs and Bett, 2010).

 

C6: Biological diversity Medium

Justification

Data on biological diversity are limited, however there are some data on seabirds, fish, and benthic megafauna, macrofauna and meiofauna (see Perez et al., 2012 for a review, and GEOMAR 2014). The south-west end of the feature has high seabird diversity, including Spectacled Petrel, Atlantic Yellow-nosed Albatross, Sooty Albatross and Atlantic Petrel. Foraging range extrapolations and satellite tracking work have highlighted the south-west part of the feature as important for the following seabird species (status on IUCN Red List 2012 is given for all): Tristan Albatross (Diomedea dabbenena) - Critically Endangered; Atlantic Yellow-nosed Albatross (Thalassarche chlororhynchos) – Endangered; Sooty Albatross (Phoebetria fusca) – Endangered; Grey Petrel (Procellaria cinerea) – Near Threatened; Atlantic Petrel (Pterodroma inerta) – Endangered; Black-browed Albatross (Thalassarche melanophrys) – Near Threatened; Cory’s Shearwater – Least Concern; Great Shearwater (Puffinus gravis) – Least Concern; Grey-headed Albatross (Thalassarche chrysostoma) – Endangered; Northern Royal Albatross (Diomedea sanfordi) – Endangered; Southern Giant-petrel (Macronectes giganteus) – Least Concern; Wandering Albatross (Diomedea exulans) – Vulnerable. Observations and the range of habitats created by the seamount complex and immediately adjacent abyssal area suggest comparatively higher diversity of ecosystems, habitats, communities, and species. This has been confirmed to some extent through bathymetric/geological surveys and biological sampling of the benthos, which revealed a variety of benthic macrofauna (GEOMAR 2014).

 

C7: Naturalness High

Justification

Human influence is largely historic, fisheries were and are mainly confined to seamount summits (SEAFO information, Clark et al., 2007, and relevant papers cited in Perez et al., 2012), and oil exploration drilling has been limited to date. Whaling has ceased in this area for several decades. Apart from seamounts that are likely to have been impacted by bottom-fishing, the remainder of the area is considered to have a high degree of naturalness. No evidence exists to suggest that the high seas portion of the EBSA is impacted to any large degree, it is therefore reasonable to assume that it is in good condition. The Namibian sections of the EBSA are largely in good condition, though some impacted areas exist on the far eastern edge (Holness et al., 2014).

Additional Information

Additional criteria BirdLife Important Bird Areas Criteria (BirdLife 2009, 2010) A1 Regular presence of threatened species A4ii >1% of the global population of a seabird.

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