Walvis Ridge seamounts benthic fauna

Abstract

The cruise is a contribution to SEAFO resulting from a co-operation between SEAFO, the EAF-Nansen project, and the FAO Deep-sea programme. The aim is to facilitate activities in support of the efforts by RFMOs to meet objectives in relation to the United Nations General Asssembly (UNGA) resolution 61/105 and the associated FAO Guidelines on Bottom Fisheries in the high seas. Furthermore, the southeastern Atlantic ABNJ was selected by the recently initiated GEF-sponsored FAO ABNJ programme (“Common Oceans”) as an area of particular attention, involving the relevant RFMO with competent to regulate bottom fishing in that region is the Southeast Atlantic Fisheries Organization (SEAFO). Despite a shortage of data on the occurrence of ‘vulnerable marine ecosystems’ (VMEs, as defined in the FAO Guidelines), SEAFO has introduced comprehensive measures to protect such ecosystems from significant adverse impacts within the convention area. The convention area consist of agreed limited ‘existing fishing areas’, i.e. areas were fisheries were conducted within a recent reference period, and more extensive ‘new fishing areas’ where this was not the case. Commercial harvesting is essentially restricted to the ‘existing fishing areas’, but further regulated by other measures such as species-specific TACs, gear restrictions and other measures aimed to conserve e.g. sharks. To conduct exploratory fisheries in ‘new fishing areas’ parties need to submit a notification and detailed plan and report to the Organization for approval. SEAFO has furthermore closed 10 subareas of the convention area to all fishing activity, a regulation most recently revised in 2011. Best available bathymetry data were compiled to locate subareas recognised in the FAO guidelines as VME features, essentially primarily seamounts. As fishing is restricted to the upper 2000m of any feature, a map of areas shallower than 2000m was the basis of further selection. In SEAFO such areas are seamounts or seamount complexes of various sizes and shapes. The spatial distribution of VME indicators such as corals and sponges (i.e. as given in FAO guidelines) is not well known in SEAFO, but the benthic and pelagic biogeographical patterns have been described. It was therefore decided to close 10 potential fishing areas that represented the range of features and the major biogeographical zones of the region. Some of these were actual fishing areas; others were either not fished or lightly fished. The end result was the set of ten closures with various shapes and sizes, ranging from single seamounts to rather wider areas comprising several seamounts or mid-Atlantic Ridge segments. While SEAFO took significant steps to ensure conservation of VMEs, the shortage of information on the distribution patterns of VME indicators remains unsatisfactory. Some new data were acquired by recent fishing experiments in existing and new fishing areas, and by Spanish-Namibian research activity on the Valdivia Bank. However, even within the relatively restricted actual fishing areas, the documentation remains scattered and limited. Similarly, the Scientific Committee of SEAFO struggles with data shortage on the fish resources actually harvested in the convention area, primarily Patagonian toothfish, pelagic armourhead, alfonsino, deep-sea red crab, and associated bycatch species. Stock assessments are generally highly uncertain and fully dependant on fisheries-generated data, hence conservation measures implemented by SEAFO remain largely precautionary rather than tuned to good estimates of abundances and productivity. Catches currently remain rather limited, i.e. collectively less than 1000t, but some species are valuable and tend to attract interest from fishing parties that would value not only enhanced opportunities but also management based on more reliable stock assessments. This project thus aims to supplement and extend knowledge and databases on VME indicators as well as fisheries resources and will accordingly conduct targeted investigations in the currently used fishing areas of the convention area, primarily on the Walvis Ridge. In addition, some of the fishing closures will be visited to document presence-absence of VME indicators and describe distribution patterns. New data from these targeted studies will enhance the quality of fish stock assessments and help analyse the appropriateness of VME closures currently implemented solely based on feature characteristics and biogeographical affinity.

Main conclusions: VMEs indicators were located in all seamounts prospected but differences in structure and development state were observed. In some seamounts coral rubble, sediment-clogged, mostly dead coral framework and living coral framework were observed from the base to the summit all along the slope. Current system associated to each seamount, depth and probably other depth-dependent environmental factors driven the presence and development of deep-water reefs. Continuous research efforts are needed to increase knowledge about biodiversity and structure. Understanding their dynamics will improve effective management and conservation of this vulnerable ecosystems.