Deep˗Sea Ecosystems Off Mauritania: An Introduction

Abstract

The waters surrounding the Islamic Republic of Mauritania, located in the Northwest African region , harbour one of the four major upwelling systems and productive areas of the world’s oceans. Along 754 km of the desert coastline, the permanent upwelling phenomena and seasonal shifting of the oceanographic front strongly influence the water mass structures , climate and species distribution in the Mauritanian EEZ . Despite oceanographic and faunistic interest and the threats of demersal deep˗sea fisheries and oil exploration , for the past few decades, marine research in Mauritania was performed to obtain information on the Banc d’Arguin and other coastal ecosystems; only recently, research is being focused on the study of slope habitats . Between 2007 and 2010, the Spanish Oceanographic Institute launched a partnership program with the Mauritanian Institute for Oceanographic Research and Fisheries and the University of Vigo (Spain) to study the resources and ecosystems in Mauritanian deep waters. Four multidisciplinary expeditions were conducted using the Spanish R/V Vizconde de Eza , and geomorphological prospection and oceanographic and biological sampling were performed. Special efforts were made to characterize the Banc d’Arguin canyon systems, the giant coral carbonate mounds barrier and a newly discovered seamount . In this monograph, we tried to compile, in one volume, the main findings of the Maurit surveys and provide a detailed description of the bathymetry , geomorphology and oceanography, and biodiversity and structure of demersal fishes and benthic communities that inhabit the soft˗ and hard˗bottom habitats of the Mauritanian slope .

General Context

The Islamic Republic of Mauritania is located in the Northwest African region between the 15 and 27 north parallels, and it has an area of 1,030,000 km². The country shares borders with Senegal and Western Sahara , Mali and Algeria and with the Atlantic Ocean (Fig. 1.1) and it forms a geographical

Fig. 1.1

Location and marine physical map of Mauritania that includes some of the main coastal features

and cultural bridge between the Arab Maghreb and western Sub–Saharan Africa.

The Mauritanian coastline spreads over approximately 754 km along the Sahara Desert , which occupies three˗quarters of its terrestrial surface and whose dust constitutes the main source of nutrient input to the seafloor, strongly affecting ocean productivity (Gelado˗Caballero 2015) (Fig. 1.2). From the Cape Blanc Peninsula, the northernmost coastal area, to the Senegal River in the south, the coast is generally sandy, flat and dotted with saltwater pools known as sebkhas . The Senegal River, the boundary between Mauritania and Senegal , is the only permanent river in Mauritania, and it fertilizes the southern part of the country, providing an important amount of sediments to the sea.

Fig. 1.2

Pictures characterizing the terrestrial and marine environments of Mauritania. Satellite image of the Saharan dust plume over Northwest Africa (1), oceanic birds in the Gulf d’Arguin (2), coastal area of the Banc d’Arguin National Park (3–5), Cape Blanc (6), aerial view of Nouadhibou (7), Puerto Cansado village (8), Cape Blanc Peninsula, where the Mauritanian Institute for Oceanographic and Fishery Research (IMROP) is located (9) [Images: (1) http://earthobservatory.nasa.gov/, (2–9) © Ana Ramos]

The most noticeable geographical feature in the Mauritanian coast is the Banc d’Arguin , a great bay that stretches from Cape Blanc Peninsula to Cape Timiris along the northern third of Mauritania’s coast. Since 1976, about 12,000 km² of shallow water and adjacent desert have been included in the Parc National du Banc d’Arguin, the first National Park established in West Africa . This natural reserve, together with the Baie de l’Étoile, harbours one of the most extensive seagrass communities in the world—covering more than 1000 km² of the surface. Besides, the Banc d’Arguin houses a major nesting and breeding area for migratory birds in the entire western Africa region, and it is one of the most important sites for this fauna worldwide (Ly 2010; Mahfoud et al. 2013) (Fig. 1.2). The surrounding waters of the park, with a wide continental shelf that reaches 110–120 km (and even 138 km in some places), have significant reserves of bivalve molluscs and are one of the richest areas for fishing resources in Western Africa (IMROP 2014). The main fishing efforts of the artisanal, coastal and industrial fleets (56–80%), targeted to the exploitation of pelagic and demersal resources, have been developed in the Banc d’Arguin zone (IMROP 2014).

Mauritanian waters are included within the Canary Current Large Marine Ecosystem (CCLME), which spreads from the Gibraltar Strait to the Sierra Leone border. The region lies hydrologically under the influence of the Canary Current, which flows southwards along the African coast between 30°N and 10°N and reaches 20°W offshore (Heileman and Tandstad 2009) (Fig. 1.3a). Northwest Africa harbours major upwelling systems that provide deep and nutrient˗rich cold waters, resulting in one of the more productive areas in the world’s oceans (Demarcq and Soumou 2015). The upwelling is permanent throughout the year north of Cape Blanc, in the Western Sahara region (21–26°N; Fig. 1.3b), and seasonal between Cape Timiris (Mauritania) and Cape Roxo (Guinea Bissau) (19–12°N), coupled with thermal front displacement (Pelegrí and Peña˗Izquierdo 2015). This front forms the boundary between warm southern tropical and colder temperate northern waters, being one of the key features of the Mauritanian EEZ that strongly influences the climate as well as species distribution (Bambayé et al. 2010; Mahfoud et al. 2013). In addition, at the Cape Blanc latitude, a giant filament exports this high productivity up to 600 km offshore towards the open sea, enriching deep waters (Gabric et al. 1993; Sangrá 2015).

Fig. 1.3

Schematic representation of the main current that affects Northwest Africa (Canary Current Large Marine Ecosystem region) (a) (Author: César Meiners); satellite image that shows the high productivity of Mauritanian waters (chlorophyll a values) (b) (Source https://svs.gsfc.nasa.gov/)

CCLME is the third most productive large marine ecosystem found worldwide, and it sustains the most productive African fisheries ; the yield during the last decade was more than two million tons a year (FAO 2014). In 2012, landings of commercial species in the CCLME represented almost one˗third (27.4%) of the total 34 FAO Areas . Catches of fishes, crustaceans and cephalopods in Mauritanian waters reached 431,000 tons in the same year (FAO 2014), which represents 20% of the total landings in the CCLME region (Fig. 1.4).

Fig. 1.4

Evolution of the historical global captures in Mauritanian EEZ between 1970 and 2014 and percentage of the Mauritanian and CCLME region captures in the total 34 FAO Area in 2012 and 2013 (source FAO 2016, FAO—Fisheries and Aquaculture Information and Statistics Branch—Accessed: 18 April 2016)

Marine fishing is effectively a social and economic activity for Mauritania, and it is considered as a strategic sector by the Government because it contributes dynamically to sustainable development (IMROP 2014) (Fig. 1.5). Fishing contributes to 20% of the budget revenues from the state and more than 18% of the export earnings of the country, creating over 40,000 direct jobs (IMROP 2014).

Fig. 1.5

Socio˗economic importance of the fishing sector in the Mauritanian economy. Industrial demersal and pelagic fisheries (1–3) and different scenes of the artisanal fisheries: pirogues in the Nouadhibou artisanal harbour and fishing offshore (8 and 10), activities on the artisanal fishing beach of Nouakchott (5, 9, 11, 13–15) and some important commercial species (4, 6, 7 and 12) [Images: (2) © Ramón G. Cancela, (3) © Greenpeace, (1, 4–15) © Ana Ramos]

Over the last 20 years, sea fishing in Mauritania has experienced a meteoric development , starting with the massive arrival of foreign industrial fleets , and, more recently, the development of national, artisanal and coastal fisheries (IMROP 2014; FAO 2016). The Mauritanian economy depends largely on fisheries agreements , particularly with the European Union, but also with China, Russia and other countries (Mahfoud et al. 2013). Between 2008 and 2012, global landings in Mauritanian EEZ have doubled from 200,000 to more than 400,000 tons, and in a period of 20 years (1991–2011), catches of pelagic industrial fleets —more than 94% on average—increased by over 300% and those of artisanal and national fleets , over 2000% (IMROP 2014) (Fig. 1.5).

This great increase in fishing effort led to an overexploitation of certain resources : In the 25 years between 1982 and 2006, the biomass of the main 24 target demersal species on the Mauritanian continental shelf was reduced by 75%, from 250,000 tonnes to less than 100,000 tonnes. Moreover, the trophic structure has been significantly modified, decreasing the mean trophic level of the catchable biomass from >3.7 to <3.5 (Gascuel et al. 2007).

In addition to the overexploitation of commercial resources , the activities of industrial trawl fishing constituted, for more than three decades, a major menace to the biodiversity of marine and coastal habitats in Mauritania. New oil exploration and extraction offshore (Colman et al. 2005) and other human activities (i.e. extension of the autonomous port of Nouadhibou) threatens the Mauritanian ecosystem s, mainly in the Banc d’Arguin area, which is the most productive and sensitive zone (Mahfoud et al. 2013).

Although the coastal protected areas constitute 12% of the total marine surface and four areas meeting the EBSA criteria (Ecologically or Biologically Significant Marine Areas ) have been proposed in Mauritania (UNEP 2014), the protected area system has not yet been properly applied for offshore waters (UNEP 2006). Currently, concerns of the Mauritanian Government and the Institute of Oceanographic Research and Fisheries (IMROP) about the impact of trawling fisheries , especially in the deep sea, have increased. In recent decades, depletion of shelf resources has caused the displacement of some trawler fleets to 800 m—or sometimes even to 1000 m—in search of new fishing grounds (FAO 2006, 2012), affecting still unknown ecosystems. Despite these strong threats , available data on the biodiversity, structure and functioning of Mauritania’s ecosystems, including benthic communities, are still fragmentary (IMROP 2014). The last Working Groups on Evaluation of Mauritanian EEZ (Mahfoud et al. 2013) strongly highlighted the need to map areas of ecological interest and define their sensitivity to disturbance; furthermore, research on marine biodiversity and the monitoring of endangered and vulnerable species along the coast of Mauritania should be strengthened.

Marine Scientific Knowledge

Scientific knowledge on environmental features and marine fauna off Mauritania has been mainly acquired in the framework of the expeditions undertaken in the 19th century and until the middle of the 20th century; the expeditions first focused on the North Atlantic and later, the Northwest African region . Particular surveys and projects were conducted in the EEZ of Mauritania only during the last few decades. Monod (1933), Maurin (1968), Van der Knaap (1985) and Chavance (2002) have provided historical revisions and inventories of major expeditions undertaken along the African coasts .

Northwest Africa

First marine invertebrates and fishes from Mauritanian waters were collected in 1872–1876 during the HMS Challenger expedition around the world. In the second half of the 19th century and first half of the 20th century, different European institutions conducted North Atlantic expeditions—some of which also included the Northwest African coasts —and collected samples in the area between the Strait of Gibraltar and Cape Verde Islands. Of these historical and emblematic campaigns, we should mention those undertaken by the Travailleur and Talisman (1880–1883), Prince Albert I of Monaco (1885–1920) and Michael Sars (1910).

Nevertheless, knowledge on Mauritanian marine fauna is mostly related to the expeditions carried out exclusively along the Northwest Africa , mainly during the 20th century. Some older expeditions in the last part of the 19th century were also particularly important, such as the German expedition on˗board the Gazelle (1874–1876) and the Chazalie schooner (1895). In 1909, the Dutch trawler Hollande VII conducted a series of experimental fishing between the Canary Islands and Angola , working for a certain period in the area of Cape Blanc. The fifth cruise of President Theodore Tissier was conducted from Villa Cisneros to Sierra Leone in 1936; the voyages of the Belgian ship Mercator sampled from Cape Boujador to Conakry in 1934–1936; the Danish expedition aboard the Atlantide worked along the African tropical coast in 1945–1946 and the French expeditions aboard the Calypso explored northwest Africa and the Gulf of Guinea in 1956.

Most of the findings of the aforementioned campaigns were disseminated in monographic series and specific publications containing mainly faunistic reports, descriptions of new species and inventories or catalogues of different groups. Because of their relevance, the report of the scientific results of the voyage of the HMS Challenger in 1873–1876, the Expéditions scientifiques du Travailleur et du Talisman pendant les années 1880, 1881, 1882, 1883, the Résultats des campagnes scientifiques accomplies sur son yacht par Albert Ier and the Atlantide Reports or the Résultats Scientifiques des Campagnes de la Calypso should be noted.

From the middle of the 20th century, biological research took a more consistent character—despite the frequent focus on fishing —acquiring new information in the fields of zoology, bionomy and ecology (Maurin 1968). In addition, during the colonial and independence periods, the Institut Fondamental de l’Afrique Noire (IFAN) and the Office de la Recherche Scientifique et Technique d’Outre˗Mer (ORSTOM), currently Institut de Recherche pour le Développement (IRD), promoted marine research , contributing significantly to information on the coastal zone. From 1939 to 1963, these local institutes developed more limited campaigns in Mauritania, Senegal and Guinea (Le Loeuff and von Cosel 1998). The Gerard Treca, a small research vessel of the IFAN, accomplished important studies and faunal collections in the region during 20 years under the direction of prestigious scientists such as Cadenat, Marche˗Marchad, Roux and Sourie.

In all the above˗mentioned surveys, most of the samples were collected from the continental shelf , and, as a result, the deep˗sea fauna of northwest Africa is among the least studied (Decker et al. 2004). The Thalassa and Discovery surveys in the 1960s and 1970s marked a turning point because they conducted samplings to a depth of 1200 m.

Similarly, oceanographic research in Northwest Africa did not start until the 1970s, when important programs were developed at a regional level. Until the 1960s, a few hydrographic sections of the eastern tropical North Atlantic Ocean had been surveyed, and the surveys were carried out by German (R/V Meteor) and United States (R/V Atlantis and Crawford) expeditions (Wüst 1964). Hence, systematic exploration of the Cape Verde Basin and, more particularly, the shelf and slope waters off Mauritania did not start until the International Decade of Ocean Exploration (IOC 1974 ) . Between 1971 and 1977, some 100 oceanographic cruises were undertaken in the area from the Strait of Gibraltar (36°N) to 10°N as part of the Cooperative Investigation of the Northern part of the Eastern Central Atlantic (CINECA) programme (Hempel 1982). Although this international multidisciplinary programme was focused on biological and fishing research , many cruises were devoted to hydrography, providing the first detailed descriptions of the physical environment in the coastal upwelling region as well as the latitudinal changes between tropical and subtropical waters.

In addition, during the 1970s, the first geomorphological explorations and publication of the first bathymetric maps of Northwest Africa , including the Mauritanian continental margin , were carried out (Jacobi 1976; Seibold and Hinz 1976; Uchupi et al. 1976; Jacobi and Hayes 1982, 1992; see Chap. 2 for references). The first sedimentological maps of Mauritania and Senegambia shelves were published by Domain (1977, 1985).

After the CINECA programme, field oceanographic research in the Cape Verde Basin remained ongoing, mainly thanks to the efforts of German scientists (Mittelstaedt 1983, 1991; Zenk et al. 1991; Müller and Siedler 1992; Siedler et al. 1992; Karstensen et al. 2008; Stramma et al. 2008; Brandt et al. 2010) and, more recently, Spanish oceanographers (Martínez˗Marrero et al. 2008; Pastor et al. 2012; Peña˗Izquierdo et al. 2012).

Mauritania

Although the first scientific survey conducted exclusively in Mauritanian waters dates back to the French Mission Gruvel—from the Bay of Nouadhibou , in the north, to the border with Senegal , in the south, in 1905—it was only in the second half of the 20th century that Mauritanian waters were noted as a zone of faunistic interest.

In 1962, 1968 and 1971, the Thalassa explored the area stretching from the Canary Islands to the northern part of Senegal River —including the Banc d’Arguin and surrounding waters. The findings provided important information on commercial species of fishes, decapods and cephalopods , description of benthic biocenosis as well as the first distribution maps from Mauritanian soft bottom s (Maurin 1968; Maurin and Bonnet 1970; Bonnet et al. 1971) (see Chaps. 4 and 7).

For many years, marine research in Mauritania was mainly focused on ornithology and fisheries in the Banc d’Arguin zone (see review of Wolff et al. 1993). The fishery research in Banc d’Arguin and offshore waters was performed by the Centre National de Recherches Océanographiques et des Pêches (now the Institut Mauritanien des Recherche Océanographiques et de Pêches, IMROP) at Nouadhibou , and the studies provided important information on shallow˗water fishes, hydrography and plankton (Wolff et al. 1993).

The benthos in the northern coast of Mauritania was extensively sampled during the CANCAP˗III survey, developed in the framework of the long˗term CANCAP programme (1974–1989) of the Netherlands (van der Land 1987). This project also had the aim of the studying the biogeography and distribution of benthos at a regional level, from the coast to a depth of 4000 m (den Hartog 1984; van der Land 1987).

The most comprehensive study of the continental shelf and slope of Mauritania was performed by the Dutch Tyro Mauritania˗ II expedition in 1988. This project was conducted to clarify the functioning of the Banc d’Arguin ecosystem and its interactions with open ocean systems (van der Land 1988; Wolff et al. 1993).

The findings of both CANCAP and Tyro Mauritania˗ II expeditions are included in more than 120 contributions published in the journals of the Rijksmuseum van Natuurlije Historie of Leiden. However, an important part of the numerous collections gathered by the oceanographic surveys remains stored in museums and scientific institutions or is currently under study.

During the last few decades, different studies have focused on specific areas or subjects related to marine research in the Mauritanian shelf and continental slope . Since the 1980s, IMROP has carried out biological and ecological research on different aspects of particular commercial species from coastal areas and littoral habitats at the Galgos Bay and Cape Blanc region, Etoile Bay and Banc d’Arguin (Maigret 1980; Diop 1988; Mint 1987; Ly 2009; Ould Baba 2010).

Although the existence of the Banc d’Arguin canyon s was known for years (Maurin 1968) and oil exploration recently led to the discovery of part of the giant cold˗water corals reef (Colman et al. 2005), information on these particular habitats and benthic biodiversity is scarce and fragmentary (Mahfoud et al. 2013).

German and Spanish multidisciplinary surveys developed on˗board the R/V Maria S. Merian (2007 and 2010) and Vizconde de Eza (2007–2010) (Hernández et al. 2008; Ramos et al. 2010; Westphal et al. 2007, 2012) focused on the research of deep ecosystem s on the Mauritanian slope only during the last decade.

The Maurit Surveys

Since 2002, the Spanish Institute of Oceanography (IEO) implemented a multidisciplinary programme that included 22 scientific surveys, some of them in deep waters, along the coasts of Atlantic and Indian African countries , where Spanish fishing fleets traditionally operated. All these surveys were developed in the framework of bilateral intergovernmental agreements between the Fisheries Ministry of Spain and the counterparts in the African countries .

From 2007 to 2010, IEO started a collaborative program with IMROP and the University of Vigo (Spain) to study the resources and ecosystem s in Mauritanian deep waters. This research was developed in the framework of a larger project, EcoAfrik , focused on the study of the biodiversity of benthic ecosystems along the African coasts .

Four multidisciplinary cruises of one˗month duration (November to December) were carried out on˗board the Spanish R/V Vizconde de Eza to study the continental margin of Mauritania (from 80 to 2000 m). The cruises performed geomorphological prospection and oceanographic and biological sampling . Special efforts were made during the two last surveys to characterize the Banc d’Arguin canyon systems, the giant coral carbonate mounds barrier and a seamount discovered to the south of Nouakchott during the surveys.

These four cruises—Maurit˗1107, Maurit˗0811, Maurit˗0911 and Maurit˗1011—performed 342 trawling and dredging operations (Fig. 1.6) and 54 plankton trawls , recorded 267 oceanographic profiles (Fig. 1.7), collected 211 sediment samples and achieved multibeam echosounder of 28,122 km² (Fig. 1.8). We caught more than three million individuals (176 tons); measured 170,000 specimens; performed biological sampling of 12,000 fishes, crustaceans and cephalopods ; and preserved more than 25,000 fishes and invertebrates for later identification and faunistic collections .

Fig. 1.6

Location of sampling stations with commercial and beam trawl gears, and rock dredging during the Maurit surveys (Author Luis Miguel Agudo)

Fig. 1.7

Location of oceanographic stations where profiles with CTD and net sensors were obtained during Maurit˗0911 and Maurit˗1011 surveys (Author Luis Miguel Agudo) (Annexe 1.1)

Fig. 1.8

Number of stations or samplings carried out with different devices during the Maurit surveys

This undoubtedly is the first and most comprehensive project along the African continental margin and in Mauritanian waters, aimed to the study of biodiversity and ecosystems. The results have provided an overview of the demersal and benthic communities living in the deep waters off Mauritania as well as information on the environmental conditions in which they develop.

Scientific Objectives

The overall aim of the two first surveys, Maurit˗1107 and Maurit˗0811, was the exploration and evaluation of existing demersal resources in the continental shelf and slope off Mauritania between 80˗ and 2000˗m depth. During the two later expeditions, Maurit˗0911 and Maurit˗1011, another goal—characterization of demersal, benthic and zooplanktonic communities and vulnerable marine ecosystems —was added to the previous ones.

The specific objectives of the surveys were as follows:

• 1. Estimation of the yields in number and weight of all demersal species, particularly those of commercial value.

• 2. Mapping of coastal and deep demersal resources of current or potential fishing interest in the Mauritanian EEZ.

• 3. Obtaining the size population structure of fishes, crustaceans and cephalopods .

• 4. Obtaining the main biological parameters of target fish and invertebrate species of commercial interest.

• 5. Study and identification of megabenthic and macrobenthic invertebrates .

• 6. Prospection of the seabed by using a multibeam echosounder and high˗resolution seismic (TOPAS) for elaboration of bathymetric and geomorphological maps and digital terrain models, to identify possible relationships between geomorphological features and biological communities.

• 7. Oceanographic characterization of the study area by using vertical CTD profiles and net sensors.

• 8. Geomorphological , oceanographic and faunal characterization of the cold˗water coral structure , northern canyons system and seamount.

• 9. Characterization of zooplanktonic communities and location of larvae of hake and other species of commercial interest.

• 10. Elaboration of comprehensive faunal inventories and characterization of the biodiversity of demersal and benthic assemblages .

• 11. Implementation of reference faunistic collections of IMROP, IEO and University of Vigo .

• 12. Fresh comprehensive photographic inventory of species and external morphological details for taxonomic identification.

• 13. Photography and video recording for dissemination of scientific knowledge on African benthic biodiversity .

The R/V Vizconde de Eza

The Vizconde de Eza is an oceanographic research vessel designed and equipped with instrumentation for fisheries and oceanographic research (Fig. 1.9). The ship, which belongs to the Spanish General Secretariat of Fisheries (former General Secretariat of the Sea) from the Ministry of Agriculture, Food and Environment, was built by M. Cíes Shipyards in Vigo (Spain) and launched in March 2000. The first oceanographic survey was performed in April 2001.

Fig. 1.9

The Spanish oceanographic vessel R/V Vizconde de Eza leaving the harbour of Santa Cruz de Tenerife after the Maurit˗1107 survey in December 2007 (© Ana Ramos)

The Vizconde de Eza presents a prow with a bulb and transom stern with a ramp, and it has the following technical characteristics: 1400 GRT; length, 53 m; beam, 13 m and 1800 HP; the ship can reach a speed of 13 knots and retains power for 40–50 days. It is equipped with a dynamic positioning system and a reinforced prow, which allows work between floating ices. Its stern davit acts as fishing gantry crane and for the tacking, recovery and trawling of equipment used in oceanographic operations.

The ship boasts six specialized laboratories (chemistry, biology, physics, acoustics, humid and informatic) for fisheries and oceanographic research , all them equipped with a fibre˗optic computer network. The Vizconde has a retractable keel where the transducers for acoustic detection equipment and control gear are installed, allowing its advanced geophysical instrumentation to carry out cartography of the seabed up to 5000˗m depth. The boat has the capacity for 35 people: 16 places for scientists and 19 for crewmen, distributed among the bridge, machines, deck and kitchen personnel.

Overall Planning of the Surveys

The four Maurit surveys lasted one month and were conducted between mid˗November and mid˗December from 2007 to 2010.

The load of the material and scientific equipment was carried out two weeks before in the ports of Vigo—where the Vizconde de Eza has its base—or Santa Cruz de Tenerife, in the case of the Maurit˗1107 survey in 2007.

The Spanish and Mauritanian scientific team embarked from the port of Nouakchott (Mauritania) in 2008 and 2009 or those of Santa Cruz de Tenerife or Las Palmas (Canary Islands) in 2007 and 2010. In the latter case, the vessel made its journey to Nouadhibou , anchoring in the Galgos Bay so that the Mauritanian researchers could come on˗board (Fig. 1.10). Surveys were divided into two legs, with a mid˗scale campaign in the ports of Nouakchott or Nouadhibou for the replacement of the Mauritanian scientific team and some Spanish researchers (Hernández et al. 2008; Ramos et al. 2010).

Fig. 1.10

Reaching the Vizconde de Eza (1). Embarking and disembarking are normally carried out through the Port de l’Amitié, the harbour in southern Nouackchott (6) or on the open sea outside the Galgos Bay (8) by using an old tugboat, the pneumatic boat of the Vizconde or other small vessels (2–4, 5, 7–11) (© Ana Ramos)

As a consequence of its multidisciplinary character, the activities performed during the expeditions were as follows:

• 1. Geomorphological prospection with the multibeam echosounder and TOPAS.

• 2. Prospection of demersal resources by trawling with a commercial gear ˗type Lofoten .

• 3. Sampling of the macrobenthos with beam trawl gear at selected transects perpendicular to the isobaths (Maurit˗0911 and Maurit˗1011).

• 4. Megabenthos sampling over the carbonate mounds barrier , northern canyons and seamount by using a rock dredge (Maurit˗0911 and Maurit˗1011).

• 5. Ichthyoplankton sampling with a Bongo net (Maurit˗0911).

• 6. Sampling of the water column with vertical CTD and net sensor (except during Maurit˗1107).

The activities were performed without interruption in 24˗h shifts. The geophysical exploration was performed overnight, from approximately 2100 to about 0600 the next day, when a rapid processing of information—to identify suitable trawling areas —was conducted. Then, at about 0700, the first trawl manoeuvres with either the commercial gear , beam trawl or rock dredge were started. Zooplankton and CTD sampling were performed at the end of the day, after fishing and benthos trawling (Fig. 1.11).

Fig. 1.11

Scientific activities performed on˗board in 24˗h shifts during the Maurit surveys : geophysical exploration (1), CTD and plankton sampling (2, 3), bottom trawling with commercial gear and beam trawl (4, 5), dredging over hard bottoms (6) and audio/visual recording (7) (© Ana Ramos)

Methodology

The complete methodology used in the surveys has been extensively described by Hernández et al. (2008) and Ramos et al. (2010), while particular methods used for the geomorphological, oceanographic and beam˗trawling research have been described in the respective chapters in this volume (Chaps. 2, 3 and 8). Here, we have described only the commonly used sampling methods used for the research whose results are presented in several chapters of this book: bottom trawling prospection with commercial gear , carried out on soft bottoms, and rock dredge sampling performed over the hard structures.

The commercial trawl samples provided the basic faunistic material for the study of fishes—bony fishes and chondrichthyans (Chaps. 4–6)—, global megabenthos (Chap. 7), decapods, cephalopods , hydrozoans and echinoderms (Chaps. 9–12). The material obtained in the rock dredge samples allowed the first benthic characterization of the main hard˗bottom habitats —coral carbonate mounds , northern canyons and seamount—(Chaps. 13–15).

Sampling with a Commercial Trawl

Trawling Operations

We used the same commercial bottom trawl —Lofoten type—in the four Maurit surveys : a fishing gear rigged for trawling over rough bottoms at great depths. The net had 17.70 and 5.5 m horizontal and vertical openings, respectively, and a 35˗mm mesh size at the codend. The trawl was equipped with sweep lines of 250 m, steel bobbins of 14″ and 32 cm in diameter, high˗strength floats (240 mm in diameter)—for fishing at great depths—and oval doors of 850 kg (Fig. 1.12).

Fig. 1.12

Pictures of the commercial Lofoten trawl gear used for demersal prospection and the fishing research activities carried out during the Maurit surveys : Arrival of net on˗board (1), bobbin details (2), door details (3, 4), codend opening (5), catch sorting of fishes and benthic invertebrates (6), specific separation of decapods (7), taxonomic identification of fish samples (8) and biological sampling of commercial cephalopods (9) (© Ana Ramos)

The geographical position of the hauls was recorded using the global positioning system (GPS). The hauls were monitored with acoustic systems (SIMRAD ITI and SCANMAR) attached to the net. These devices provided information on the trawl openings, distance between doors, bottom contact and distance between the footrope and the bottom.

The area prospected with the commercial trawl in the four surveys spread along the entire coast of Mauritania, between the latitudes of Cape Blanc (20°50′N) and the Senegal border in the south (16°04′N), covering a total area of 34,500 km².

We used the swept˗area method and a stratified sampling design on three latitudinal sectors—North (20°50′N–19°23′N), Centre (19°23′N–17°40′N) and South (17°40′N–16°04′N)—and six bathymetric strata (80–200, 200–400, 400–800, 800–1200, 1200–1500 and 1500–2000 m) (Table 1.1). The latitudinal sectors were based on the criteria followed for the evaluation of demersal resources by IMROP (sectors) and the stratification, on the available information on the bathymetric distribution of the faunal communities (strata). On the basis of the available bathymetry and new data acquired during the expeditions, we estimated the surface of each of the six bathymetric strata in the three geographic sectors and performed random selection for grid sampling . A 10′ × 10′ grid was established as the basic sampling unit.

The trawling time in the coastal zone (80–400 m) was 30 min, while that in the deep zone (400–2000 m)—to try to offset the characteristic low densities from the deep sea—as one hour. Hauls that lasted less than 15 min (in the coastal strata) or 30 min (in the deep strata) were considered invalid and not included in the statistical analysis.

We sampled 291 trawling stations between 80˗ and 1860˗m depth, of which 283 were valid (Fig. 1.6). The trawl station data—date, geographical coordinates and depth—are presented in Annexe 1.2.

Canyons and enormous soft sediment deposits occupy large areas of the Mauritanian seabed. This prevented us from carrying out trawling and forced us to seek alternative areas . Therefore, it was not always possible to rigorously follow the sampling methodology on this slope .

Sampling on˗Board and Data Treatment

At each station, all fish and benthic invertebrate species were sorted. In the case of fishes, their identification was performed on˗board; the megabenthos was first split into high˗range taxa and subsequently sorted to the morphospecies level (Figs. 1.12 and 1.13).

Fig. 1.13

Activities related to the benthos sampling: Sorting the invertebrates collected by the commercial trawl (1–3) and dredging (4, 5), sieving on deck (6–8) and taxonomic identification in the laboratory on˗board (9) (© Ana Ramos)

We obtained data on numerical abundance and biomass (wet weigh) for each species or morphospecies . In the case of colonial benthic taxa, numerical abundance was expressed as colony number. When the large volume of the capture made exhaustive sorting impossible, we obtained a subsample of the total catch. In this case, total abundance and biomass were estimated using a weighting coefficient .

All valid stations were treated as representative for the relevant bathymetric strata where the species or group of species were caught, assuming a constant catchability coefficient. For each haul , we calculated the swept area on the basis of the SCANMAR estimated measures (horizontal opening  = 20.4 m and towing speed  = 3.1 knots) and haul duration . Data for numerical abundance and weight by species and station were standardized to the sweep area and expressed by 0.1 km² because this area is close to the real surface swept during each trawling .

Megabenthos Sampling

Although the R/V Vizconde de Eza is devoted to the evaluation and prospecting of demersal resources , this research vessel is a suitable platform for the study of megafauna associated as bycatch to fishing trawling catches. Besides, knowing that deep˗water surveys are rarely focused specifically on benthic research (Menot et al. 2010), we used the excellent opportunity offered by the Vizconde de Eza expeditions. Thus, the sampling of megabenthos and subsequent taxonomic identification were included as an important objective of the Maurit surveys (Hernández et al. 2008; Ramos et al. 2010).

We followed on˗board the protocol for megabenthos sampling established to be carried out during all demersal trawling surveys in the African Atlantic and Indian oceans, from the beginning of the African surveys Program of the Spanish Institute of Oceanography .

In the four Maurit surveys , the scientific team of EcoAfrik specialists—belonging to IEO and the University of Vigo—developed the on˗board benthos program, which included the later taxonomic study of the faunistic collections and data analysis of benthic invertebrates (Fig. 1.13).

Because of the particular efforts undertaken for benthos sampling during the four Maurit surveys , a significant part of the chapters of this monographic volume are devoted to the results on the biodiversity and assemblages of different megabenthic taxa.

Sampling with a Rock Dredge

Hard substrata —namely, rocks and boulders —cannot be sampled using an otter trawl ; therefore, we collected megabenthos from the coral carbonate mounds , canyons and seamount by using a rock dredge , a specialized sampler for hard bottoms . The dredge had 0.80 m horizontal opening and 0.30 m vertical opening, and a 10˗mm mesh in the collector bag (Fig. 1.11). The device was symmetric and had two leather pieces to avoid net breaking and sample loss during the hauls over rough sea bottoms . We obtained one 60˗L sample, which was sieved through a tower with 10˗, 5˗ and 1˗mm meshes. The fauna retained in each fraction was sorted to morphospecies level and then counted and weighed (Fig. 1.13).

At each station, the total catch, rocks and boulder pieces, sieved fractions, specimens and details of all living benthic invertebrates and dead fauna were photographed.

We performed 26 dredging operations on hard structures—13 along the coral barrier , 11 over the canyons in the northern area and two at Wolof’s Seamount —(Figs. 1.6 and 1.13; Annexe 1.2).

Faunistic Collections

Fish specimens of all species collected using the commercial gear and beam trawl were preserved on˗board—frozen or in 4–10% formaldehyde solution—and transported to laboratories in Spain to perform species identification. Specimens have been deposited at the Marine Fauna Collection, Oceanographic Centre of Málaga (CFM˗IEOMA) (see Chap. 16).

In the case of megabenthos , at each station, we fixed and preserved representative specimens for all morphospecies in 70% ethanol or 4% formaldehyde solutions for further taxonomic study . After each survey, the materials were transported to Vigo. Decapods and stomatopods were later housed in the Crustacean Collection at the Oceanographic Centre of Cádiz (CCDE˗IEOCD), while cephalopods , together with a representative collection of fishes and crustaceans , were deposited at the Oceanographic Centre of Tenerife (Canary Islands; IEO).

We conserved almost 25,000 specimens during the four Maurit surveys , and the distribution on the basis of taxa by the Spanish research centre is listed in Table 1.2. The most important collection of benthic invertebrates , composed of around 23,000 specimens, has been deposited at the University of Vigo ; since 2011, it is being studied in the framework of the EcoAfrik project.

Table 1.1 Distribution of the estimated surface (in km²) on the basis of the depth of the strata in the three geographical areas (North, Centre and South)

Table 1.2 Number of specimens of different taxa collected during the Maurit surveys and deposited at different research centres of the Spanish Institute of Oceanography (IEO) and University of Vigo

Monograph

When we decided to publish this book, our objective was to compile, in one unique volume, the main findings of the four joint surveys carried out by the IEO and IMROP in the deep shelf and upper slope off Mauritania between 2007 and 2010.

The monograph has 17 chapters that include, in addition to this introductory chapter, a detailed description of the bathymetric , geomorphological and oceanographic features (Chaps. 2–3), followed by chapters with the findings on the biodiversity and distribution of the main demersal fishes and benthic invertebrates . We paid particular attention to outlining the main components, assemblages and habitats of the benthic realm. Thus, Chaps. 7–12 focus on the biodiversity of megabenthos and epibenthos , as a whole, and of decapods, cephalopods , hydrozoans and echinoderms that inhabit the soft bottom s. Chapters 13–15 describe and characterize the physical environment and benthic fauna of the most important hard˗bottom habitats on the Mauritanian slope —the coral carbonate mounds barrier , the northern canyons and the recently discovered Wolof’s seamount —Chap. 16 is devoted to the zoological collections obtained during the Maurit surveys , and the last Chap. 17 is an integrate synthesis of the main results described in the previous chapters.

In Chap. 2, we have offered an unprecedented and detailed description and cartography of the bathymetry and geomorphology of the Mauritanian continental margin between 80˗ and 2000˗m depth. On the basis of our results, it has been defined into three geomorphological provinces, Arguin , Nouakchott and Senegal River , and it has more than 70 canyons and large gullies grouped into 10 systems. Likewise, the 580˗km length of the coral carbonate mounds barrier, named the Mauritanian Slide Complex , the pockmark areas and the isolated Wolof’s Seamount (about 200 m in height) have been described and mapped. Our exhaustive geophysical work provides the first and detailed cartography of the seabed, compiled in 25 maps, and it can be very useful for future habitat studies and conservation of vulnerable areas .

In Chap. 3, we have described the hydrological conditions of the Cape Verde Basin—and particularly, its eastern boundary , the Mauritania Slope Ocean—, a largely unexplored but dynamic oceanic region where fronts and currents split up and connect tropical and subtropical waters. This exhaustive description is based on the review of previous studies, analyses of historical, satellite and numerical data, and our hydrographic data collected from the Mauritanian continental slope . Coastal and offshore upwellings , high primary production and low oxygen concentrations are characteristics of the entire region and have important implications on fisheries and biodiversity .

Chapters 4–6 present the main findings with respect to demersal fishes, as a whole, and chondrichthyans and hakes, separately. Chapter 4 tackles the biodiversity and composition of the six major fish assemblages with 403 species and 139 families that constitute the demersal fish fauna of the Mauritanian deep waters. We have widely discussed the transitional character of the ichthyofauna composed of tropical and subtropical species , in addition to cosmopolitan and wide˗distribution elements, as well as the observed changes in its distribution and composition, probably related to global warming and intense fishing exploitation .

In Chap. 5, besides the analysis of the biodiversity , assemblages , distribution patterns and demographic structure of elasmobranches —a group of species that play a key ecological role in the balance and dynamics of the Mauritanian deep˗sea ecosystem —we present biological information on the most abundant species. Spatial patterns of abundance , average sizes , length distribution and sex ratio by bathymetric range of Centroscymnus coelolepis, Centroselachus crepidater , Deania calcea and Centroscyllium fabricii have been described, and the influence of the environment and direct and indirect fishing impact s have been extensively discussed.

Chapter 6 deals with the distribution of hakes, the most relevant fishery resource in the Mauritanian deep waters. Two species— Merluccius polli and Merluccius senegalensis —are distributed in Northwest African waters and are so far landed and managed jointly as Merluccius spp . We analysed the influence of different environmental variables on the distribution and abundance of these two sympatric species that, despite their co˗occurrence , are ecologically different, live completely segregated on Mauritanian bottom s and show significant inter˗annual variations that are probably related to climatic variability. The Maurit’s findings described in this Chapter could help in improving the future management of these hake species separately as two different stocks .

Chapters 7–8 analyse the distribution , composition and structure of megabenthic and epi benthic assemblages on the basis of the sampling carried out over slope soft bottom s with two demersal trawling gears—commercial and beam trawl —and two different methodological approaches —random stratified sampling procedure and bathymetric transects —. In both chapters, we have discussed the composition and biodiversity distribution patterns at latitudinal and bathymetric levels , as well as the structure of the main assemblages . In addition, we have discussed the role that the coincidence of the coral barrier and minimum oxygen zone at the same depth range plays in the faunistic discontinuity observed in the Mauritanian continental margin , which acts as a boundary between shallow and deeper benthic assemblages .

The next four Chaps. 9–12 focus on four of the most representative taxa from the Mauritanian slope megabenthos : decapods, cephalopods , hydrozoans and echinoderms. We analysed the diversity, occurrence and composition of each taxonomic group and its corresponding families . Besides, the effects of environmental variables on the diversity and structure of the assemblages have been discussed and compared to those of other previously studied marine areas .

In Chap. 9, we analysed the structure of decapod assemblages, and we particularly outlined the special role played by the reef and minimum oxygen zone, which seems to favour or negatively affect certain decapod species; we described for the first time the existence of a particular crustacean community living in a low˗oxygen environment in Northwest Africa .

In Chap. 10, we have presented a checklist of 132 neritic and benthic cephalopod species that comprise the Mauritanian fauna, remarking on new geographical records and providing the first data on benthic octopod species that inhabit the deep waters of Northwest Africa . In addition, the possible role of the very abundant ommastrephid species as a potential and important fishery resource—not currently exploited in the area—has been discussed.

Chapter 11 offers an overview of the biodiversity , distribution and abundance of the hydroid fauna in Mauritania. This chapter also includes an interesting discussion on eurybathy and particular strategies developed by different hydroid species to colonize the soft bottom s: hydrorhizal adaptations to anchor the colony to the sediment and epizoism . We also analysed the biogeographical components of the Mauritanian hydroid fauna, without endemic species and with strong affinities to the Atlantic˗Mediterranean region.

In Chap. 12, the biodiversity and assemblages of echinoderms—the most important megabenthic taxa in Mauritanian deep waters—have been widely described. We analysed the specific richness , occurrence , abundance and biomass of 82 species belonging to five classes: Ophiuroidea , Asteroidea , Holothuroidea , Echinoidea and Crinoidea . Latitudinal and bathymetric patterns have also been described, being noticeable the finding of the highest densities and biomass in the southern slope , as well as the scarcity and low diversity of the group between 300 and 700 m.

Chapters 13–15 describe the environmental and faunistic characteristics of the three most important and vulnerable ecosystem s found along the Mauritanian continental slope : the giant coral carbonate mounds barrier , submarine canyon systems of the Banc d’Arguin zone and the small Wolof’s Seamount discovered during the Maurit surveys . In the three chapters, we have described and mapped the geomorphologic structures on the basis of multibeam echosounder results, and we characterized the water masses by using CTD profiles. Rock dredge samplings were used to achieve the first identification of the megabenthos living in these hard˗bottom habitats , and particular attention was paid to sessile suspension˗feeder fauna (cold˗water corals , sponges and gorgonians, among others).

We considered it important to add a chapter (Chap. 16) on the biological reference collections because they constituted one of the main objectives of the Maurit scientific program and extraordinary efforts were undertaken to accomplish this objective. The 25,000 specimens of demersal fishes and benthic invertebrates —currently preserved in the IEO and University of Vigo collections—constitute the largest and most important natural archive of biodiversity of the Mauritanian deep˗sea fauna.

In Chap. 17, we have summarized the main findings of the four Maurit surveys , providing the first integrated overview of the demersal and benthic communities that inhabit the Mauritanian deep waters. On the basis of international conventions , we identified areas of particular ecological and/or biological interest and vulnerable habitats, and presented our conclusions and recommendations concerning the interest and protective needs of the most important and vulnerable habitats identified.

Annexes

Annexe 1.1 Characteristics of the oceanographic stations carried out with vertical CTD (SBE25) and net sensor (SBE37) in Mauritanian waters during Maurit-0811, Maurit-0911 and Maurit-1011 surveys

Annexe 1.2 Characteristics of the stations carried out with commercial and beam trawl over soft bottoms , and with rock dredge over hard structures in Mauritanian slope during the four Maurit surveys (n.a.: not available data, null station; in rock dredging’s only start position was taken)