The Gulf of Cadiz is located in the North-East Atlantic Ocean, to the southwest of the Iberian Peninsula. Its eastern boundary is the Strait of Gibraltar, at the western border of the Mediterranean Sea. Its complex physiographic is characterized by irregular reliefs and a diversity of geomorphological features, including the continental shelf of the Spanish coast, channels, numerous mud volcanoes and the deep basin.

In the Gulf of Cádiz, oceanographic circulation follows an anti-cyclonic gyre (Pelegrí et al. 2005) and is controlled by the exchange of water masses through the Strait of Gibraltar: a surface flow of Atlantic origin enters the Mediterranean Sea, while another deep flow of Mediterranean origin circulates under the former towards the Atlantic Ocean.

The upper thermocline water mass is the North Atlantic Central Water (NACW), located at 300–600m water depth (Machín et al. 2006). Two intermediate water masses are found between 600 and 1,500 m: the low-salinity Antarctic Intermediate Water (AAIW) and the Mediterranean water mass out into the Atlantic (Mediterranean Outflow Water, MOW). Below 1,500 m occurs the North Atlantic Deep Water (NADW). MOW circulation is poorly constrained and flows in three main branches: an intermediate branch towards the northwest, a principal branch towards the west, and a southern branch that plunges as far as the Canary Islands. The latter has been reported at 800 m along the Moroccan margin (Pelegrí et al. 2005), possibly transported through eddies (Ambar et al. 1999).

The MOW exerts a greater influence on the bottom of the area as it circulates in contact with the friction surface of the seabed. This interaction with the seabed causes very particular small-scale hydrodynamics, producing subdivisions of the main flow as current energy is dissipated at greater depths.

The area is located to the southwest of the Iberian Peninsula. Its eastern boundary is the Strait of Gibraltar, on the western border of the Mediterranean Sea. It is bounded by the parallels (37º 00'N and 35º 56'N) and meridians (6º 00'W and 7º 24'W).

•          The area includes a variety of benthic habitats that are considered hotspots of biodiversity, including mud volcanoes.

Unique and significant geomorphological features are present in the continental margins, and they are known as mud volcanoes (León et al., 2012; Díaz del Río et al., 2014; Mascle et al., 2014). Mud volcanoes (MVs) are defined as conic edifices constructed by surface extrusion of cold fluids containing mud, saline water and/or gases expelled from a pressurized deep source upwards through structurally controlled conduits (e.g., Brown, 1990; Milkov, 2000; Dimitrov, 2002; Kopf 2002). This process causes substantial changes to the surface of deposits, significantly changing the existing reliefs and generating new carbonated structures. In this way, these bottoms become consolidated surfaces or surfaces of a mixed nature, composed of fragments of new carbonate rock created by the bacterial consumption of methane. The active process of the expulsion of fluid saturated gas through them causes high levels of biological diversity in the benthic ecosystems, which in turn determines the development of important deep-water habitats. The community associated with these bottoms is composed of symbiont species, such as polychaetes, bivalves and decapods, that excavate galleries, but also of other species not strictly linked to the emissions, and which are characteristic of the bathyal sludge, such as molluscs, sea pens, polychaetes and echinoderms. The communities of sea pens and excavator megafauna are widely distributed across different areas adjacent to the mud volcanoes, presenting high densities (as in the case of Tarshish and Pipoca volcanoes) and low densities (Anastasya) of sea pens (Funiculina quadrangularis, Kophobelemnon stelliferum, Pennatula cf. aculeata). Other species that are part of this community are the sponge Thenea muricata, molluscs, decapods, echinoderms and fish (Díaz del Río, 2014, ATLAS, 2019).

Many other benthic habitats occur in this area, both on soft and rocky bottoms. Among them, there are mud with mixed communities such as bamboo corals (Isidella elongate), gorgonian (Radicipes fragilis),  hexactinellid sponges (Pheronema carpenter), crinoids of the genus Leptometra, cnidarians (Flabellum chunii); and rocky bottoms with aggregations of gorgonians (Acanthogorgia, Swiftia, Gymnosarca bathybius, Placogorgia spp., Callogorgia verticillata, Viminella flagellum, Paramuricea clavata), black corals (Leiopathes, Stichopathes, Anthipathella) and scleratinians (Madrepora oculata dominates, Lophelia pertusa and Dendrophyllia cornigera) (e.g., Aguilar et al., 2010; Cúrdia et al., 2012; Fonseca et a., 2013; Díaz del Río, 2014; Boavida et al., 2016) as well as assemblages of the red coral (Corallium rubrum) deep reefs (Boavida et al., 2016).

•          The area includes habitats for endangered, threatened and declining species.

Many species recorded in the area are considered endangered, threatened and/or declining species, according to, for example, the IUCN, OSPAR, ICES and the EU HABITAT DIRECTIVE.

Table 1 shows a list of species that are considered endangered, threatened and/or declining by different laws and conventions. Additionally, some species that are not currently protected have been proposed for inclusion (Aguilar et al., 2010).

The following habitats are also endangered or threatened and are considered by different laws and conventions:

OSPAR Habitats

Coral gardens

Deep-sea sponge aggregations

Seamounts

Sea-Pen & Burrowing Megafauna Communities

Habitat Directive Habitats

1170 Reefs

1180 Submarine structures made by leaking gases

•          The area is important for cetaceans.

This Atlantic–Mediterranean water interface is considered a biogeographic boundary (Sanjuán et al. 1994).  Nevertheless, there is substantial transport of organisms across this ecotone, and different cetaceans species are present in the waters of the Gulf of Cádiz and Strait of Gibraltar: short-beaked common dolphins (Delphinus delphis), striped dolphins (Stenella coeruleoalba), bottlenose dolphins (Tursiops truncatus), long-finned pilot whales (Globicephala melas), sperm whales (Physeter macrocephalus) and killer whales (Orcinus orca) (De Stephanis et al., 2008).

During spring and summer this area provides essential feeding and nursing habitat for killer whales (Orcinus orca). The small seasonal resident population of 39 killer whales, which are genetically and ecologically distinct from killer whales in the Atlantic Ocean, use the area, and the same individuals have been re-sighted annually from 1999 to 2016. They belong to five social pods, which were stable over the study period (Esteban et al., 2014; 2016). Esteban et al. (2014) showed, using model predictions, that killer whale occurrence in the Strait is related to the migration of their main prey, Atlantic bluefin tuna (Thunnus thynnus). In spring, whale distribution was restricted to shallow waters off the western coast of the Strait, where all pods were observed actively hunting tuna. In summer, the whales were observed towards the shallow central waters of the Strait. A relatively new feeding strategy has been observed among two of the five pods. These two pods interact with an artisanal drop-line fishery. Pods predating the fishery had access to larger tuna in comparison with pods that were actively hunting. The Strait of Gibraltar killer whales are socially and ecologically different from individuals in the Canary Islands, where genetic research has indicated that there is little or no female-mediated gene migration between these areas (Esteban et al., 2016).

The Strait of Gibraltar subpopulation of killer whales is considered vulnerable in the Spanish National Catalogue of Endangered Species but may be considered endangered based upon other monitoring studies. In 2016 the area of the Strait of Gibraltar and Gulf of Cádiz was classified as an Important Marine Mammal Area (IMMA) resulting from the assessment of experts within the IUCN joint SSC/WCPA Marine Mammals Taskforce (IUCN MMPATF, 2017; IUCN MMPATF, 2019).

•          The area is also a seasonal migratory pathway for a large migratory pelagic species: Atlantic bluefin tuna (Thunnus thynnus).

The Atlantic bluefin tuna (Thunnus thynnus) (Linnaeus, 1758) is the largest of all tunas (ICCAT 2006–2014) and one of the most highly prized fish species in the world (Ottolenghi et al., 2004). In spring, Atlantic bluefin tuna perform long seasonal reproductive migrations between feeding areas in the Atlantic Ocean and spawning grounds, either in the Gulf of Mexico (western stock) or the Mediterranean Sea (eastern stock). Like all bluefin tuna stocks, both stocks of the Atlantic bluefin tuna are threatened by overfishing.

The bluefin tuna reproductive season in the Mediterranean Sea extends from May to July. In correlation with a progressive east-to-west increase of the sea surface temperature, the spawning process begins in the Levantine Sea, shifts to the southern Tyrrhenian-Malta region and eventually to the Balearic Sea (Heinisch et al., 2008). Like the eastern spawning area, the reproductive season is known to last approximately three months (April-June) in the Gulf of Mexico (Baglin et al., 1982). 

In addition, the Strait of Gibraltar has been identified as a transiting area for satellite-tagged fin whales (Balaenoptera physalus) moving between the Gulf of Cádiz and the Ligurian Sea area of the northern Mediterranean (Gauffier et al. 2009,2018; Cotte et al., 2011; Notarbartolo di Sciara et al., 2016).

The waters of the Gulf of Cádiz are impacted by fishing, shipping and pollution.

Fishing activities: probably the fishing activity that has the greatest impact is bottom trawling, which is responsible for the destruction of some ecosystems. This type of non-selective fishing causes changes in the composition of ecosystems, affecting the long-term productivity of the fishery. The physical consequences of bottom trawling are the alteration and/or direct destruction of habitat and the re-suspension of sediment, increasing turbidity and changing the geochemical composition of the deposits.

Shipping: due to its proximity to the Strait of Gibraltar and the Cape of San Vicente there are important navigation routes that pass over this area, with a high intensity of large-tonnage vessels that mainly transport oil and containers. Maritime traffic is an important source of pollution both because of the potential risk of accidental spillage and because of the intense noise that it generates.

- Water pollution: the main sources of pollution are ships and cities located on the coast (mostly in summer when the intensity of tourism in some coastal areas increases).

Conversely, some actions to protect the area and to ensure the conservation of its biodiversity are being carried out, and one specific area has been protected in accordance with international and Spanish regulations and conventions: "The Gulf of Cádiz mud volcanoes" is located in the bathymetric range between 300 and 1,200 m, placing it on the upper middle part of the continental slope and the southern Iberian continental margin.

Three basic types of habitats have been identified, catalogued and described within the generic Habitats Directive habitat type 1180: (1) the "Mud volcanoes" subtype, which is widespread in the area; (2) the subtype "Collapsed depressions", located next to the volcanoes Anastasya, Pipoca, Hesperides, Almazan, Aveiro and San Petersburg, and (3) the "Pockmarks" subtype, which is widespread throughout the area, especially in the south, being a very diffuse phenomenon in the more distal areas of the slope (112 locations have been mapped). Other habitats at different levels, within the generic 1180 habitat type, include the "Structures produced by leaking gases with carbonate substrates of chemosynthetic origin", which is extensive in the area of gas emission, as well as the designation "Structures produced by leaking gases with chemosynthetic species", which has been identified in the volcanoes Albolote, Gazul, Anastasya, Pipoca, Tarsis, Hesperides, Almazan, Aveiro and St. Petersburg.

In addition, and of equal importance, nine subtypes of habitats linked to the habitat type 1170 "Reefs" have been identified. These are: (1) Bathyal rock with Acanthogorgia hirsuta, on Pipoca; (2) reef of deep coral Lophelia pertusa and/or Madrepora oculata, on bottoms of carbonate rocks and accumulations of compressed dead coral on the slopes of the Gazul mud volcano, which presents significantly more active hydrodynamics than in other areas of the SCI, as well as a low level of dragnet fishing activity; (3) deep rocky bottoms with antipataria, of the genus Leiopathes, Antipathes and Stichopathes, have been found in the environment of the volcanoes Gazul, Hesperides and Almazan; (4) bathyal rock with large hexactinellid sponges (Asconema setubalense), in the surroundings of Chica and Enmedio; (5) bathyal sedimentary rock with Bebryce mollis, found only on Gazul; (6) bathyal rock with Callogorgia verticillata in specific areas of the Chica complex; (7) bathyal rock with Callogorgia and Demospongiae, in the area around Enmedio; (8) deep rocky bottoms with aggregations of Demospongiae, identified in Gazul, Magallanes, Enano, Enmedio and Chica, and (9) deposits of dead coral with remains of escleractinias (e.g., Lophelia pertusa, Madrepora oculata, Dendrophyllia alternata), colonized by small octocorals (e.g., Swiftia, Bebryce, Placogorgia) scattered around the volcanoes Albolote, Gazul, Hesperides, Almazan and Aveiro. Between them, these reef habitats occupy a surface area of approximately 2,063 hectares.

The management plan for the area is being developed in the framework of the INTEMARES project.

Apart from conservation projects, every year the Instituto Español de Oceanografía (IEO) carries out a bottom trawling survey on the Gulf of Cádiz named ARSA. This survey aims to provide data for the assessment of demersal commercial fish species and benthic ecosystems on the area. This survey is part of an international effort to monitor marine ecosystems and is coordinated by the International Bottom Trawling Surveys (IBTS) working group of the International Council for the Exploration of the Sea (ICES).