Improving Scientific Knowledge of Mallorca Channel Seamounts (Western Mediterranean) within the Framework of Natura 2000 Network

Abstract

The scientific exploration of Mallorca Channel seamounts (western Mediterranean) is improving the knowledge of the Ses Olives (SO), Ausias March (AM), and Emile Baudot (EB) seamounts for their inclusion in the Natura 2000 network. The aims are to map and characterize benthic species and habitats by means of a geological and biological multidisciplinary approach: high-resolution acoustics, sediment and rock dredges, beam trawl, bottom trawl, and underwater imagery. Among the seamounts, 15 different morphological features were differentiated, highlighting the presence of 4000 pockmarks, which are seafloor rounded depressions indicators of focused fluid flow escapes, usually gas and/or water, from beneath the seabed sediments. So far, a total of 547 species or taxa have been inventoried, with sponges, fishes, mollusks, and crustaceans the most diverse groups including new taxa and new geographical records. Up to 29 categories of benthic habitats have been found, highlighting those included in the Habitats Directive: maërl beds on the summits of AM and EB, pockmarks around the seamounts and coral reefs in their rocky escarpments as well as fields of Isidella elongata on sedimentary bathyal bottoms. Trawling is the main demersal fishery developed around SO and AM, which are targeted to deep water crustaceans: Parapenaeus longirostris, Nephrops norvegicus, and Aristeus antennatus. This study provides scientific information for the proposal of the Mallorca Channel seamounts as a Site of Community Importance and for its final declaration as a Special Area of Conservation.

1. Introduction

The protection of marine species and ecosystems is especially relevant in the Mediterranean, which has been described as a hot spot of biodiversity [1]. Marine protected areas (MPAs) are recognized as useful tools for managing and enhancing marine species and ecosystems. MPAs can constitute a globally connected system for safeguarding biodiversity and maintaining the health of marine ecosystems and the services they provide. Through the Protocol Concerning Specially Protected Areas and Biological Diversity in the Mediterranean (SPA/BD Protocol), the Contracting Parties to the Barcelona Convention promote cooperation in the management and conservation of natural areas as well as in the protection of threatened species and their habitats. The Marine Strategy Framework Directive (MSFD) also includes a requirement for the European countries of the Mediterranean to establish an ecologically coherent network of MPAs to help protect vulnerable species and habitats [2]. In the European Union, the main instrument for protecting biodiversity is the Natura 2000 network, which seeks the stable maintenance or, where appropriate, the restoration to a favorable status of certain habitats and species including the marine environment.

The Natura 2000 network is composed of Sites of Community Importance (SCI), which are subsequently declared as Special Areas of Conservation (SAC). These protection regimes seek to ensure the long-term preservation of these areas and their flora and fauna as well as the sustainability of human activities carried out therein through the implementation of management plans. As a result of the LIFE INDEMARES project (https://www.indemares.es/en (accessed on 15 December 2021)) developed between 2009 and 2014 in Spain, 10 large marine areas were declared as SCI, half of them sited in the Mediterranean. With this, the total protected sea surface off Spain increased from <1% to >8%, thus contributing to the objective of the Convention on Biological Diversity to protect 10% of marine regions by 2020.

The current LIFE IP INTEMARES project (https://intemares.es/en (accessed on 15 December 2021)) has the aim to complete this work. The scientific exploration of seamounts in the Mallorca Channel (Balearic Islands, western Mediterranean), developed within this project, is to improve the scientific knowledge of this area for its inclusion in the Natura 2000 network. The main objective is to map and characterize the benthic habitats and species of special interest for conservation, the most important human threats, and the vulnerability of the area to propose it as a SCI for the subsequent development of management plans and its final declaration as a SAC.

Seamounts are isolated undersea topographical elevations on continental margins and oceanic domains, which are considered as hotspots of biological activity and biodiversity in the deep-sea [3]. These relevant seafloor reliefs span a broad depth range, being influenced by different oceanographic processes [4] and located in diverse geodynamic settings. Therefore, they comprise heterogeneous habitat types [5], some of them structured by fragile, sessile, slow-growing, and long-lived species sensitive to fishing and other types of disturbance, being internationally recognized as Vulnerable Marine Ecosystems [6]. The scientific knowledge on Mediterranean seamounts is marked by large gaps and an asymmetry between the number of geological studies and biological ones [5].

Up to 60 seamounts and seamount-like structures have been identified in the western Mediterranean [7,8]. Among these are the Ses Olives (SO), Ausias March (AM), and Emile Baudot (EB) seamounts, currently studied within the INTEMARES project (Figure 1). Previous studies on these seamounts have analyzed the demersal fisheries targeted on deep water decapods crustaceans [9,10], the geomorphology and geodynamics [11,12], and the benthic species and habitats [13,14,15,16,17], suggesting their high ecological value. For this, the protection of these seamounts is recommended [18]. The present study includes the first results obtained in the INTEMARES project regarding the mapping and characterization of seafloor, benthic species, and habitats as well as fishing activity on SO, AM, and EB seamounts and adjacent bottoms.

2. Study Area

The Mallorca Channel corresponds to a seaway between the Ibiza and Mallorca islands, located southwest of the Balearic Promontory between the Valencia Trough to the west and the abyssal domain to the east (western Mediterranean). It can be described as an asymmetric channel, whose width varies between 100 and 200 km, narrowing toward the north and deepening up to 1050 m. It is characterized by the presence of a variety of morphological features such as seamounts, scarps, and depressions [8,19]. The three studied seamounts are located in this area, being situated east off Ibiza and the Formentera Islands in the case of SO and AM and south off Mallorca and the Cabrera Islands in the case of EB (Figure 1).

The Balearic promontory delimits the Balearic and Algerian sub-basins in the north and the south, respectively (Figure 1), with different oceanographic conditions [20]. The Balearic sub-basin is more influenced by atmospheric forcing and Mediterranean waters, which are colder and more saline, whereas the Algerian sub-basin is basically affected by density gradients and receives warmer and less saline Atlantic waters [21]. Different water masses can be found in both sub-basins [21,22]. The surface waters, coming from the Atlantic and called the Atlantic Waters (AW), have high seasonal temperature variation, ranging from 13 °C during winter to 26 °C during summer, when a strong vertical temperature gradient is established between a 50 and 100 m depth. The Western Mediterranean Intermediate Water (WMIW) is found at 100–300 m depths and exhibits variable thickness. It is formed during winter in the Gulf of Lions by deep convection, when sea–air heat flux losses are high enough, being characterized by a minimum temperature (~12.5 °C). The Levantine Intermediate Water (LIW), originating in the eastern Mediterranean, reaches the Balearic Islands after circulating through the northern part of the western Mediterranean. It shows maximum temperature and salinity (~13.3 °C and ~38.5, respectively) and is found at 200–700 m depths, just above the Western Mediterranean Deep Water (WMDW), which is located in the deeper part of the water column.

The regional circulation in the western Mediterranean is dominated by the Northern Current, which carries down these intermediate waters along the continental slope of the Iberian Peninsula and bifurcates when reaching the Ibiza Channel [21,23]. One significant part crosses this channel flowing southward, and the other part cyclonically returns along the northern Balearic Islands, forming the Balearic Current (Figure 1). The composition of the waters passing through the Balearic channels are subject to inter-annual variations, depending on the amount of these waters reaching and passing these channels and the flows of the Atlantic Waters passing northward through Ibiza and Mallorca Channel [21,24,25].

Within the general oligotrophic environment of the Mediterranean, the waters around the Balearic Islands show more pronounced oligotrophy than the adjacent waters off the Iberian Peninsula and the Gulf of Lions, due to the lack of supply of nutrients from land runoff [26,27]. Frontal meso-scale events between Mediterranean and Atlantic waters [28] and input of old northern water into the channels [29] can act as external fertilization mechanisms that enhance productivity off the Balearic Islands.

These distinct hydrodynamic scenarios in the northern and southern Balearic Archipelago [30] could be on the basis of some differences observed in deep water ecosystems between the Algerian and the Balearic sub-basins: (i) trophic webs are supported more by plankton biomass than by benthic productivity, while supra-benthos plays a more important role, respectively [31,32]; and (ii) body condition of species is lower in the Algerian sub-basin than in the Balearic sub-basin, not only at an individual species level but also considering the whole assemblage [33]. The interannual variability in the meso-scale circulation above explained can influence the population dynamics of two of the most important demersal resources of the Mediterranean, the hake and the red shrimp as well as their accessibility to fishing exploitation [34,35].

Some demersal fisheries are developed in the Mallorca Channel, mainly focused on the deep water decapod crustaceans red shrimp (Aristeus antennatus) and the pandalid shrimp Plesionika edwardsi using bottom trawl in the adjacent bottoms of SO and AM and traps at the flanks and summits of the three seamounts, respectively [9,10], where commercial and recreational fishing fleets also operate more sporadically using bottom long-line and hand-lines, respectively, to capture large sparids and serranids. In all areas, there are also pelagic fisheries, mainly targeted to swordfish (Xiphias gladius) using pelagic and semi-pelagic long-lines [36] and to bluefin tuna (Thunnus thynnus) using purse-seine [37].

3. Materials and Methods

We developed a multidisciplinary approach including both geological and biological sampling, monitoring of the fishing fleet, and compilation and review of information from existing databases on fishing landings (Figure 2).

3.1. Research Surveys

Between 2018 and 2020, four INTEMARES research surveys were developed (

Table 1. Summary of the research surveys developed in the Ses Olives, Ausias March, and Emile Baudot seamounts and adjacent bottoms of the Mallorca Channel (Balearic Islands, western Mediterranean) during the INTEMARES and MEDITS projects, showing the methods applied to obtain the data and samples: multibeam (MB) and parametric (P) echosounders, Shipek (SK), Box–Corer (BC) and rock (RD) dredges, beam trawl (BT), the experimental bottom trawl GOC-73 (GOC), photogrammetric sledge (ROTV), and remote operated vehicle (ROV).

Survey	Period	Research Vessel	Methods
INTEMARES_A22B_0718	25 July–8 August 2018	Ángeles Alvariño	MB, P, SK, BC, RD, BT
INTEMARES_A22B_1019	11–30 October 2019	Ángeles Alvariño	MB, P, SK, BC, RD, BT, GOC, ROTV
MEDITS_ES_GSA5_2020	24 June 2020	Miguel Oliver	GOC
INTEMARES_A22B_0720	19–29 July 2020	Ángeles Alvariño	MB, P, RD, BT
INTEMARES_A22B_0820	21–31 August 2020	Sarmiento de Gamboa	P, SK, BC, ROV
MEDITS_ES_GSA5_2021	23 June 2021	Miguel Oliver	GOC
MEDITS-PITIÜSES-2021	18, 19 and 25 August 2021	Miguel Oliver	GOC

). High resolution geophysical techniques were applied to study the seafloor and dredges, where beam trawl and an experimental bottom trawl were used for sampling sediments, rocks, epi-benthic and nekton-benthic organisms as well as demersal fishing resources. A photogrammetric sledge and a remote operated vehicle (ROV) were also used to take videos of the seafloor communities. In 2020 and 2021, samples from the experimental bottom trawl were also collected during the three MEDITS surveys (Table 1).

3.1.1. Geophysical Methods

Bathymetric and backscatter data were obtained on board the R/V Angeles Alvariño, which is equipped with a Kongsberg EM710 multibeam echosounder transmitting from 40 to 100 kHz, depending on the changes in depth. During the acquisition, a sound velocity correction was applied using sound velocity profiles of the full water column (SVP+ from AML). An area of 4506 km² has been prospected, from 86 to 1720 m depths along 3250 km of parallel navigation lines (Figure 3A) with full coverage. At the same time, ca. 3000 km of high-resolution parametric profiles were acquired on board R/V Angeles Alvariño and R/V Sarmiento de Gamboa (Figure 3B) using Kongsberg TOPAS PS018 and Atlas Parasound P-35 sub-bottom profilers, respectively. These data allowed us to analyze the geomorphological features of the area.

3.1.2. Sediments and Rocks

A total of 137 surface sediment samples were collected using Shipek and Box–Corer grabs between 86 and 1062 m depths (Figure 3C, Appendix A). Recovered sediments were photographed and described on board. The topmost 5 cm layer of sediments recovered using the Box–Corer grab were sub-sampled using two sterilized bottles of 50 g each, which were stored at −18 °C for subsequent analysis in the laboratory.

A total of 55 samples were taken using a rock dredge between 89 and 1191 m depth, mainly at the summit and upper flanks of the seamounts (Figure 3D, Appendix B). This dredge is composed of a metallic rectangular mouth with beveled edges, equipped with a 1 cm mesh cod-end, protected by another net of 2 cm meshes and leather covers on bottom and top sides. It was trawled in an upward direction over the seafloor, collecting rock fragments, together with the associated flora and fauna. Sampling was conducted at 0.5–1 knots, with an effective duration from 5 to 10 min.

3.1.3. Epi-benthos

Samples were collected with a standard beam trawl described by Jennings et al. (1999) [38], and efficiency was estimated by Reiss et al. (2006) [39]. It has horizontal and vertical openings of 2 and 0.5 m, respectively, and a cod-end mesh size of 5 mm. Sampling was conducted at 2 knots and between 5 and 15 min of effective sampling duration. A total of 85 sampling stations were covered between 99 and 764 m depths (Figure 3E, Appendix C).

The megafauna was sorted on board, identified to species level or to the lowest possible taxonomic level, counted, and weighed. For the calculation of the abundance of colonial ascidians or cnidarians, a foot or colony was counted as one unit or individual. Some species of sponges and algae appeared fragmented and only their biomass was estimated. In the case of calcareous algae, only the biomass of living rhodoliths was measured.

Unidentified specimens were preserved in absolute ethanol or formaldehyde depending on the taxon for further identification at the laboratory. Abundance and biomass of living organisms were standardized by species or taxon to 500 m² using the horizontal opening of the net and the effective towing distance over the bottom in each haul. This distance was estimated using a global positioning system (GPS) and a SCANMAR net probe attached to the headline of the beam trawl to control depth and its arrival and departure to the bottom.

3.1.4. Nekto-Benthos and Demersal Resources

Samples were collected using the experimental bottom trawl GOC-73, widely used along the northern Mediterranean by the MEDITS program to estimate the abundance and distribution of demersal resources and the impact of the fishing activity on the ecosystems [40,41]. This gear has horizontal and vertical net openings ranging 18–22 and 2.5–3 m, respectively, and a cod-end mesh size of 10 mm. Its sampling efficiency has been estimated by Dremière et al. (1999) [42] and Fiorentini et al. (1999) [43]. Sampling was conducted at 2.8 knots and between 45 and 60 min of effective sampling duration, depending on depth. A total of 29 sampling stations were covered between 237 and 1028 m depths in the adjacent fishing grounds of AM and EB (Figure 3E, Appendix D).

Samples were sorted on board, identified to species level, counted, and weighed following the above-mentioned criteria. Length frequency sampling of fishes, decapod crustaceans, and cephalopod mollusks was also estimated. Abundance and biomass of species were standardized to one square km, using the horizontal opening of the net and the distance covered in each haul, obtained using the SCANMAR system and the GPS, respectively.

3.1.5. Visual Transects

Habitat and benthic communities were high resolution filmed from transects developed with the TASIFE photogrammetric sledge, a remotely operated towed vehicle (ROTV), and the ROV Liropus 2000. Each covered a different objective: The ROTV filmed sedimentary and flat areas, while the ROV filmed rockier areas and steeper slopes.

The ROTV transects were carried out with the vehicle moving at 0.5 knots and flying from 0.5 to 2.5 m above the seafloor. It was equipped with a Nano SeaCam piloting camera installed forward and a Nikon D800 video recording camera in the zenithal position, a spotlight system to illuminate the seafloor and three green laser beams, with a distance between them of 10 and 24 cm. Its accurate location over the bottom was obtained from the HiPAP^® acoustic positioning of the R/V. The ROTV was also equipped with a precision altimeter and a SBE50 pressure sensor to control its distance to the bottom and depth, respectively. A total of 48 transects from 15 to 20 min were recorded with a ROTV between 87 and 708 m depths (Figure 3F, Appendix E), providing 13 h of video and a total explored area of 30,066 m²: 8304 m² in SO, 19,124 m² in AM, and 2638 m² in EB.

The ROV transects were carried out with the vehicle moving at <0.3 knots and flying from 0.5 to 2.5 m above the seafloor. This ROV is equipped with a full HD color camera and a pal color camera installed forward and a mini camera in the rear part. It was also equipped with a CTD SBE37Microcat, two laser pointers, a dual frequency SONAR Seaking DST, an altimeter (LPA200), an acoustic Beacon MST 324, two hydraulic manipulators, and a boxes system to store collected samples. The navigation system of this ROV includes a Tether Management System (TMS) and a Launch and Recovery System (LARS). The TMS is equipped with an extra low light back and white camera, a CTD, a current meter Midas Valeport, and an acoustic beacon MST 324. A total of 29 transects from one to four hours were recorded with ROV between 89 and 1162 m depth (Figure 3F, Appendix F), providing 52 h of video and a total explored area of 17,322 m².

3.2. Fishing Activity

The most important demersal fishery operating within the study area was assessed from Vessel Monitoring by satellite System (VMS) data of the bottom trawl fleet. The VMS database consists of records that contain data on the geographic position, date, time, and instantaneous velocity for each boat, approximately every two hours. In the study area, trawlers are only allowed to work 12–13 h per day (05:00–17:00 the insular fleet and 05:00–18:00 the peninsular fleet) and five days a week, from Monday to Friday. In order to remove VMS signals from boats transiting to fishing grounds or ports, only records with an instantaneous vessel velocity from 2 to 3.5 knots were selected, making sure vessels were fishing at the time of the emitted signal.

After filtering, a total of 115,764 VMS signals were retained during the period 2016–2019. These signals were used to estimate the trawling effort in the study area. Each signal was assigned to one of the trawl fishing grounds previously mapped by Guijarro et al. (2020) [44] around the Balearic Islands. Then, the fishing effort in each fishing ground was calculated as the number of fishing trips per year. In addition, data on the landings and their economic value were obtained from daily sales bills of the bottom trawl fleet. The marketing of their catches takes place the same day or the day after the catches, depending on the ports. These sheets detail, for each vessel, the kilograms auctioned by species commercial category as well as their first sale value. The daily VMS data of the vessels allowed us to assign their sales sheets, and therefore the landings, to the exploited fishing grounds. To assess the bottom trawling around the studied seamounts, we estimated the number of fishing days developed by trawlers in the fishing grounds closest to them as well as the catches extracted and revenues obtained.

3.3. Analysis of Samples in the Laboratory and Data Processing

3.3.1. Geomorphology

Bathymetric raw data were imported in a single project using CARIS HIPS and SIPS V. 11.3 software (© Teledyne) and were georeferenced to create a gridded base surface of a 2 × 2 m cell size in the shallower zones of the summit of the seamounts, of 8 × 8 m in the whole seamounts, and 16 × 16 in the deepest zones of the seafloor. The CUBE algorithm was used to create the surface and data were manually inspected and cleaned using the subset editor module. Tide correction was applied and the final processed data were exported as geotiff raster files. After cleaning, the backscatter mosaic was obtained using the SIPS backscatter module and Geocoder algorithm and exported as a geotiff raster with the same resolution. Bathymetric and backscatter processed data were integrated into an ArcGIS v.10.8 (© ESRI) project where the geomorphological analysis was conducted.

Parametric profiles were loaded in a Kingdom IHS Markit software for their interpretation. Time-to-depth conversion was conducted assuming a sound velocity of 1600 m/s for unconsolidated sediments [45].

The identification and counting of pockmarks were carried out using a sequence of well-defined ESRI ArcGIS tools for mapping and spatially delineated these features in individual polygons, which represent the areas of the seabed where pockmarks occur. The methodology used was based on the study developed in other pockmark fields located in the central North Sea [46].

3.3.2. Sediments

The sedimentological analysis for grain size distribution was carried out on 10–15 g of sediment pre-treated with 10% H₂O₂ to remove organic matter and sodium hexametaphosphate as a dispersing agent. Samples were wet sieved to separate the coarse fraction (gravel) using a 2 mm mesh size sieve. Particles <2 mm (sand, silt, and clay) were determined by using a laser diffraction analyzer (Mastersizer 3000, Malvern^® Panalytical, Fuengirola, Spain). The textural classification of the sediments was based on Folk (1954) [47] ternary diagrams.

The organic matter (OM) and carbonate contents were obtained by the loss on ignition method (LOI) [48] in dry sediment samples (60 °C for 72 h). The percentages of OM and carbonates were estimated as the weight loss after the first (550 °C for 4 h) and second (950 °C for 2 h) ignitions, respectively.

3.3.3. Biological Communities and Fishing Resources

The standardized abundance and/or biomass by species or taxon at each beam trawl and experimental bottom trawl station were used to construct benthic and nekton-benthic species matrices, respectively. In the case of rock dredge stations, for which standardization was not possible, the data matrix only included the presence/absence data by species or taxon. Additionally, the length frequency distribution of the red shrimp Aristeus antennatus, the target fishing resource for the deep-water trawl fishery in the whole western Mediterranean [49], was also estimated from the experimental bottom trawl samples.

For multivariate analysis, data were square-root transformed and similarity between samples was calculated using the Bray–Curtis index. Cluster analysis and non-metric multidimensional scaling (MDS) were conducted to identify assemblages. The similarity percentage analysis (SIMPER) and the analysis of similarity (ANOSIM) were applied to characterize the species composition of assemblages and to test for differences in their composition, respectively.

For each assemblage, we calculated the following community and diversity indicators: mean standardized total abundance and biomass, number of species (S), Shannon–Wiener (H’), and Pielou evenness (J’). These analyses were performed with the PRIMER-E 6 and PERMANOVA software [50]. The index of diversity N90, especially sensitive to the fishing impact [51,52,53], was also applied to detect differences between assemblages. This was calculated following the R procedure described in Farriols et al. (2021) [54]. For statistical comparisons, the Student’s t-test was used. The Shapiro–Wilk test was applied to check for normality. When this assumption was not met, a Kruskal–Wallis non-parametric test was applied.

3.3.4. Habitat Identification from Images

The analysis of video transects carried out thus far has been qualitative. Both ROTV and ROV were viewed using VLC Media Player 3.0.16 for Windows software. Video fragments not allowing for accurate identification of habitats or species, containing blurry images or not showing the two laser pointers, were considered not valid. Video recorded while the ROV stopped or was too far or too close from the seabed to properly visualize it was also considered not reliable for analysis.

In the case of ROTV, the coverage percentage of each habitat type was estimated with the time observed within a width of 0.5 m (based on the laser beams). The video fragments were divided into sections that showed only one habitat at a constant speed and the same distance from the seafloor. These fragments were considered the sampling units. The covered area was calculated by multiplying the sampling unit length by the field of vision width of the ROV camera, estimated from the laser pointers for scaling.

On each sampling unit, habitat and substrate type categories (fine sand, medium sand to gravel, cobbles and pebbles, rhodoliths, and rock) were defined and the biota was identified to the lowest possible taxonomic level and counted, with special focus on taxa considered vulnerable, as a conservation target and habitat-forming species. In some cases, especially for sponges, cnidarians and tunicates were catalogued in morphotype categories.

The habitat identification was carried out considering those included in the Habitats Directive 92/43/EEC such as sandbanks that are slightly covered by sea water all the time (Habitat code 1110), reef (Habitat code 1170), and underwater structures formed by gas leaking (Habitat code 1180). When none of these habitats was observed, it was categorized according to the Spanish Inventory of Marine Habitats and Species [55], guidelines for inventorying and monitoring dark habitats in the Mediterranean [56], and previous studies in the Balearic Islands [13,14,57,58,59].

4. Results

4.1. Geomorphological Features of the Seafloor

Six main morphological feature groups characterized the geodiversity of the Mallorca Channel (Figure 4). Based on their origin, these features were classified as (i) structural; (ii) fluid escape-related; (iii) volcanic; (iv) mass movement-related; (v) bottom current-related; and (vi) biogenic-related (Figure 4A). The near surface morphology and the sub-bottom sedimentary structure of these features as well as their location in each seamount and adjacent seafloor are explained below.

• Structural features

Features related to tectonics, with a morphological expression on the seafloor, were mapped in the entire study area. The main structures were seamounts, highs, ridges, tectonic depressions, and fault scarps.

Both SO and AM are NNE–SSW trending seamounts made up sedimentary rocky materials, corresponding to Balearic Promontory basement uplifted by tectonics. A linear fault scarp runs longitudinally across the summit of AM at 86–150 m depth (Figure 4C). It is 8.6 km long, up to 64 m deep in its SW edge and 23 m in its NE edge, with 32° of slope (Figure 4B). The sub-bottom profiles indicated a relatively thin sedimentary cover (<15 m) at the summits of the seamounts (Figure 4F).

Close to these seamounts, two minor highs named Greixonera and Dimoni are located, showing sharp flanks up to 40° of slope (Figure 4D). Greixonera, 230 m high and 6 km long, is located in western SO, whilst Dimoni is a 300 m high and 5 km long spike located at the edge of a structural spur in northern AM.

Moreover, two NE–SW ridges were located to the north and central area of the Mallorca Channel, having lengths of 10 to 12 km, respectively, and moderate slopes (Figure 4A,B). Several depressions and fault scarps with NNE–SSW to N–S trends are located to the northeast of the northern ridge, and to the north, south, and east of the central ridge, most likely associated with structural control.

• Fluid escape-related features

Pockmarks are the main feature related to fluid seepage, being mapped more than 3950 in a 300–1000 m depth range. They are extended in the whole study area, with the exception of the deep central basin area, which only presents some individual depressions. Most of these pockmarks had circular shapes with U to V-shaped cross sections (Figure 4D). Their length ranged from 10 to 500 m and up to 40 m in deep. Although most of them appeared randomly distributed, some were aligned, forming strings in mainly NW–SE, NE–SW, and N–S trends. In some cases, these strings developed elongated depressions and were emplaced on normal faults, recognized in the parametric profiles (Figure 4G).

• Volcanic features

The main volcanic element is the EB seamount that corresponds to a NNE–SSW oriented volcanic guyot, whose summit is located between 94 and 150 m in depth. It is constituted by the coalescence of several volcanic buildings, partially visible on the eroded summit of the seamount, which is also characterized by several terraced levels at 100–150 m depth and a volcanic cone of 130 m high in its northeastern edge.

In addition, a volcanic cone field was identified on the flanks and adjacent seafloor of EB between 215 and 915 m depths (Figure 4A,E). It comprises at least 170 spike and flat-topped conical edifices that rise from 25 to 420 m, with maximum widths and lengths of 140 to 1785 m and slopes up to 50°. These are mostly circular, although some have irregular geometries.

• Mass-movement related features

Mass-movement features were one of the most widespread features in the Mallorca Channel. They comprise both erosive and depositional elements such as slide scars and mass-transport deposits (MTD). In addition, some gullies related to these features have been differentiated.

Erosive surfaces and gullies developed in the upper sector of the eastern flanks of EB and AM, forming a network of drainage that erodes their walls. They appear as narrow V incisions, separated by moderate to sharp ridges up to 30 m in depth. They have different orders of magnitude, being larger in EB than in AM. In general, they are 1 to 5 km long and have NW–SE and NE–SW to N–S trends, respectively (Figure 4C), with moderate slopes. Their heads are mainly sub-circular in shape and coalesce, forming major amphitheater scarps such as the one located northwestern EB, up to 4 km long (Figure 4C).

Slide scars were identified on the eastern flanks of SO and the western flank of EB as well as in the adjacent seafloor (Figure 4A,E). They have amphitheater geometry and high slopes of 40°, lengths of 1.5 to 2.2 km in SO, and up to 5 km long in EB. Those developed in the northern Mallorca Channel are evenly affected by pockmarks at the sharp walls.

MTDs were present along the Mallorca Channel slope, mainly at the foot of the slope of the seamounts at different depths, generating scarps of up to 20 m high at the seafloor. In parametric profiles, it was observed that most part of these deposits has nowadays been buried, but recent deposits affecting the seabed were also observed. Those MTDs were up to 50 m thick and recognized by the disappearance of sediment packages and the presence of sedimentary features. Moreover, some buried MDTs appeared stacked, representing at least three different events (Figure 4G).

• Bottom current related features

Bottom current features were mainly mapped at the base of AM. They comprised erosive elements such as contourite moats and furrows and depositional ones such as contourite drifts and sediment waves.

Contourite moats were elongated depressions located around seamounts. They are asymmetric and have U–shaped cross sections that deepen up to 30 m of incision and are mainly NE–SW oriented. In addition, a major 2 km long and 35 m deep moat was identified locally, associated with the western edge of AM. It is asymmetric, half-moon shaped, and NE–SW oriented (Figure 4C).

Several contourite drifts were identified associated with the moats, depressions, and the seamounts. These are mainly mounded and plastered drifts attached to the edges or bases of these features. These contourite drifts are occasionally disturbed by pockmarks and slide scars, and in some cases, older MTDs appear under the youngest drift deposits (Figure 4F).

Small scale sediment waves were identified in the southern AM at 300–400 m depths. They comprise slightly sinuous crests with NE–SW to N–S trends and occupy a total area of 17 km² (Figure 4C,H).

• Biogenic related features

Biogenic features were identified in the summits and upper flanks of SO, AM, and EB, being well represented in the western area of AM and central area of EB. They are mound-shaped to ridge features up to 2 to 15 m high and around 200 m long, that when coalesced reach lengths up to 1 km (Figure 4A,D). Biogenic mounds were formed by hard substrates, coming from bioclastic accumulations of fossil and contemporary calcareous framework-building organisms such as coralline algae (e.g., rhodoliths) and other skeleton-supported reefs of scleractianians and octorals as well as bivalve cement-supported reefs.

4.2. Sediment Characterization

Sediments at the summit of AM were coarse and mixed (Figure 5A,B) with a texture ranging from gravelly sand (up to 35% gravel) to gravelly muddy sand (up to 28% mud). The nearest surrounding areas of this seamount were muddy to silty sand (up to 36% silt), evolving to a finer texture of sandy silt (up to 61% silt) toward the Dimoni high. The pockmark field at the southern area of the seamount was mostly sandy mud to sandy silt (53% average silt), with 22 and 25% clay and sand content, respectively (Figure 5A,B).

Sediments at the summit of SO show an average sand content of 90%, thus they were classified as sand and muddy sand becoming less sandy (68% on average) and more muddy (32%) toward the flanks (Figure 5A,B). The pockmark field observed at the northwest of this seamount was sandy mud in texture, where the silt content (40% on average) was higher than the clay (23%) and the sand (37%).

EB was quite heterogeneous in sediment texture, ranging from coarse sediments of gravelly sand (up to 92% sand) and mixed sediments of gravelly mud (up to 83% mud) at the summit, toward sand (up to 98%) to muddy sand (up to 48% mud) in some areas of the summit, the flanks, and in the nearest area of volcanic cones (Figure 5A,B). The pockmark field at the north of this seamount is sandy mud that evolves to coarser sediment, predominantly muddy sand toward northern areas. On average, the sediment was characterized by 42, 36, and 22% of sand, silt, and clay, respectively.

In general, the coarser sediments were observed in the summit of AM, followed by EB and SO. The main difference among the pockmark fields is the content of silt and sand, since the clay was similar in all of them. The coarser textures were observed in the pockmark field in northern EB, while the finer textures were present in the pockmark field in southern AM. Some samples in the central basin showed sediments of sandy mud texture (Figure 5A,B), with silt (up to 50%) as the dominant fine fraction.

The organic matter content in surface sediments ranged from 4 to 14.3% with a mean value of 10.4% (Figure 5C). The lowest values (4.6–9.3%) were observed on the summits of the three seamounts, extending along their flanks to 300–350 m depths. The rest of the studied area showed intermediate to high values of organic matter content (9.3–14.3%), with the highest values observed in the central basin.

The carbonate (inorganic carbon) content values of the surface sediments ranged from 19.5 to 52.2%, with an average value of 27.9% (Figure 5D). The spatial distribution showed maximum values (>43%) at the summits of AM and EB, extending on their flanks up to 250 m in depth. In general, the distribution was opposite to that of the organic matter content. The intermediate values (43–34%) were distributed from 250 to 350 m depths including the summit of SO. The rest of the studied area, from a 350 m depth onward, was covered by sediments with low carbonate content (<34%), reaching minimum values in the central basin.

4.3. Biodiversity, Species Assemblages and Fishing Resources

So far, a total of 547 different species or taxa have been identified (Appendix G), most of them identified from beam trawl (68%), while 30, 29, and 25% were identified from ROV, bottom trawl, and rock dredge sampling, respectively. There were also differences in the number of species or taxa identified by seamount, being 184 in SO, 413 in AM, and 369 in EB. The more diverse groups were sponges, followed by teleost fishes, mollusks, crustaceans, and echinoderms with 118 (22%), 105 (19%), 96 (18%), 91 (17%), and 49 (9%) species or taxa identified, respectively.

The cluster and MDS analyses of standardized biomass from beam trawl samples identified three epi-benthic assemblages on sedimentary bottoms, which were strongly influenced by depth (Figure 6A): (BT-a) the shallowest samples, between 102 and 169 m, at the summits of AM and EM; (BT-b) a group of samples from intermediate depths, between 227 and 574 m, at the summit of SO and flanks of SO, AM, and EB; and (BT-c) the deepest samples, between 500 and 756 m, at the base and adjacent bottoms of these three seamounts. The ANOSIM results (R = 0.77; p < 0.01) confirmed significant differences between these assemblages. The mean values for the estimated ecological parameters showed differences between these assemblages (

Table 2. Mean values (µ) and standard errors (SE) of standardized abundance and biomass, species richness (S), Shannon–Wiener (H’), and Pielou evenness (J’), estimated for each of the assemblages identified from multi-variant analysis of beam trawl, rock dredge, and experimental bottom trawl samples obtained at the Ses Olives, Ausias March, and Emile Baudot seamounts and adjacent area of the Mallorca Channel (Balearic Islands, western Mediterranean). The code (see Figure 7), number of samples analyzed (n), depth (D), number of species (Spp.) of each assemblage, and the significant differences (Kruskal–Wallis test; p < 0.001) between all assemblages (*) or between pairs of assemblages (1–2, 1–3) are also shown. In the case of beam trawl sample assemblages, mean (AvN₉₀) and standard deviation (SDN₉₀) values of the N90 diversity index are also shown, jointly with the associated average (AvSim) and the standard deviation (SDSim) values of within-group similarity.

				n/500 m2		g/500 m2		S		H’		J’
Code	n	D (m)	Spp.	µ	SE	µ	SE	µ	SE	µ	SE	µ	SE	AvN90	SDN90	AvSim	SDSim
Beam trawl (BT)
BT-a1	25	99–156	407	33(*)	7	208.6(1,3)	69.2	52.0(*)	16.2	2.6(1,3)	0.7	0.7	0.2	45.62	1.08	11.43	0.48
BT-b2	40	195–574	354	10.3(*)	1.4	16.8(1,2)	4.8	38.9(*)	16	2.2	0.5	0.6	0.2	28.48	0.98	11.53	0.42
BT-c3	17	501–759	124	3.4(*)	0.9	9.7	2.8	20.5(*)	7.2	1.9	0.6	0.6	0.2	8.71	0.59	21.88	0.88
Rock dredge (RD)
RD-a	20	90–193	139					15.15	2.11
RD-b	12	242–609	64					8.25	1.55
RD-c	10	209–1081	56					9.8	1.9
Experimental bottom trawl (GOC)
GOC-a	21	542–768	76	3.5 × 103	485.4	270.5	45.2	22	0.8	2.3	0	0.8	0
GOC-b	4	444–510	66	15.1 × 103	3283.1	206.9	73.8	41.3	2.4	2.9	0.3	0.7	0.1
GOC-c	2	328–393	60	44.8 × 103	20,958.7	1157	427.6	42	2	2.3	0.1	0.6	0
GOC-d	1	237	25	6.3 × 103	-	749.2	-	25	-	2	-	0.6	-
GOC-e	1	1028	4	150.1	-	0.42	-	4	-	1.1	-	0.8	-

). Both standardized abundance and biomass and the diversity indices S, H’, and N₉₀ decreased with depth. In contrast, the three assemblages showed similar values of equitability (J’).

SIMPER results (

Table 3. Summary of SIMPER results of the assemblages (see codes in Figure 7) identified from multi-variant analysis of beam trawl (BT), rock dredge (RD), and experimental bottom trawl (GOC) samples obtained in sedimentary and rocky bottoms of the Ses Olives (SO), Ausias March (AM), and Emile Baudot (EB) seamounts and adjacent area of the Mallorca Channel (Balearic Islands, western Mediterranean), showing the percentage of within-group similarity (Sim) and the number of species (Spp.) contributing up to 90% to this similarity. The percentage of between-group dissimilarity (Diss) comparing geographic differences (by seamount) in the identified beam trawl sample assemblages as well the number of species (Spp.) contributing up to 90% to this dissimilarity, is also shown.

Codes	Sim	Spp.	Areas	Diss	Spp.
BT-a	24.0	64	AM vs. EB	79.3	230
BT-b	21.9	59	SO vs. AM	79.7	144
			SO vs. EB	79.3	171
			AM vs. EB	78.7	170
BT-c	33.3	16	SO vs. AM	67.7	64
			SO vs. EB	67.4	68
			AM vs. EB	70.5	53
RD-a	23.4	13
RD-b	23.6	7
RD-c	15.4	8
GOC-a	57.1	13	AM vs. EB	46.2	38
GOC-b	52.1	24
GOC-c	53.4	14

; Appendix H) showed that the main species contributing to within-group similarity in the BT-a assemblage were coralline red algae (10%), while the contribution of a high number of decapod crustaceans, sponges, brachiopods, and echinoderms, both sea urchins and sea stars, was much lower (1–3%). No species contributed much more than the others to the similarity of the BT-b assemblage, ranging between 2 and 7% of the contribution of ten species of crustaceans (decapods and the peracarid Lophogaster typicus), sponges, the brachiopod Gryphus vitreus, an echinoderm (the brittle star Ophiura (Dictenophiura) carnea), and the cephalopod mollusk Sepietta oweniana. Decapod crustaceans were the main species contributing to similarity of the BT-c assemblage, with only three species (Geryon longipes, Polycheles typhlops, and Calocaris macandreae) summing more than 50% of this similarity.

The geographic differences (by seamount) were also analyzed. SIMPER results (Table 3; Appendix H) showed an average dissimilarity of 79.3% between AM and EB summit samples, being coralline red algae and sponges (e.g., Hexadella sp.), both more abundant in AM, as the species with a higher contribution to this dissimilarity. The average dissimilarity values by comparing SO summit and flanks of the three seamount samples were 79% in all cases, being G. vitreus and Desmacella inornata, with larger biomass in AM and EB, respectively, the main species that contribute to this dissimilarity. The comparison of samples obtained in the base and adjacent bottoms of the seamounts showed lower values of dissimilarity: 67.7% (SO-AM), 67.4% (SO-EB), and 70.5% (AM-EB). The presence of Isidella elongata at SO and the higher abundance of the fishes Nezumia aequalis and Lepidorhombus boscii at AM and G. vitreus and G. longipes at EB contributed mostly to this dissimilarity.

The cluster and MDS analysis of the presence/absence matrix from the rock dredge also identified three benthic assemblages on rocky bottoms (Figure 6B): (RD-a) the shallowest samples between 90 and 193 m depths at EM and AM summits; (RD-b) samples from 242 to 609 m depths at SO, AM, and EB flanks; and (RD-c) a more heterogeneous group, between 240 and 1081 m depths, at the flanks of the three seamounts and volcanic cones surrounding EB. The ANOSIM results (R = 0.64; p = 0.001) confirmed significant differences between these assemblages. SIMPER results (Table 3; Appendix H) showed that main species contributing to within-group similarity of the RD-a assemblage were coralline red algae and the brachiopods Megerlia truncata and Argyrotheca cordata, summing up to 70% of similarity. The decapod crustaceans of the genus Plesionika (three species summing up to 45.7%) and the bivalve mollusk Asperarca nodulosa (31%) were the main species in the RD-b assemblage. The sponges Haliclona poecillastroides, Hamacantha (Hamacantha) sp. 1, Ancorinidae sp. 1, Poecillastra compressa, and other not identified sponges, summed up to 77.5% of similarity of the RD-c assemblage.

The cluster and MDS analysis of standardized abundance from the experimental bottom trawl GOC samples on the deep water trawl fishing grounds adjacent to the seamounts identified an assemblage between 542 and 768 m in depth at AM and EB (GOC-a), which is clearly separated from four samples at 444 and 510 m depth in AM (GOC-b), the two samples at 328 and 393 m depth in AM (GOC-c), and the shallowest and deepest samples at a 237 m depth in AM (GOC-d) and at a 1028 m depth in EB (GOC-e), respectively (Figure 6C). The ANOSIM results (R = 0.71; p < 0.01) confirmed significant differences between these assemblages. The mean values for the ecological parameters analyzed also showed differences between these assemblages (Table 2). While standardized abundance and the species richness (S) clearly decreased with depth, the standardized biomass and the other diversity indices H’ and J’ did not show this trend. In the GOC-a assemblage, the ANOSIM results showed low geographic differences between AM and EB (R = 0.24, p < 0.002). The dissimilarity between AM and EB in this group was 42.16% and the main species that contributed to this dissimilarity were the elasmobranch Galeus melastomus (7.9%) and the decapod crustaceans Aristeus antennatus (6.8%), Geryon longipes (5.9%), and Phasiphaea multidentata (5.3%).

The SIMPER results (Table 3; Appendix H) showed that the main species contributing to within-group similarity in the GOC-b assemblage were decapod crustaceans, teleost fishes, and one cephalopod mollusk, some of them of commercial interest: Plesionika martia, Nephrops norvegicus, Parapenaeus longirostris, Phycis blennoides, Helicolenus dactylopterus, Lepidorhombus boscii, and Merluccius merluccius. The main species that contributed to within-group similarity in the GOC-a assemblage were also decapod crustaceans, teleost fishes, and the elasmobranch G. melastomus. Some of these species (P. martia, Hymenocephalus italicus, P. blennoides, and Hoplostethus mediterraneus) were the same as the previous assemblage, but contributed with different percentages. Species of commercial interest also contributed to the similarity of the GOC-a assemblage: the target A. antennatus and its by-catch P. martia, G. longipes, G. melastomus, and P. blennoides.

The data obtained from the experimental bottom trawl GOC-37 samples from 542 to 768 m in depth at AM and EB allowed us to compare the density and population structure of red shrimp (A. antennatus) between the two fishing grounds adjacent to these seamounts (Figure 7). No significant differences were detected in the standardized abundance and biomass. However, length frequency distributions showed larger males in EB and smaller females in EB.

4.4. Bottom Trawling

In recent years, three different trawl fleets operate in the identified three fishing grounds around Ibiza and Formentera Islands, possibly impacting the SO and AM seamounts (Figure 8A): (i) up to nine local vessels from the ports of Sant Antoni de Portmany, Eivissa, and La Savina that focus their activity mainly on the continental shelf; (ii) up to 29 vessels from the ports of Denia, Calp, Altea, La Vila Joiosa, and Santa Pola on the Iberian Peninsula, that can carry out trips of 3–5 days to fish below 150 m depth; and (iii) only three vessels from the port of Andratx on Mallorca, that carry out daily trips to fish sporadically on the northern slope off Ibiza Island. In contrast, no trawling activity has been detected in adjacent bottoms of EB.

Three different fishing grounds were identified in the vicinity of SO and AM (Figure 8B): (i) one situated east and northeast of Ibiza Island, with its southern part including upper and middle slope bottoms adjacent to SO and AM; and (ii) two situated east of Formentera, one including upper slope bottoms and the other including middle slope bottoms, in both cases adjacent to AM. These fishing grounds correspond to slope bottoms, where the insular trawling fleets of Ibiza and Formentera do not operate. They are mainly exploited by the trawling fleet from the Iberian Peninsula targeting deep water decapod crustaceans of high economic value such as rose shrimp (P. longirostris) and Norway lobster (N. norvegicus) on the upper slope and the red shrimp on the middle slope.

On average, these three fishing grounds represent 16% of the fishing days conducted by the trawl fleet around Ibiza and Formentera. They concentrated 28% of the fishing days conducted by the Iberian Peninsula fleet around these two islands and 13% of its fishing days with respect to the whole fishing area of this fleet including both insular and peninsular fishing grounds. The fleet from Mallorca only operates in the northernmost part of the fishing ground in eastern and northeastern Ibiza, which on average concentrates up to 45% of the fishing days developed by this fleet when operating near Ibiza and less than 6% of its fishing days with respect to its whole fishing area, mainly western and southern Mallorca.

Up to 16 species or commercial categories were identified as the most important catches of the trawling fleet operating on slope bottoms around Ibiza and the Formentera Islands (

Table 4. Estimated annual landings, in terms of biomass (kg) and economic value (€ from first sale), for the main species or commercial categories extracted from the three bottom trawl fishing grounds in adjacent bottoms of the Ses Olives and Ausias March seamounts in the Mallorca Channel (Balearic Islands, western Mediterranean), and average values (± standard error) during the period 2016–2019. The location of these fishing grounds is shown in Figure 9B.

	2016		2017		2018		2019		Average Whole Period
Species or Category	kg	€	kg	€	kg	€	kg	€	µ (kg)	SD (kg)	µ (€)	SD (€)
Argentinidae	318	849	4005	9223	6136	16,381	4726	14,224	3796	2482	10,169	6899
Aristeus antennatus	3481	94,361	3633	93,128	6064	188,003	15,126	427,550	7076	5496	200,760	157,588
Citharus linguatula	2	4	1881	5527	10,837	40,892	8611	40,829	5333	5208	21,813	22,109
Galeus melastomus	98	368	96	492	875	2988	1493	5171	640	677	2255	2288
Geryon longipes	5657	27,320	8589	39,188	7026	32,608	9713	27,199	7746	1776	31,578	5665
Helicolenus dactylopterus	499	1717	2876	5131	9340	22,458	10,490	24,994	5801	4871	13,575	11,849
Lepidorhombus spp.	1582	8009	1675	8358	1616	7345	2489	14,066	1840	434	9445	3110
Lophius spp.	2453	23,449	4065	28,998	10,268	70,846	12,847	90,179	7408	4949	53,368	32,402
Merluccius merluccius	1672	11,949	3055	20,633	8992	46,242	13,416	90,701	6784	5444	42,381	35,350
Micromesistius poutassou	805	2264	2468	8704	2369	8810	13,400	45,650	4761	5810	16,357	19,767
Nephrops norvegicus	2977	72,615	5840	148,117	16,302	445,533	20,547	496,826	11,417	8358	290,773	211,623
Ommastrephidae	323	525	3242	8913	8929	23,541	5585	20,363	4520	3643	13,335	10,602
Pandalidae	2302	17,213	2957	20,352	4518	30,692	5784	43,262	3890	1568	27,880	11,761
Parapenaeus longirostris	136	1419	4541	50,727	20,935	249,009	19,137	258,024	11,187	10,401	139,795	132,899
Phycis blennoides	2005	4984	5191	15,486	11,318	30,043	13,132	34,524	7912	5200	21,259	13,556
Rajidae	285	958	1924	2676	2996	8085	4813	11,485	2505	1900	5801	4856
TOTAL	24,593	268,004	56,037	465,652	128,523	1,223,475	161,309	1,645,048	92,616	63,175	900,545	644,931

): rose shrimp, Norway lobster, red shrimp, the deep water crab G. longipes and other category of decapod crustaceans composed by species of the genus Plesionika, a category of cephalopod mollusk composed by the Ommastrephidae species Illex coindetii, Todaropsis eblanae and Todarodes sagittatus, the teleost fishes hake (M. merluccius), spotted flounder (Citharus linguatula), blackbelly rosefish (H. dactylopterus), blue whiting (Micromesistius poutassou), greater forkbeard (Phycis blennoides), monkfishes (Lophius budegassa and L. piscatorius), megrims (L. boscii and L. whiffiagonis), a category composed by species of the family Argentinidae (Glossanodon leioglossus and Argentina sphyraena), the elasmobranch blackmouth catshark (G. melastomus), and a category composed of species of the family Rajidae. These species or commercial categories represent >90% of total landings in terms of biomass and >92% in terms of economic value.

On average, the annual catches of these species or commercial categories obtained by the trawling fleet from the three fishing grounds adjacent to the SO and AM seamounts represent 24% of their landings from all trawl fishing grounds around Ibiza and Formentera and 25% in terms of their economic value (Table 4). These landings represent 35% of the annual biomass of these species extracted by the fleet from the Iberian Peninsula on the fishing grounds around Ibiza and Formentera, and 7% from their landings obtained both on insular and peninsular fishing grounds. In terms of economic value, these figures were 32 and 7%, respectively (Table 4). Regarding the vessels from Mallorca Island, their landing obtained in the northernmost part of the fishing ground in eastern and northeastern Ibiza represent up to 83% of the annual biomass extracted by these vessels from the fishing grounds in this area and 84% of its economic value, but they only represented 2 and 1.5% of their total landings in terms of biomass and economic value, respectively (Table 4).

4.5. Habitats

Up to 29 different categories of benthic habitats were identified from ROTV and ROV video transects (

Table 5. Categories of benthic habitats identified from ROTV and ROV video transects in th Ses Olives, Ausias March, and Emile Baudot seamounts of the Mallorca Channel (Balearic Islands, western Mediterranean) during the INTEMARES project. Their name, code, and hierarchical organization level (HOL; ranging from 1 for the more generalist and least detailed one to 5 for the level with the highest detail and knowledge) were assigned according to the Habitats Directive, with some exceptions (*) identified during the previous INDEMARES project (https://www.indemares.es/en (accessed on 15 December 2021)).

Habitat Name	Code	HOL	Habitat Assignment
Sandbanks which are slightly covered by sea water all the time	1110		Rhodoliths beds *
			Infralittoral and circalittoral detritic beds with rhodoliths dominated by invertebrates *
			Circalittoral detritic beds with Alcyonium palmatum and Paralcyonium spinulosum *
		5	Infralittoral and circalittoral detritic beds with rhodoliths dominated by invertebrates with sponges dominance *
		2	Circalittoral detritic bottoms
			Circalittoral and infralittoral detritic biogenic habitats *
			Circalittoral and infralittoral detritic biogenic habitats with Phyllophora crispa *
		3	Bathyal detritic bottoms
			Bathyal shelf-edge sedimentary bottoms with Brachiopoda (Gryphus vitreus) *
			Bathyal mud and sandy mud bottoms dominated by burrowing megafauna *
Reefs	1170		Bathyal rock with Scleractinia *
		5	Bathyal rock with Alctyonacea (Paramuricea hirsuta)
		4	Dead coral framework
		5	Dead coral mounds
		4	Bathyal rock with Anthipataria (Leiopathes glaberrima)
		4	Bathyal rock with Alcyonacea (Callogorgia verticillata)
		4	Bathyal rock with coarse sediments with Bebryce mollis
		4	Bathyal rock with coarse sediments with Leptometra celtica
		3	Coralligenous rock dominated by invertebrates
		3	Circalittoral rock invertebrate-dominated
		3	Bathyal rocky bottoms with sponges aggregations
		4	Bathyal rock with coarse sediments dominated by sponges
		5	Upper bathyal biogenic Thanatocoenosis of giant ostreids
		2	Bathyal muds
			Bathyal muds with small sponges (Thenea muricata) *
		4	Bathyal compact muds with Alcyonacea (Isidella elongata)
			Escarpments, rocky walls and slopes of seamounts with anthozoans (scleractinians, gorgonians, and antipatharians)
Submarine structures made by leaking gases	1180	3	Pockmarks

; Figure 9). Two of them are considered protected habitats: rhodoliths beds and coralligenous bottoms. Five of them were designated as sensitive habitats: (i) bathyal muds with Isidella elongata; (ii) facies with crinoids, (iii) facies with red algae of the genus Peyssonnelia; (iv) rhodoliths beds; and (v) communities of bathyal detritic sands with Gryphus vitreus.

The analysis of video transects obtained with ROTV (Figure 10) showed that dominant habitats in SO were soft bottoms. Bathyal mud with burrowing mega-fauna dominated around the seamount and detritic bottoms on the summit, both habitats summing up 87.5% coverage. On the flanks, hard bottoms with bathyal rock, dominated by sponges were found, with 11.5% coverage. In the summit of AM there were rhodolith beds (16%), alternating with detritic bottoms (30%), while in the base, soft bottom with pockmarks (13%) and bathyal detritic bottoms (30%) predominated. On flanks, escarpments, rocky walls, and slopes with anthozoans and/or small sponges such as Thenea muricata were also found. Rhodolith beds with invertebrates, especially anthozoans (alcyonarians and gorgonians) and sponges, predominated on the EB summit (67% coverage), while muddy bottoms were found at the base and adjacent areas.

The analysis of the ROV (Figure 10) found that the SO seafloor consisted mostly of bathyal muds (69% of covered area), in some areas with burrowing megafauna, and to a lesser extent, detritic bathyal bottoms and rocky slopes covered by sponges (10 and 7% coverage, respectively). Pockmarks, soft bottoms with G. vitreus or T. muricata, and rocky areas dominated by the crinoid Leptometra celtica were also found. The circa-littoral seafloor of AM was defined by rhodolith beds (33%) and detritic sand (7%), dominated by alcyonids and sponges, while its bathyal areas were widely covered by sand or muddy sediments (41%), some of them dominated by the brachiopod G. vitreus (3%). The rocky slopes and escarps of AM were covered mainly by sponges (10%), but also by the cnidarians Paramuricea hirsuta (1.6%) and Bebryce mollys (1%). EB, with the widest depth range of visual deployments, showed a circa-littoral domain with detritic soft bottoms (38%), some dominated by the soft red algae Phyllophora crispa, the alcyonids Alcyonium palmatum and Paralcyonium spinulosum, and rhodolith beds. The bathyal transects showed mainly muddy or soft detritic sediments (22% and 38%, respectively), with dead coral mounds and pockmarks. The hard substrates were dominated by sponges, the crinoid L. celtica, and black corals.

The geographic distribution of the habitats (Figure 10) showed that the lowest number of habitats was observed in SO (11) and the highest in EB (21). AM presented an intermediate number of habitats (16), despite being the seamount with less video transect sampling. In general, thanatocoenosis of giant ostreids seemed to be distributed around the three seamounts and dead coral framework, and mounds were also found in some bathyal areas of their flanks.

5. Discussion

The present paper includes a preview of the results obtained during the INTEMARES project regarding the mapping and characterization of seafloor, benthic species, and habitats as well as fishing activity on the SO, AM, and EB seamounts and adjacent bottoms. This multidisciplinary approach has greatly improved the scientific knowledge on the geological, biological, and habitat diversity of these seamounts in the Mallorca Channel, which constitutes the first step for their inclusion in the Natura 2000 network. It provides new baseline information on the diversity patterns in the area and useful details of the seascape distribution, which can be used for future ecological assessments.

5.1. Geodiversity

The new geomorphological mapping has enhanced between six and 20 times the bathymetric detail of the seabed. From this improvement, we differentiated, among the seamounts, 15 different morphological types: minor highs, ridges, tectonic depressions, fault scarps, pockmarks, volcanic cones, gullies, slide scars and mass-transport deposits, contourite moats, furrows and drifts, sediment waves, and numerous biogenic mounds. This great geomorphological variety of features shows the importance of the interplay of several geological (structural and fluid flow processes), oceanographic (bottom current related processes), and biogenic (bioaccumulation of reef-building organisms) processes in shaping the seafloor and influencing substrate types and benthic habitats of the Mallorca Channel.

The presence of biogenic mounds and mass-movement related features is widespread at the summits and flanks of all the seamounts and adjacent bottoms, with AM the most affected seamount by both processes. Biogenic mounds and patch settlements strongly depend on the availability of hard substrates [61] such as the rocky outcrops identified at these summits, occurring in at least half of the summit surface of AM and EB, but being less represented at the summit and upper flanks of SO. They were previously reported by OCEANA (2011, 2015) [13,14], although their distribution was more than double that described, probably due to the widest depth range analyzed in the present study. All seamounts have flat-topped summits and some develop terraced levels, suggesting that they were once islands that later on became submerged edifices associated with different mechanisms such as wave erosion at the sea surface, water mass interaction, or affected by subsidence [62,63]. These processes, together with other environmental conditions such as the hydrodynamic regime and the sufficient productivity, have modulated the morphology of these structures.

The seafloor surface affected by sedimentary instabilities is 12% of the study area (~600 km²), a value double that of that previously estimated for the Balearic Promontory by Acosta (2005) [64]. At the same time, they are related to zones of fragility associated with structural and fluid flow processes such as active faulting, folding, and pockmark development, as has been suggested by Iglesias et al. (2010) and Palomino et al. (2011) [4,65] in the Cantabrian and Alboran seas, respectively.

Pockmarks have been categorized as habitat type 1180 “Submarine structures made by leaking gases” in the Habitats Directive 92/43/EEC, which has a restricted distribution in European waters, with the Mediterranean one of these areas where this habitat is located. However, it remains unrepresented within the Mediterranean Natura 2000 network [18]. Previous studies have identified some pockmarks in the Mallorca and Ibiza Channel and Iberian Peninsula area [66], with our results in line with these findings. However, we highlighted the presence of almost 4000 pockmarks that largely developed surrounding the three seamounts with deep bottoms up to 1000 m in depth. These pockmarks occur in areas with great sedimentary thickness, where the higher sedimentation rates favor the burial of organic matter and make it more prone to anaerobic digestion. In this sense, the location of the large pockmark fields displays the highest organic matter values in superficial sediments of the study area.

The formation of pockmarks has been univocally proposed in the literature as caused by the existence of fluid escape processes, water or gas, preferably gas such as methane from the subsoil [67] whose expulsion would favor the erosion of sediments. These seafloor depressions can also be affected by bottom currents, which may favor their erosion and genesis. In the Mallorca Channel, these fluid flow features can be found in different evolutionary phases, although in some cases, the occurrence of underneath acoustic chimneys in the subsoil has been located in the high resolution parametric profiles. The origin of these acoustic masking features has been proposed in the literature as amplitude anomalies related to free gas that is migrating upward through the sediments toward the seabed (e.g., [68]).

Another feature to remark on is the volcanic cone field (up to 170 edifices) restricted to the flanks and adjacent bottoms of EB, a seamount that unlike SO and AM is of volcanic origin [11]. The presence of numerous volcanic cones suggests a multiple focused extrusion paths towards the seafloor. High-resolution sub-bottom profiles show low penetration on them, indicating the absence of a recent unconsolidated sedimentary cover, that point out to the availability of hard substrates at these structures for reef-forming organisms, as reported for the seamounts. Furthermore, volcanic cones and pockmarks are spatially interspersed along the periphery of EB, fact that could influence the fluid flow process development onto the seafloor.

5.2. Biodiversity, Communities, and Habitats

The flora and fauna inventoried, with up to 547 species or taxa, have also contributed to improve the knowledge of the biodiversity of the study area. In contrast to previous studies, developed exclusively from visual censuses and samples of benthic organisms using ROV [13,14,17], the combination of sampling methods used in the present study (epi-benthic sledge, bottom trawl, rock dredge, and ROV) has allowed us to cover not only a wide range of species including small-sized benthic organisms, species difficult to identify only from images and highly mobile nekton-benthic fishes, but also to achieve a more precise identification of them by obtaining more samples to be analyzed in the laboratory.

Some of the identified species up to date have been new to science and new records in the study area or even in the Mediterranean. This is the case of the discovery of the new genus of sponge Foraminospongia, whose type species F. balearica is one of the most abundant sponges at the AM and EB summits and the other two new sponge species F. minuta and Paratimea masuttii [69]. Moreover, new geographical records have been published for another 16 sponges [69] and one ophiuroid [70], with this last species also abundant in the study area. Some species have been found at depths where they had not been previously recorded, which was the case of two little known decapod crustaceans: the alpheids Alpheus platydactylus and Alpheus cf. dentipes. Up until now, the first species had been reported at depths of 120–791 m [71,72,73], but we collected a male and an ovigerous female at 88 m depth in the coralligenous bottoms of EB. The second was collected at a 305 m depth in SO, but this species had always been reported at shallow infra-littoral bottoms inside sponges, rock cavities, and among calcareous algae [71,74,75,76,77]. Although the species was identified as A. cf. dentipes according to Noël (1992) [78], these differences in bathymetry cause doubt about its specific assignment, pending future studies. The report of the sepiolid Stoloteuthis leucoptera in the fishing grounds adjacent to AM must also be pointed out. This species is a deep-sea cephalopod, whose presence in the Mediterranean is very rarely known [79].

These invertebrate groups are good examples of the limitations regarding the identification of species only from images. Since Pitcher et al. (2007) [80], the assessment of benthic species richness on seamounts can be strongly influenced by the sampling methodology applied, with extractive sampling yielding broader estimation of biodiversity. Moreover, with these sampling methods, it is possible to obtain individuals and perform the detailed morphological and genetic analyses needed for the description and identification of new species or records [81]. This is clear from the number of species inventoried exclusively using one or another sampling method. From the 537 species or taxa detected in the Mallorca Channel seamounts, only 110 have been detected using both images and one of the three sampling gears. The majority of these species have been exclusively detected using gears, up to 484, whereas only 54 of them were exclusively detected from the images. The most effective sampling gear was the beam trawl, with up to 184 species detected exclusively using this gear, whereas 51 and 41 species were exclusively detected using bottom trawl and rock dredge, respectively.

However, ROV images are very useful for sampling rocky bottoms and to improve the information collected with epi-benthic sledge and bottom trawling on sedimentary bottoms. In rocky bottoms, images and samples from ROV can allow for the estimation of the standardized density of benthic flora and fauna and to detect highly mobile nekton-benthic species. This was the case of Trachyscorpia cristulata echinata and Pontinus kuhlii, observed in EB from our study and OCEANA (2011) [13], respectively. Both scorpionfishes are poorly known in the Mediterranean, probably because their preferential habitat is not accessible to the more conventional and widespread sampling in the area for nekto-benthic species, developed from bottom trawl gears. In fact, these findings represent the second report of both species in the Balearic Islands [82,83]. In the case of sedimentary bottoms, ROV or photogrammetric sledge images can provide information on the spatial distribution of benthic species (e.g., patchiness) and the tridimensional structure of habitats, thus providing a more realistic picture. All these results emphasize the importance of combining complementary sampling methods to assess the diversity of seamounts.

In most seamount studies, depth has been shown to be the most important environmental factor in determining the structure of benthic assemblages, which generates their distribution as bands encircling the seamounts [84,85]. Our results are not an exception, and the assemblages of benthic and nekton-benthic species identified both in sedimentary and rocky bottoms of the Mallorca Channel follow a clear bathymetric distribution, with different communities in the summit, flanks and base that is also related to the substrate type. Albeit to a lesser degree, we have also detected differences in epi-benthic species composition between the seamounts, both at summits, characterized by coarser sediment, high content in inorganic carbon, and low content in organic matter as well as in flanks and bases, mainly dominated by finer sediments, low content in organic carbon, and high content in organic matter. These differences were lower for the nekton-benthic assemblages in the trawl fishing grounds adjacent to AM and EB. This result should be highlighted considering the different level of exploitation of the fishing grounds compared. While AM has been exploited by the bottom trawl fleet targeted to red shrimp (A. antennatus) for more than 50 years [86], this fishery has not been developed in EB during the last two decades because of its large distance from any port, and more recently, the high fuel cost [44]. Despite this, the only difference that could be attributable to the impact of fishing is the slightly greater abundance of the elasmobranch G. melastomus observed in EB compared to AM (on average, 21.0 vs. 17.2 individuals/km², respectively), although the other elasmobranch Etmopterus spinax showed a contrary situation (on average, 6.2 vs. 1.2 individuals/km², respectively). In contrast, red shrimp did not show differences in its abundance, only in its length frequency.

The gradient of habitats found also followed the depth range. In the circalittoral summits of AM and EB, there are detritic bottoms with rhodolith beds and coralligenous outcrops, dominated by communities of sponges and alcyonarians and gorgonian anthozoans. As a consequence of the extreme transparency of the water in these areas, these rhodolith beds have been found quite well structured down to a 137 m depth, most likely the deepest depth of this habitat in the western Mediterranean. As above-mentioned, most of SO summit is flat and is covered by detritic bottoms, which is in contrast to the seafloor around this seamount, containing mud and sandy mud beds dominated by burrowing fauna, as occur in the Gulf of Cadiz [87]. Sponges and corals colonize the rocky bottoms of the flanks, in the upper slope of the three seamounts. These filtering species seem to be more frequent and abundant in the flanks facing the main current directions, probably as a consequence of a current-mediated increase in food availability, an aspect that should be further investigated in studies of habitat and species modeling. Other habitats in this bathymetric range were some crinoid beds and thanatocoenosis of giant ostreids, which seems to surround each seamount between 260 and 415 m in depth. In the less steep flaks and bathyal terraces of the upper and middle slope were muddy soft sediments accumulating facies of the brachiopod G. vitreus, burrowing megafauna and/or dead coral debris. The deepest areas of the middle slope at the base of seamounts are dominated by the finest muddy sediments and the presence of pockmarks. In these bottoms, facies with the corals Callogorgia verticillata and Isidella elongata, the sponge Thenea muricata, and the bryozoan Kinetoskias sp. have also been found.

5.3. Fisheries

Currently, deep water bottom trawl fishing activity is developed on adjacent bottoms of SO and AM. The comparison of the epi-benthic and nekton-benthic assemblages and one of the main fishing resources (red shrimp) between these fishing grounds and a fishing ground adjacent to EB that has not been exploited by the trawling fleet for 20 years did not show clear differences. However, these results must be considered as very preliminary and further studies to assess the impact of fishing activities on species and habitats will be necessary.

These studies should also consider the direct impact of trawling gears on the seafloor, because bottom trawling has also been reported as an important driver of sediment resuspension, caused by the passage of the fishing gear through bottoms, becoming an important seafloor micro-morphology disturbing process in muddy and moderate-energy continental shelves [88] and a driver of deep seascape evolution [89,90]. Sediment resuspended, as a result of trawl fishing, also has a wide variety of additional effects including the smothering of feeding and respiratory organs [91], which can affect the settlement and feeding of the biota. Hence, the potential effect of these sediments reaching and settling in the seamounts should be assessed, considering the high diversity and density of filtering benthic species inhabiting both the sedimentary and rocky bottoms of the SO and AM seamounts and adjacent areas.

The potential impact of other demersal fishing gears should also be considered in these studies. This was the case for two commercial fleets operating with traps and bottom long-lines on the summits and flanks of the three seamounts and the recreational fleet operating with hand-lines. The activity and catches of this last fishery is largely unknown. Although traps and long-lines are more selective than bottom trawl and their impact is much less, it can still be significant not only on their target species, but also on benthic habitats [92]. Moreover, it must be taken into account that these fishing gears operate in areas not accessible to trawling.

5.4. Ecological Value of Mallorca Channel Seamounts

Most of these habitats are included in the Habitats Directive (HD) as being of community interest (habitat codes 1110, 1170, and 1180) and are of high ecological value, not only because of the high species diversity they house, some of which are threatened or declining, but also because some of them are considered as sensitive or vulnerable habitats and, for that reason, they have been protected by both environmental and fisheries regulations. That is the case of maërl or rhodoliths and coralligenous beds, which the Council Regulation No. 1967/2006, concerning management measures for the sustainable exploitation of fishery resources in the Mediterranean Sea, considers as protected habitats and prohibits fishing with bottom trawls on these bottoms. To implement this, in 2014, the Spanish Ministry for Agriculture, Fisheries, and Food declared the summits of AM and EB as fishing protection zones in which trawling was forbidden. Until then, the AM summit had hosted some trawl fishing grounds, which are currently not exploited. Maërl/rhodolith beds have also been considered as Essential Fish Habitats because they are necessary for the development of critical life stages of exploited fish species, and require special protection to improve stock status and the long-term sustainability of fisheries [93].

Some of the inventoried species are considered of conservation interest, according to Annex IV of the HD (species that need strict protection), Annex II of the Barcelona Convention (endangered or threatened species), and the Spanish List of Wild Species under Special Protection Regime (Law 42/2007 on Natural Heritage and Biodiversity), which include species, subspecies, and populations deserving of attention and particular protection based on the scientific, ecological, and/or cultural value due to its uniqueness, rarity, or degree of threat as well as species that appear as protected in directives and international conventions ratified by Spain, and the Balearic Catalog of Threatened and Special Protection Species (Decree 75/2005). This is the case of the Corallinaceae red algae Lithothamnium coralloides and Phymatholithon calcareum, the sponges Axinella polypoides and Tethya sp., the gastropod mollusk Ranella olearium, and the corals Callogorgia verticillata, Dendrophyllia cornigera, and Madrepora oculata. Other anthozoans such as the bamboo coral I. elongata, the sea pen Funiculina quadrangularis, and the whip coral Viminella flagellum, not included in the previous regulations but catalogued by the International Union for the Conservation of Nature (IUCN) as Critical Endangered, Vulnerable and Near Threatened, respectively [94] have also been observed. In addition, the elasmobranch Centrophorus uyato, catalogued by IUCN as Endangered [95], has also been recorded. To these benthic and nekton-benthic species must be added especially protected pelagic species that have also been reported in the seamounts of the Mallorca Channel. This is the case of the sea turtle Caretta and the cetaceans Delphinus delphis, Stenella coeruleoalba, Tursiops truncates, and Physeter macrocephalus [13]. Recent studies have also suggested that these seamounts and the area around them are an important enclave for this last species and have reported the presence of two other cetaceans: Grampus griseus and Globicephala melaena (Unpublished data, Fundación TURSIOPS).

The high heterogeneity of habitats found is in concordance with previous studies in the area [13,14] and encompasses similar values in the nearby Menorca Channel [59,96,97,98,99] and other Mediterranean seamounts [100,101]. However, the number of habitats identified in the Mallorca Channel seamounts is higher than that of the Seco de los Olivos seamount [101,102], one of the closest and recently studied seamounts in the western Mediterranean. This could be due to the widest depth range analyzed, the special oceanographic characteristics of the SO, AM, and EB seamounts in between the Balearic and Algerian sub-basins [20], and the large heterogeneity of environments, both hydrographic and geo-morphological, as has been found in other seamounts [84,85,103]. Other explanatory factors may include biotic (e.g., availability of food or space for attachment and competition) and abiotic characteristics, taking into account the different origin of SO and AM, made up of carbonate materials like most of geological units of the Balearic Promontory, with respect to EB of volcanic origin, which increase the availability of different substrate types, promoting a wide variety of habitats.

Our results agree with Galil and Zibrowius (1998) [104] who suggested that Mediterranean seamounts can be considered as isolated refuges for relict populations of species that have disappeared from a previously larger distribution range [70] and that also provide an excellent habitat for rich communities of filter-feeding animals such as sponges [69]. This fact, together with the presence of species and habitats of special interest for their protection, justify the inclusion of the seamounts of the Mallorca Channel within the Natura 2000 network. This will complement the marine SCIs of the Balearic Islands because all of them are sited in coastal areas, with the only exception of the Menorca Channel, which also includes circa-littoral and bathyal bottoms [96,105]. This will also expand the SCIs that include seamounts in Mediterranean waters off Spain, until now represented only by the Seco de los Olivos in the Alboran Sea [101,102] and the deep-sea habitats corresponding to 1170 and 1180 types, which are not well-represented in the Mediterranean Natura 2000 network [18].

To do this, benthic species and habitat modeling as well as mapping of fishing and other human activities in the area (e.g., shipping) that can also affect sea turtles and cetaceans should be made. These studies, together with the assessment of their impact in terms of species and habitat degradation and loss of diversity, both geological and biological, will provide the required scientific information to propose the seamounts of the Mallorca Channel as a SCI and to provide advice to develop the management plan required for its final declaration as a SAC, with the objective to maintain not only their biodiversity and ecosystems, but also the services they provide.