Introduction

Due to their vertical range and habitat diversity, seamounts often support high fish diversity, as compared to the surrounding ocean, and some are known as hotspots of endemic species (e.g. Shank 2010; Stocks et al. 2012). Seamounts are considered to act as “stepping stones” for species dispersal (Almada et al. 2001; Ávila and Malaquias 2003; Santos et al. 1995; Shank 2010; Xavier and van Soest 2007), bridging large oceanic areas in particular for shelf and slope dwelling species. Recently, studies of seamount ecosystems have received a great deal of attention by the scientific community, due to their role as habitat providers for benthopelagic fishes and important spots for great pelagics (e.g. Clark et al. 2010; Morato et al. 2010). There is an ongoing concern that seamount stocks are overfished, and fishing is impairing the benthic communities (e.g. Clark 2001; 2010; Clark and Koslow 2007; Clark and Rowden 2009; Clark et al. 2006; Norse et al. 2012).

Little has been known about the ichthyofauna at most NE Atlantic seamounts, in particular in the deeper regions. Although commercial fisheries have targeted many banks and seamounts, detailed scientific studies of the demersal and benthopelagic fish fauna are available only for a few of them. In particular, extensive sampling programmes were conducted at Great Meteor Bank and other seamounts south of the Azores (Ehrich 1977; Kukuev 2004; Maul 1976 and literature cited therein), and at Seine and Sedlo Seamounts (Christiansen et al. 2009; Menezes et al. 2009, 2012). Knowledge of the fish communities at Ampère Seamount is currently based on visual observations by Russian researchers covering the upper and middle slopes, but there is also anecdotal evidence that Ampère Seamount is a fishing ground for commercially valuable species, such as the black scabbardfish Aphanopus carbo (Bordalo-Machado et al. 2009) and the silver scabbardfish Lepidopus caudatus (personal observation).

The fish survey at Ampère Seamount reported here was part of a comprehensive, multidisciplinary sampling programme, which also included hydrography, phyto- and zooplankton, benthos, biogeochemistry and sedimentology. The studies aimed at an advanced understanding of the seamount ecology, in comparison with other seamounts in the NE Atlantic and the Eastern Mediterranean, and addressed, for example, the relationships between flow field and plankton distribution, fluxes of organic matter, the trophic structure and the biodiversity of benthic and pelagic seamount communities. First results show that the oligotrophic nature of the sea area around Ampère is reflected in low zooplankton standing stocks, with no indications of an enhanced biomass at the seamount as compared to the surrounding ocean (Denda and Christiansen 2013). The major aim of the fish studies conducted on the P384 cruise of R.V. Poseidon and the M83/2 cruise of R.V. Meteor was to characterize the benthopelagic fish community at different depths, using various fishing gears and considering several locations at and around Ampère Seamount, including the summit, the slopes and an abyssal reference station. The results are compared to fish assemblages found at other NE Atlantic seamounts.

Materials and methods

Study area

Ampère Seamount is part of the Horseshoe Seamounts Chain and located at 35°N, 13°W between the island of Madeira and the Portuguese southern coast, to the west of the Exclusive Economic Zone of Morocco (Fig. 1). Ampère rises from a base depth of ca. 4,500 to 55 m below the surface. It is separated from the neighbouring Coral Patch Seamount by a deep valley of 3,400 m depth. The seamount has a conical shape with an elongated base and a small, rough summit plateau at 110–200 m, with a single narrow peak reaching to 55 m (Fig. 2). The slopes are steep and rocky with canyon-like structures particularly at the northern, eastern and southern sides (Halbach et al. 1993; Hatzky 2005; Kuhn et al. 1996), but sediment-covered flat areas exist as well. For comparison, a reference station on a flat, sedimentary abyssal plain in 4,400 m depth, located ca. 70 nm south of Ampère Seamount at 33°56′N, 013°16′W, was also sampled (Fig. 1).

Fig. 1
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The Horseshoe seamount chain. The location of the reference station south of Ampère Seamount is marked by a triangle. Depth contour interval 500 m. Bathymetric data source: GEBCO (IOC et al. 2003)

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Fig. 2
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Central area of Ampère Seamount with locations of stationary fishing gear and beam trawl tracks during cruise M83/2. Credit bathymetric data: J. Hatzky, AWI

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Sampling

The inventory of demersal fishes on Ampère Seamount was derived from sampling during the P384 cruise of R.V. Poseidon in May 2009 and the M83/2 cruise of R.V. Meteor in November/December 2010, using hooks-and-line, longlines, baited traps, beam trawl and otter trawl. The variety of sampling methods aimed at broadening the collection of fishes caught to several feeding types. However, the rough and steep terrain of the seamount made benthopelagic trawling in the upper 2,000 m extremely risky and led to the damage of the beam trawl during all tows and to the complete loss of one beam trawl. The otter trawl was deployed only below 2,000 m. Stationary fishing gears (longlines, hook-and-line and traps) were used down to a maximum depth of 1,200 m, except for small traps which were exposed in connection with lander deployments at 1,700–2,200 m.

During the P384 cruise, hook-and-line fishing was performed opportunistically in the vicinity of the summit area at depths between 70 and 213 m (Fig. 2). Different hooks and various types of bait were used, including commercial fish lures, aluminium foil and pieces of sardines. Fish traps (“Madeira type”, Fig. 3; Biscoito 1993) were baited with sardines and deployed for several hours, also in the summit area. A 2-m beam trawl with 6-mm mesh (see Christiansen et al. 2009 for details) was towed twice at 123–133 m depth on the summit plateau (Fig. 2); bottom time was 10–15 min each.

Fig. 3
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Trap types used during cruises P322 and M83/2. Upper left “Madeira type” (Biscoito 1993); upper right “Apollo” eel trap; lower left crab trap; lower right flatfish trap

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During the M83/2 cruise, a total of 12 demersal longline sets were deployed between 60 m and 1,200 m depth on the summit and on the upper slopes of the seamount (Fig. 2; Table 1). The basic design of the longline sets is shown in Fig. 4. From the third deployment on, a drift anchor was used instead of the weight at the end of the longline. In haul 7, a different design using 30 m long branch lines with 10 snoods each was employed, similar to the longline “Type B” used by Menezes et al. (2009). In hauls 8–12, small buoys and weights were alternately attached to the main rope every 10 hooks. A total of 48–83 hooks of different size (straight # 3 and # 6; twisted # 6/0 and # 8/0) per set were baited with mackerels and sardines, and up to 8 traps of different design (Madeira-type fish trap, eel trap, crab trap and flatfish trap; Fig. 3) were attached either to the anchor weight or to the end of the main rope. Soak time was usually 5–6 h. Of the 12 longline/trap deployments, one longline was completely entangled and did not fish, and two longlines were lost. The total number of recovered hooks was ca. 500. Small traps made from 1.5-l plastic bottles were also attached to a lander system in four deployments at depths from 1,700 to 2,172 m.

Table 1 Features of the longline deployments during cruise M83/2
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Fig. 4
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Basic design of the longline used during cruise M83/2

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A 15-m semiballoon otter trawl was towed on a single warp at 2–3 knots on the western lower slope of the seamount (approximately 2,000–2,100 m depth, bottom time approximately 30 min) and on the abyssal plain at the reference station south of Ampère Seamount (two hauls, both at 4,415 m depth, bottom time approximately 180 and 120 min, respectively). The estimated horizontal net opening of the trawl was 8.6 m; the mesh size was 44 mm in the front part and 37 mm in the intermediate and rear parts, with a 13-mm inner liner in the cod end.

Immediately after recovery of the catch, specimens were identified to the lowest taxonomic level possible using the keys in Whitehead et al. (1986). Total and standard lengths and weights of all specimens were measured and, where possible, stomachs were dissected and tissue samples taken for later analysis of DNA and isotopic ratios. Fishes from cruise P384 could not be landed due to lack of freezing facilities during transport, whereas all fishes from M83/2 were frozen at −20 °C to be transported to the University of Hamburg. Here, the on-board identifications were checked using a variety of taxonomic keys and descriptions (Franco et al. 2009; Froese and Pauly 2013; Nielsen et al. 1999; Sulak 1977; Sulak and Shcherbachev 1997; Whitehead et al. 1986). Less common species are being finally deposited at the Zoological Museum, Hamburg.

Results

The list of fishes (Table 2) collected at Ampère Seamount includes a total of 239 specimens (pelagic bycatch in the otter trawl not included). Thirty-four species belonging to 23 families were identified. Three species were Chondrichthyes, and 31 were Actinopterygii. Perciformes (seven species), Anguilliformes and Gadiformes (five species each) were the most speciose orders in the collections.

Table 2 Fishes collected at and around Ampère Seamount, arranged according to depth of occurrence
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The collections showed a vertical zonation with little overlap. The upper depth guild, which included longline, hook-and-line, trap and beam trawl catches from 60 to 300 m, was the most diverse and comprised a total of 11 species belonging to 10 families. The most frequently caught species were the pelagic or benthopelagic horse mackerel Trachurus picturatus with 16 specimens, silver scabbard fish Lepidopus caudatus with 13 species, and the two mackerel species, Scomber colias and Scomber scombrus with 11 and 10 specimens, respectively. The scorpionfish Pontinus kuhlii (9 specimens), the Mediterranean moray Muraena helena (7) and the blacktailed comber Serranus atricauda (6) were also relatively abundant in the catches, whereas the other species were rare with only two or three specimens each.

The next depth guild from 300 to 500 m was covered by longline and traps. A total of five species were captured in this depth range including Trachurus picturatus, which was also caught above 300 m. Apart from the blackbelly rosefish Helicolenus dactylopterus with five specimens, only single fishes were caught from each family.

Longlines and traps were also employed between 900 and 1,200 m. The predominant species in this depth layer was the common mora Mora moro with 37 specimens. Further, a few specimens of the black scabbardfish Aphanopus carbo, Kaup’s arrowhead Synaphobranchus kaupii, and two shark species, the birdbeak dogfish Deania calcea and the smooth lanternshark Etmopterus pusillus, were captured.

Synaphobrancus. kaupii, was also caught on the lower slope of the seamount at about 2,000 m, where it was the predominant species in the otter trawl catch with 30 individuals, corresponding to nearly 75 % of all specimens. Other families included Alepocephalidae, Bythitidae, Ipnopidae, Ophidiidae and Halosauridae with one species and 1–3 specimens each. The small traps attached to the lander system at 1,700 m in two deployments captured six specimens of the Synaphobranchidae Simenchelys parasitica.

A total of 11 species were collected in the otter trawl on the abyssal plain at a depth of ca. 4,400 m. The most abundant families were Macrouridae with 26 specimens (3 species), Ipnopidae with 22 specimens (3 species) and Alepocephalidae with 12 specimens (2 species). The families Synaphobranchidae and Ophidiidae were caught with one species and one specimen each. The most common species were the abyssal grenadier Coryphaenoides armatus and the abyssal spiderfish Bathypterois longipes, with 15 specimens each. One species was found in the otter trawl collections from both 2,400 and 4,400 m, the salmon smooth-head Conocara salmoneum.

In addition to the benthopelagic species, the otter trawl also collected pelagic species. Only a few particularly rare pelagic species have been analysed in detail so far, such as the first record in the North Atlantic of the cetomimid Cetichthys indagator (Vieira et al. 2012) and two large females of the anglerfish Gigantactis vanhoeffeni, and one unidentified Gigantactis female, which represent, to our best knowledge, the first records of adult Gigantactis females in the region (Bertelsen 1986; Santos et al. 1997; Swinney 1995).

Discussion

Contrary to the relatively well-known fish fauna of seamounts and islands around the Azores and Madeira (Arkhipov et al. 2004; Christiansen et al. 2009; Ehrich 1977; Kukuev 2004; Maul 1976; Menezes et al. 2006, 2009, 2012; Pakhorukov 2008; Santos et al. 1997; Shcherbachev et al. 1985), the ichthyofauna of the Horseshoe Seamount Chain (Fig. 1) and in particular of Ampère Seamount has been little studied until now. Visual observations were conducted on the summit area of Gorringe Bank (Abecasis et al. 2009; Gonçalves et al. 2004) and on the upper and middle slopes at Ampère Seamount and Josephine Bank; the latter was also trawled (Pakhorukov 2008). Recently, Wienberg et al. (2013) reported some fish species at Coral Patch Seamount from ROV observations above the middle slope.

The results presented here increase the number of fish species described from Ampère seamount substantially. Most of the 34 species collected at and around the seamount in this study are widely distributed in the Atlantic and in the Mediterranean (e.g. Froese and Pauly 2013; Haedrich and Merrett 1988; Menezes et al. 2006, 2009, 2012; Merrett 1992; Merrett and Marshall 1981; Pakhorukov 2008). Endemic species were not found. Except for the abyssal fishes Conocara salmoneum, Bathymicrops regis, Coryphaenoides profundiculus and Echinomacrurus mollis, all species have also been reported from waters around the Azores (Menezes et al. 2006, 2012; Santos et al. 1997). Of all species listed here, Conocara salmoneum is reported explicitly in Portuguese waters for the first time, but data from Merrett (1992) already suggest its presence in the Madeiran EEZ.

In the 1980s, Soviet investigators surveyed Ampère Seamount together with Josephine Bank and most of the seamounts south of the Azores down to 1,200 m depth by a submersible, the so-called underwater inhabited device (UID), and partly also with bottom trawls (Pakhorukov 2008). Table 3 lists all fish species at Ampère Seamount reported from Pakhorukov (2008) and the present study. Species are organized according to the main depth of catch/observation in these studies and include occurrence and depth range at other NE Atlantic seamounts. The resulting inventory comprises a total of 52 identified species of benthic and benthopelagic fishes. From the visual observations at Ampère, Pakhorukov (2008) reports of 24 fish species, complemented by observations of their behaviour and the type of habitat. Only 6 of these species were recovered in this study, but on the other hand, an additional 14 species were found between summit and middle slope at 1,200 m. On the lower slope and surrounding abyssal plain at 1,700, 2,050 and 4,400 m depth, a further 14 species add to the ichthyocoenoses of Ampère Seamount. The little overlap between Pakhorukov’s (2008) and the results from this study in the upper layers points to the importance of applying a wide variety of methods in order to assess the ichthyofauna as completely as possible.

Table 3 Inventory of fish species recorded at Ampère Seamount from the present study (C) and from Pakhorukov (2008, P), and comparative occurrences and depth ranges at NE Atlantic seamounts
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The results from this study suggest a clear vertical zonation of the fishes at Ampère; however, part of this can probably be attributed to the rather small sample size and the few discrete sampling depths in our study, particularly in the upper 1,200 m. Information which includes depth ranges at other NE Atlantic seamounts shows that many of the same species occur along a wide depth range (Table 3), thus, making the allocation of single species to vertically discrete assemblages difficult.

Menezes et al. (Menezes et al. 2006, 2009 and literature quoted therein) identified vertically structured fish assemblages around the Azores archipelago and at Seine Seamount and related them to the water mass properties in the areas. During the Meteor cruise in December 2010 (Christiansen et al. 2012), Eastern North Atlantic Central water (ENACW; 10–20 °C, 35.2–36.7 °C) characterized the upper 500 m below a 50–80 m thick, warm mixed layer. Between 600 and 1,300 m depth Mediterranean Outflow Water (MOW; 13 °C, 38.4 PSU) was found, and underneath a mixing zone from 1,400 to 2,000 m, North Atlantic Deep water (NADW; 1.5–4 °C, 34.8–35 PSU) and probably Antarctic Bottom water (ANBW) formed the water layers in the lower bathyal and abyssal zones (Fig. 5).

Fig. 5
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Profiles of temperature, salinity and oxygen concentration at the southern slope of Ampère Seamount

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Both the summit and upper slope regions of Ampère Seamount down to about 600 m are under influence of the Eastern Atlantic Central Water/Azores current. The benthic and benthopelagic fish communities in this depth stratum comprise 35 species (Table 3). Among this shallow fish community, 21 fish species seem to occur exclusively in this depth range, with four species being confined to the summit area down to 200 m. The remaining 14 species, including commercially exploited fish like alfonsino Beryx splendens Lowe, silver scabbardfish Lepidopus caudatus and silver roughy Hoplostethus mediterraneus, have also been found deeper in the Mediterranean Outflow water between 600 and 1,300 m. Only two species, Ruvettus pretiosus and Alepocephalus bairdii, seem to be restricted to this water mass of Mediterranean origin; however, this is based on only a small number of observations and should be judged with some caution. Generally, indications for an affinity of species to certain water masses are vague. Most of the fishes on the upper and middle slope of Ampère Seamount occur in a wide range of temperatures, salinities and oxygen concentrations.

The majority of the shallower species at Ampère were found in common with Gorringe Bank on the northern arc of the Horseshoes seamounts: Conger conger, Muraena helena, Capros aper, Coris julis and Serranus atricauda. However, Pontinus kuhlii (also observed at Seine and Great Meteor, Christiansen et al. 2009; Ehrich 1977; Menezes et al. 2009) and the widely distributed Lepidopus caudatus, which has been known from other NE Atlantic seamounts like the neighbouring Seine and Josephine and the more southern Great Meteor, Atlantis, Flamingo and Hyères (Ehrich 1977; Menezes et al. 2009; Pakhorukov 2008), were not observed at Gorringe (Abecasis et al. 2009), see Table 3. On the other hand, several species common at Gorringe were not found at Ampère, most of them being littoral fishes like Blenniidae or Labridae. However, the survey at Gorringe employed visual observations from divers and covered only the peaks at depths from 29 to 50 m, so the different methodologies involved do not allow for a comprehensive comparison of the fish communities between these seamounts, and Lepidopus caudatus, for example, has been known from commercial catches in the Gorringe area.

The upper and mid-slope collections included species that are usually found at the continental and island slopes, and, with the exception of the blue skate Dipturus batis (taxonomic status of this species is under discussion, see Iglésias et al. 2010), have been reported also from other seamounts in the NE Atlantic (Table 3). In particular, Helicolenus dactylopterus, Aphanopus carbo, Mora moro and Deania calcea have been observed at several seamounts in the Horseshoe region and to the north and south of the Azores (Abecasis et al. 2009; Menezes et al. 2009; Pakhorukov 2008). Synaphobranchus kaupii were also observed at Seine and Sedlo Seamounts (Menezes et al. 2009), but not at other Horseshoe seamounts or at seamounts south of the Azores, which is probably due to the limited depth range of the samples there, since Almeida and Biscoito (2007) reported their presence at the Lucky Strike vent field on the mid-Atlantic Ridge south of the Azores.

Due to the depth limitation to 1,200 m in the observations of Pakhorukov (2008), the ichthyocoenoses found in this study on the lower slope of Ampère (1,700 and ∼2,000 m depth) and on the adjacent deep-sea abyssal plain (4,400 m) supplement the species described from the regional seamounts substantially. With the exception of Cataetyx laticeps, which is restricted to the Atlantic and Mediterranean, all of the fishes caught on the lower slope are species with a circumglobal distribution (Froese and Pauly 2013). They are usually found at bathyal depths, often with a wide depth range. However, no overlap was observed with the upper/mid-slope species, except for the predominating species on the lower slope of Ampère, Synaphobranchus kaupii, which has been reported, for example, from Porcupine Seabight at depths from 230 to 2,420 m (Merrett et al. 1991).

The Alepocephalidae Conocara salmoneum was the only species present on both the lower slope and the abyssal plain adjacent to Ampère Seamount. The fishes found here are typical abyssal species belonging to the northern and intermediate deep-sea fish assemblages as proposed for the NE Atlantic by Merrett (1992). Only the Ophidiidae Holcomycteronus squamosus was not included in those assemblages, but it is a particularly rare species known from a few scattered locations in the North Atlantic, with a certainly incomplete distribution record.

Despite the general similarities of the ichthyofaunal assemblages of the Northeast Atlantic seamounts, the comparisons also show some differences. These can partly be attributed to differing methodologies, as already indicated above, and to sampling effort. For example, the higher number of species collected with longlines by Menezes et al. (2009) at Seine Seamount is most probably due to the much higher fishing effort (>14,000 hooks at Seine Seamount, as compared to ca. 500 recovered hooks at Ampère), and to the deeper-reaching sets (max. 2,000 vs. 1,200 m). On the other hand, despite the differences in sampling effort, seven of the 20 species captured down to 1,200 m at Ampère Seamount in this study had not been collected at Seine (all gears considered). Apart from possible dissimilarities in gear selectivity, true differences between the two seamounts like summit depth, topography, substrate or coral cover may play a role. For example, most of the species missing at Seine, including Muraena helena, Serranus atricauda, Coris julis and Capros aper, were captured at depths shallower than the Seine summit plateau. Furthermore, the much more rugged topography with basaltic outcrops of the Ampère summit area, as compared to the flat, sedimentary summit plateau of Seine, may provide suitable habitat for species, such as the moray eel M. helena, which depend on or prefer richly structured, rocky substrate (e.g. Bauchot 1986).

The relatively similar inventory of shelf and slope fish species in the summit and upper slope regions of the seamounts of the Horseshoe Chain and of the seamounts around the Azores supports the hypothesis of these topographic features being “stepping stones” for species dispersal within the larger NE Atlantic region, possibly powered by the large-scale current system (Azores Current, Canary Current) and westward propagating meddies. This may also apply to the benthopelagic deep-water fishes found at the seamount slopes, which are also dependent on suitable substrate at appropriate depths. However, a higher sampling effort and the use of genetic tools will be necessary to evaluate the biogeographic significance of Ampère Seamount and to assess the biogeographic links between the NE Atlantic seamounts.

This effort will also be important for designing conservation and management measures in order to mitigate eventually unsustainable impacts of fishing activities on the commercially targeted species such as several species found at Ampère, which are important in the southern Portuguese longline fisheries (Borges 2007), like Lepidopus caudatus, Aphanopus carbo, Mora moro, Helicolenus dactylopterus and Pontinus kuhlii, and, usually restricted to coastal areas, Conger conger and Muraena helena.

In conclusion, the data show, despite the comparatively low sampling effort, that Ampère Seamount hosts a rich and diverse fish community. Generally, the seamounts in the subtropical NE Atlantic appear to be very similar with respect to their ichthyofauna. However, the differences, for example between the adjacent and relatively well-sampled Ampère and Seine Seamounts, point to the role of properties like substrate type and habitat complexity in shaping the final expression of community composition.