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Life on Seamounts

Mireille Consalvey1, Malcolm R. Clark1, Ashley A. Rowden1, Karen I. Stocks2

1National Institute of Water and Atmospheric Research, Wellington, New Zealand
2San Diego Supercomputer Center, University of California San Diego, La Jolla, California, USA


7.1. Introduction: A History of Seamount Research

The rugged terrain and vast mountain ranges that rise from our continents inspire a strong passion, perhaps epitomized by the first climbing of Mount Everest by Sir Edmund Hillary and Sherpa Tenzing Norgay in 1953. In the same decade marine scientists Bruce Heezen and Marie Tharp were looking down, deep into our oceans, mapping the Atlantic seafloor. Painstakingly assembling echo-sounded data, they revealed for the first time the extent of the mid-Atlantic ridge. At 20,000 km, it easily surpasses the length of the Himalayas, Andes, and Rockies combined, and is the longest mountain range on Earth. On this ridge and elsewhere in the oceans stand undersea mountains, or seamounts (Box 7.1), the largest of which rise many kilometers from the sea floor. 

Box 7.1 Seamounts Seamounts are prominent features of the world's underwater topography, found in every ocean basin (Fig. 7.1). They are generally volcanic in origin, and often conical in shape. Over geological time seamounts sink (through isostatic adjustment) and erode to become less regular. The topography of seamounts can be complex and within any seamount one may find terraces, canyons, pinnacles, crevices, and craters.

Seamounts are traditionally defined by geologists as having an elevation greater than 1,000 m above the seabed (Menard 1964). Biologists now widely include peaks less than 1,000 m in their definitions, for there is no known ecological reason for this cutoff height. Pitcher et al. (2007) defined a seamount as any topographically distinct seafloor feature that is greater than 100 m but which does not break the sea surface to become an island. This definition excludes large banks and shoals (as they differ in size) and topographic features on continental shelves (because of their proximity to other shallow topography).


The bathymetry of our oceans is now resolved at a scale and detail unimaginable by early pioneers. Yet despite advances in ocean mapping we are still unable to answer seemingly simple questions such as how many seamounts there are. To even begin to estimate the global number of seamounts requires advanced computational technologies. In 2007 Hillier & Watts took 40 million kilometers’ worth of echosounder depth measurements and predicted the occurrence of around 40,000 seamounts over 1,000 m tall, most not yet discovered. Widening their scope to include seamounts >100 m high, the authors predicted about 200,000, and speculated there could be as many as 3 million seamounts (Fig. 7.1).

 Figure Figure 7.1 Location of 63,000 seamounts collated from verified regional datasets, or estimated from satellite altimetry or vessel track sounding data (CenSeam 2009).

Biological research on seamounts has been limited, and we have a poor understanding of global seamount biodiversity. So far, fewer than 300 seamounts have been studied in sufficient biological detail to describe adequately the assemblage composition of seabed organisms. Furthermore, sampling has been biased toward larger fauna such as fishes, crustaceans, and corals (SeamountsOnline; Stocks 2009).

Carl L. Hubbs (1959) was one of the first biologists to work on seamounts, and the questions he posed in 1959 remain relevant half a century later. What species inhabit seamounts and with what regularity and abundance? How did these species disperse to, and establish on, seamounts? What bearing may the determined constitution of these isolated faunas have on our ideas concerning past and present oceanic circulation and temperatures? Do banks and seamounts provide stepping stones for trans-oceanic dispersal? To what degree has isolation led to speciation? What factors are responsible for the abundance of life over seamounts?

However, one of Hubbs’ questions was to be answered quickly: are demersal or pelagic fishes sufficiently abundant on seamounts to provide profitable fisheries? Seamounts host significant commercial fisheries in many parts of the world. Traditional handline fisheries were likely the first fisheries associated with seamounts (Marques da Silva & Pinho 2007) as far back as the fourteenth century (Brewin et al. 2007), and continue to the present day. In the 1970s deep-sea trawling began in earnest, targeting large seamount associated fish aggregations (Clark et al. 2007a) with nations sending hundreds of vessels around the world's oceans. So far, at least 77 commercially valuable fish species have been fished on seamounts (Rogers 1994). Since the 1960s the total international catch of demersal fishes on seamounts by distant-water fishing fleets is estimated to be over 2.25 million tonnes (Clark et al. 2007a), although the true extent of trawling on seamounts may never be known through a combination of catches not being reported, or catches coming from wider areas than just seamounts (Watson et al. 2007).

Historically seamount ecosystems have not been well protected (Probert et al. 2007) and have been affected by fishing activities that can cause declines in fish stocks and visible damage to benthic habitat (Davies et al. 2007). A great deal of fishing effort has, and continues to, occur on the high seas and many fisheries proceeded largely unregulated, falling outside of any nation's jurisdiction. Although the United Nations and regional fisheries management organizations are becoming more effective, enforcement of regulations on the high seas remains a challenge.

Emergent threats such as deep-sea mineral extraction and indirect threats to all deep-sea habitats are also increasingly being considered, such as rising CO 2 (Guinotte et al. 2006). High-profile governmental and non-governmental initiatives have elevated the position of seamounts in the public eye.

Recognizing it is not feasible to sample all of the world's seamounts, research efforts needed to be coordinated to assess the current state of knowledge, fill critical knowledge gaps, and target understudied regions and seamount types. The Global Census of Marine Life on Seamounts (CenSeam) has provided a focal point for coordinating global research and for communicating research results to stakeholders seeking scientific advice and guidance. To mark the end of the first Census of Marine Life, this chapter addresses some of the core research questions that have faced seamount researchers, including those of the CenSeam project, over the past five years. It also indicates where seamount research is likely to be directed in the future.


7.2. A Global Census of Marine Life on Seamounts (CenSeam)

The field of seamount biology has grown in recent decades, as shown by the increasing number of scientific publications each year (Brewin et al. 2007). The Census field project CenSeam started in 2005, and has served to bring together more than 500 seamount researchers, policy makers, environmental managers, and conservationists from every continent. At the outset of CenSeam, our understanding of seamount ecosystems was hampered by significant gaps in global sampling, a variety of approaches and sampling methods, and a lack of large-scale synthesis; scientific attention was not yet consistent with their potential biological and ecological value (Stocks et al. 2004). CenSeam has aimed to do the following: (1) synthesize and analyze existing data (Box 7.2); (2) coordinate and expand existing and planned research (Box 7.3); (3) communicate the findings through public education and outreach; and (4) identify priority areas for research and foster scientific expeditions to these regions. CenSeam researchers have augmented sampling efforts and analyses in the well studied Southwest Pacific and Northern Atlantic. CenSeam has also identified three key undersampled regions: the Indian Ocean, the South Atlantic, the Western and Southern Central Pacific, and researchers have worked toward securing funding to sample these regions. So far, CenSeam-linked scientists have participated in over 20 voyages. 

Box 7.2  SeamountsOnline: Providing Researchers and Managers with Tools for Finding and Accessing Information on the Biological Communities That Live on Seamounts
Since 2005, SeamountsOnline (Stocks 2009) has been collecting data on species that have been recorded from seamounts globally, and making them available through a free online data portal [more]
Box 7.3  Sampling Seamounts
On seamount voyages researchers will typically conduct a bathymetric survey (usually using multibeam sonar) of the target seamount. The resulting baythmetric map provides the basis for more detailed planning of the sampling program: plans that will take into account factors such as seamount size, shape, and depth. Echosounder information can also be used to identify substrate type and can guide sampling to target soft and hard bottoms.[more]


To help focus global research efforts, the CenSeam community identified two overarching priority themes: (1) What factors drive community composition and diversity on seamounts, including any differences between seamounts and other habitat types? (2) What are the impacts of human activities on seamount community structure and function? (Fig. 7.2). Within these themes key questions were developed to address where more science was needed to improve our understanding of the structure and functioning of seamount ecosystems and to inform management and conservation objectives. These questions will be used below to present what is known about seamounts so far, and how CenSeam has contributed to this knowledge. The CenSeam research effort has focused on seamount mega- and macroinvertebrates.

 Figure Figure 7.2 The CenSeam field program aims to investigate what factors drive community composition and diversity on seamounts, and to understand better the impacts of human activities such as fishing on seamounts (Erika Mackay, National Institute of Water and Atmospheric Research).

7.3. What Factors Drive Community Composition and Diversity on Seamounts?

Effective management of any seamount ecosystem must be based on a solid understanding of the seamount community, and associated physical and biological processes. Furthermore, it is important to determine the interactions of seamount communities with those in the wider deep-sea realm.


7.3.1. Seamount Community Composition and Diversity

The dominant large fauna of hard substrate on many deep-sea seamounts are attached, sessile organisms that feed on particles of food suspended in the water (Fig. 7.3). These suspension feeders are predominantly from the phylum Cnidaria, which includes stony corals, gorgonian corals, black corals, sea anemones, sea pens, and hydroids. Deep-sea (or cold water) corals are one of the most studied groups, and CenSeam has promoted research on their global distributions (Clark et al. 2006; Rogers et al. 2007; Tittensor et al. 2009). Corals can grow as individual colonies, or can coalesce as reefs; potentially providing complex three-dimensional habitat for a wide range of other animals, providing more refuge, an enhanced supply of food, surface area for settlement, and microhabitat variability to support a greater faunal diversity than less complex habitat. However, the role of biogenic habitat in the deep sea has only recently emerged as an area of both academic and conservation interest, and only a few quantitative studies have been made of the relationship between biogenic habitats and the composition of seamount fauna (see, for example, O'Hara et al. 2008).

 Figure Figure 7.3 Corals on seamounts can grow as reefs or as individual colonies. (A) Solenosmilia variabilis on Ghoul Seamount (approximately 1,000 m; New Zealand; National Institute of Water and Atmospheric Research). (B) Paragorgia arborea and a dense population of basket stars Gorgonocephalus sp. on San Juan Seamount (USA; courtesy of the Monterey Bay Aquarium Research Institute). (C) Viminella on the summit of Condor Seamount (200 m; Azores, North Atlantic; © Greenpeace/Gavin Newman). (D) Paragorgiid, acanthogorgiid, and chrysogorgiid corals on Pioneer Seamount (approximately 1,700–1,800 m; Northwestern Hawaiian Islands; 2003 NWHI exploration team: Amy Baco-Taylor, Chris Kelley, John Smith, and pilot Terry Kerby, NOAA Office of Ocean Exploration and Hawaii Undersea Research Laboratory).
 Figure Figure 7.3(continued) Corals on seamounts can grow as reefs or as individual colonies. (A) Solenosmilia variabilis on Ghoul Seamount (approximately 1,000 m; New Zealand; National Institute of Water and Atmospheric Research). (B) Paragorgia arborea and a dense population of basket stars Gorgonocephalus sp. on San Juan Seamount (USA; courtesy of the Monterey Bay Aquarium Research Institute). (C) Viminella on the summit of Condor Seamount (200 m; Azores, North Atlantic; © Greenpeace/Gavin Newman). (D) Paragorgiid, acanthogorgiid, and chrysogorgiid corals on Pioneer Seamount (approximately 1,700–1,800 m; Northwestern Hawaiian Islands; 2003 NWHI exploration team: Amy Baco-Taylor, Chris Kelley, John Smith, and pilot Terry Kerby, NOAA Office of Ocean Exploration and Hawaii Undersea Research Laboratory).
 Figure Figure 7.3(continued) Corals on seamounts can grow as reefs or as individual colonies. (A) Solenosmilia variabilis on Ghoul Seamount (approximately 1,000 m; New Zealand; National Institute of Water and Atmospheric Research). (B) Paragorgia arborea and a dense population of basket stars Gorgonocephalus sp. on San Juan Seamount (USA; courtesy of the Monterey Bay Aquarium Research Institute). (C) Viminella on the summit of Condor Seamount (200 m; Azores, North Atlantic; © Greenpeace/Gavin Newman). (D) Paragorgiid, acanthogorgiid, and chrysogorgiid corals on Pioneer Seamount (approximately 1,700–1,800 m; Northwestern Hawaiian Islands; 2003 NWHI exploration team: Amy Baco-Taylor, Chris Kelley, John Smith, and pilot Terry Kerby, NOAA Office of Ocean Exploration and Hawaii Undersea Research Laboratory).

In the literature there are many studies that describe the fish that live on seamounts. The most recent review by Morato et al. (2004) identified a total of 798 species of seamount fish, though exactly how to define a “seamount fish” is not straightforward. Commonly cited examples include the orange roughy (Hoplostethus atlanticus), alfonsino (Beryx splendens), Patagonian toothfish (Dissostichus eleginoides), oreos (Pseudocyttus maculatus, Allocyttus niger), and pelagic armourhead (Pseudopentaceros wheeleri) (Table 7.1). Sharks and tuna are also reported as occurring on seamounts, and in the waters above some shallow seamounts serranids (including sea basses and the groupers) and jacks are observed to spawn (Morato & Clark 2007).

Table 7.1 Distribution of main commercial fish species on seamounts (North Atlantic (NA); South Atlantic (SA); North Pacific (NP); South Pacific (SP); Indian Ocean (IO); Southern Ocean (SO)) and the depth range commonly fished

Common name Scientific name Distribution Main depth range (m)

Alfonsino Beryx splendens NA, SA, NP, SP, IO 300–600
Black cardinalfish Epigonus telescopus NA, SA, SP, IO 500–800
Rubyfish Plagiogenion rubiginosum SA, SP, IO 250–450
Black scabbardfish Aphanopus carbo NA 600–800
Redbait Emmelichthys nitidus SA, SP, IO 200–400
Sablefish Anoplopoma fimbria NP 500–1,000
Pink Maomao Caprodon spp. NP, SP 300–450
Southern boarfish Pseudopentaceros richardsoni SA, SP, IO 600–900
Pelagic armorhead Pseudopentaceros wheeleri NP 250–600
Orange roughy Hoplostethus atlanticus NA, SA, SP, IO 600–1,200
Oreos Pseudocyttus maculatus, Allocyttus niger SA, SP, IO, SO 600–1,200
Bluenose Hyperoglyphe antarctica SA, SP, IO 300–700
Redfish Sebastes spp. (S. marinus, S. mentella, S. proriger) NA, NP 400–800
Roundnose grenadier Coryphaenoides rupestris NA 800–1,000
Toothfish Dissostichus spp. SA, SP, IO, SO 500–1,500
Notothenid cods Notothenia spp. SA, SP, IO, SO 200–600

Seamounts are popularly referred to as hot spots of high species richness in the deep sea. However, many researchers are failing to find support for this premise. Stocks & Hart (2007) report variability but no overall trend of elevated species richness across approximately 18 studies comparing seamounts to either surrounding deep sea or nearby continental margins. However, sampling-related issues complicate such comparisons, an issue that has been addressed within the CenSeam Data Analysis Working Group (DAWG). Taking sampling factors into account, DAWG member O'Hara (2007) compared levels of ophiuroid species richness between seamount and non-seamount areas (for the latter by randomly generating populations from areas and depth ranges that reflected the typical sampling profile of seamounts) and concluded that seamounts do not show elevated levels of species richness.

At macro-ecological scales, the fauna of individual seamounts have been found to broadly reflect the species pools present on neighboring seamounts and continental margins (see, for example, Samadi et al. 2006; Stocks & Hart 2007; McClain et al. 2009; Clark et al. 2010; Brewin et al. 2009). Although the main body of evidence suggests that broad assemblage composition may be similar to surrounding deep-sea environments, community structure may differ between habitats. For example McClain et al. (2009) present preliminary evidence that the faunal communities of Davidson Seamount (off the west coast of the USA), although similar in composition to adjacent canyon habitat, are structurally different, particularly in the frequency of occurrence of particular species.

Seamounts are hypothesized to serve as biogeographical “islands” that could also function as shallow stepping stones across the abyssal plains. The isolated nature of many seamounts has fueled the hypothesis that seamounts can support high levels of endemicity, and numerous studies have supported this hypothesis (Richer de Forges et al. 2000; see review by Stocks & Hart 2007). However, so far, it is unlikely that we have identified enough of the regional or global deep-sea fauna to use the term endemic with any confidence, and apparently high levels of endemism may be an artifact of sampling species-rich communities or uneven sampling effort. Furthermore, many seamount fauna are recorded to have global or near-global distributions including reef-building scleractinian corals (Lophelia pertusa, Solenosmilia variabilis, and Madrepora oculata) (Roberts et al. 2006) and fish species such as orange roughy (Francis & Clark 2005).

In summary, seamounts can host abundant and diverse benthic communities. However, in many instances community composition is similar to those of adjacent habitats including continental slope. Today the concept of seamounts being islands in the sea has little support, but more sampling is required to be able to address this idea fully.


7.3.2. Connectivity of Seamount Populations

Differences in the connectivity of faunal populations among seamounts is almost certainly an important determinant of community composition on seamounts, a potential driver of endemicity, and a major consideration for the management of seamount ecosystems. The dispersal capabilities of deep-sea fauna depend primarily on whether species can disperse as adults, or only as eggs, larvae, and/or post-larvae; however, one cannot fully predict the distribution of a species based on larval life history alone (see Johannesson's (1988) paradox of Rockall). Relatively little is known about the life-history traits of deep-sea organisms, including those found on seamounts. Studies so far have been restricted to a single seamount (Parker & Tunnicliffe 1994) or limited taxa (see, for example, Calder (2000) on hydroids). Distance from shore, or degree of isolation from other seamounts, is widely proposed as an important factor determining community composition and richness. Leal & Bouchet (1991) report a significant decline in species richness of prosobranch gastropods moving offshore along the Vitória-Trindade seamount chain, but could not attribute this to any differences in larval life histories and posed that species may be passively dispersed along the chain through “island hopping”. In fact a suite of physical and biological factors also influence dispersal, and hence connectivity, over space and time. In a major CenSeam review paper, Clark et al. (2010) break these factors down: (1) physical ocean structure (for example hydrographic retention, large-scale and local currents), (2) factors influencing larval development time (for example temperature, food availability, predation), (3) habitat availability for larval settlement, and (4) post-settlement survival; with interactions thereof driving variations in the dispersal capabilities of fauna among seamounts.

Genetic studies are essential to our understanding of connectivity, but so far have been limited to few fauna and by the sensitivity of the current techniques. Historically, seamount genetic connectivity studies focused on commercially fished fauna but in recent years efforts have expanded to non-commercial fauna. No consistent pattern has emerged, with mixed results indicating both genetic differentiation and genetic homogeneity between some commercially fished fauna on seamounts, and those on oceanic islands and the continental margins at both oceanic and regional scales (Aboim et al. 2005; Stockley et al. 2005).

Baco & Shank (2005) discovered relatively high levels of genetic diversity, as well as low yet significant levels of population differentiation, for the precious coral Corallium lauuense among several Hawaiian seamounts and islands. They suggested that C. lauuense are primarily self-recruiting with occasional long-distance dispersal events maintaining genetic connectivity between sites. In contrast, Smith et al. (2004) provided evidence of widespread distribution of bamboo coral species in the Pacific which were not endemic to seamounts. However, the authors could not rule out that the mitochondrial markers they used in their analysis were insensitive to recent speciation events. Samadi et al. (2006) determined that populations of a gastropod with dispersive larvae were more similar than populations of non-planktotrophic gastropod species. Samadi et al. (2006) also determined that dispersive squat lobster species were genetically similar among populations on seamounts and the adjacent island slope.

The potential ability of certain seamount fauna to disperse widely is perhaps not surprising when compared with the finding of dispersive fauna at other isolated deep-sea habitats, such as hydrothermal vents or cold seeps (Samadi et al. 2007; see Chapter ).

As well as understanding the dispersal characteristics of seamount fauna, it is vital to set these in the context of both large-scale oceanic circulation and localized hydrological phenomena. For example, Taylor cones have been cited as a possible trapping mechanism that may drive endemism. Mullineaux & Mills (1997) recorded larval concentrations above and around Fieberling Guyot to be consistent with modeled tidally rectified recirculation over the seamount. Parker & Tunnicliffe (1994) proposed the presence of a modified Taylor cap on Cobb Seamount was important for trapping short-lived larvae, but because water mass is replaced approximately every 17 days, concluded that medium and long-lived larvae would be dispersed. Recent research by DAWG members has concluded, for some faunal groups, that seamount-scaled oceanographic retention is weak compared with other ecological drivers of community diversity on seamounts (Brewin et al. 2009).

To conclude, current understanding of the dispersal capabilities of seamount fauna, and the role of large and smaller-scale oceanographic processes, is limited and as such we cannot assess the role that dispersal may play in producing spatial differences between communities. The premise that seamounts may be dominated by short-lived or non-planktonic larval phased fauna has not been widely tested.


7.3.3. Environmental Factors Driving Differences in Diversity and Species Composition of Seamount Fauna

Seamount communities, as with slope and abyssal faunas, may exhibit latitudinal turnover in species composition. For example, O'Hara (2007) reports a clear biogeographical gradient for both seamount and non-seamount ophiuroid fauna from the tropics to the sub-Antarctic. Though incomplete, research so far has demonstrated that environmental factors can vary at large spatial scales, hence, can have the potential to influence community composition of deep-sea fauna.

Seamounts differ in their location, depth, elevation, and geological history (Rowden et al. 2005), all factors that may alter environmental conditions on large and small spatial scales and, in turn, influence seamount biodiversity and species composition. Clark et al. (2010) list as among the main factors that may determine the character of seamount benthic assemblages: seamount geomorphology, geological origin and age; local hydrodynamic regime (all preceding influence substrate type); light levels; water chemistry (for example oxygen); productivity of the overlying water (which relates to food availability); as well as the presence of volcanic/hydrothermal activity (see Chapter 9), temperature, and pressure. All these factors may operate in tandem and can create a unique set of conditions for a region, for any given seamount, and within a seamount.

Most marine animals are restricted to a limited bathymetric range (see, for example, Rex et al. 1999), and recent work by CenSeam-linked researchers has demonstrated that seamount assemblages can be depth stratified (O'Hara 2007; Lundsten et al. 2009). Work on the very deep slopes of seamounts has been limited but new research indicates that these can support distinct assemblages (see, for example, Baco 2007). However, the depth-related patterns, and the drivers thereof, remain largely unexplored for seamounts, but environmental gradients that correlate with depth such as temperature, oxygen concentration, food availability, and pressure are likely to be as important as they are for other deep-sea habitats (Clark et al. 2010).

Large seamount chains can divert major currents, and individual seamounts can affect localized hydrographic events including the formation of a rotating body of water retained over the summit of a seamount (Taylor cone, which may flatten to a cap) and the generation or interaction with internal waves (White et al. 2007). These may influence the faunal composition through larval transport (section 7.3.2). Additionally currents can be amplified around seamounts creating favorable conditions for suspension feeders as the waters bring an increased particle supply, as well as removing sediments (Genin 2004). Suspension feeders (for example corals, sponges, hydroids, crinoids, anemones, sea pens, feather stars, and brittlestars) have been found to dominate some seamounts (particularly their peaks) (Genin et al. 1986; Wilson & Kaufmann 1987), and large sessile fauna can, in turn, form structural habitat for a diverse range of smaller, mobile fauna (section 7.3.1).

Exposed rock surfaces are limited in the deep sea and seamounts represent a significant source of this substrate (Gage & Tyler 1991). Soft sediments can also dominate some seamounts, particularly flat-topped seamounts, called guyots, and in these circumstances community composition can switch from suspension to deposit feeders, similar to neighboring continental slopes (see, for example, Ávila & Malaquias 2003; Lundsten et al. 2009). The composition of infaunal communities on seamounts have not been well studied but include a wide diversity of polychaetes, crustaceans, mollusks, ribbon worms, peanut worms, and oligochaetes, as well as meiofaunal organisms such as nematode worms, loriciferans, and kinorhynchs (Samadi et al. 2007).

Although depth-related factors and substrate type (including biogenic habitat) are important drivers of community composition on seamounts, more research is required to describe and explore large-scale biogeographic patterns on seamounts.


7.3.4. Productivity on Seamounts

Enhanced productivity at seamounts is a widely cited phenomenon, and seamounts appear to support relatively large planktonic and higher consumer (fish) biomass when compared with surrounding ocean waters, particularly so in oligotrophic oceans (Genin & Dower 2007). Seamounts can each have their own local oceanographic regimes (section 7.3.3), which could influence seamount productivity.

Elevated phytoplankton concentrations have been observed on some seamounts (see, for example, Genin & Boehlert 1985; Dower et al. 1992; Mouriño et al. 2001), and it has been theorized that nutrient-rich upwelled waters and eddies around a seamount enhance surface primary productivity which leads to an energy transfer to higher trophic levels. However, the persistence of upwelling does not generally seem sufficient for such a transfer, making this an unlikely explanation for the elevated zooplankton and fish biomasses found over seamounts (Genin & Dower 2007).

It is now proposed that the high biomass on some seamounts may be fueled not by enhancement of primary production but instead by a trophic subsidy to carnivores (Genin & Dower 2007). These authors proposed food inputs by the following routes: (1) bottom trapping of vertically migrating zooplankton, (2) greatly enhanced horizontal fluxes of suspended food through current acceleration, and (3) amplification of internal waves increasing horizontal fluxes of planktonic prey. Porteiro & Sutton (2007) proposed that fish behavior may have evolved to capitalize on the regular planktonic food supply passing over a seamount, enabling them to convert mid-trophic level biomass efficiently to higher trophic levels.

The importance of biogenic structure in supporting higher fish densities has been widely cited but work so far has yielded mixed results (see, for example, Husebø et al. 2002; Krieger & Wing 2002; Ross & Quattrini 2007). Hydrographic factors around deep-sea coral reefs may increase zooplankton density (Dower & Mackas 1996; Husebø et al. 2002) in turn benefiting planktivorous fish, but there is no consistent explanation for enhanced fish productivity over seamounts. Fish aggregations may occur independently of biogenic fauna; for example, orange roughy are observed to spawn over seamounts but do not feed during spawning (Morato & Clark 2007). Clearly, more information on the life histories of many seamount fish (for example larval stages, juvenile fish grounds) is needed.

In summary, enhanced secondary productivity over seamounts is most likely attributable to a food supply exported from elsewhere, and not locally enhanced primary productivity. The influence of bio-physical coupling, and the potentially complicated and varied interactions of forcing mechanisms and seamount topographies, is uncertain. Vital to future research on seamount-related productivity will be the establishment of long-term monitoring programs, in concert with the development of physical and trophic models.


7.3.5. Trophic Architecture of Seamount Communities

Large suspension feeders such as corals, sponges, and crinoids can dominate the biomass of the seamount megabenthos on hard substrates. A dominance of suspension feeders may suggest that most animals are consuming similar resources at a low trophic level; hence, it might be thought seamounts have short food chains and low guild complexity.

CenSeam-linked researchers Samadi et al. (2007) used isotopic analysis to report that the benthic food webs on seamounts of the Norfolk Ridge have a diverse trophic architecture, with food-chain lengths (four to five trophic levels) toward the upper end of reported values from other aquatic systems, and broadly similar to other deep-sea food webs. Samadi et al. (2007) excluded the larger predatory fish which would further elongate seamount food chain length.

Pelagic food webs are likely to be a key part of the seamount ecosystem. Midwater fishes represent an important link from zooplankton to higher trophic level predators such as seabirds, squids, piscivorous fishes (for example tunas), sharks, and marine mammals (see Morato & Clark 2007; Porteiro & Sutton 2007). Many benthopelagic and demersal fish also feed on zooplankton, hence, there exists the possibility of numerous benthopelagic couplings in the water column around a seamount.

Food supply may vary as a result of the complex topographic and oceanographic patterns around seamounts, and feeding flexibility is also probably instrumental in enhancing trophic complexity on seamounts; for example, sponges are highly efficient at capturing ultraplankton (Pile & Young 2006) but also include carnivorous forms that prey on copepods (Watling 2007).

To conclude, despite a dominance of filter feeders, seamount communities are no less complex than other marine communities. However, considerably more research is required to gain a broad-scale understanding of the trophic architecture of seamounts.


7.4. What are the Impacts of Human Activities on Seamount Community Structure and Function?

Anthropogenic impacts in the deep sea are indisputable and this environment is more sensitive to human and natural impacts than previously thought (Davies et al. 2007). Human-induced changes are likely to be more intense, and occur over a shorter time period, than natural events, especially in the deep sea.


7.4.1. Vulnerability of Seamount Communities to Fishing

In contrast to nearshore communities there have been limited studies investigating the impacts of deep-sea fishing on seamounts (Koslow et al. 2001; Clark & O'Driscoll 2003; Althaus et al. 2009; Clark & Rowden 2009). As distinct geological features, seamounts can provide a focal point for both fish to aggregate, and for intensive fishing effort (Clark 1999). The concentration of trawling on a seamount can be much higher than on the continental slope where activities might be more diffuse (O'Driscoll & Clark 2005).

The aggregating nature of seamount fish can facilitate large catch volumes (Fig. 7.4A). Seamount fisheries have often been boom and bust (Clark et al. 2007a) and under current management practice most deep-sea fisheries are not sustainable in the long term (Glover & Smith 2003). Deep-sea fish are typically less productive than shallow water shelf species and are highly vulnerable to overfishing. Deep-sea fish are often slow growing, long lived (for example orange roughy have been aged at 100 years or more (Andrews et al. 2009)), and slow to mature with low fecundity and sporadic reproduction (see, for example, reviews by Morato & Clark 2007; Clark 2009). Furthermore, deep-sea fish have low natural rates of mortality. The aforementioned are all factors that can limit recovery and resilience. So far many deep-sea fish populations have shown no signs of recovery and indeed it is uncertain if they can while commercial fisheries remain (Clark 2001; Dunn 2007).

 Figure Figure 7.4 (A) Orange roughy catch on deck. (B) Orange roughy swimming over a trawled area of a seamount (Malcolm Clark/New Zealand's National Institute of Water and Atmospheric Research).

The effects of fishing on seamounts must be considered beyond the impact on the target catch (Koslow et al. 2000). Longlines, gillnets, traps, and pots can all have some effect on non-target fauna, but bottom trawling is widely recognized as the primary threat to the seabed communities of seamounts (Fig. 7.4B).

The initial composition of the community will largely affect the scale of impact, with attributes of the component fauna such as fragility, size, and mobility as key traits determining the potential resistance (ability to withstand change and/or avoid trawl damage) of the community to trawling (Probert et al. 1997). Habitat-forming fauna such as stony corals are particularly vulnerable to trawling damage. Comparing eight seamounts on the Chatham Rise, New Zealand, Clark & Rowden ( 2009) found unfished seamounts to possess a relatively large amount of stony coral habitat (Solenosmilia variabilis and Madrepora oculata predominantly on the seamount peaks) compared with fished seamounts with relatively little coral habitat, and they reported significant differences in assemblage composition between fished and unfished seamounts. Overall, fishing can impact benthic species composition, abundance, age composition, size structure, and overall structural complexity of the benthic habitat (Clark & Koslow 2007).

Researchers agree that communities on seamounts are vulnerable to human activities, with fishing the major impact so far. The effects of fishing are now widely acknowledged to extend beyond the target species. A better understanding of the entire seamount ecosystem is vital if we are to mitigate human impacts.


7.4.2. Recovery of Seamount Communities from Human-Induced Disturbance

The life-history characters of many seamount species (such as limited mobility, long generation time, low larval output, and limited dispersal capability) predispose many seamount ecosystems to recover over a scale of decades to centuries. Recovery will also be influenced by substrate type (and any changes associated with trawling), seamount location (for example proximity to seed populations), and prevailing oceanographic conditions.

Althaus et al. (2009) reported no evidence of the structure forming taxa (primarily stony corals) recovering a decade after cessation of trawling on seamounts off Tasmania, Australia. Furthermore, the long-term loss of biogenic faunas such as corals and sponges may ultimately mean that impacted communities never return to their pre-disturbance state.

However, some fauna have shown either resistance to damage or capacity for rapid recovery and have been reported as more abundant on trawled seamounts (e.g., hydrocorals, gold corals, bryozoans, and some anemones) (Althaus et al. 2009; Clark & Rowden 2009). Within a given seamount there can exist refuges, such as areas that are too rugged for nets, which may help conserve biodiversity on an individual seamount and serve as source populations for recovery.

So far, there has been limited research on the recovery of seamount communities after disturbance. However, seamount biological communities have been considered among the least resilient in the marine environment (Clark et al. 2010) and timescales of recovery for some taxa exceed human life expectancies. The closures of previously fished seamounts to fishing operations will present future opportunity to monitor recovery, and provide valuable information to help guide future management initiatives.


7.5. Knowledge Transfer to Stakeholders

The scientific basis necessary for the successful management, protection, and restoration of deep-sea habitats such as seamounts is limited at national and international levels (Davies et al. 2007), and the variability in seamounts and their associated communities dictates that no single management model can be applicable to all seamounts. Crucial to increasing our understanding of seamount ecosystems is an open, honest dialogue and a free exchange of information between all seamount stakeholders.

Future research must focus on addressing urgent management needs. Seamount fisheries on the high seas remain poorly regulated with no unified single managing authority or mechanism in place. Despite gaps in global coverage and inconsistent measures to prevent damage or destruction to vulnerable habitats such as seamounts, regional fisheries management organizations provide the best option for the management/protection of seamount ecosystems. CenSeam researchers have contributed extensively to recent guidelines from the Food and Agriculture Organization of the United Nations (see Rogers et al. 2008) to assist in making future management of deep-sea seamount habitats more effective. These cover the need for careful and controlled development of any fishery, and Rogers et al. ( 2008) outline options for controlling initial exploitation levels (encompassing effort and catch limits). The instruments to protect seamounts are available (for example marine protected areas, closed areas, site-based effort control, licensing, gear restrictions) but examples of their implementation are rare (Alder & Wood 2004; Probert et al. 2007). The most effective practice for seamounts is thought to be closure of areas to trawling (Clark & Koslow 2007), and many regions are moving forward to designate marine protected areas in the deep sea (Davies et al. 2007). This places increasing pressure on scientists to deliver information to aid the selection process. Limitations to knowledge should not restrict efforts toward seamount conservation and, recognizing that seamounts are under-sampled, it is vital to develop means of predicting what communities might occur where (for example CenSeam publications: Clark et al. 2006; Tittensor et al. 2009) as well as robust means of classifying seamounts (Rowden et al. 2005) that may ultimately lead to the design of networks of marine protected areas balancing conservation and exploitation (Leathwick et al. 2008).

CenSeam has facilitated the first global seamount classification based on “biologically meaningful” physical variables (M.R. Clark et al. personal communication). This global classification of seamounts first uses a general biogeographic classification for the bathyal depth zone (near-surface to 3,500 m; UNESCO 2009) and then four key environmental variables (overlying export production, summit depth, oxygen levels, and seamount proximity) to group seamounts with similar characteristics. The classification determined 194 seamount classes throughout the world's oceans. The development of such classifications is vital for enabling the transparent selection of seamounts as candidates for protection, as well as to help guide researchers in strategically targeting seamounts for research.


7.6. Moving Beyond 2010: Emerging Issues

So far, the major threat to seamount ecosystems has been deep-sea bottom fishing (Probert et al. 2007). The maximum depths which can be fished are 2,000 m for trawling and 3,000 m for longline, but future generations may see technological advances in the depths and bottom types that can be fished. Hence the current “footprint” of fishing may expand into totally new areas.

The search for minerals in the deep sea has emerged as a potential threat to deep-sea biodiversity, including that of seamounts. Seamounts can have thick cobalt-rich ferromanganese crusts, manganese nodules, and polymetallic sulfide accumulations, which could be exploited for base metals, such as copper, zinc, and lead, or for precious metals such as gold and silver (see, for example, Hein 2002; Glover & Smith 2003; Davies et al. 2007). Regions of interest for seamount mining have been identified in the Pacific Ocean; cobalt-rich ferromanganese crusts off Hawaii, Micronesia, and the Marshall Islands, and polymetallic sulfide deposits around Papua New Guinea, New Zealand, and Vanuatu.

Direct physical disturbance of the seabed by mining equipment, and associated indirect and direct effects of sediment suspension and deposition, have been shown to influence the composition of macrofaunal assemblages that live on or in seabed substrates (Baker et al. 2001; Glover & Smith 2003). Glover & Smith ( 2003) cite our limited knowledge of the taxonomy, species structure, biogeography, and basic natural history of deep-sea animals as preventing any accurate assessment of the risk of species extinctions from large-scale seabed mining. Research is critical in this current era of prospecting to inform the formulation of environmental guidelines effectively before commercial mining begins. Researchers from CenSeam and ChEss (Biogeography of Deep-Water Chemosynthetic Ecosystems) have provided input to the environmental guidelines produced by the International Seabed Authority and those initiated by the mining industry itself (Clark et al. 2007b).

Rising levels of carbon dioxide in our atmosphere may lower the pH and calcium carbonate saturation of the oceans. A reduction in carbonate saturation inhibits the ability of marine organisms to build calcium carbonate skeletons, shells, and tests. Guinotte et al. (2006) predicted that within this century we might see substantial changes in the distribution of deep-sea corals, a dominant component fauna of communities on seamounts.


7.7. Moving Forward: The Next Decade of Seamount Research

Half a century after Hubbs (1959) posed his initial seamount questions, most remain unanswered. However, researchers have moved from a census of seamount fauna to examining the structure and function of seamount communities, recognizing the need for cross-habitat and multidisciplinary research.

So far, limited sampling prevents broad statements being made about seamount ecosystems, and it is most likely that high variability will ultimately prevent broad generalizations. However, expanding the global sampling effort will increase our chances of being better able to understand seamount ecosystems, and to predict global patterns in faunal diversity and distribution. At the outset of CenSeam, seamount researchers identified the Indian Ocean, the South Atlantic, and the Western and Southern Central Pacific as priority undersampled regions, and in the CenSeam lifetime new expeditions have been launched that will sample seamounts in most of these regions.

Just as we have recognized the geographical confines of our research, we must also recognize bathymetric limitations. The summits of seamounts remain much more intensively sampled than their slopes and bases. Technical limitations have constrained our abilities to sample deep seamounts; sampling deeper than 2,000 m requires specialized equipment such as remotely operated vehicles and submersibles. In the coming decades new sampling capabilities will start to extend the depth boundaries of what we currently think possible.

Virtually every seamount voyage that sails will return with species new to science, and a common challenge across the entire deep-sea realm is to overcome the so-called “taxonomic impediment” (Giangrande 2003). An ever-declining number of taxonomic experts face the challenge of updating faunal inventories, and the considerable time and effort taken to formally name and describe a species limits the rate at which we can census the marine realm. However, new methods, such as barcoding and metagenomics, may make inventories of a broader range of taxa, as well as taxonomically difficult organisms, more tractable. Advances in genetic technologies can also help us better determine the extent to which species, populations, and communities on seamounts are isolated.

So far, the specific relations between environmental drivers and faunal distributions are generally unresolved. CenSeam researchers have been at the forefront of developing new modeling methodologies to extrapolate from the known to the unknown, and expand our knowledge of the distributions of seamount communities. Rigorous testing of the prevailing hypotheses about what environmental factors may affect species survival is still required and, one day, improvements in aquarium design may enable long-term maintenance of deep-sea populations to test future climate change scenarios.

There is a growing need to understand temporal as well as spatial changes in species distributions, recognizing both natural and anthropogenic drivers of such changes, and to feed this information into management approaches (Probert et al. 2007). The effectiveness of marine protected areas in the deep sea is largely untested, and researchers need to be afforded the opportunities to study the recovery of deep-sea communities and feed the results back into future management strategies. Long-term observation programs are no longer a pipe dream with recent methodological advances. The Global Ocean Observing System (GOOS) and its national counterparts are instrumenting the ocean with a suite of physical, chemical, geological, and biological sensors that will provide valuable new information to explain drivers and patterns of marine biodiversity better.

Economics is a major driver in seamount research, and it is likely that, just as fishing operations provided the foundations of much of our current understanding, activities of the future, such as seabed mining, may drive future research direction. Although the negative impacts of fishing activities on some seamount communities are undisputed, fisheries have also increased our knowledge of seamount communities. Industries such as oil and mining are recognizing that they can provide a valuable platform for science. Initiatives such as SERPENT (Scientific and Environmental ROV Partnership using Existing Industrial Technology; have already proven their worth in providing a research capacity not previously available

One of the principal legacies of CenSeam will be to have challenged and changed the seamount paradigms that reigned at its inception. The project has played a role in moving deep-sea ecology and conservation issues into the mainstream, and it is likely that the call for better management of our deep-sea resources will grow ever louder. At the global scale, researchers must work together to align and standardize sampling and analysis approaches, ultimately strengthening our capacity to conduct cross-regional analyses. Our increased understanding of seamount ecosystems has highlighted the crucial need for seamount research to be set in a broad ecological context and include other habitats to increase our understanding of the deep sea in general.


7.8. Acknowledgments

The authors are the secretariat of CenSeam. We thank and acknowledge the many seamount researchers who have contributed to CenSeam as well as the entire CenSeam Steering Committee and Data Analysis Working Group participants for their role throughout the life of the project; for having given so freely of their time and ideas toward developing the core themes and questions. Without their enthusiasm the success achieved so far would not have been possible. The National Institute of Water and Atmospheric Research (NIWA) hosts the CenSeam secretariat. SeamountsOnline has been supported by NSF grants DBI 0074498 and OCE 0340839.

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