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An Ecosystem Approach to Management of Seamounts in the Southern Indian Ocean Volume 1 – Overview of Seamount Ecosystems and Biodiversity

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An Ecosystem Approach to Management of Seamounts in the Southern Indian Ocean Volume 1 – Overview of Seamount Ecosystems and Biodiversity An Ecosystem Approach to Management of Seamounts in the Southern Indian Ocean Volume 1 – Overview of Seamount Ecosystems and Biodiversity Alex D. Rogers IUCN GLOBAL MARINE AND POLAR PROGRAMME The designation of geographical entities in this publication, and the presentation of the material, do not imply the expression of any opinion whatsoever on the part of IUCN concerning the legal status of any country, territory, or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries. The views expressed in this publication do not necessarily reflect those of IUCN. Published by: IUCN, Gland, Switzerland Copyright: © 2012 International Union for Conservation of Nature and Natural Resources Reproduction of this publication for educational or other non-commercial purposes is authorized without prior written permission from the copyright holder provided the source is fully acknowledged. Reproduction of this publication for resale or other commercial purposes is prohibited without prior written permission of the copyright holder. Citation: Rogers, A.D. (2012). An Ecosystem Approach to Management of Seamounts in the Southern Indian Ocean. Volume 1 – Overview of Seamount Ecosystems and Biodiversity. Gland, Switzerland: IUCN. 18 + iipp. This paper is to be read in conjunction with two others: one on threats to seamount ecosystems (Volume 2) and one on a legal and instituional gap analysis (Volume 3). ISBN: 978-2-8317-1561-2 Cover photos: Front: Unidentified octopus, South West Indian Ocean, R/V Fridtjof Nansen Cruise, Nov-Dec 2009. Back: Chaunacidae – Sea toads, South West Indian Ocean, R/V Fridtjof Nansen Cruise, Nov-Dec 2009. Layout by: Tim Davis, DJEnvironmental, UK Available from: IUCN (International Union for Conservation of Nature) Publications Services Rue Mauverney 28 1196 Gland Switzerland Tel +41 22 999 0000 Fax +41 22 999 0020 [email protected] www.iucn.org/publications An Ecosystem Approach to Management of Seamounts in the Southern Indian Ocean Volume 1 – Overview of Seamount Ecosystems and Biodiversity Alex D. Rogers IUCN SIO Seamount Governance i TABLE OF CONTENTS I. General: About Seamount Ecosystems....................................................................................1 II. Specific: The Southern Indian Ocean and its Seamounts.......................................................5 A. Biology of seamounts in the Indian Ocean.............................................................................6 B. Deep-sea fisheries on seamounts in the Indian Ocean...........................................................9 C. Management of deep-water fisheries on the high seas of the Indian Ocean...........................9 III. Knowledge Gaps .....................................................................................................................11 IV. References...............................................................................................................................12 I. G ENERAL : A BOUT SEAMOUNT ECOSYSTEMS eamounts are topographic rises of the demersal fish are important as spawning sites seabed with a limited extent across the (e.g., Pankhurst et al., 1987; Boehlert and Sasaki, Ssummit. Originally this definition held that 1988; Pankhurst, 1988; Tsukamoto et al., 2003; seamounts had an elevation of ≥1,000 m and that Tsukamoto, 2006). they were intraplate features, reflecting a contrasting geological origin to those associated To date (July 2010), 798 species of fish have with mid-ocean ridges and other settings been listed as occurring at seamounts (Morato et (Staudigel et al., 2010). However, smaller al., 2004). Deep-sea fisheries target a relatively submarine knolls (elevation 500-1,000 m) and hills small fraction of these that aggregate around (elevation >500 m) share many of the seamounts. Most of these species have been environmental characteristics of larger features, identified as forming a specific guild of robust- and given that the size distribution of such bodied demersal fish species, generally elevations are continuous, the term seamount is characterized by a strong swimming ability, high used interchangeably for most features >100 m in food consumption and energy expenditure, and elevation (Wessel, 2007; Staudigel et al., 2010). low rates of growth and productivity (e.g., Koslow Large seamounts usually originate as volcanoes, et al., 2000). These species include orange although some, such as Atlantis Bank in the roughy (Hoplostethus atlanticus), pelagic South West Indian Ocean, are formed by tectonic armourhead ( Pseudopentaceros wheeleri; uplift, or even from serpentinite mud (e.g., Fryer et Pseudopentaceros richardsoni ), rockfish al., 1992). Most large seamounts are associated (Sebastes spp., Helicolenus spp.), oreos with intraplate hotspots, mid-ocean ridges or (Oreosomatidae), cardinal fish ( Epigonus spp.) island arcs. There may be up to 100,000- roundnose grenadier ( Coryphaenoides rupestris ), 200,000 large seamounts in the oceans, with Patagonian toothfish ( Dissostichus eleginoides ) perhaps >25 million (with a range of error of 8-80 and alfonsinos ( Beryx splendens, Beryx million) with an elevation ≥100 m (Wessel, 2007; decadactylus ). These fish are specifically Wessel et al., 2010). Most seamounts are located associated with seamounts, although they also in the Pacific Ocean, with lower numbers in the occur on continental slopes and the slopes of Indian, Atlantic, Arctic and Southern Oceans oceanic islands. (Wessel, 2007). Overall this adds up to a globally significant area of habitat with an estimate of Why seamounts host abundant populations of fish nearly 10 million km 2, comparable with tropical and other pelagic and aquatic predators is still humid forest, temperate broad-leafed forest or all uncertain. Early observations suggested that the world’s wetlands (Etnoyer et al., 2010). chlorophyll concentrations may be higher above seamounts than in non-seamount locations There is evidence that seamounts form hotspots (Bezrukov and Natarov, 1976; Lophukin, 1986; of biological activity in the oceans. Over large Boehlert and Genin, 1987; Dower et al., 1992; geographic scales ocean predators appear to be Mouriño et al., 2001). This gave some support to associated with seamounts and other features. a hypothesis of enhanced primary productivity Tuna, billfish, sharks, cetaceans, pinnipeds, turtles above seamounts, resulting from nutrient and seabirds may all be associated with enrichment of the euphotic zone by localized seamounts, and high biomass and abundance of upwelling induced by current-topography such predators have been observed in the vicinity interactions, especially the formation of Taylor of seamounts (Holland and Grubs, 2007; columns. Seamounts may also be associated Kaschner, 2007; Litvinov, 2007; Santos et al., with turbulent mixing caused by internal or 2007; Thompson, 2007). This is thought to be baroclinic waves that can also induce upwelling of because of enhanced levels of prey or feeding nutrients (Kunze and Sanford, 1997; Toole et al., opportunities for these species. Seamounts may 1997). However, it has been argued more recently also act as ‘navigational waypoints’ for oceanic that evidence for enhancement of primary species that undergo large-scale annual or production, beyond sporadic and temporary ontogenetic migrations (e.g., Holland and Grubs, blooms of phytoplankton, are lacking (Genin and 2007) and for some species of catadromous and Dower, 2007; Clark et al., 2010). Although such SIO Seamount Ecosystems and Biodiversity 1 GENERAL : A BOUT SEAMOUNT ECOSYSTEMS phenomena may enhance patchiness of micronekton (Rogers, 1994; Genin and Dower, phytoplankton downstream of seamounts, they 2007; Porteiro and Sutton, 2007). These layers, are unlikely to significantly enhance the known as the deep-scattering layer (DSL) productivity of the seamounts themselves. because it is seen as a layer of acoustic reflectivity in echosounders, undergo migrations towards the It is now recognized that several other physical surface at dusk and then back down into deeper processes associated with seamounts may waters at dawn. During the night the DSL may be enhance productivity or food supply in their advected over a seamount and at dawn, when vicinity. These include tidal rectification (generation the organisms descend, their passage is of mean residual currents by tidal flow), flow obstructed by the raised topography and they are acceleration and the formation of internal waves, preyed on by fish and other planktivores living on and can enhance vertical mixing around a the seamount (Genin and Dower, 2007). The DSL seamount, causing upwelling of nutrient-rich deep may be enhanced by organisms that are water and enhancement of primary production associated with the seabed on the seamount but (White et al., 2007; Lavelle and Mohn, 2010). The which undertake migrations into the water column water above a seamount may become stabilized to feed at night (Boehlert and Seki, 1984; by high-density stratification, promoting Vereshaka, 1995). Evidence that the DSL is an productivity (Comeau et al., 1995). Seamounts important food source for seamount-resident or may also interact with eddies shed from frontal visiting species of predators has been observed zones, or Meddies, lenses of warm salty water through analyses of stomach contents for released from the Mediterranean into the colder demersal and pelagic species (Genin et al., 1988; waters of the Atlantic which can interact with Seki and Somerton, 1994; Fock et al., 2002; seamounts. Eddies have been observed
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