Polar Biol (2004) 27: 321–338 DOI 10.1007/s00300-004-0599-z REVIEW M. La Mesa Æ J. T. Eastman Æ M. Vacchi The role of notothenioid fish in the food web of the Ross Sea shelf waters: a review Received: 27 June 2003 / Accepted: 23 January 2004 / Published online: 12 March 2004 Ó Springer-Verlag 2004 Abstract The Ross Sea, a large, high-latitude (72–78°S) are benthic predators on fish while smaller species feed embayment of the Antarctic continental shelf, averages mainly on benthic crustaceans. Channichthyids are less 500 m deep, with troughs to 1,200 m and the shelf break dependent on the bottom for food than other notothe- at 700 m. It is covered by pack ice for 9 months of the nioids. Some species combine benthic and pelagic life year. The fish fauna of about 80 species includes pri- styles; others are predominantly pelagic and all consume marily 4 families and 53 species of the endemic perci- euphausiids and/or fish. South polar skuas, Antarctic form suborder Notothenioidei. This review focuses on petrels, Ade´lie and emperor penguins, Weddell seals and the diet and role in the food web of notothenioids and minke and killer whales are the higher vertebrate com- top-level bird and mammal predators, and also includes ponents of the food web, and all prey on notothenioids new information on the diets of artedidraconids and to some extent. Based on the frequency of occurrence of bathydraconids. Although principally a benthic group, prey items in the stomachs of fish, bird and mammal notothenioids have diversified to form an adaptive predators, P. antarcticum and ice krill E. crystallorophias radiation that includes pelagic and semipelagic species. are the key species in the food web of the Ross Sea. In the southern Ross Sea, notothenioids dominate the P. antarcticum is a component of the diet of at least 11 fish fauna at levels of abundance and biomass >90% species of nototheniid, bathydraconid and channichth- and are, therefore, inordinately important in the food yid fish and, at frequencies of occurrence from 71 to web. Antarctic krill (Euphausia superba) and mesope- 100%, is especially important for Dissostichus mawsoni, lagic fishes are virtually absent from the shelf waters of Gvozdarus svetovidovi and some channichthyids. At least the Ross Sea. Of the four notothenioid families, noto- 16 species of notothenioids serve as prey for bird and theniids show the most ecological and dietary diversifi- mammal predators, but P. antarcticum is the most cation, with pelagic, cryopelagic, epibenthic and benthic important and is a major component of the diet of south species. Neutrally buoyant Pleuragramma antarcticum polar skua, Ade´lie and emperor penguins and Weddell constitutes >90% of both the abundance and biomass seals, at frequencies of occurrence from 26 to 100%. of the midwater fish fauna. Most benthic nototheniids E. crystallorophias is consumed by some nototheniid and are opportunistic and feed on seasonally or locally channichthyid fish and can be of importance in the diet abundant zooplanktonic prey. Artedidraconids are of emperor and Ade´lie penguins, although in the latter benthic sit-and-wait predators. Larger bathydraconids case, this is dependent on location and time of year. Unlike the linear phytoplankton fi E. super- ba fi consumers of the E. superba food chain hypothe- M. La Mesa (&) sized for much of the Southern Ocean, the food web of Istituto di Ricerche sulla Pesca Marittima, CNR, the Ross Sea shelf is non-linear, with complex prey- Largo Fiera della Pesca 1, predator interactions. Notothenioid fish play a key role: 60125 Ancona, Italy as predators, they occupy most of the trophic niches E-mail: [email protected] Fax: +39-071-55313 available in the ecosystem, relying on benthic, zoo- planktonic and nektonic organisms; as prey, they are J. T. Eastman important food resources for each other and for most Department of Biomedical Sciences, College of Osteopathic Medicine, top predators living and foraging on the shelf. They also Ohio University, Athens, OH 45701-2979, USA constitute the major link between lower (invertebrates) M. Vacchi and higher (birds and mammals) levels of the food web. ICRAM, Istituto Centrale per la Ricerca Scientifica e Tecnologica This is especially true for P. antarcticum. Along with Applicata al Mare, Via Casalotti 300, 00166 Rome, Italy E. crystallorophias, its ecological role in the Ross Sea is 322 equivalent to that of myctophids and E. superba else- the genera Notothenia and Lepidonotothen, as well as by where in the Southern Ocean. harpagiferids, bathydraconids and channichthyids. Finally, the High-Antarctic Zone is that region immediately adjacent to the continent which is covered by ice for most of the year. The fish fauna is similar to Introduction that of the Seasonal Pack-ice Zone, at least for the pe- lagic fauna. However, the benthic fauna is greatly re- The Southern Ocean consists of the southern parts of stricted in diversity of higher taxa, and characterized by the Atlantic, Indian and Pacific oceans (Lutjeharms nototheniids of the genus Trematomus and a high pro- 1990). The northern boundary of the Southern Ocean portion of other notothenioids, such as artedidraconids, is the Antarctic Polar Front, located at about 50°Sin bathydraconids and channichthyids. In the Antarctic, the Atlantic and Indian sectors and 60°S in the Pacific fish species diversity, especially that of notothenioids, sector (Hureau 1994; Lutjeharms 1990). The Antarctic generally increases with latitude, although biomass Polar Front is characterized by a sudden drop of sur- decreases farther south. For example, fish biomass is 10- face temperature of 3–4°C, as well as by changes in to 20-fold higher in the Seasonal Pack-ice Zone near other oceanographic parameters like salinity. At the Elephant Island than in the High-Antarctic southern front, northward-flowing Antarctic Surface Water sinks Weddell Sea, but indices of species diversity and even- beneath warmer Subantarctic Surface Water, generat- ness are lower at Elephant Island than at comparable ing a transition zone between surface water masses of depths on the High-Antarctic shelf (Tiedtke and Kock different temperatures (Eastman 1993). From an 1989). oceanographic perspective, the surface circulation In high-latitude shelf areas like the Ross Sea, the around Antarctica consists of two main currents. Close overwhelming dominance by a single taxonomic group is to the Antarctic Continent, easterly winds generate a unparalleled in the fish fauna of any other marine eco- counterclockwise water circulation, i.e. the Antarctic system. As documented by benthic trawling at depths Coastal Current or East Wind Drift. North of 60°S, of 107–1,191 m in the southwestern Ross Sea (73–77°S), westerly winds produce a clockwise and northward notothenioids comprise 76.6% of species, 91.6% of water flow, known as Antarctic Circumpolar Current abundance and 91.2% of biomass (Eastman and Hubold or West Wind Drift, representing the major current in 1999). Non-notothenioids are represented by Rajidae, the Southern Ocean. In large embayments of the Ant- Muraenolepidae, Liparidae and Zoarcidae, accounting arctic Continent, particularly the Weddell and Ross for 8.4% of abundance and 8.5% of frequency of Seas, the Antarctic Coastal Current forms clockwise occurrence (Eastman and Hubold 1999). Since noto- gyres, which probably concentrate nutrients like sili- thenioids are the only fish group of significant biomass, cate, phosphate and nitrate (Lutjeharms 1990). Unlike they obviously play a major role in the food web. other areas of Antarctica, the continental shelves of the There is a historical or evolutionary hypothesis for Weddell and Ross Seas are wide and 500–600 m deep, this phenomenon—notothenioids reach these levels of with inner shelf depressions reaching depths of over dominance because they occupy niches filled by taxo- 1,200 m (Anderson 1999). Furthermore, much of the nomically diverse groups of fishes in temperate shelf Antarctic coastline is covered by ice shelves, which are areas, groups not represented on the Antarctic shelf exceptionally large in the Weddell and Ross Seas. (Eastman 2000). The fossil record indicates that over the During much of the year, the adjacent shelf waters are past 40 million years, there has been a nearly complete covered by pack ice. replacement of the Antarctic fish fauna. A diverse cos- Based on the distribution of the pelagic and demersal mopolitan temperate fauna from the late Eocene was fishes, three main ichthyofaunal subregions are recog- replaced by the highly endemic, cold-adapted modern nized in the Southern Ocean: from north to south, the fauna. Although the specific factors responsible for the Ice-free Zone, the Seasonal Pack-ice Zone and the High- replacement are not completely understood, the Antarctic Zone (Kock 1992). grounding of the ice sheet on the continental shelf and The Ice-free Zone, where ice cover is absent all year, changing trophic conditions may have played a role. In a lies between the Antarctic Polar Front and the northern habitat with few other fishes and reduced competition, limit of the pack ice in winter/spring. It is deep sea, notothenioids underwent a depth-related diversification almost exclusively inhabited by mesopelagic and directed away from the ancestral benthic habitat toward bathypelagic fish, and most of these are cosmopolitan pelagic or partially pelagic zooplanktivory and pisci- rather than members of the indigenous Antarctic perci- vory. Although they lack swim bladders, in a few spe- form suborder Notothenioidei. cies, density reduction to neutral buoyancy has been The Seasonal Pack-ice Zone extends between the achieved through a combination of reduced skeletal limit of pack ice in winter/spring and in autumn/sum- mineralization and lipid deposition, with paedomor- mer, including the islands north of the maximum limit of phosis being one of the mechanisms facilitating such a the pack ice.