The Antarctic toothfish (Dissostichus mawsoni), also known as the Antarctic cod, is a large, black or brown fish found in very cold (even subzero) waters of the Southern Ocean near Antarctica. It is the largest species of bony fish in the Southern Ocean, feeding largely on smaller fishes and crustaceans, and, in turn, preyed on by orcas, other toothed whales, and seals. It is caught for food and marketed as Chilean sea bass together with its sister species, the more northerly Patagonian toothfish (D. eliginoides). Often mistakenly called "Antarctic cod", the Antarctic toothfish is a species in the (Nototheniidae), a family of fishes abundant in subantarctic waters.
Name and taxonomy The common name "toothfish" refers to the two rows of teeth in the upper jaw, thought to give it a shark-like appearance.
The genus name Dissostichus is from the Greek (twofold) and stichus (line) and refers to the presence of two long lateral lines that enable the fish to sense prey. The species name, mawsoni, honors the Australian geologist Douglas Mawson who led the 1911–1914 Australasian Antarctic Expedition that explored the Antarctic coast and obtained the species' type specimen.
The Antarctic toothfish was first formally described in 1937 from the English ichthyologist John Roxborough Norman with the type locality given as off MacRobertson Land at 66°45'S, 62°03'E in Antarctica.
Description Fully grown, antarctic toothfish, (like their warmer-water relatives, the Patagonian toothfish, D. eleginoides) may grow to more than in length and 135 kg in mass, twice as large as the next-largest Antarctic species of fishes. Being large, and consistent with the unstructured food webs of the ocean (i.e., big fish eating little fish regardless of identity, even eating their own offspring), the Antarctic toothfish has been characterized as a voracious predator. Furthermore, by being by far the largest midwater fish in the Southern Ocean, it is thought to fill the ecological role that sharks play in other oceans.Andriashev, A.P. (1962). On the systematic position of the giant nototheniid fish (Pisces, Nototheniidae) from the McMurdo Sound, Antarctica. Zool. Zhur. 41:1048–1050 (in Russian; English translation available from National Institute of Oceanography, Wormley, Godalming, Surrey, UK, No. NIOT/1132, June 1970).DeVries, A.L.; and Eastman JT (1998) Brief review of the biology of Dissostichus mawsoni. CCAMLR Document WG-FSA-98/49, Hobart, Australia. As an adaptation to that role, the Antarctic toothfish is one of only five notothenioid species that, as adults, are neutrally buoyant. This buoyancy is attained at 100–120 cm in length and enables them to spend time above the bottom without wasting energy.Yukhov, V.L. (1971). The range of Dissostichus mawsoni Norman and some features of its biology. Journal of Ichthyology 11: 8–18. Both bottom-dwelling and mid-water prey are, therefore, available to them. Most other notothenioid fish and the majority of all Antarctic fishes, including smaller toothfish, are confined to the bottom. Coloring is black to olive brown, sometimes lighter on the undersides, with a mottled pattern on body and fins. Small fish blend in very well among the benthic sponges and corals. The species has a broad head, an elongated body, long dorsal and anal fins, large pectoral fins, and a rudder-like caudal fin. They typically move slowly, but are capable of speed bursts that may elude predatory seals.
Feeding ecology Over the continental shelf, Antarctic toothfish feed on shrimp (Nauticaris spp.) and small fish, principally another neutrally buoyant nototheniid, the Antarctic silverfish (Pleuragramma antarcticum). This loosely schooling species is also a major prey of Adélie (Pygoscelis adeliae) and emperor penguins (Aptenodytes forsteri), Weddell seals (Leptonychotes weddellii) and Antarctic minke whales (Balaenoptera bonaerensis). Therefore, competition for prey among toothfish and these other mesopredators (middle trophic level predators) could be very important. The large Antarctic toothfish are eaten by sperm whales (Physeter macrocephalus), killer whales (Orcinus orca), Weddell seals, and possibly colossal squid (Mesonychoteuthis hamiltoni). Toothfish that are dwelling on the bottom, particularly those caught during the summer on the continental slope, eat mainly grenadiers (Macrouridae), but also feed on other smaller fish species and skates (Raja spp.).Fenaughty, J.M.; Stevens, D.W., Hanchet, S.M. (2003). Diet of the Antarctic toothfish (Dissostichus mawsoni) from the Ross Sea, Antarctica (CCAMLR Statistical Subarea 88.1). CCAMLR Sci. 10:1–11. They also feed on the colossal squid. Antarctic toothfish have been caught to depths of 2200 m, though based on commercial fishing effort, few occur that deep.Hanchet, S.M.; Rickard, G.J.; Fenaughty, J.M.; Dunn, A.; and Williams, M.J.H. (2008). Hypothetical life cycle for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region. CCAMLR Sci. 15:35–53.
Aging and reproduction Aging data indicate Antarctic toothfish are relatively fast-growing when young, but then growth slows later in life. They reach about one-third of maximum size after 5 years, and half maximum by 10 years, after which growth slows considerably.Brooks, C.M.; Andrews, A.H.; Ashford, J.R.; Ramanna, N.; Jones, C.D.; Lundstrom, C.C.; and Cailliet, G.M. (2010). Age estimation and lead–radium dating of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea. Polar Biology . To grow fast when small is an adaptation of most predatory fish, e.g., sharks, so as not to be small for very long. The maximum age recorded so far has been 48 years.Hanchet, S.M.; Stevenson, M.L.; Phillips, N.L.; and Dunn, A. (2005) A characterisation of the toothfish fishery in Subareas 88.1 and 88.2 from 1997/98 to 2004/05. CCAMLR WG-FSA-05/29. Hobart, Australia. Antarctic toothfish take a long time to mature (13 years for males, 17 years for females) and once mature may not spawn every year, though the actual spawning interval is unknown.Parker, S.J.; and Grimes, P.J. (2010). Length- and age-at-spawning of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea. CCAMLR Sci. 17: 53–73. Only a few Antarctic toothfish with mature eggs have ever been caught, meaning knowledge is sparse about fecundity.Hanchet, S.M. (2010) Updated species profile for Antarctic toothfish (Dissostichus mawsoni). CCAMLR WG-FSA-10/24. Hobart, Australia. They spawn sometime during winter.Brooks, C.M.; Ashford, J.R. (2008) Spatial distribution and age structure of the Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea, Antarctica. CCAMLR WG-FSA-08-18. Hobart, Australia. Large, mature, older fish have been caught among the seamounts of the Pacific-Antarctic Ridge, a location thus thought to be important for spawning. Smaller, subadult Antarctic toothfish tend to concentrate in shallower waters on the continental shelf, while a large portion of the older fish are found on in the continental slope. This sequestering by size and age could be another adaptation for small fish to avoid being eaten by large ones. The recruitment potential of Antarctic toothfish, a measure of both fecundity and survival to spawning age, is not known.
Anatomy and physiology The Antarctic toothfish has a lightweight, partially cartilaginous skeleton, lacks a swim bladder, and has fatty deposits which act as a stored energy source, particularly during spawning. This fat also makes large toothfish neutrally buoyant. Many toothfish caught over the seamounts are very depleted of fat, and this is thought perhaps to be related to spawning and spawning migration, which are energy-demanding activities.Fenaughty, J.M.; Eastman, J.T.; and Sidell, B.D. (2008). Biological implications of low condition factor “axe handle” specimens of the Antarctic toothfish, Dissostichus mawsoni, from the Ross Sea. Antarctic Science 20:537–551. It is not known what happens to these fat-depleted fish, including whether they reach, or how long it takes them to reach, breeding condition again; this ostensibly occurs upon returning to continental-slope waters. Antarctic toothfish have vision and lateral line systems well adapted to find prey in low light levels.Eastman, JT; Lannoo, MJ. (2011). Divergence of brain and retinal anatomy and histology in pelagic Antarctic notothenioid fishes of the sister taxa Dissostichus and Pleuragramma. Journal of Morphology 272:419-441. Since ice covers the surface of the ocean where Antarctic toothfish occur even in summer, these sensory specializations likely evolved to enable survival in the reduced light levels found under ice and in the Antarctic winter, as well as at deep depths. Antarctic toothfish also have a very well developed sense of smell, which is why they are easily caught by baited hooks and also scavenge the remains of penguins killed by other predators.
Cold adaptation The Antarctic toothfish lives in subzero degree water below latitude 60°S. It is noteworthy, like most other Antarctic notothenioids, for producing antifreeze glycoproteins, a feature not seen in its closest relative, the Patagonian toothfish, which typically inhabits slightly warmer waters. The presence of antifreeze glycoproteins allows the Antarctic toothfish (and other notothenioids) to thrive in subzero waters of the Southern Ocean surrounding Antarctica. The Antarctic toothfish's voracious appetite also is important in coping with cold water.Roberts, J.; Xavier, J.C.; and Agnew, D.L. (2011). The diet of toothfish species Dissostichus eleginoides and Dissostichus mawsoni with overlapping distributions. Journal of Fish Biology 79: 138–154. It is mainly caught in the Ross Sea in the austral summer, but has also been recorded from Antarctic coastal waters south of the Indian Ocean sector, in the vicinity of the Antarctic Peninsula, and near the South Sandwich Islands.
Fishery and associated ecosystem