Dissostichus, the toothfish, is a genus of marine ray-finned fish belonging to the family Nototheniidae, the notothens or cod icefish. These fish are found in the Southern Hemisphere. Toothfish are marketed for consumption in the United States as Chilean sea bass (or Chilean seabass) or less frequently as white cod. "Chilean sea bass" is a marketing name coined in 1977 by Lee Lantz, a fish wholesaler who wanted a more attractive name for selling the Patagonian toothfish to Americans. Retrieved on 16 May 2015.G. Bruce Knecht, Hooked: Pirates, Poaching, And the Perfect Fish", 2006. . p. 9 In 1994, the U.S. Food and Drug Administration (FDA) accepted "Chilean sea bass" as an "alternative market name" for Patagonian toothfish. The toothfish was remarkably successful in the United States, Europe and Asia, and earned the nickname "white gold" within the market. Toothfish are vital to the ecological structure of Southern Ocean ecosystems. For this reason, on 4 September a national day is dedicated to the toothfish in South Georgia.
Taxonomy Dissostichus was first described as a genus in 1898 by the Swedish zoologist Fredrik Adam Smitt, he was describing a new species from waters off Tierra del Fuego, Dissostichus eleginoides, which he placed as the only species in the new genus. Some authorities place this taxon in the subfamily Pleuragrammatinae, but the 5th edition of Fishes of the World does not include subfamilies in the Nototheniidae. The name of the genus Dissostichus is a compound of dissos which means "twofold" or "double" and stichus which means "row" or "line", an allusion to the two lateral lines of D. eleginoides.
Species Two species in this genus are recognized: Dissostichus eleginoides Smitt, 1898 (Patagonian toothfish) Dissostichus mawsoni Norman, 1937 (Antarctic toothfish)
Description The Patagonian toothfish and the Antarctic toothfish are markedly similar in appearance, but can be identified via several morphological features. The Patagonian toothfish has a characteristic scaleless patch between the eyes and a longer visible lateral line.Fischer W. and Hureau J.C. (1985). "FAO Species identification sheets for fishery purposes". Southern Ocean: Fishing Areas 48, 58, and 88 (CCAMLR Convention Area) 2, 233–470.
Toothfishes are large in size, with both species reaching maximum sizes exceeding 100 kg.Eastman J.T. and DeVries A.L. (2000). "Aspects of body size and gonadal histology in the Antarctic toothfish, Dissostichus mawsoni, from McMurdo Sound, Antarctica". Pol. Bio. 23, 189–195. This trait distinguishes the toothfishes from other notothenioids.
Distribution The Patagonian toothfish is distributed circumpolarly near the Antarctic Convergence, spanning the Atlantic, Pacific and Indian sectors of the Southern Ocean, with a few populations near the Antarctic Peninsula as well as the coasts of Chile and Argentina. The Antarctic toothfish is distributed around the Antarctic continental shelf and in the Ross Sea, generally south of 60°S.
Both species are benthopelagic as adults. The Patagonian toothfish has been found at depths between 600 and 1500 m, and the Antarctic toothfish tends to be found at depths between 1300 and 1900 m.Petrov A.F. (2011). "Distribution and biological characteristics of two species of toothfishes of the genus Dissostichus (Fam. Nototheniidae) at Bouvet Island". J. Ichth. 51, 848–853. The Patagonian and Antarctic toothfish populations overlap geographically in the southern Indian Ocean, north of the Ross Sea and several south Atlantic islands.Roberts, J.; et al. (2011). "The diet of toothfish species" Dissostichus eleginoides and Dissostichus mawsoni with overlapping distributions. J. Fish Bio. 79, 138–154. Since the Southern Ocean has not yet been fully sampled, both species may extend further throughout the region.Collins M.A., et al. (2010). "The Patagonian toothfish: biology, ecology and fishery". Adv. Mar. Bio. 58, 227–300.
Biology Both species occupy similar ecological niches. They are piscivorous at all life stages.McKenna J.E. (1991). Trophic relationships within the Antarctic demersal fish community of South Georgia Island. Fish. Bul. 89, 643-654. Juveniles mostly prey on smaller notothenioids, krill, squid and pelagic fishes. Prey range increases with maturity. As adults, both species consume a variety of fishes, cephalopods and crustaceans. The Patagonian toothfish is opportunistic, and sometimes scavenges penguin and fish remains that sink from the epipelagic zone.Stevens D.W. (2004). Stomach contents of the Antarctic toothfish (Dissostichus mawsoni) from the Western Ross Sea, Antarctica. Hobart: CCAMLR, WG-FSA 04/31. The toothfishes may compete with Adélie penguins for prey such as silverfish and krill.Ainley D.G., et al. (2006). Competition among penguins and cetaceans reveals trophic cascades in the Western Ross Sea, Antarctica. Ecol. 87, 2080-2093.
Toothfish are important prey for large mammals. Patagonian toothfish predators include elephant seals, sperm whales, and killer whales. Antarctic toothfish predators include Weddell seals and whales, especially killer whales.Ainley D.G. and Siniff D.B. (2009). The importance of Antarctic toothfish as prey of Weddell Seals in the Ross Sea. Antarc. Sci. 21, 317-327. Some data suggests that the overfishing of toothfish may stress whale populations. Hucke-Gaete et al. observed that when toothfish were removed from via longline capture, killer whales were more likely to prey on nearby sperm whales.Hucke-Gaete et al. (2004). Operational interactions of sperm whales and killer whales with the Patagonian toothfish industrial fishery off southern Chile. CCAMLR Sci. 11, 127-140.
Life history Much remains unknown regarding the life cycles of both toothfish species, particularly the Antarctic toothfish. Much of the existing data has been collected by reports from commercial fishing vessels. Current research suggests that both the Patagonian and Antarctic toothfishes tend to form discontinuous populations, meaning that little gene flow exists between populations.Kuhn K.L. and Gaffney P.M. (2008). Population subdivision in the Antarctic toothfish (Dissostichus mawsoni) and revealed by mitochondrial and nuclear single nucleotide polymorphisms (SNPs). Antarctic Sci. 1-12.Smith P. and McVeagh M. (2000). Allozyme and microsatellite DNA markers of toothfish population structure in the Southern Ocean. J. Fish Bio. 57, 72-83.
Both species migrate during spawning season. However, the specific duration and distance of these migrations have not been fully characterized. Tag-and-release experiments on the Patagonian toothfish have indicated that most individuals migrate an average of 15 miles from their habitat, and that any migratory movement generally occurs after the fishes reach maturity.Williams R. et al. (2002). Movement, growth and available abundance to the fishery of Dissostichus eleginoides Smitt, 1898 at Heard Island, derived from tagging experiments. CCAMLR Sci. 9, 33-48. However, some anecdotal evidence demonstrates that the Patagonian toothfish may be capable of migrating between the northern and southern poles, likely by submerging beneath tropical and temperate waters.Moller P.R. et al. (2003). Fish migration: Patagonian toothfish found off Greenland. Nature 421, 599.
Both species are long-lived and relatively slow-growing. The Patagonian toothfish can live up to at least 50 years of age and the Antarctic toothfish can live to at least 35 years of age.Horn P.L. (2002). "Age and growth of Patagonian toothfish (Dissostichus eleginoides) and Antarctic toothfish (D. mawsoni) in waters from the New Zealand subantarctic to the Ross Sea, Antarctica". Fish. Res.56, 275–287. Both species show the fastest growth in the first 10 years of life, and reach maximum body size around 20 years of age. The Patagonian toothfish grows at an average rate of 2 cm and 1 kg per year.Ainley D.G., et al. (2013). "Decadal trends in abundance, size and condition of Antarctic toothfish in McMurdo Sound, Antarctica, 1972–2011". Fish and Fisheries 14, 343–363.
Patagonian toothfish
The Patagonian toothfish spawns in deep waters during the austral winter and early spring, between June and September. Discontinuous oogenesis development patterns suggest that Patagonian toothfish do not spawn annually. Individuals migrate to spawn in shallower waters (800–1000 m), although migration patterns can vary widely between different geographic populations.Laptikhovsky V. (2006). Latitudinal and bathymetric trends in egg size variation: a new look at Thorson's and Rass's rules. Mar. Ecol.27, 7-14. Some data suggest that spawning yields decrease at higher latitudes.Arana P.M. (2009). Reproductive aspects of the Patagonian toothfish (Dissostichus eleginoides) off southern Chile. Lat. Am. J. Aqua. Res. 37, 381-394.
Egg development occurs over a period of approximately three months, with hatching occurring in October or November.Evseenko S.A. (1995). Early life history of the Patagonian toothfish, Dissostichus eleginoides Smitt, 1898 in the Atlantic sector of the Southern Ocean. Antarc. Sci. 7, 221-226. After hatching, the planktonic larvae drift for a period of time, likely until February. Around this time, the fish reach a critical size and become bathypelagic. They gradually inhabit deeper waters as they grow, migrating down the continental slope. Adult fish are generally found at depths greater than 500 m.
Antarctic toothfish
The life cycle of the Antarctic toothfish is less well-characterized. Researchers and fisheries are limited to collecting this species during the austral summer and autumn (December to May) due to harsh polar conditions during the austral winter (June to August). Scientists have not yet collected any juvenile or larvae Antarctic toothfishYoung, P. (2012). The last ocean: The toothfish and the battle for Antarctica’s soul. Fisheye Films. Documentary. and have not yet verified the location, season or frequency of spawning activity. Hanchet et al. used all previously collected data from 1996 to 2008 to generate an updated model for the life cycle of the Antarctic toothfish within the Ross Sea fishery, although the authors note that much of this model is based on speculation and much of the data has not yet been collected fully.Hanchet S.M. et al. (2008). A hypothetical life cycle for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region. CCAMLR Sci. 15, 35-53. They estimated that the Antarctic toothfish adults spawn between June and November, and that spawning likely occurs north of 70°S, beyond the Ross continental shelf and slope.
Eggs likely have a development period of four to five months and hatch between November and February, developing more slowly than their Patagonian toothfish counterparts. Current data suggests that spawning requires a large amount of reproductive tissue, and may not occur annually. Planktonic larvae undergo larval drift for a period of several months, likely into late summer or early autumn. Researchers predict that the Antarctic toothfish inhabit benthic regions after reaching 15 cm. Sub-adult individuals develop within the Ross Sea and gradually inhabit deeper waters over a period of two to three years.Hanchet S.M. et al. (2003). Characterisation of the exploratory fishery for toothfish (Dissostichus mawsoni and D. eleginoides) in the Ross Sea, and approaches to the assessment of the stocks. N.Z. Fish. Assess. Report 43, 3-27.
Relation to humans