Dissostichus mawsoni, the Antarctic toothfish, is a species of cod icefish native to the Southern Ocean. It is often mistakenly referred to as an Antarctic cod, consistent with the misnaming of other notothenioid Antarctic fish as rock cods. However, notothenioid fishes are unrelated to cods, which are in another taxonomic order, the Gadiformes. The generic name Dissostichus is from the Greek dissos (twofold) and stihcus (line) and refers to the presence of two long lateral lines, which are very important to the species’ ecology. The common name toothfish refers to the presence of biserial dentition in the upper jaw, thought wrongly to give it a shark-like appearance. The habitat of the Antarctic toothfish is in subzero degree water below latitude 60°S.
Fully grown, these fish (and their warmer-water relative, the Patagonian toothfish, D. eleginoides) can grow in excess of 2 m in length and 135 kg in weight, twice as large as the next largest Antarctic fish. Being large, and consistent with the unstructured food webs of the ocean (i.e., big fish eat 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. Aiding in 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 expending extra energy. Both bottom-dwelling and mid-water prey are therefore available to them. Most other notothenioid fish and the majority of all Antarctic fish, 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. They have 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 can elude predatory seals. 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 colossal squid Mesonychoteuthis hamiltoni. Toothfish that are dwelling on the bottom, especially in ice free waters of the continental slope, eat grenadiers (Macrouridae) and skates (Raja spp.). Antarctic toothfish have been caught to depths of 2200m, though based on commercial fishing effort, few occur that deep.
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 five years, and half maximum by 10 years, after which growth slows considerably. 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. 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. Only a few Antarctic toothfish with mature eggs have ever been caught, meaning knowledge is sparse about fecundity. They spawn sometime during winter. 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 on the bottom hidden among sponges and corals of the continental shelf, and a large portion of the larger ones remain in the particularly food-rich waters of 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. 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. 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.
The Antarctic toothfish 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 glycroproteins allows the Antarctic toothfish (and other notothenioids) to thrive in sub-zero waters of the Southern Ocean surrounding Antarctica. The Antarctic toothfish’s voracious appetite also is important in coping with cold water. 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.
A fishery for Antarctic toothfish, managed by the Convention for the Conservation of Antarctic Marine Living Resources(CCAMLR), has existed since 1996. The existence of this fishery in the Ross Sea, the area where most Antarctic toothfish are caught, is very contentious owing to the lack of accurate population parameters, such as original stock size, fecundity, and recruitment. Moreover, the main fishing grounds cover the area through which the entire stock of Antarctic toothfish passes, adding further challenge to management owing to the indiscriminate nature of benthic longline fishing. The bycatch of other fish can also be substantial, with the ratio to tonnes of toothfish caught ranging from 5–35%. CCAMLR is allowing the original, hypothetical spawning biomass to be reduced by 50%, which is done by targeting the oldest, largest, and potentially most fecund fish in the stock. In just 15 years, the average age of fish has decreased about 10 years.
Consistent with this loss of large fish, an independent study has detected the disappearance of large fish at the southern periphery of its range. In accord with decreasing number and size of fish, other studies have shown that the prevalence of fish-eating killer whales has been decreasing in the southern Ross Sea, foraging efficiency of Weddell seals is decreasing, and numbers of Adélie penguins (competitors for silverfish) have been increasing. These trends indicate the problem of managing this fishery in the best interests of the ecosystem in the face of a paucity of life history information.
The total catch of Antarctic toothfish in 2011/12 was around 3,800 tonnes. Over 3,500 tonnes of this was taken from the Ross Sea (FAO Statistical Divisions 88.1 and 88.2) with the remainder taken from other high seas areas within the CCAMLR convention area.
Environment and bycatch
CCAMLR imposes stringent environmental protection and bycatch mitigation measures to Antarctic toothfish fisheries, including:
- Monitoring of daytime setting and movement of vessels from the fishery should any vessel catch more than three seabirds 
- Use of streamer lines during setting to keep birds away from baited hooks 
- Weighting of lines to ensure fast sink rates to prevent seabirds from accessing baited hooks 
- The use of bird exclusion devices to prevent birds from accessing hooks whilst lines are being hauled 
- Limitations on the release of fish offal overboard at the same time as setting and hauling of lines to avoid attracting seabirds. An additional requirement prohibits the dumping of all offal south of 60°S, the region where Antarctic toothfish are caught 
- Prohibition on the dumping of oil, plastic, garbage, food waste, poultry, eggs or eggshells, sewage, and ash by fishing vessels 
- Prohibition of the use of plastic packaging bands on fishing vessels 
Incidental mortality of seabirds as a result of fishing has fallen to near-zero levels in the CCAMLR convention area. No mortality of seabirds or marine mammals was recorded as a result of fishing for Antarctic toothfish in 2011/12 and only one seabird has been killed as a result of fishing in the Ross Sea since 1996/97.
Bycatch of fish species is poorly managed in this fishery. For example, mitigation for bycatch of grenadiers (Macrourus spp.), a major prey item for toothfish, is unsatisfactory, with the take and likely abundance of these fish having decreased dramatically since initiation of the toothfish fishery. Similarly, moray cod (Muraenolepis evseenkoi) are caught as bycatch by the toothfish fleet at levels that may be leading to its extirpation.
Compliance measures adopted by CCAMLR apply to all Antarctic toothfish fisheries. These include:
- At-sea inspections of fishing vessels 
- Vessel licensing 
- Port inspections of fishing vessels
- Continuous reporting of fishing vessel positions via satellite linked Vessel Monitoring Systems (VMS)
- Catch Documentation Scheme for Toothfish which tracks toothfish from the point of landing through to the final point of sale and requires verification and authorisation by government authorities at each step
In November 2010, the Marine Stewardship Council certified the Ross Sea Antarctic toothfish fishery as a sustainable and well-managed fishery. This, too, is contentious, with many conservation groups protesting the certification due to the paucity of information needed to reliably manage the fishery, and the fact that only six of about 15 vessels in the fishery are certified. Moreover, MSC-certified Antarctic toothfish comprised only 1% of total toothfish caught in the Southern Ocean in 2010.
The small portion of Antarctic toothfish certified, the high price it commands (sold under its commercial name of Chilean sea bass, it fetches at least US$25 per pound at retail prices) and the remote areas where a large proportion of the fish are caught, encourages illegal, unreported and unregulated (IUU) fishing and mislabeling. A recent genetic study of MSC-labeled Antarctic toothfish found in markets revealed a significant proportion were not from the MSC-certified stock, and many were not even toothfish at all. However, the claims of this study were unable to be substantiated as the data were not made available for independent examination. The MSC had conducted an earlier study using more robust methodology which found no evidence of mislabelling. The MSC conducts an annual audit of the fishery which includes sampling of certified product.
Due to the intense challenges that faced toothfish management in the 1990s and early 2000s (e.g., IUU fishing, mislabelling, inadequate data for management), consumer seafood guides such as the Monterey Bay Seafood Watch placed toothfish (Chilean seabass) on their red, or “avoid”, list., however in light of up to date, internationally peer-reviewed scientific information, in April 2013, Seafood Watch upgraded the Ross Sea Antarctic toothfish fishery to a "good alternative".
Greenpeace International also added the Antarctic toothfish to its seafood red list in 2010.  This approach is at variance with the high score given the fishery when it was granted certification by the Marine Stewardship Council, though the high score itself is controversial.
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