Antarctic toothfish

The Antarctic toothfish (Dissostichus mawsoni) is a species of notothen 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 stichus (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 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 to more than 1.7 m (5 ft 7 in) 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 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 can elude predatory seals. 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.). 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. 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. 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 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. 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 subzero 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 1997. The existence of this fishery in the Ross Sea, the area where most Antarctic toothfish are caught, is very contentious - the main argument proposed for this is the lack of accurate population parameters, such as original stock size, fecundity, and recruitment. Moreover, the main fishing grounds are presumed by some researchers to cover the area through which the entire stock of Antarctic toothfish pass. Typically, the fishing season has finished in the area by the end of February and for the remainder of the year, much of the area is covered by sea ice, providing a natural impediment to fishing. This fishery is characterised by opponents as being a challenge to manage owing to the nature of benthic longline fishing. The bycatch of other fish can also be significant, with the ratio of toothfish caught ranging from 4.5% to 17.9% and averaging 9.3% from the 1999/2000 fishing season to 2013/14 in CCAMLR Subarea 88.1 when the toothfish catch first exceeded 50 tonnes and from 2.3% to 24.5% averaging 12.4% in CCAMLR Subarea 88.2 up to the latest publicly available figure from 2013/14. The bycatch of other fish species is also regulated to a maximum amount annually by CCAMLR. CCAMLR decision rules are based on determining the catch level that will ensure that the median estimated spawning stock biomass (not total biomass) is greater than or equal to 50% of the average pre-exploitation spawning biomass after a further 35 years of fishing (i.e. 35 years from each year of assessment), with the additional condition that the probability is less than a 10% that the spawning biomass will decline below 20% of the pre-exploitation level at any time during this period. Current spawning stock biomass for Antarctic toothfish in the Ross Sea Region is estimated to be at 75% of the pre-exploitation level (95% Bayesian probability interval 71–78%), well above the 50% target reference point. An independent study was reported to have detected the disappearance of large fish at the southern periphery of its range in the McMurdo Sound area and was postulated to be consistent with this apparent loss of large fish. However, more recent work has shown this was not the case in 2014. Some studies have reported that the prevalence of fish-eating killer whales has been apparently decreasing in the southern Ross Sea, foraging efficiency of Weddell seals is decreasing, and numbers of Adélie penguins (competitors for silverfish) have been increasing. More recent studies have confirmed visual sightings of Weddell seals and Type-C killer whales holding and consuming large toothfish in the McMurdo Sound area and raise questions over the previously assumed importance of assumed dominance of Antarctic silverfish (Pleuragramma antarcticum) in the diet of Weddell seal and Type-C killer whales. These reports highlight the importance of managing this fishery in the best interests of the ecosystem by continuing to collect information on both Antarctic toothfish life history and the interaction of that species with predators and prey. An important research programme in this regard is the annual 'Shelf' survey carried out annually since 2012, which is designed to monitor the abundance of subadult Antarctic toothfish in areas where subadult-sized fish have been regularly found (e.g., in the southern Ross Sea) has been designed provide data to better estimate recruitment variability and provide an important early-warning signal of changes in toothfish recruitment. The project also is used for additional targeted data collection to better understand the lifecycle and ecosystem role of Antarctic toothfish.