Antarctic toothfish

The Antarctic toothfish (''Dissostichus mawsoni'') is a large, black or brown fish found in very cold (subzero) waters of the Southern Ocean near Antarctica. It is the largest fish in the Southern Ocean, feeding on shrimp and smaller fish, and preyed on by whales, orcas, 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 belongs to the notothen family (Nototheniidae), a group of fish species abundant near Antarctica.

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 by 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, these fish (and their warmer-water relative, the Patagonian toothfish, ''D. eleginoides'') can grow to more than 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.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 can 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.). 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. 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.Hanchet, S.M. (2010) Updated species profile for Antarctic toothfish (''Dissostichus mawson''i). 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. 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. 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

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

HOME


Content is Copyleft
Website design, code, and AI is Copyrighted (c) 2014-2017 by Stephen Payne


Consider donating to Wikimedia



As an Amazon Associate I earn from qualifying purchases