Instruct: Given a question, retrieve passages that can help answer the question.
Query: How big can a dusky shark get?
The dusky shark ("Carcharhinus obscurus") is a species of requiem shark, in the family Carcharhinidae, occurring in tropical and warm-temperate continental seas worldwide. A generalist apex predator, the dusky shark can be found from the coast to the outer continental shelf and adjacent pelagic waters, and has been recorded from a depth of 400 m (1,300 ft). Populations migrate seasonally towards the poles in the summer and towards the equator in the winter, traveling hundreds to thousands of kilometers. One of the largest members of its genus, the dusky shark reaches 4.2 m (14 ft) in length and 347 kg (765 lb) in weight. It has a slender, streamlined body and can be identified by its short round snout, long sickle-shaped pectoral fins, ridge between the first and second dorsal fins, and faintly marked fins.
Swells dropped to 1–1.5 m on 28 June 2011, allowing more netting of sardines. Sardines were netted at Amanzimtoti; on the main beach and at Chain Rocks. A 22-year-old American marine biology student (research diver) named Paulo Edward Stanchi was attacked by a large dusky shark while diving at Aliwal Shoal Marine Protected Area. The group of divers had encountered a pocket of sardines when a 3 m long dusky shark bit Mr Stanchi on his left leg and hands. Mr Stanchi managed to free himself from the shark, and was treated on the diving boat before being transported to Rocky Bay, where medics stabilised him. He was then airlifted to Nkosi Albert Luthuli Hospital, where he underwent surgery. Dusky sharks generally live offshore, but come closer to the shore during the sardine run. The annual sardine run allowed more dusky sharks in the Aliwal Shoal MPA than usual, but there was no reason for them to show any more interest in divers than usual. Mr Stanchi had been wearing split fins with black and grey stripes, and this may have looked like a small shoal of fish to the shark. Meanwhile, a woman in her 40s broke her leg in the frenzy at Amanzimtoti when the sardines were netted. The woman is believed to have been trying to get some of the sardines when she "stepped wrong" and fractured her leg. Paramedics stabilized her before transporting her to hospital.
Stocks off the eastern United States are severely overfished; a 2006 stock assessment survey by the U.S. National Marine Fisheries Service (NMFS) showed that its population had dropped to 15–20% of 1970s levels. In 1997, the dusky shark was identified as a Species of Concern by the NMFS, meaning that it warranted conservation concern but there was insufficient information for listing on the U.S. Endangered Species Act (ESA). Commercial and recreational retention of dusky sharks was prohibited in 1998, but this has been of limited effectiveness due to high bycatch mortality on multi-species gear. In addition, some 2,000 dusky sharks were caught by recreational fishers in 2003 despite the ban. In 2005, North Carolina implemented a time/area closure to reduce the impact of recreational fishing. To aid conservation efforts, molecular techniques using polymerase chain reaction (PCR) have been developed that can identify whether marketed shark parts (e.g. fins) are from prohibited species like the dusky shark, versus similar allowed species such as the sandbar shark.
The great white shark has a robust, large, conical snout. The upper and lower lobes on the tail fin are approximately the same size which is similar to some mackerel sharks. A great white displays countershading, by having a white underside and a grey dorsal area (sometimes in a brown or blue shade) that gives an overall mottled appearance. The coloration makes it difficult for prey to spot the shark because it breaks up the shark's outline when seen from the side. From above, the darker shade blends with the sea and from below it exposes a minimal silhouette against the sunlight. Leucism is extremely rare in this species, but has been documented in one great white shark (a pup that washed ashore in Australia and died). Great white sharks, like many other sharks, have rows of serrated teeth behind the main ones, ready to replace any that break off. When the shark bites, it shakes its head side-to-side, helping the teeth saw off large chunks of flesh. Great white sharks, like other mackerel sharks, have larger eyes than other shark species in proportion to their body size. The iris of the eye is a deep blue instead of black.
The Bahamas sawsharks ("Pristiophorus schroeder") have very little information on them. Studies are being done daily to learn more about the deep sea dweller. They are located near Cuba, Florida, and the Bahamas (hence their name) where they dwell in the depths of 400–1000 m. As far as their appearance they can be identified by their snouts with teeth which appear as a saw, as well as their length, they are averaged at 80 cm in length.
Saw sharks and sawfish are cartilaginous fish possessing large saws. These are the only two fish that have a long blade-like snout. Although they are similar in appearances, saw sharks are distinct from sawfish. Sawfish are not sharks, but a type of ray. The gill slits of the sawfishes are positioned on the underside like a ray, but the gill slits of the saw shark are positioned on the side like a shark. Sawfish can have a much larger size, lack barbels, and have evenly sized teeth rather than alternating teeth of the saw shark. Clear difference is that a sawfish has no barbels and a saw shark has a prominent pair halfway along the saw. The saw shark uses these like other bottom fish, as a kind of antennae, feeling the way along the ocean bottom until it finds some prey of interest. Both the saw shark and the sawfish utilize the electroreceptors on the saw, ampullae of Lorenzini, to detect the electric field given off by buried prey.
The Caribbean roughshark ("Oxynotus caribbaeus") is a rough shark of the family Oxynotidae, found on the upper continental slopes of the Caribbean Sea, at depths between . It reaches a length around .
Juvenile Caribbean reef sharks are preyed upon by larger sharks such as the tiger shark ("Galeocerdo cuvier") and the bull shark ("C. leucas"). Few parasites are known for this species; one is a dark variegated leech often seen trailing from its first dorsal fin. Off northern Brazil, juveniles seek out cleaning stations occupied by yellownose gobies ("Elacatinus randalli"), which clean the sharks of parasites while they lie still on the bottom. Horse-eye jacks ("Caranx latus") and bar jacks ("Carangoides ruber") routinely school around Caribbean reef sharks.
The heaviest known cephalopod, and the largest living invertebrate, is the colossal squid. The largest recorded specimen of this species, caught in the Ross Sea in 2007, weighed . However, its beak is not the largest known from this species; even bigger colossal squid beaks have been recovered from the stomachs of sperm whales, indicating that this species can grow larger still.
Sea urchins are established in most seabed habitats from the intertidal downwards, at an extremely wide range of depths. Some species, such as "Cidaris abyssicola", can live at depths of several thousands metres. Many genera are only found in the abyssal zone, including many cidaroids, most of the genera in the Echinothuriidae family, and the "cactus urchins" "Dermechinus". One of the deepest-living families is the "Pourtalesiidae", strange bottle-shaped irregular sea urchins that live only in the hadal zone and have been collected as deep as 6850 metres beneath the surface in the Sunda Trench. Nevertheless, this makes sea urchin the class of echinoderms living the least deep, compared to brittle stars, starfish and crinoids that remain abundant below and sea cucumbers which have been recorded from .
Ommastrephinae includes the largest species of squids belonging to the family Ommastrephidae, Humboldt squid ("Dosidicus gigas") which can grow to in mantle length (ML). It also contains the smallest squid species belonging to the family, the glass squid ("Hyaloteuthis pelagica") which has a mantle length of only up to . Ommastrephinae are mostly pelagic members of the family Ommastrephidae. All species of this subfamily contain photophores whose locations vary by species. Some species of the subfamily (notably "Sthenoteuthis" and "Ommastrephes") are known for their behavior of leaping out of the water (hence the common names 'flying squid').
Sawfishes, also known as carpenter sharks, are a family of rays characterized by a long, narrow, flattened rostrum, or nose extension, lined with sharp transverse teeth, arranged in a way that resembles a saw. They are among the largest fish with some species reaching lengths of about . They are found worldwide in tropical and subtropical regions in coastal marine and brackish estuarine waters, as well as freshwater rivers and lakes.
Marine biota in the region have representatives of both the Indian and Pacific oceans which were caught in the Caribbean before the emergence of the Isthmus of Panama four million years ago. In the Caribbean Sea there are around 1,000 documented species of fish, including sharks (bull shark, tiger shark, silky shark and Caribbean reef shark), flying fish, giant oceanic manta ray, angel fish, spotfin butterflyfish, parrotfish, Atlantic Goliath grouper, tarpon and moray eels. Throughout the Caribbean there is industrial catching of lobster and sardines (off the coast of Yucatán Peninsula).
The blacknose shark inhabits the continental and insular shelves off the eastern coast of the Americas, as far north as North Carolina and as far south as southern Brazil, including the Bahamas, the Gulf of Mexico, and the Caribbean Sea. They frequent coastal waters over beds of seagrass, sandy flats, and shell or coral rubble. This species is spatially segregated by size and sex. Generally, only young sharks are encountered in shallow water, as the adults prefer depths greater than and is most common at . Blacknose sharks in the South Atlantic Bight (off the Atlantic coast of the southern United States) migrate northward in the summer and southward (or possibly offshore) in the winter; a similar migration occurs for sharks in the Gulf of Mexico.
At present, hang baits are illegal off Isla Guadalupe and reputable dive operators do not use them. Operators in South Africa and Australia continue to use hang baits and pinniped decoys. In South Australia, playing rock music recordings underwater, including the AC/DC album "Back in Black" has also been used experimentally to attract sharks.
Greenland sharks migrate annually based on depth and temperature rather than distance, although some do travel. During the winter, the sharks congregate in the shallows (up to 80° north) for warmth but migrate separately in summer to the deeps or even farther south. The species has been observed at a depth of by a submersible investigating the wreck of the SS "Central America" that lies about east of Cape Hatteras, North Carolina. Daily vertical migration between shallower and deeper waters has also been recorded.
Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon) to the abyssal and even hadal depths of the deepest oceans (e.g., gulpers and anglerfish), although no species has yet been documented in the deepest 25% of the ocean. With 33,600 described species, fish exhibit greater species diversity than any other group of vertebrates.
The largest recorded specimen was captured in 2007 by a New Zealand fishing boat off Antarctica. It was initially estimated to measure in length and weigh . The squid was taken back to New Zealand for scientific study. A study on the specimen later showed its actual weight was , but it only measured in total length as a result of the tentacles' shrinking "post mortem".
The Yangtze giant softshell turtle is noted for its deep head with a pig-like snout and dorsally placed eyes. This critically endangered species holds the title of being the largest freshwater turtle in the world. Although hard to compare due to the extremely small number of remaining specimens, its average and maximum sizes seem to exceed other large freshwater turtles such as alligator snapping turtles and related giant softshells such as other "Rafetus" and "Pelochelys" species. It measures over in overall length and to in width, and can easily weigh up to . The average size of large turtle specimens (i.e. turtles weighing over ) that could be collected in the Yangtze River per one study was , although not all specimens were definitively identified as "Rafetus". Its carapace, or shell, can grow larger than , with the largest shell that could be found having measured in length. However, along the curve carapace lengths of up to have been reported and it is possible for the largest specimens (largely reported in Vietnam) to have weighed up . Its head can measure over in length and in width. The male is generally smaller than the female and has a longer, larger tail.
Leatherback turtles are one of the deepest-diving marine animals. Individuals have been recorded diving to depths as great as .
Typical dive durations are between 3 and 8 minutes, with dives of 30–70 minutes occurring infrequently.
Sawfish are mostly found in relatively shallow waters, typically at depths less than , and occasionally less than . Young prefer very shallow places and are often found in water only deep. Sawfish can occur offshore, but are rare deeper than . An unidentified sawfish (either a largetooth or smalltooth sawfish) was captured off Central America at a depth in excess of .
The knifetooth sawfish ("Anoxypristis cuspidata"), also known as the pointed sawfish or narrow sawfish, is a species of sawfish in the family Pristidae, part of the Batoidea, a superorder of cartilaginous fish that includes the rays and skates. Sawfish display a circumglobal distribution in warm marine and freshwater habitats. Their extant biodiversity is limited to five species belonging to two genera (Pristis and Anoxypristis). The sawfishes are characterised by the long, narrow, flattened rostrum or extension on their snout. This is lined with sharp transverse teeth, arranged in a way that resembles the teeth of a saw. It is found in the shallow coastal waters and estuaries of the Indo-West Pacific, ranging from the Red Sea and the Persian Gulf to southern Japan, Papua New Guinea and northern Australia. It is the only living member of the genus Anoxypristis, but was previously included in the genus "Pristis". Compared to that genus, "Anoxypristis" has a narrower rostral saw with numerous teeth on the distal part and no teeth on the basal one-quarter (toothless section about one-sixth in juveniles). This endangered species reaches a length of up to .
Newborn dusky sharks measure long; pup size increases with female size, and decreases with litter size. There is evidence that females can determine the size at which their pups are born, so as to improve their chances of survival across better or worse environmental conditions. Females also provision their young with energy reserves, stored in a liver that comprises one-fifth of the pup's weight, which sustains the newborn until it learns to hunt for itself. The dusky shark is one of the slowest-growing shark species, reaching sexual maturity only at a substantial size and age (see table). Various studies have found growth rates to be largely similar across geographical regions and between sexes. The annual growth rate is over the first five years of life. The maximum lifespan is believed to be 40–50 years or more.
In the northwestern Atlantic, around 60% of the dusky shark's diet consists of bony fishes, from over ten families with bluefish ("Pomatomus saltatrix") and summer flounder ("Paralichthys dentatus") being especially important. Cartilaginous fishes, mainly skates and their egg cases, are the second-most important dietary component, while the lady crab ("Ovalipes ocellatus") is also a relatively significant food source. In South African and Australian waters, bony fishes are again the most important prey type. Newborn and juvenile sharks subsist mainly on small pelagic prey such as sardines and squid; older sharks over long broaden their diets to include larger bony and cartilaginous fishes. The run of the southern African pilchard ("Sardinops sagax"), occurring off the eastern coast of South Africa every winter, is attended by medium and large-sized dusky sharks. Pregnant and post-partum females do not join, possibly because the energy cost of gestation leaves them unable to pursue such swift prey. One South African study reported that 0.2% of the sharks examined had preyed upon bottlenose dolphins ("Tursiops truncatus").
One of the largest members of its genus, the dusky shark commonly reaches a length of and a weight of ; the maximum recorded length and weight are and respectively. Females grow larger than males. This shark has a slender, streamlined body with a broadly rounded snout no longer than the width of the mouth. The nostrils are preceded by barely developed flaps of skin. The medium-sized, circular eyes are equipped with nictitating membranes (protective third eyelids). The mouth has very short, subtle furrows at the corners and contains 13-15 (typically 14) tooth rows on either side of both jaws. The upper teeth are distinctively broad, triangular, and slightly oblique with strong, coarse serrations, while the lower teeth are narrower and upright, with finer serrations. The five pairs of gill slits are fairly long.
The dusky shark is considered to be potentially dangerous to humans because of its large size, though little is known of how it behaves towards people underwater. As of 2009, the International Shark Attack File lists it as responsible for six attacks on people and boats, three of them unprovoked and one fatal. However, attacks attributed to this species off Bermuda and other islands were probably in reality caused by Galapagos sharks. Shark nets used to protect beaches in South Africa and Australia entangle adult and larger juvenile dusky sharks in some numbers. From 1978 to 1999, an average of 256 individuals were caught annually in nets off KwaZulu-Natal; species-specific data is not available for nets off Australia. Young dusky sharks adapt well to display in public aquariums.
The dusky shark is one of the most sought-after species for shark fin trade, as its fins are large and contain a high number of internal rays (ceratotrichia). In addition, the meat is sold fresh, frozen, dried and salted, or smoked, the skin is made into leather, and the liver oil is processed for vitamins. Dusky sharks are taken by targeted commercial fisheries operating off eastern North America, southwestern Australia, and eastern South Africa using multi-species longlines and gillnets. The southwestern Australian fishery began in the 1940s and expanded in the 1970s to yield 500–600 tons per year. The fishery utilizes selective demersal gillnets that take almost exclusively young sharks under three years old, with 18–28% of all newborns captured in their first year. Demographic models suggest that the fishery is sustainable, provided that the mortality rate of sharks over 2 m (6.6 ft) long is under 4%.