Instruct: Given a question, retrieve passages that 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 (1,300ft). 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 4.2m (14ft) in length and 347kg (765lb) 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.
When hunting in shallower waters in New Zealand and Argentina, dusky dolphins tend to forage during the day. The dolphins chase schools of fish or squid and herd them into stationary balls. They may control the school with light reflected from their white bellies. Dolphins herd prey against the surface, but also horizontally against the shore, a point of land, or the hull of a boat. During these times, dusky dolphins are believed to increase prey availability for other predators, including other dolphins, seabirds, sharks, and pinnipeds. In Argentina, dusky dolphins may use bird aggregations to locate and herd prey. Conversely, pinnipeds and sharks take advantage of the dolphin hunts. Dusky dolphins are themselves preyed on by killer whales and large sharks. Dolphins avoid killer whales by swimming into shallower water. Dusky dolphins are also susceptible to parasitism by certain nematode, cestode, and trematode species, mostly the genera "Nasitrema" and "Anisakis", and "Phyllobothrium delphini", "Braunina cordiformis", and "Pholeter gasterophilus".
In 1982, Jack Garrick published a phylogenetic analysis of Carcharhinus based on morphology, in which he placed the dusky shark and the Galapagos shark (C. galapagensis) at the center of the "obscurus group". The group consisted of large, triangular-toothed sharks with a ridge between the dorsal fins, and also included the bignose shark (C. altimus), the Caribbean reef shark (C. perezi), the sandbar shark (C. plumbeus), and the oceanic whitetip shark.[15] This interpretation was largely upheld by Leonard Compagno in his 1988 phenetic study,[16] and by Gavin Naylor in his 1992 allozyme sequence study. Naylor was able to further resolve the interrelationships of the "ridge-backed" branch of Carcharhinus, finding that the dusky shark, Galapagos shark, oceanic whitetip shark, and blue shark (Prionace glauca) comprise its most derived clade.[8]
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.
Because of its slow reproductive rate, the dusky shark is very vulnerable to human-caused population depletion. This species is highly valued by commercial fisheries for its fins, used in shark fin soup, and for its meat, skin, and liver oil. It is also esteemed by recreational fishers. The International Union for Conservation of Nature (IUCN) has assessed this species as Near Threatened worldwide and Vulnerable off the eastern United States, where populations have dropped to 15–20% of 1970s levels. The dusky shark is regarded as potentially dangerous to humans due to its large size, but there are few attacks attributable to it.
The known diet of the dusky shark encompasses pelagic fishes, including herring and anchovies, tuna and mackerel, billfish, jacks, needlefish and flyingfish, threadfins, hairtails, lancetfish, and lanternfish; demersal fishes, including mullets, porgies, grunts, and flatheads, eels, lizardfish, cusk eels, gurnards, and flatfish; reef fishes, including barracudas, goatfish, spadefish, groupers, scorpionfish, and porcupinefish; cartilaginous fishes, including dogfish, sawsharks, angel sharks, catsharks, thresher sharks, smoothhounds, smaller requiem sharks, sawfish, guitarfish, skates, stingrays, and butterfly rays; and invertebrates, including cephalopods, decapod crustaceans, barnacles, and sea stars. Very rarely, the largest dusky sharks may also consume sea turtles, marine mammals (mainly as carrion), and human refuse.[4][5][38][41]
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).[51] 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.[7] 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 2m (6.6ft) long is under 4%.[51]
How Big Can You Get?: The Music of Cab Calloway is the sixth studio album released by Big Bad Voodoo Daddy.
Adult dusky sharks have a broad and varied diet, consisting mostly of bony fishes, sharks and rays, and cephalopods, but also occasionally crustaceans, sea stars, bryozoans, sea turtles, marine mammals, carrion, and garbage. This species is viviparous with a three-year reproductive cycle; females bear litters of 3–14 young after a gestation period of 22–24 months, after which there is a year of rest before they become pregnant again. Females are capable of storing sperm for long periods, as their encounters with suitable mates may be few and far between due to their nomadic lifestyle and low overall abundance. Dusky sharks are one of the slowest-growing and latest-maturing sharks, not reaching adulthood until around 20 years of age.
Full-grown dusky sharks have no significant natural predators.[20] Major predators of young sharks include the ragged tooth shark (Carcharias taurus), the great white shark (Carcharodon carcharias), the bull shark (C. leucas), and the tiger shark (Galeocerdo cuvier). Off KwaZulu-Natal, the use of shark nets to protect beaches has reduced the populations of these large predators, leading to a dramatic increase in the number of juvenile dusky sharks (a phenomenon called "predator release"). In turn, the juvenile sharks have decimated populations of small bony fishes, with negative consequences for the biodiversity of the local ecosystem.[5][37]
Residing off continental coastlines from the surf zone to the outer continental shelf and adjacent oceanic waters, the dusky shark occupies an intermediate habitat that overlaps with its more specialized relatives, such as the inshore sandbar shark, the pelagic silky shark (C. falciformis) and oceanic whitetip shark, the deepwater bignose shark, and the islandic Galapagos shark and silvertip shark (C. albimarginatus).[5] One tracking study in the northern Gulf of Mexico found that it spends most of its time at depths of 10–80m (33–262ft), while making occasional forays below 200m (660ft); this species has been known to dive as deep as 400m (1,300ft). It prefers water temperatures of 19–28°C (66–82°F), and avoids areas of low salinity such as estuaries.[4][18]
Newborn dusky sharks measure 0.7–1.0m (2.3–3.3ft) long;[1] 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.[39] The dusky shark is one of the slowest-growing shark species, reaching sexual maturity only at a substantial size and age (see table).[51] Various studies have found growth rates to be largely similar across geographical regions and between sexes.[46][48][49] The annual growth rate is 8–11cm (3.1–4.3in) over the first five years of life.[52] The maximum lifespan is believed to be 40–50 years or more.[51]
Garrick (1982) placed the Galapagos shark and the dusky shark at the center of the "obscurus group", one of two major groupings within "Carcharhinus". The group consisted of the bignose shark ("C. altimus"), Caribbean reef shark ("C. perezi"), sandbar shark ("C. plumbeus"), dusky shark ("C. obscurus"), and oceanic whitetip shark ("C. longimanus"), all large, triangular-toothed sharks and is defined by the presence of a ridge between the two dorsal fins. Based on allozyme data, Naylor (1992) reaffirmed the integrity of this group, with the additions of the silky shark ("C. falciformis") and the blue shark ("Prionace glauca"). The closest relatives of the Galapagos shark were found to be the dusky, oceanic whitetip, and blue sharks.
The International Union for Conservation of Nature (IUCN) has assessed this species as Near Threatened worldwide and Vulnerable in the northwestern Atlantic and the Gulf of Mexico. The American Fisheries Society has also assessed North American dusky shark populations as Vulnerable.[20] Its very low reproductive rate renders the dusky shark extremely susceptible to overfishing.
Carcharhinus altimus Carcharhinus plumbeus Carcharhinus falciformis Carcharhinus perezi Carcharhinus galapagensis Carcharhinus obscurus Carcharhinus longimanus Prionace glaucaPhylogenetic relationships of the dusky shark, based on allozyme sequences.[8]
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.[5] 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.[53] However, attacks attributed to this species off Bermuda and other islands were probably in reality caused by Galapagos sharks.[5] 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.[51] Young dusky sharks adapt well to display in public aquariums.[5]
In addition to commercial shark fisheries, dusky sharks are also caught as bycatch on longlines meant for tuna and swordfish (and usually kept for its valuable fins), and by recreational fishers. Large numbers of dusky sharks, mostly juveniles, are caught by sport fishers off South Africa and eastern Australia. This shark was once one of the most important species in the Florida trophy shark tournaments, before the population collapsed.[51]
The first dorsal fin is tall and moderately falcate (sickle-shaped), with the origin over the pectoral fin rear tips. It is followed by a low midline ridge running to the second dorsal fin. The second dorsal fin originates over the anal fin. The pectoral fins are large with pointed tips. The coloration is brownish gray above and white below, with a faint white stripe on the sides. The edges of the fins are darker but not prominently marked. The Galapagos shark can be distinguished from the dusky shark in having taller first and second dorsal fins and larger teeth, and it can be distinguished from the grey reef shark in having a less robust body and less pointed first dorsal fin tip. However, these characters can be difficult to discern in the field. These similar species also have different numbers of precaudal (before the tail) vertebrae: 58 in the Galapagos shark, 86–97 in the dusky shark, 110–119 in the grey reef shark.
Teeth belonging to the dusky shark are fairly well represented in the fossil record, though assigning Carcharhinus teeth to species can be problematic.[9] Dusky shark teeth dating to the Miocene (23-5.3 Ma) have been recovered from the Kendeace and Grand Bay formations in Carriacou, the Grenadines,[10] the Moghra Formation in Egypt,[11] Polk County, Florida,[12] and possibly Cerro La Cruz in northern Venezuela.[13] Teeth dating to the Late Miocene or Early Pliocene (11.6-3.6 Ma) are abundant in the Yorktown Formation and the Pungo River, North Carolina, and from the Chesapeake Bay region; these teeth differ slightly from the modern dusky shark, and have often been misidentified as belonging to the oceanic whitetip shark (C. longimanus).[9] Dusky shark teeth have also been recovered from the vicinity of two baleen whales in North Carolina, one preserved in Goose Creek Limestone dating to the Late Pliocene (c. 3.5 Ma), and the other in mud dating to the Pleistocene-Holocene (c. 12,000 years ago).[14]
The dusky shark is nomadic and strongly migratory, undertaking recorded movements of up to 3,800km (2,400mi); adults generally move longer distances than juveniles. Sharks along both coasts of North America shift northward with warmer summer temperatures, and retreat back towards the equator in winter.[1] Off South Africa, young males and females over 0.9m (3.0ft) long disperse southward and northward respectively (with some overlap) from the nursery area off KwaZulu-Natal; they join the adults several years later by a yet-unidentified route. In addition, juveniles spend spring and summer in the surf zone and fall and winter in offshore waters, and as they approach 2.2m (7.2ft) in length begin to conduct a north-south migration between KwaZulu-Natal in the winter and the Western Cape in summer. Still-larger sharks, over 2.8m (9.2ft) long, migrate as far as southern Mozambique.[1][5][19] Off Western Australia, adult and juvenile dusky sharks migrate towards the coast in summer and fall, though not to the inshore nurseries occupied by newborns.[1]
As an apex predator positioned at the highest level of the trophic web, the dusky shark is generally less abundant than other sharks that share its range.[5] However, high concentrations of individuals, especially juveniles, can be found at particular locations.[4] Adults are often found following ships far from land, such as in the Agulhas Current.[19] A tracking study off the mouth of the Cape Fear River in North Carolina reported an average swimming speed of 0.8km/h (0.50mph).[24] The dusky shark is one of the hosts of the sharksucker (Echeneis naucrates).[25] Known parasites of this species include the tapeworms Anthobothrium laciniatum,[26] Dasyrhynchus pacificus,[27] Platybothrium kirstenae,[28] Floriceps saccatus,[29] Tentacularia coryphaenae,[30] and Triloculatum triloculatum,[31] the monogeneans Dermophthirius carcharhini[32] and Loimos salpinggoides,[33] the leech Stibarobdella macrothela,[34] the copepods Alebion sp., Pandarus cranchii,[35] P. sinuatus,[20] and P. smithii, the praniza larvae of gnathiid isopods,[35] and the sea lamprey (Petromyzon marinus).[36]
The Caribbean reef shark ("Carcharhinus perezii") is a species of requiem shark, belonging to the family Carcharhinidae. It is found in the tropical waters of the western Atlantic Ocean from Florida to Brazil, and is the most commonly encountered reef shark in the Caribbean Sea. With a robust, streamlined body typical of the requiem sharks, this species is difficult to tell apart from other large members of its family such as the dusky shark ("C. obscurus") and the silky shark ("C. falciformis"). Distinguishing characteristics include dusky-colored fins without prominent markings, a short free rear tip on the second dorsal fin, and tooth shape and number.
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).[54] 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.[55] 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.[56]
The range of the dusky shark extends worldwide, albeit discontinuously, in tropical and warm-temperate waters. In the western Atlantic Ocean, it is found from Massachusetts and the Georges Bank to southern Brazil, including the Bahamas and Cuba. In the eastern Atlantic Ocean, it has been reported from the western and central Mediterranean Sea, the Canary Islands, Cape Verde, Senegal, Sierra Leone, and possibly elsewhere including Portugal, Spain, Morocco, and Madeira. In the Indian Ocean, it is found off South Africa, Mozambique, and Madagascar, with sporadic records in the Arabian Sea, the Bay of Bengal, and perhaps the Red Sea. In the Pacific Ocean, it occurs off Japan, mainland China and Taiwan, Vietnam, Australia, and New Caledonia in the west, and from southern California to the Gulf of California, around Revillagigedo, and possibly off northern Chile in the east. Records of dusky sharks from the northeastern and eastern central Atlantic, and around tropical islands, may in fact be of Galapagos sharks.[1][2] Mitochondrial DNA and microsatellite evidence suggest that Indonesian and Australian sharks represent distinct populations.[17]
For most of the smaller species of dolphins, only a few of the larger sharks, such as the bull shark, dusky shark, tiger shark, and great white shark, are a potential risk, especially for calves. Dolphins can tolerate and recover from extreme injuries (including shark bites) although the exact methods used to achieve this are not known. The healing process is rapid and even very deep wounds do not cause dolphins to hemorrhage to death. Even gaping wounds restore in such a way that the animal's body shape is restored, and infection of such large wounds are rare.