Instruct: Given a question, retrieve relevant documents that can help answer the question.
Query: Can you eat a dusky shark?
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 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.
The dusky shark is a generalist that takes a wide variety of prey from all levels of the water column, though it favors hunting near the bottom.[23][38] A large individual can consume over a tenth of its body weight at a single sitting.[39] The bite force exerted by a 2m (6.6ft) long dusky shark has been measured at 60kg (130lb) over the 2mm2 (0.0031in2) area at the tip of a tooth. This is the highest figure thus far measured from any shark, though it also reflects the concentration of force at the tooth tip.[40] Dense aggregations of young sharks, forming in response to feeding opportunities, have been documented in the Indian Ocean.[1]
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.
Many sharks will outgrow most home aquariums and/or adapt poorly to captivity. However, numerous coastal and coral reef sharks do well in good aquarium surroundings although you should have experience in keeping other saltwater fish before trying to keep sharks as they are more difficult to care for. In a shark aquarium setup (preferably an oval shaped tank for more active species), there should be lots of surface area (wide and long tanks with good gas exchange/more room for biological filtration and room for sharks to swim, glide, and turn with little constraint opposed to tall, thin tanks), fine substrate (coarse substrate can irritate the shark's underside), little décor and rockwork (which should be secure) for swimming space (sharks in the orders Orectolobiformes and Heterodontiformes however, feel more secure in tanks with caves and ledges), excellent filtration (sharks are messy eaters and need good water conditions), protected heaters, filter intakes, etc. by surrounding them in polyurethane foam barriers (unprotected equipment can be dangerous to active sharks), and a secure canopy (sharks can jump out of the water) as well as, strong, steady, linear water flow (10+ x the volume of the aquarium per hour) moving in a gyre circling the aquarium, no dead spots in the circulation of the tank, dissolved oxygen levels of 7-8ppm (slightly more if you are using ozone), a salinity of 1.025-1.026 specific gravity, low light levels, and no stray electrical currents/amounts of metal in the aquarium water. Many sharks feed on invertebrates to a great degree along with fish (even ones that are larger than themselves), and although they don't eat coral, they can knock them over and rest on them. There are also many fish and invertebrates that can harm/irritate sharks such as Scorpionfish, Butterflyfish, Angelfish (large), Filefish, Triggerfish, Pufferfish, Suckerfish (over time), Porcupinefish, certain other Sharks, large Crabs, Hermit crabs, sea anemones, and stinging corals and Eels and Frogfish can be threats to juvenile sharks. Also, many sharks are sensitive to copper, sharks need iodine which can be provided through regular water changes or supplements for sharks (iodine deficiencies and possibly the buildup of nitrates can result in goiter), and feeding frequency is species-specific.
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.
Some large shark species, such as the tiger shark, the dusky shark, the great white shark and the bull shark, prey on the bottlenose dolphin, especially calves. The bottlenose dolphin is capable of defending itself by charging the predator; dolphin 'mobbing' behavior of sharks can occasionally prove fatal for the shark. Targeting a single adult dolphin can be dangerous for a shark of similar size. Killer whale populations in New Zealand and Peru have been observed preying on bottlenose dolphins, but this seems rare, and other orcas may swim with dolphins. Swimming in pods allows dolphins to better defend themselves against predators. Bottlenose dolphins either use complex evasive strategies to outswim their predators, or mobbing techniques to batter the predator to death or force it to flee.
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 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]
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]
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]
Sharks are opportunistic feeders, and the bull shark is no exception to this, as it is part of the "Carcharhinus" family of sharks. Normally, sharks eat in short bursts, and when food is scarce, sharks digest for a much longer period of time in order to avoid starvation. As part of their survival mechanism, bull sharks will regurgitate the food in their stomachs in order to escape from a predator. This is a distraction tactic; if the predator moves to eat the regurgitated food the bull shark can use the opportunity to escape.
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]
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.
Many early sources gave the scientific name of the dusky shark as Carcharias (later Carcharhinus) lamiella, which originated from an 1882 account by David Starr Jordan and Charles Henry Gilbert. Although Jordan and Gilbert referred to a set of jaws that came from a dusky shark, the type specimen they designated was later discovered to be a copper shark (C. brachyurus). Therefore, C. lamiella is not considered a synonym of C. obscurus but rather of C. brachyurus.[4][6] Other common names for this species include bay shark, black whaler, brown common gray shark, brown dusky shark, brown shark, common whaler, dusky ground shark, dusky whaler, river whaler, shovelnose, and slender whaler shark.[7]
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]
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]
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.
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]
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.
The diet of the bird can be varied. They eat various forms of foliage and other grassy material that they find on the ground on in trees and shrubs. Dusky woodswallows have been seen eating termites, butterflies and other insects. They also eat nectar from flowers. One notable aspect of their feeding habits is the way they hunt flying insects, which is done by picking them up on their wing. They do, however, also eat their prey from the ground, and they often find inconspicuous places to perch while waiting for prey, such as utility lines and the like. They have also been observed engaging in kleptoparasitism, working as a group to rob a restless flycatcher of its prey. Kleptoparasitism is extremely unusual in passerine birds, as is cooperative kleptoparasitsim in general.
The New Zealand Department of Conservation has classified the dusky shark as "Migrant" with the qualifier "Secure Overseas" under the New Zealand Threat Classification System.[57]
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]
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.