C.+Obscurus



**CARCHARINUS OBSCURUS - C. OBSCURUS** (DUSKY SHARK)

__CLASSIFICATION __ Kingdom: Animalia Phylum: Chordata Class: Chondrichthyes Subclass: Elasmobranchii Order: Carcharhuiniformes Family: Carchafhinidae Genus: Carcharhinus Species: Carcharinus Obscurus



 __ANATOMY & PHYSIOLOGY__ The dusky shark has a long streamlined body that is brown or gray above and white below. There is a faint lateral stripe that extends to the pelvic fin. Juveniles have a dark coloration on the tip of the caudal fin and the underside of the pectoral fins that is less pronounced in adults. The first dorsal fin is much larger than the second with a dermal ridge found between the two dorsal fins. The upper lobe of the caudal fin is larger than the lower lobe. The pectoral fins are long and curved on the trailing edge. A large shark, C. obscurus can attain a length of about 400 cm (12.5 ft). Average size and weight are 320 cm (10 ft) and 160-180 kg (350-395 lbs), respectively. Males mature at about 280 cm (8.5 ft) and females the same or slightly larger. Size at birth ranges from 70-100 cm (33-39 in). Dusky sharks are very slow growing, mature at about 20 years and may live as long as 45 years. The upper teeth of the dusky shark are broad and triangular with a serrated edge, but while the front teeth are virtually erect, the others are slightly oblique. The lower teeth differ since they have narrow, erect cusps with more finely serrated edges than the upper teeth.

__HABITAT__ The dusky shark is found in subtropical waters between 43°N-43°S in the western Atlantic from southern Massachusetts to Florida in the USA and in the Bahamas, Cuba, northern Gulf of Mexico, Nicaragua, and southern Brazil. In the eastern Atlantic, the dusky shark is found in the Canary Islands, Cape Verde, Senegal, and Sierra Leone. They are also found in the Indo-West Pacific, Red Sea, Mozambique, South Africa, Japan, China, Vietnam and Australia. Juveniles are often found in shallower waters. It occurs on continental and insular shelves and the oceanic waters adjacent to them, ranging from close inshore in the surf zone to well out to sea and from the surface to 400 m depth. It avoids estuaries and other reduced salinity areas. Adult dusky sharks are often seen offshore and are known to follow ships.

World distribution map for the dusky shark

__INTERESTING FACT __ The strongest shark bite belongs to the dusky shark (Carcharhinus obscurus); its jaws have been measured to exert 132 pounds of force.

__FOOD CHAIN__ The diet of this shark includes small schooling fish such as sardines and anchovies, as well as larger fish like tuna, mackerel, flatfish and eels. The dusky shark also eats dogfish, catsharks, skates and rays. Diet includes a variety of reef, bottom, and pelagic bony fishes, along with other shark species, rays, crustaceans, cephalapods, barnacles, and whale meat. **Predators:** Young dusky sharks may fall prey to larger sharks including the bull shark. However, mature dusky sharks have few if any predators. **Parasites:** Pandarus sinuatus, a copepod, is parasitic on the body surface of the dusky shark.

__MIGRATION PATTERNS__ The dusky shark is a highly migratory species generally found in coastal and offshore waters at depths between 200-400 meters. They move north during the warmer summer months and retreating south when water temperatures drop.

__ POPULATION ESTIMATES __ No specific information is available on the exact population of the dusky shark. However, the population in the northwestern Atlantic and Gulf of Mexico is now probably at 15-20% of its mid-1970s abundance.

__REPRODUCTION__ In the western Atlantic, mating occurs in the spring. Due to the presence of two class sizes of young found in pregnant females off the coast of Florida, it is believed that females of this species only mate every second year. These different class sizes suggest alternating birth seasons every two years with a gestation period of about 8 months or a single season with a longer gestation period of about 16 months. As with other carcharhinids, developing embryos are nourished via a pseudo-placental sac, a reproductive strategy known as viviparity. In the western Atlantic, the number of young per liter ranges from 6-10 with an average of 8, whereas in the southeastern Atlantic, numbers are slightly higher (range 6-14, average 10). In both cases, both sexes are represented in a 1:1 ratio. Populations off the coast of Africa show no distinct breeding season although it appears that the young are born throughout the year with an increase in births from April to June. In the western North Atlantic, females give birth in the shallow bays and estuaries along the southeastern United States leave the area shortly after. This shallow water habitat provides a 'nursery' area for young sharks where they are afforded protection from larger sharks.

__THREATS & REASON FOR ENDANGERMENT__ 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 World Conservation Union 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. A combination of slow growth rates, late maturation, and long gestation period result in this species having a very low intrinsic rate of increase. This increases the shark's vulnerability to fishery activities. Off Natal, South Africa, an efficient shark gillnetting program set up to protect bathing beaches has apparently resulted in an increase in juvenile dusky sharks there.

__CONSERVATION EFFORTS__ On a global scale, dusky shark populations are considered at-risk, with the World Conservation Union (IUCN) assessing the species as "Near Threatened." However, an ongoing decline in numbers indicated by low catch rates in the western North Atlantic has prompted a ban on the harvesting of dusky sharks by U.S. commercial fishermen and has led to this regional population being placed on the 2000 IUCN's Redlist of threatened species. Presently, the dusky shark in the western North Atlantic and Gulf of Mexico is listed as "vulnerable" due to a population reduction of 20% over the last ten years. In a recent assessment of fish stocks at risk of extinction by the AFS (American Fisheries Society), populations of dusky sharks in the western Atlantic and eastern Pacific are both considered vulnerable (not endangered or threatened severely but at possible risk of falling into one of these categories in the near future).

__ SOLUTIONS FOR CONSERVATION __ 1) Place heavy fines on fisherman who catch dusky sharks. 2) Place several dusky sharks in captivity to encourage mating to increase the population. 3) Place heavy fines on businesses that purchase dusky shark parts to use in their products.

__SCAVENGER HUNT__ 1) What year was the dusky shark placed on the redlist of threatened species? 2) How many years can the dusky shark live up to? 3) How often do females mate?

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