Dusky Shark

Quick Take

• Shark nets installed to protect swimmers off South Africa triggered an unexpected chain reaction that damaged the entire local ecosystem, with dusky sharks sitting at the center of it all. Shark nets and ecosystem impact →
• Female dusky sharks have an unusual reproductive trick they rely on precisely because of how rarely they encounter a mate. Dusky shark reproduction details →
• Dusky shark populations in the Atlantic have collapsed to a fraction of what they were just decades ago, with their own biology making a comeback nearly impossible. Atlantic population collapse →
• When a female dusky shark is ready to give birth, she abandons the group migration and heads to a very specific destination tied to her own origins. Birth location and origins →

The dusky shark is one of the largest sharks and has a long, streamlined body. Its back is brownish-gray, and it has a pale white underbelly.

Dusky sharks are in high demand in the shark fin trade. Unfortunately, their fins are large and valuable, making them victims of human capture, especially in China. In addition, their skins are used to make leather, and liver oil is extracted for use in vitamins.

Dusky sharks partake in massive migrations; however, the females won’t join if they are about to give birth. Instead, they will venture back to the area where they were born.

Dusky Shark Facts

• Their massive size makes them dangerous to humans.
• They are solitary.
• Female dusky sharks sometimes store sperm because they don’t come across potential mates often.
• They are slow growers and only reach adulthood at 20 years old.
• Dusky sharks can reach 50 years of age.
• In one sitting, they can eat up to a tenth of their total weight, and they sometimes eat trash discarded by humans.
• According to the International Shark Attack File, dusky sharks have attacked people and boats a total of six times. Three were unprovoked attacks, and one of the attacks was fatal.

Classification and Scientific Name

The dusky shark’s scientific name is Carcharhinus obscurus, and it belongs to the family Carcharhinidae and the class Chondrichthyes.

This shark species has many names, which include:

• Brown shark
• Bronze whaler shark
• Common Whaler shark
• Shovelnose shark
• Bay Shark
• River Whaler

In 1818, French naturalist Charles Alexandre Lesueur first documented the dusky shark in the Journal of the Academy of Natural Sciences of Philadelphia. At first, he placed it in the genus Squalus, but its genus was changed to Carcharhinus later. The specific epithet (the second part of their scientific name) is obscurus, which means ‘dim’ or ‘dark’ because of their coloration.

Appearance

Dusky sharks have slender builds and long, sickle-shaped pectoral fins. However, their most distinctive feature is their round snouts. They can also be identified by the interdorsal ridges on their backs.

They vary from a blue-gray to a dark-gray color on the dorsal side, and they have white underbellies. Their sharp teeth are triangular and saw-edged. They are apex predators because they are so big and can weigh up to 400 pounds.

When dusky sharks reach adulthood, they generally measure 10 feet long; however, the maximum length ever recorded was 14 feet.

Behavior

These sharks are predominantly solitary but seasonally migrate in groups, often covering thousands of miles per event. The dusky sharks that inhabit the Atlantic and eastern Pacific move north during the warmer months and south in the winter.

They are potentially dangerous to humans because of their large size and the shallow coastal waters they inhabit, and recorded attacks on humans have occurred.

In Australia and South Africa, dusky sharks are frequently caught in shark nets, which, along with overfishing and other pressures, have contributed to significant declines in their populations.

Habitat

Dusky sharks prefer tropical and temperate waters and occur globally in temperatures between 66 and 82°F. Though they prefer shallow water, they do venture off the coast to the outer continental shelf all the way to the adjacent pelagic waters at depths of 1,300 feet.

They are widely distributed in the Canary Islands through the western and central Mediterranean Sea. Dusky sharks also occur in:

• Senegal
• Sierra Leone
• Spain
• Portugal
• Madeira
• Morocco

Additionally, they have been spotted in the Bay of Bengal, the Arabian Sea, the coasts of Vietnam, China, Taiwan, Japan, Australia, and South Africa.

Diet

The dusky shark has a wide and varied diet. Their preferred prey includes:

• Cephalopods
• Bony fish like bluefish
• Stingrays
• Small sharks
• Sea turtles
• Crustaceans
• Carrion

However, their preferred pelagic prey includes:

• Tuna
• Mackerel
• Herrings
• Anchovies
• Flying fish

Dusky sharks are also known to eat human garbage.

Predators and Threats

Because of their size, adult dusky sharks don’t have any natural predators. However, their pups are vulnerable to larger sharks like the:

• Ragged-tooth shark
• Great white shark
• Bull shark
• Tiger shark

Off the coast of South Africa, along the shorelines of KwaZulu-Natal, shark nets are used to protect people swimming in the bay. These nets have reduced the population size of adult dusky sharks. This allowed the juvenile population to flourish, and as a result, the juveniles have decimated bony fish populations, harming the local ecosystem and biodiversity.

Another threat facing the dusky shark is capture for the shark fin trade. Their valuable fins make them highly sought after, especially in China, where they use their fins in soup and meat in traditional medicine.

In addition, people use their skins to make leather, and also extract liver oil used in vitamins. Unfortunately, in the past, recreational anglers caught them in shark-fishing tournaments, though this practice has since ceased.

Due to population declines from overfishing and the shark fin trade, the IUCN currently lists the dusky shark (Carcharhinus obscurus) as endangered.

Reproduction, Babies, and Lifespan

Dusky sharks are viviparous, meaning they give live birth to their young. However, the eggs are fertilized in the uterus, and the embryos feed off a yolk inside the egg for nourishment.

after a gestation period of 18 to 22 months, the females give birth to a litter of between 3 and 16 pups every two to three years. When they are ready to give birth, the females swim to shallow water just off the coastline. Pups measure around 33 to 39 inches at birth.

Dusky sharks have a slow growth rate, and it takes males between 16 and 20 years to reach maturity, depending on location. The females have to wait 16 to 22 years before they are fully mature.

Population

The dusky shark population is currently decreasing because of its prolonged growth rate, late age of maturity, and small litter sizes, all of which make the species vulnerable to further decline.

In the Northwest and Western Central Atlantic, dusky shark populations have declined significantly. A recent study revealed that their current population represents only 15 to 20% of their numbers in the 1970s.

Fishing and Cooking

Dusky sharks are one of the most desired shark species for their fins used in shark-fin soup and liver oil. In addition, their skin is used to make leather. As a result, they are targeted by commercial and recreational fisheries and are frequently taken as bycatch.

Because their numbers are declining off the coasts of the USA, both commercial and recreational retention of dusky sharks is prohibited.

Dusky Shark In the Aquarium

Dusky sharks do not do well in captivity due to their large size. However, two aquariums house them:

• Okinawa Churaumi Aquarium, Japan
• SeaWorld Marine Park, Gold Coast, Australia