Dusky Shark

The dusky shark (Carcharhinus obscurus), a member of the Carcharhinidae family, is a widely distributed species found in tropical and warm-temperate continental seas globally. As a generalist apex predator, this shark inhabits areas ranging from coastal regions to the outer continental shelf and adjacent pelagic waters, recorded at depths of up to 400 meters (1,300 feet). Its populations are known for their seasonal migration patterns, moving towards the poles in the summer and towards the equator in the winter, covering extensive distances ranging from hundreds to thousands of kilometers. The dusky shark is one of the largest species in its genus, reaching lengths over 4 meters (13 feet) and weights of 350 kilograms (770 pounds). It is characterized by a slender, streamlined body, a short rounded snout, long pectoral fins shaped like a sickle, a ridge between the dorsal fins, and faint markings on the fins.

Adult dusky sharks maintain a diverse diet predominantly composed of bony fishes, sharks, rays, and cephalopods, with occasional consumption of crustaceans, sea stars, bryozoans, sea turtles, marine mammals, carrion, and debris. The species exhibits viviparous reproduction with a three-year cycle; females give birth to litters of 3–14 offspring following a gestation period of 22–24 months, followed by a one-year rest period before becoming pregnant again. Known for its long gestation period, rivaled only by the Spiny dogfish, this reproductive strategy makes the dusky shark susceptible to population declines. Females possess the ability to store sperm for extended periods, due to their nomadic lifestyle and the resulting infrequency of encountering suitable mates. Dusky sharks are among the slowest-growing and latest-maturing sharks, with individuals reaching adulthood at approximately 20 years of age.

With its slow reproductive rate, the dusky shark is particularly vulnerable to human-induced population reductions. The species is highly sought after by commercial fisheries for its fins, utilized in shark fin soup, as well as for its meat, skin, and liver oil, and is also valued by recreational fishers. The International Union for Conservation of Nature (IUCN) classifies this species as Endangered worldwide and Vulnerable along the eastern 🇺🇸 United States, where populations have decreased to 15–20% of their levels in the 1970s. Despite its size, the dusky shark is considered potentially dangerous to humans, though verified attacks are scarce.

The geographic range of the dusky shark spans across tropical and warm-temperate waters globally, though it is not continuous. In the western Atlantic Ocean, it is found from Massachusetts to southern 🇧🇷 Brazil, including the 🇧🇸 Bahamas and Cuba. In the eastern Atlantic Ocean, sightings include the western and central Mediterranean Sea, the Canary Islands, Cape Verde, 🇸🇳 Senegal, Sierra Leone, and possibly regions such as Portugal, Spain, Morocco, and Madeira. In the 🌊 Indian Ocean, it has been documented off 🇿🇦 South Africa, 🇲🇿 Mozambique, and 🇲🇬 Madagascar, with occasional reports from the Arabian Sea, Bay of Bengal, and possibly the 🌊 Red Sea. In the 🌊 Pacific Ocean, its distribution extends from 🇯🇵 Japan to mainland 🇨🇳 China and 🇹🇼 Taiwan, Vietnam, 🇦🇺 Australia, 🇳🇨 New Caledonia (🇫🇷 Overseas France), southern California to the Gulf of California, Revillagigedo, and potentially northern 🇨🇱 Chile. Records from the northeastern and eastern central Atlantic and around tropical islands may actually pertain to Galapagos sharks. Genetic analyses suggest that populations in 🇮🇩 Indonesia and 🇦🇺 Australia may be distinct.

The dusky shark resides along continental coastlines, from the surf zone to the outer continental shelf and adjacent oceanic waters, sharing habitats with closely related species such as the inshore sandbar shark, pelagic silky shark, oceanic whitetip shark, deepwater bignose shark, and islandic Galapagos and silvertip sharks. A tracking study in the northern 🌊 Gulf of Mexico indicated that this species predominantly occupies depths of 10–80 meters (33–262 feet), occasionally venturing to depths below 200 meters (660 feet), and reaching up to 400 meters (1,300 feet). It prefers water temperatures between 19–28 °C (66–82 °F) and avoids low-salinity environments, such as estuaries.

The diet of the dusky shark is wide-ranging, encompassing pelagic fishes (e.g., herring, anchovies, tuna, mackerel, and jacks), demersal fishes (e.g., mullets and eels), reef fishes (e.g., barracudas and groupers), cartilaginous fishes (e.g., dogfish, sawsharks, and stingrays), and invertebrates (e.g., gastropods and cephalopods). Infrequently, large individuals may also consume sea turtles, marine mammals (primarily as carrion), and human waste.

As in other requiem sharks, the dusky shark exhibits viviparous reproduction, with embryos initially nourished by a yolk sac which is eventually replaced by a placental connection. Mating occurs in the spring in the northwestern Atlantic, though other regions, such as 🇿🇦 South Africa, lack distinct reproductive seasonality. Females can retain sperm from multiple males for several months to years, leveraging this capability as a strategy to counter their itinerant nature and low population abundance, which lead to unpredictability in mate encounters. With an extended gestation period of 22–24 months and a year of rest between pregnancies, females produce offspring once every three years at most.

Though potentially dangerous to humans due to its size, the behavior of the dusky shark towards people remains largely undocumented. As of 2009, the International Shark Attack File attributes six incidents to this species, three unprovoked and one fatal, though some may have been misattributed and caused by Galapagos sharks instead. In regions with beach protective measures such as shark nets, like 🇿🇦 South Africa and 🇦🇺 Australia, the dusky shark is sometimes entangled, with KwaZulu-Natal (🇿🇦 South Africa) nets capturing an average of 256 individuals annually from 1978 to 1999. Specific data related to shark nets in 🇦🇺 Australia is currently unavailable.