The journeying nomad
The Dusky shark (Carcharhinus obscurus) is a species of requiem shark, in the family Carcharhinidae, that can be found in tropical and warm-temperate continental seas worldwide. Dusky sharks can oftentimes be found with Sandbar sharks and Silky sharks. Here in Jupiter that is true, and we see the three species together often. Dusky sharks are seasonal here in Jupiter, check out our videos here to see them alongside Silkies, Sandbars, Lemons and Bulls! Dusky sharks travel long migratory journeys distances up to 2400 miles.
Family: Carcharhinidae – Requiem sharks
Common Name– Ground Sharks
Common Name– Requiem Sharks
Status: IUCN Red List VULNERABLE
Average Size and Length: The average length of a dusky shark is 10 feet, but there have been some reports of larger female Dusky sharks reaching 14 feet long.
Average Weight: The average weight of a Dusky shark is 350-400 pounds. One account of a large female was 765 pounds.
Teeth and Jaw: The mouth of a Dusky shark has very short, subtle furrows at the corners and contains 13-15 (normally 14) tooth rows on either side of both jaws. The upper teeth are broad, triangular, and slightly oblique with strong, coarse serrations, while the lower teeth are narrower and upright, with finer serrations.
Head: The Dusky shark has a broadly rounded snout no longer than the width of the mouth. The nostrils have small skin flaps, and they have circular eyes that are medium in size with nictitating membranes.
Denticles: The dermal denticles of a Dusky shark are diamond-shaped and closely set, each has five horizontal ridges leading to teeth on the posterior margin.
Tail: The caudal fin is large and high, with a well-developed lower lobe and a ventral notch near the tip of the upper lobe.
Demographic, Distribution, Habitat, Environment and Range: The Dusky shark can be found worldwide in warm-temperate tropical waters. The Dusky shark spends most of its time at depths of 33–262 feet, while making occasional ventures below 660 feet. The Dusky shark has been recorded as deep as 1,300 feet. The preferred temperature is between 66-82 degrees F. They do not care for low salinity waterways.
Mitochondrial DNA and microsatellite evidence suggest that Indonesian and Australian sharks represent distinct populations.
In the eastern Atlantic Ocean, it has been reported from the western and central Mediterranean Sea, the Canary Islands, Cape Verde, Sierra Leone, and Senegal. There have been some accounts in Madeira, Portugal, Spain, and Morocco.
In the western Atlantic Ocean, it is found from Massachusetts and the Georges Bank to southern Brazil, including Cuba and the Bahamas.
In the Indian Ocean, the Dusky shark can be found off South Africa, Mozambique, and Madagascar, with some records in the Arabian Sea, the Bay of Bengal, and even the Red Sea.
In the Pacific Ocean, the Dusky shark can be found off the coast of 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 even possibly off northern Chile.
The Dusky shark is often found with relatives the Sandbar shark, the Silky shark, the Oceanic Whitetip shark and several others in other locations. Here in Florida it is typical to see Silkies, Sandbar sharks and Dusky sharks together, and on occasion, we have seen Oceanic Whitetips with the. Check out our videos with these species together! Dusky sharks reside off continental coastlines from the surf zone to the outer continental shelf and adjacent oceanic waters. The Dusky shark is commonly mistaken, and therefore some accounts of Dusky sharks from the northeastern and eastern central Atlantic, and around tropical islands, may be of Galapagos sharks. Some locations commonly mistake Sandbar sharks for Dusky sharks.
The Dusky shark is nomadic and strongly migratory, with recorded distances of up to 2,400 miles. Juveniles do not travel as far as adult Dusky sharks, but overall, they are quite the traveling shark. Dusky sharks along both coasts of North America shift northward with warmer summer temperatures, and withdrawal back towards the equator in winter.
Off Western Australia, adult and juvenile Dusky sharks migrate towards the coast in summer and fall. They do not go inshore to the newborn nurseries.
Off South Africa, young males and females over 3.0 feet long scatter southward and northward respectively from the nursery area off KwaZulu-Natal; they join the adults several years later by a yet-unidentified route. Juveniles spend spring and summer in the surf zone and fall and winter in offshore waters, and as they approach 7.2 feet in length begin to conduct a north-south migration between KwaZulu-Natal in the winter and the Western Cape in summer. Larger Dusky sharks, over 9.2 feet long, migrate as far as southern Mozambique.
Diet: The Dusky shark will eat a variety of food, and from all water column levels; it surely isn’t picky. The Dusky shark does however prefer hunting towards the bottom. They eat a lot. Larger Dusky sharks can devour a tenth of its body weight in one feeding.
Dusky sharks are known to eat pelagic fishes, including herring and anchovies. They also eat tuna and mackerel, billfish, jacks, needlefish, flyingfish, threadfins, hairtails, lanternfish, and lancetfish. Dusky sharks also eat demersal fishes, including mullets, porgies, grunts, and flatheads, eels, lizardfish, cusk eels, gurnards, and flatfish. They eat reef fishes like, barracudas, goatfish, spadefish, groupers, scorpionfish, and porcupinefish. And they eat cartilaginous fishes like Dogfish, Sawsharks, Angel sharks, Catsharks, Thresher sharks, Smoothhounds, Sawfish, Guitarfish, Skates, Butterfly Rays, Stingrays, and even smaller requiem sharks. The known invertebrates in their diet are cephalopods, decapod crustaceans, sea stars and barnacles. On occasion, the larger Dusky sharks will eat sea turtles, marine mammals (mainly as carrion), and human refuse.
In the northwestern Atlantic, about 60% of the Dusky shark’s diet consists of bony fishes, with bluefish and summer flounder being particularly important. Cartilaginous fishes, mainly skates and their egg cases, are the second-most important, and the lady crab is also a favorite.
In South African and Australian waters, bony fishes are again the most important prey type. Newborn and juvenile sharks subsist mainly on small pelagic prey such as sardines and squid; older sharks over 6.6 feet long will start to eat larger bony and cartilaginous fishes. Dusky sharks are a part of the annual Sardine Run off the eastern coast of South Africa every winter. Mostly medium and large Dusky sharks will be seen. However, pregnant females, or post-partum females do not participate. Some research suggests this could be due to their nee to conserve energy. The Sardine run is full of energy from the smallest being to the largest. In one interesting South African study, recorded data confirmed that 0.2% of the sharks examined had preyed on Bottlenose dolphins.
Off KwaZulu-Natal, the use of shark nets to protect beaches has reduced the populations of these larger predators, leading to a studied increase in the number of juvenile Dusky sharks (a phenomenon called “predator release”). Due to these actions, the juvenile Dusky sharks have decimated populations of small bony fishes, with negative consequences for the biodiversity of the local ecosystem.
Ram-Suction Index: The bite force recorded of a 6.6-foot-long Dusky shark has been measured at 130 pounds over the 0.0031 in2 area at the tip of a tooth. This is the highest figure so far measured from any shark, though it also reflects the concentration of force at the tooth tip. (Smale, M.J. (1991). “Occurrence and feeding of three shark species, Carcharhinus brachyurus, C. obscurus and Sphyrna zygaena, on the eastern Cape coast of South Africa“. South African Journal of Marine Science.)
Aesthetic Identification: One of the first things you will notice about the Dusky shark is its slender, streamlined body. It has 5 pairs of long gill slits. It is bronzy to bluish gray above and counter-shaded white below, which extends onto the flanks as a faint stripe. The fins, mainly the underside of the pectoral fins and the lower caudal fin lobe darken towards the tips. This is more noticeable in juveniles. The large pectoral fins measure around one-fifth as long as the body, and have a falcate shape tapering to a point. The first dorsal fin is of moderate size and somewhat falcate, with a pointed apex and a strongly concave rear margin; its origin lies over the pectoral fin free rear tips. The second dorsal fin is much smaller and is positioned opposite the anal fin. A low dorsal ridge is present between the dorsal fins.
Biology and Reproduction: Known parasites of the Dusky shark are the tapeworms: Anthobothrium laciniatum, Dasyrhynchus pacificus, Platybothrium kirstenae, Floriceps saccatus, Tentacularia coryphaenae, and Triloculatum triloculatum. Also, the monogeneans Dermophthirius carcharhinid and Loimos salpinggoides. They also carry the leech Stibarobdella macrothela. Dusky sharks also carry the copepods Alebion sp., Pandarus cranchii, P. sinuatus, and P. smithii. And also, the praniza larvae of gnathiid isopods. and the Dusky sharks also carry the sea lamprey Petromyzon marinus, and are the hose of the sharksucker Echeneis naucrates.
The Dusky shark is viviparous. In the northwestern Atlantic, the mating season of Dusky sharks is during spring time. In South Africa, there is no trace of seasonality.
Based on a study in 1993 by Pratt, females are capable of storing masses of sperm, possibly from multiple males, for months to years within their nidamental glands (an organ that secretes egg cases). This would be advantageous given the sharks’ itinerant natures and low natural abundance, which would make encounters with suitable mates infrequent and unpredictable. (Pratt, H.L. (Jr.) (1993). “The storage of spermatozoa in the oviducal glands of western North Atlantic sharks“. Environmental Biology of Fishes).
With a gestation period estimated at up to 22–24 months and a one-year resting period between pregnancies, female Dusky sharks have at most one litter of young every three years. The litter size of Dusky shark’s ranges from 3 to 16, with 6 to 12 being the average. This doesn’t seem to associate with the size of the female. Dusky sharks in the western Atlantic seem to produce slightly smaller litters than those from the southeastern Atlantic. They average 8 pups instead of 10 pups.
Depending on region, birthing may happen throughout the year or over a span of several months: newborn sharks have been reported from late winter to summer in the northwestern Atlantic, in summer and fall off Western Australia, and throughout the year with a peak in fall off southern Africa.
Females move into shallow inshore habitats like lagoons to give birth. These nurseries offer their pups abundant food supplies and shelter from predation. The females leave immediately after. Some of these nursery areas are known along the coasts of KwaZulu-Natal, southwestern Australia, western Baja California, and the eastern United States from New Jersey to North Carolina.
Newborn Dusky sharks measure between 2.3–3.3 feet long. The pup size does increase with female size or decreases with litter size. One interesting theory suggests that female Dusky sharks can determine the size 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. It is one of the slowest growing shark species. The annual estimated growth rate is 3.1–4.3 inches over the first five years of its life.
The maximum lifespan of a Dusky shark is believed to be 40–50 years or more.
Behavioral Traits, Sensing and Intelligence: Dense aggregations of young sharks, forming in response to feeding opportunities, have been documented in the Indian Ocean.
The Dusky shark is positioned as an apex predator at the top trophic web. Due to its position in the ecosystem, it is usually less abundant than the other sharks that share its range. It is interesting that in some locations, despite this fact, some areas and locations seem to have much higher concentrations of Dusky sharks.
Dusky sharks can sometimes be found following ships far from land. Another area where we have seen higher concentrations would be at offshore oil rigs.
Speed: A tracking study off the mouth of the Cape Fear River in North Carolina reported an average swimming speed of a Dusky shark at 0.50 mph.
Dusky Shark Future and Conservation: Dusky sharks have a low reproductive rate, and are extremely susceptible to overfishing. Bycatch and recreational fishing have produced very negative consequences to the Dusky shark.
In 2005, North Carolina implemented a time/area closure to reduce the impact of recreational fishing. To support 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. (Pank, M.; Stanhope, M.; Natanson, L.; Kohler, N. & Shivji, M. (May 2001). “Rapid and Simultaneous Identification of Body Parts from the Morphologically Similar Sharks Carcharhinus obscurus and Carcharhinus plumbeus (Carcharhinidae) Using Multiplex PCR“. Marine Biotechnology.)
The Dusky shark is one of the most desirable species for shark fin trade, as its fins are large and contain a high number of internal rays. 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.
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. They are also caught as bycatch on longlines meant for tuna and swordfish. Juvenile Dusky sharks are caught as sport fishing in South Africa and eastern Australia.
The Dusky shark was once one of the most important species in the Florida trophy shark tournaments before the population collapsed.
Dusky Shark Recorded Attacks on Humans: The Dusky shark is potentially dangerous to humans.
Little is known or studied about its behavior towards humans underwater. From our personal experience, on one occasion, a Dusky shark off the coast of Texas at an oil rig behaved in an inquisitive manner, swimming in and out of a group of divers cautiously, then after about 15 minutes, swam off. On another occasion here in Jupiter, a migratory returning resident Dusky shark local call Rhonda, seems to always hold interest in divers, and has a hard time finding any other interest when divers are present. Based on our observational studies here with Rhonda, her behavior may have been influenced by external factors.
Shark nets have had a negative impact on Dusky sharks. Humans use shark nets to protect beaches in South Africa and Australia, and they entangle adult and larger juvenile Dusky sharks in large quantities. From 1978 to 1999, an average of 256 individual Dusky sharks were caught annually in nets off KwaZulu-Natal.