Dusky Shark, Carcharhinus obscurus
Dusky Shark, Carcharhinus obscurus. Fish caught from coastal waters off Sebastian, Florida, June 2022. Length: 1.88 m (6 feet 2 inches). Catch, photograph, and identification courtesy of Ben Cantrell, Sebastian, Florida.
Dusky Shark, Carcharhinus obscurus. Fish caught from coastal waters off Ft. Lauderdale, Florida, June 2022. Length: 1.90 m (6 feet 3 inches). Catch, photograph and identification courtesy of Luke Ovgard, Klamath Falls, Oregon.
The Dusky Shark, Carcharhinus obscurus, is a member of the Requiem Shark or Carcharhinidae Family, that are also known as bronze or black whalers and in Mexico as tiburón gambuso. Globally, there are thirty-five species in the genus Carcharhinus, of which seventeen are found in Mexican waters, seven in the Atlantic, four in the Pacific, and six in both the Atlantic and Pacific Oceans.
The Dusky Shark has a moderately slender body. They are a dark blue-gray dorsally transitioning to white ventrally. The lower lobe of the caudal fin, and their pectoral and pelvic fins are dusky as juveniles and plain in adults. Their head has a broadly rounded snout with large round eyes and large nasal flaps. Their upper teeth are triangular and slightly oblique with serrated edges; their lower teeth are erect, narrowly cusped and more finely serrated. They are characterized by having a strongly asymmetrical caudal fin that has a well-developed lower lobe and a notch under the tip of the top lobe, low rounded dorsal fins, a sloping moderately large first dorsal fin that originates over or slightly before the free tips of the pectoral fins, a second dorsal fin that originates above the anal fin that is low with a long base and a free tip that is less than twice the fin height, large falcate curved pectoral fins and an interdorsal ridge. Their pectoral fins are long, curved with pointed tips and of modest size, and their pelvic fins origin is slightly behind the free rear tip of the first dorsal fin. Their body is covered with large, closely imbricated dermal denticles.
The Dusky Shark is a coastal and pelagic shark that has a patchy distribution in tropical and warm temperate seas where it occurs from the surf zone to well offshore, and from the surface down to depths of 500 m (1,640 feet). The avoid area of low salinity and rarely enter estuaries. They make long-distance seasonal migrations (up to 2,000 nautical miles), traveling in schools separated by size and sex, to cooler waters. They reach a maximum of 4.20 m (13 feet 9 inches) in length and 346.5 kg (762 pounds) in weight. As of January 1, 2023, the International Game Fish Association world record stood at 346.5 kg (764 lbs) with the fish caught in coastal waters off Longboat Key, Florida in May 1982. Dusky Sharks prey primarily on bony and cartilaginous fishes as well as crustaceans and cephalopods. In turn they are preyed upon by various larger sharks. They have very slow growth rates, a late maturity age with both sexes requiring 20 years to reach maturity. Reproduction is viviparous and has a long cycle that involves a three-year cycle with a 22-month gestation period with the young nourished in utero with yolk-sac placenta. The small litter sizes of 6 to 12 are born as live miniature adults. The females return and give birth to their offspring in the same well defined well-known regions in which they were born. Parturition occurs in shallow water. At birth, the pups measure between 70 cm (2 feet 7 inches) and 1.0 m (3 feet 3 inches) in length and the remain in shallow bays and coastal lagoons for extended periods of time to avoid predation. They have lifespans of up to forty years.
The Dusky Shark is a resident of Mexican waters of both the Atlantic and Pacific Ocean. In the Atlantic Ocean they are found throughout the Gulf of Mexico; they are absent from along the east coast of the Yucatán Peninsula in the Caribbean. In the Pacific Ocean that are limited to with waters north of Mazatlán along the west coast of the mainland, throughout the Sea of Cortez and along the west coasts of Baja California and Baja California Sur.
The Dusky Shark is easily confused with the Galapagos Shark, Carcharhinus galapagensis (origin of first dorsal fin over inner margin and before the tip of the pectoral fins; pelvic fin origins well behind the free rear tip of the first dorsal fin, found in much deeper waters).
From a conservation perspective the Dusky Shark is currently classified to be ENDANGERED, with significant declines in their populations that are estimated to be >80% over the last 100 years. As a result, the Dusky Shark has become a major focus of conservation efforts. The United States has banned the retention of these fish. However, in most of their territories they remain poorly monitored, unregulated and catch levels are undocumented are remain a focus of artisanal, recreational fishermen and commercial fishermen utilizing longline and nets for their food value, leather, and liver oil. Approximately 750,000 Dusky Sharks are caught each year by commercial fishermen. They have high mortality rates with a post release mortality approaching 70%. As birthing occurs in the same small areas they become very susceptible to localized overfishing pressure. They have been fished to extinction in certain areas and their long-term viability is currently threatened by overfishing and inshore habitat destruction caused by human development. They are considered to be potentially dangerous to humans (traumatogenic) due to their size and presence in shallow waters but very few attacks on humans have been documented.