Dusky Shark Populations Dwindling
Despite being identified as a vulnerable species for several years, dusky shark populations in the U.S. and around the globe continue to plunge as these slow-growing sharks face pressures from fisheries, the shark fin trade and a number of other human activities.
During the last 40 years U.S. dusky shark populations have decreased by 70 to 98 percent in some areas. In the North Atlantic and Gulf of Mexico, populations have declined by as much as 20 percent in the last decade, according to the Florida Museum of Natural History.
Dusky shark numbers are also plummeting around the world. A recent assessment off the coast of southwestern Australia determined that between 1970 and 2004, the dusky shark population in the area declined by over 75 percent. Data from highly fished areas in other parts of the world is unavailable.
Also known as bronze or black whalers, dusky sharks (Carcharhinus obscurus) are a species of requiem shark, found in tropical and temperate waters throughout the world. There are important inshore nursery areas along the coast of South Africa, the southeast U.S and southwestern Australia.
Dusky sharks have a rounded snout and a low interdorsal ridge. They grow to an average size of 11.8 feet, but can be as long as 14 feet. An average adult weighs around 400 pounds. Extremely long-lived, dusky sharks can survive between 40 to 50 years.
Dusky sharks are known for swimming long distances and can be found in shallow coastal waters as well as far offshore, diving to a depth of 1,300 feet. They are not commonly found in estuaries, as they tend to avoid waters with low salinity. They feed on bony fish, other sharks, rays, skates, rays, cephalopods, gastropods and crustaceans. Dusky sharks have been known to eat garbage.
Despite their extensive migratory patterns, female dusky sharks return to the place they were born to give birth to their own live pups.
“This natal site fidelity means that dusky sharks around the globe live in distinct populations that are not replenished by wandering or migrating animals,” says National Geographic’sdusky shark page. “Such local pride has a downside, however, because the isolated communities are more susceptible to localized overfishing pressures.”
Dusky sharks are especially vulnerable to overfishing pressures because they take up to 20 years to reach sexual maturity and have small litters once every three years after a long gestation period.
Dusky sharks were identified as a “species of concern” by the National Oceanic and Atmospheric Administration’s (NOAA) National Marine Fisheries Service in 1997, and global populations are currently designated as vulnerable under the International Union for Conservation of Nature’s (IUCN) Red List and American Fisheries Society. Additionally, the IUCN has designated the U.S. dusky shark population in the Northwestern Atlantic and Gulf of Mexico as endangered.
Despite several laws in the U.S. that prohibit the landing of dusky sharks, including a 2000 law that prohibits any commercial or recreational possession of a dusky shark, numbers continue to decline. According to the IUCN, while protection laws may have been successful in increasing juvenile numbers, adult populations are still falling.
NOAA has identified illegal landings by commercial and recreational shark fisheries and bycatch as the main factors for the continuing decline in dusky shark populations in the U.S.
Bycatch is the unintended catch of marine life when fishing for a target species. Bycatch includes other fish, crustaceans, marine mammals, turtles and birds. A recent report by Oceana reveals that nearly 20 percent of the U.S. catch is thrown away as bycatch.
Bycatch is a major threat to dusky sharks in U.S. waters. In 2010, for example, over 3,400 dusky sharks were caught and discarded as bycatch by two of the numerous fisheries in the southeastern U.S., according to a recent op-ed by Oceana marine scientist Amanda Keledjian.
Dusky sharks are targeted for their fins, the main ingredient in shark fin soup, popular in a number of Asian countries and Asian restaurants around the world. Dusky shark fins are especially prized for their high fin needle content and large size. Recent estimates suggest that up to 750,000 dusky sharks are killed for the fin trade every year, according to National Geographic.
These sharks are also targeted for their skin, used in luxury items like wallets and shoes, as well as the oils in their livers, used in vitamins and traditional medicine. Beach meshing programs also pose a significant threat to dusky sharks in Australia and South Africa.
Keledjian argues that there are several ways to reduce numbers of dusky sharks caught as bycatch. She advocates the use of a “count, cap and control” approach, as well as the use of less destructive gear and implementation of bycatch-reducing initiatives like catch-monitoring programs.
“There’s no reason dusky sharks should remain overfished when regulations have prohibited their capture for a decade,” she writes. “With the sharks’ numbers dwindling, the National Marine Fisheries Service must implement science-based management approaches to minimize bycatch.”