The Dusky shark swims in tropical and temperate oceans worldwide, cruising from depths of 1,300 feet (400 meters) near the continental shelf all the way in to the surf zone and other shallow inshore waters.
Dusky sharks are long-distance swimmers known for seasonal, temperature-driven migrations that males and females undertake in separate groups. Local patterns vary but the sharks often head toward the Poles in summer and return to the Equator in winter on sea voyages that have been known to top 2,000 nautical miles.
Despite their wanderlust, adult female dusky sharks are homebodies when it comes to reproduction. These animals give birth in the same continental regions where they were born. This natal site fidelity means that dusky sharks around the globe live in distinct populations that are not replenished by wandering or migrating animals. Such local pride has a downside, however, because the isolated communities are more susceptible to localized overfishing pressures.
This threat has materialized into a serious danger in the western Atlantic and Gulf of Mexico. Studies suggest that population numbers in this region are only 15 to 20 percent of their mid-1970s levels. Commercial and recreational fishing for these sharks in the western Atlantic and Gulf was banned in 2000 but they are often accidentally caught on longlines and other fishing gear—with high mortality rates. Elsewhere the dusky shark is still targeted for trade in shark fin soup, with devastating results. The shark fin trade is difficult to quantify but recent studies have suggested as many as 750,000 dusky sharks could be caught in the trade each year.
Dusky sharks are extremely long-lived and may survive up to half a century, but they are slow to grow and to reproduce. The sharks don’t reach sexual maturity until about 20 years of age. Dusky sharks may have a gestation period of up to 22 months, so females only give birth once every three years. The young sharks, some 6 to 12 per litter, congregate at inshore nursery areas along the coasts of South Africa, the southeast United States, and southwest Australia.
Off South Africa, young males and females over 0.9 m (3.0 ft) long disperse southward and northward respectively (with some overlap) from the nursery area off KwaZulu-Natal; they join the adults several years later by a yet-unidentified route. In addition, juveniles spend spring and summer in the surf zone and fall and winter in offshore waters, and as they approach 2.2 m (7.2 ft) in length begin to conduct a north-south migration between KwaZulu-Natal in the winter and the Western Cape in summer. Still-larger sharks, over 2.8 m (9.2 ft) long, migrate as far as southern Mozambique. During the Sardine Run large dusky sharks are frequently encountered gorging themselves on bait-balls of Sardine and other baitfish. They can readily be recognised from blacktip sharks by their more muted colouration, relatively smaller dorsal fin, and absence of distinguishing black pigmentation on their pectoral and dorsal fin.