Dusky shark
(Carcharhinus obscurus)

Carcharhinus obscurus (Lesueur, 1818)

Diagnosis

A large, but rather slender shark with a short, rounded snout and low interdorsal ridge. Eyes circular and moderately large. Anterior nasal flaps not prominent as in C. altimus. First dorsal fin height averaging 8.2% of TL (range 5.8-10.4%) with a pointed or narrowly-rounded apex, origin approximately above or just behind the pectoral free tips. Anal fin larger than 2nd dorsal fin and situated directly below; 2nd dorsal fin low; pectoral fins long, falcate with quite acute apices, their anterior margins measuring 16.8 - 23% of TL.

Upper-jaw teeth broadly triangular and slightly oblique with strong serrations; lower teeth narrower and erect, with broad, low basal crowns. Colour slate-blue to mid-grey dorsally, fading to white ventrally with an indistinct horizontal band of lighter pigment invading the flanks; fin tips dusky but not prominently marked, with the darkness more pronounced on the paired fins of young specimens.


Size

To 365cm and possibly 400 cm TL; size at birth 69-100cm.


Status and Distribution

Mediterranean Sea: Rather rare but status scantily known; Gibraltar, Morocco and Spain (Catalon Sea southwards); eastwards through the southern Alboran Sea to the Sicilian Channel; three large females (330-349 cm TL) examined there in the summer of 1983 from Mazara del Vallo (Sicily) by Cigala Fulgosi (1983 and pers. comm.) and also at Sidi Daoud, Cape Bon (Tunisia), a gravid female caught 3km offshore reported by Capape et al. (1979); a newer record and range extension originates from near Filfla, Malta - a 311cm TL male taken on 07.12.95 (Fergusson et al., in press); probably more cosmopolitan in the Central Mediterranean and liable to be misidentified in fisheries with other other superficially-similar carcharhinid


Biology

A coastal species in warm-temperate and tropical waters, found on continental and insular shelves, both offshore and inshore from the surface down to at least 400m; caught over shallower waters within the central Mediterranean (30 to 200 m depth) and in clear waters.

Adults may readily follow ships far offshore. Strongly migratory in other parts of the globe, with northerly movements during the summer and southwards in winter; Mediterranean incursions from the Atlantic may be solely related to reproduction.

Dusky sharks are active, powerful predators and scavengers with a cosmopolitan diet that includes numerous bony fish such as mackerels, tunas, sardines, herrings, jacks, garfish, groupers and flatfish; also a number of sharks including Centrophorus spp., Mustelus spp., Squalus spp., Carcharhinus limbatus and C. brevipinna; numerous rays, octopi, other cephalopods and crustaceans.

This species will also scavenge upon cetacean carcasses and may attack free-swimming juvenile (and possibly adult or ailing) dolphins, and should be considered (along with Carcharodon carcharias and adult Isurus oxyrinchus) as a potential, if sporadic, predator of Mediterranean odontocetes.

Viviparous; litter size is 3-14 pups; females moving briefly inshore to give birth and apparently mating each alternate year, with parturition occuring over a period of several months. The gravid Tunisian example observed by Capape et al. (1979) contained 7 fully-formed foetuses and was caught near-shore, implying that parturition was imminent.

Males mature at 280 cm, females at between 257-300cm, the 3 females examined by Cigala-Fulgosi in Sicily were apparently mature based on TL.