Bigeye Thresher Shark

fieldmarks: Long dorsal caudal lobe nearly as long as rest of shark, notched or helmeted contour of head, huge eyes extending onto dorsal surface of head, falcate but rather broad-tipped pectoral fins. Head broad in dorsal and ventral view, with a notched dorsolateral profile. Snout moderately long, bulbous. Eyes greatly enlarged in young and adults, expanded onto dorsal surface of head and with a vertical, binocular field of view; interorbital space nearly flat. Labial furrows absent. Teeth large and in 22 to 27/20 to 24 rows (total for both jaws 42 to 51 rows); posterior tooth rows 1 to 3; no symphysial or intermediate teeth. Strong nuchal grooves present above branchial region. Pectoral fins falcate with curved and moderately broad tips. Claspers moderately slender and not whip-like. First dorsal midbase closer to pelvic-fin bases than to pectoral-fin bases. Caudal tip broad with wide terminal lobe. Ribs of monospondylous precaudal vertebrae fused ventrally to form a canal extending nearly to the occiput. Total vertebral count 219 to 319. Intestinal valve count 43 to 45. Colour: body purplish grey or grey-brown on upper surface and sides with underside grey to white, light colour of abdomen not extending over pectoral-fin bases; no white dot on upper pectoral-fin tips.

Oceanic and coastal, virtually circumglobal in tropical and temperate seas.Western Atlantic (including Gulf of Mexico): USA (Atlantic coast from New York to Florida, Gulf of Mexico off Florida, Mississippi and Texas), Mexico (Veracruz to Yucatan), Bahamas, Cuba, Venezuela, central and southern Brazil. Eastern Atlantic: Portugal, Spain, Madeira, near Azores, Morocco, Canary Islands, Senegal, Guinea to Sierra Leone, Angola, South Africa (Western Cape): also western and central Mediterranean Sea. Indian Ocean: South Africa (Eastern Cape and KwaZulu-Natal), Madagascar, Arabian Sea (Somalia), Gulf of Aden, Maldives, Sri Lanka. Western Pacific: Southern Japan (including Okinawa), Taiwan (Province of China), Viet Nam, between northern Mariana Islands and Wake Island, Northwestern Submarine Rise, New Caledonia, Australia (northwestern coast), New Zealand. Central Pacific: Area betweenWake, Marshall, Howland and Baker, Palmyra, Johnston, and Hawaiian Islands; north and south of Hawaiian Islands, off east of Line Islands, and between Marquesas and Galapagos Islands. Eastern Pacific: USA (California), Mexico (Gulf of California) and west of Galapagos Islands (Ecuador); possibly off Peru and northern Chile.

Maximum total length about 461 cm. Size at birth 100 to 140 cm, with full term foetuses at 105 or 106 cm and free-swimming individuals down to 155 cm. Males immature up to 316 cm, maturing at about 279 to 300 cm; adult males as small as 276 cm and reaching about 410 cm with an estimated maximum of 421 cm. Females immature up to 350 cm, and maturing at about 294 to 355 cm; adult females as short as 341 cm and reaching at least 458 cm. Length-weight equations for this species are given by Kohler, Casey and Turner (1995) for fork length: W (kg) = 9.1069 x 10-6 x FL (cm)3.0802 ( n = 55; both sexes) where FL (cm) = 0.5598 x TL (cm) + 17.666 (n = 56) Liu, Chiang, and Chen (1998) give equations for total length: Females: W (kg) = 1.02 x 10-5 TL (cm)2.78 (n = 175) Males: W (kg) = 3.73 x 10-5 TL (cm)2.57 (n = 65)

Habitat: Found in coastal waters over the continental shelves, sometimes close inshore in shallow waters, and on the high seas in the epipelagic zone far from land; also caught near the bottom in deep water on the continental slopes. Ranges from the surface and in the intertidal to at least 500 m deep, but mostly below 100 m depth.An epipelagic, neritic, and epibenthic shark, apparently strong-swimming but little-known behaviourally. Ovoviviparous, with uterine cannibalism (oophagy), number of young usually two per litter, but sometimes with 3 or 4. Sex ratio of foetuses 1:1. Larger females apparently do not have larger term foetuses than smaller females. Birth may occur throughout the year although in the eastern Atlantic more females may give birth in autumn and winter than other times of year. The gestation period may be 12 months long but remains uncertain because of lack of birthing seasonality. In the eastern Atlantic a nursery area occurs off the Straits of Gibraltar, but similar areas no doubt occur elsewhere in the vast range of this species. Bigeye threshers have been aged with annular growth rings in vertebral centra, assuming one ring per year (Liu, Chiang and Chen, 1998). Males mature at about 9 or 10 years old and females at about 12 or 13 years old. Maximum number of growth bands, and maximum known age, about 19 for males and 20 for females. The bigeye thresher feeds on pelagic fishes, including lancetfishes (Alepisauridae), herring (Clupeidae), mackerel (Scombridae), and small billfishes (Istiophoridae) and bottom fishes including hake (Merluccidae); also squids (Ommastrephidae). Apparently this species stuns its prey with its long caudal fin, as individuals are often tail-hooked on longlines. The arrangement of the eyes, with keyhole-shaped orbits extending onto the dorsal surface of the head, suggest that this species has a dorsal, vertical binocular field of vision (unlike other threshers) which may be related to fixating on prey and striking them with its tail from below. Michael (1993) observed sea lampreys attached near the cloaca of a bigeye thresher.

Caught or formerly caught in the oceanic longline fisheries operated by the former USSR, Japan, Taiwan (Province of China), Spain, the USA, Brazil, Uruguay, Mexico, and probably other countries. Especially important areas for these fisheries are the northwestern Indian Ocean, western and Central Pacific, eastern North Pacific, and North Atlantic. The bigeye thresher was formerly a very important component of the Cuban longlines fishery, and more recently has been taken in considerable numbers by longliners off the northeastern USA and by gillnets vessels off southern California (USA) and the eastern Atlantic (by Spanish vessels), and by longliners off Taiwan (Province of China; where about 220 t per year are landed). This species is also taken as incidental bycatch in fixed bottom and pelagic gill nets, in trawls, and as a rare catch of anti-shark nets off KwaZulu-Natal, South Africa. It has been caught by anglers with sportsfishing gear (rod-and-reel) in the USA, South Africa, and New Zealand, in some instances by anglers targeting swordfish at night and using luminous lures. It is listed as a record fish along with other threshers by the International Game Fish Association. Its meat is utilized fresh, smoked and dried-salted for human consumption, its liver oil is processed for vitamin A, its skin for leather, and fins for shark-fin soup.