Featured Elasmobranch

November 2004

PELAGIC THRESHER SHARK

ALOPIAS PELAGICUS (NAKAMURA, 1935)

FAMILY ALOPIIDAE (THRESHER SHARKS)

Identification: The pelagic thresher, like all thresher sharks, is distinguished by an extremely long tail fin that is as long as the rest of the body, moderately large eyes, but not extending onto the surface of the head, first dorsal fin about equidistant between the pectoral and pelvic fin bases, and pectoral fins with rounded apices. Coloration is a brilliant blue above, white below, but fading to gray rapidly after death.

Size: Maximum reported length is 387 cm.

Distribution: Oceanic and wide-ranging in the tropical and warm-temperate Pacific and Indian Oceans, but not found in the Atlantic.

Habitat: An active, strong-swimming shark ranging in depth from the surface down to at least 152 m. Although primarily an oceanic species it has been observed near coral reefs, in large lagoons, and over seamounts. It is highly migratory, but long-range movements for this shark are not well known. Genetic studies indicate a separation of stocks between the eastern and western Pacific Ocean. In southern California, it is associated with sea surface temperatures over 21°C.

Biology: Oophagous, a reproductive mode whereby embryos developing in the uterus feed on ovulated mature eggs. The size at birth is 158-190 cm. This species has unusually large young, with the largest fetus 41% of the length of the largest adult female. Males mature between 267-276 cm, with a maximum length of 347 cm. Females mature between 282-292 cm, and grow to a maximum length of at least 383 cm. Males mature between 7-8 years and live about 20 years. Females mature at 8-9 years and live about 29 years. Gestation time is unknown, and they do not appear to have a defined breeding season, as pregnant females have embryos at different developmental stages throughout the year. Behavior is poorly known, but the pelagic thresher has been witnessed repeatedly leaping (breaching) out of the water. This shark feeds on small pelagic fishes and squid, often using its large tail as a whip to stun or kill prey.

General interest: An important species off Taiwan, with about 222 t landed yearly, and in the central Pacific. It enters the commercial shark catch off California and is sometimes caught in considerable numbers off the Pacific coast of Mexico and Gulf of California. The pelagic thresher is utilized for its meat (for human consumption), liver oil for vitamin-A extraction, hides for leather, and fins for shark-fin soup. Divers have viewed and photographed this shark on coral reefs and seamounts in the Gulf of California, the Red Sea, Indonesia and Micronesia. Recently, the Filipino island of Malapascua has become a center for sightings of pelagic threshers, which can be seen regularly on the edge of an underwater shoal just offshore from the island. The conservation status of this shark is uncertain, but it is considered vulnerable to overexploitation do to its very low fecundity and relatively high age at maturation. Despite its size, these sharks have small teeth and are relatively harmless to people.

By Tonatiuh Trejo

Pacific Shark Research Center, Moss Landing Marine Laboratories
8272 Moss Landing Road, Moss Landing, CA 95039
ttrejo@mlml.calstate.edu