The thresher shark is easily recognizable by its large upper caudal fin. This tail fin may often be 50 percent of the total length of the shark. It has a short snout and large eyes placed forward on the head. The second dorsal fin is much smaller than the first. The thresher is a strong swimmer and can leap clear of the water. The jaws are small with small, curved sharp teeth without basal cusps or serration. Colour varies from brown to black with metallic hues from above and irregular white markings on the underside.
The thresher ranges through all warm and temperate areas of the world’s oceans. Its northernmost range in the western Atlantic is eastern Newfoundland and it ranges all the way down the Atlantic to the West Indies and northern South America. The thresher shark is a summer visitor to the Canadian Atlantic region. They have been captured from July to November, but most frequently during August and September.
The thresher often swims at the surface of coastal waters. However it can also occur at depths of 350 meters (1,150 feet) or more. The young may be found inshore in shallow water. They prefer temperate to tropical waters.
In Canadian waters sizes have ranged from 3.3 to 5.5 meters (10.8 to 18 feet) long. The maximum size recorded for this species is 6.1 meters (20 feet), however they are generally between 2 to 5 meters (10 to 16.5 feet) in length. Maximum lifespan is about 45 to 50 years. They are seasonally migratory with no evidence of transoceanic migrations.
The thresher shark eats schooling fish, such as herring and mackerel, and cephalopods such as squid. The large caudal fin is used to slap the surface of the water forcing fish to form tighter schools; the tail can then be used as a whip to stun or kill the prey.
This shark is ovoviviparous, with the eggs being hatched inside the female, and 2 to 6 live pups being delivered at a size of 1.5 meters (5 feet) long. During development the young may cannibalize their siblings within the uterine chamber. They apparently use inshore areas as nursery grounds. This shark matures between 3 and 8 years of age with males maturing at 3.3 m and females at 4.2 m. Gestation lasts for about 9 months with birth occurring in the spring.
Interaction with People
Mainly sought after as a sport fish, the thresher poses little threat to people. It is not considered harmful but can be a nuisance to mackerel fishermen.