Known by a variety of names, the thresher shark is characterized by its well-muscled tail, the upper lobe of which is usually as long as the rest of the body. These sharks use their tails to herd baitfish into a mass by slapping or thrashing the water, then stunning or injuring fish before swallowing them.
Grayish to dark charcoal in color, the thresher shark turns abruptly white on the belly and may be mottled on the lower half of the body. The thresher is further identified by the absence of a keel on the caudal peduncle; by its small, pointed, and broad-based teeth; and by its comparatively smooth skin.
Longtail and pelagic threshers have moderate-size eyes, and the first dorsal fin is set almost directly in the middle of their backs and far ahead of the beginning of the pelvic fins. The Atlantic and the Pacific bigeye threshers have much larger eyes, and the rear margins of the dorsal fins are located at least as far back as the origin of the pelvic fins.
Threshers are excellent food fish, comparable to mako and swordfish, and they are outstanding fighters (the longtail has been known to leap out of the water). Thresher sharks were more popular than makos off California until recently and are a relatively rare catch along the U.S. Atlantic coast, although specimens in the 300- to 600-pound class are the most common size encountered from New Jersey to Massachusetts.
The largest threshers have come from New Zealand, where they've been boated in excess of 800 pounds. The all-tackle world record for A. vulpinus is a 767-pound, 3-ounce fish taken off New Zealand in 1983.