Big-Eye Thresher Shark

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ly cool and oxygen levels are low. Their enormous eyes are positioned on top of their head to enable the shark to focus on species specifics prey that is silhouetted by the down- welled light. Similar to many pelagic species, the big-eye thresher vertically BY CHUGEY SEPULVEDA, Ph.D., migrates at night to feed in the productive AND SCOTT AALBERS, M.S. surface waters. This feeding strategy allows them to inhabit very productive forage zones both during the day and night. Although the feeding ecology of the big-eye thresher is not well studied, it is hypothesized that this species, just like the common thresher, uses its tail to sub- due prey before it is consumed. Support for this comes from long-line data show- ing that they are often tail-hooked, as well as reports of big-eye threshers hooked in the tail while slow-trolling BIG-EYE baited lures in Southern California. Prey consists primarily of a wide variety THRESHER of fish and squid that make up the deep scattering layer, including hake, lantern- SHARK fish, and some other pelagic species. (Alopias Reproduction superciliosus) The big-eye thresher gives birth to only two, well-developed pups at a time. Pups are born at a large size (greater than four feet), and despite precise gestation data, it is believed that gestation is around one year. Similar to many pelagic sharks, males Angler Jack Sepulveda –Angler Jack Photo courtesy of PIER reach maturity at a slightly smaller size orldwide, there are three species of thresher shark Because its meat does not keep well than the females, which mature at around (Family Alopiidae), all of which possess an extremely and is not as firm as that of the com- 11 feet in total length. mon thresher, the big-eye thresher long, whip-like tail. Although the common thresher shark has a low market value; therefore, Conservation Status (Alopias vulpinus), pelagic thresher (A. pelagicus), they are not targeted in California and Fortunately for the big-eye thresher, there W A. superciliosus and big-eye thresher ( ) sharks typically segregate are not usually retained for sale. Out- are no large-scale targeted fisheries for side of California, big-eye threshers are this species. Further, because they spend based on different environmental pref- farther back on the body and a tail that caught on tuna and swordfish long-line so much of their existence well beneath erences (i.e., water temperature, depth), is slightly shorter than that of the com- gear, particularly deep-set operations the surface waters, there is limited inter- the waters off southern California are mon thresher. targeting big-eye tuna. Again, most of action with most of the commercial gear unique in that all three species can be Ranging from tropical to temperate the industrialized operations around types used today. Nonetheless, this caught off our productive coast during seas, big-eye threshers have a broad dis- the world do not retain big-eye thresh- species is slow-growing, gives birth to the same season. tribution, and they frequent Southern er sharks because there are many other only a couple of pups, and has long ges- In Southern California the most California waters in the summer and fall more lucrative species that can fill the tation periods, characteristics that make abundant species is by far the common months. Typically, these sharks occur at valuable hold space. However, in many all pelagic sharks extremely vulnerable to thresher, a species that has bounced back great depths, well below the surface of the artisanal fisheries of Latin Amer- overexploitation. from heavy exploitation during the during the day, and therefore are rarely ica, the big-eye thresher is a welcome 1980s and now supports a growing rec- encountered by recreational fishers. catch that receives a similar price to PIER is a non-profit 501(c)3 research insti- reational fishery. The second most ab- other shark species. tute dedicated to scientific research, educa- undant thresher is the big-eye or brown Fisheries tion, and the sustainable manage ment of the thresher. Big-eye threshers are easily Although big-eye threshers are occa- Ecology marine environment. Special thanks are distinguishable from the common thre- sionally caught on hook and line (usu- In southern California the big-eye offered to Mr. Thomas Pfleger and Family, the sher by the presence of lateral grooves ally while fishing at night or deep- thresher has a similar ecological niche to George T. Pfleger Foundation, the Harris along the top of the head, which give dropping during the day), the vast that of the swordfish. During the day, Foundation, Tommy Fullam, Billy Seiler, Lor- the appearance of a helmet-like head majority of those landed in California these sharks remain well beneath the raine Bohnet, and Vicki Wintrode. To read with very large, upward-looking eyes. are caught in the drift gillnet fishery thermocline, hunting prey at great more about PIER research projects please visit They also have a dorsal fin that is set for swordfish. depths, where temperatures are extreme- us at www.pier.org.