Biology of the Common Thresher

(Alopias vulpinus)

Field Marks Upper caudal lobe long and curving, about as long as rest of shark; labial furrows at mouth corners; teeth without basal cusplets; eyes relatively small, not expanded onto top of head; no deep groove behind eyes; pectoral fins falcate with pointed tips; first dorsal fin base closer to pectoral fin free rear tips than pelvic fin origins; white color of abdomen extending over pectoral fin bases as conspicuous patch. Color black above shading to brown or blue-grey on flanks, with metallic luster.
Size Pups at birth are 45-61 in (114-155 cm) in length and 11-13 lb (5-6 kg) in weight; specimens 6.5-16 ft (2-5 m) long and up to 500 lb (230 kg) are common; maximum length at least 18 ft (5.5 m), and possibly to 20 ft (6 m).
Range Virtually circumglobal in warm seas.
Habitat Coastal over continental and insular shelves and epipelagic far from land in tropical and warm temperate waters; young often close inshore and in shallow bays. Populations in northwestern Indian Ocean show spatial and depth segregation by sex January-May (pupping season). Depth range from the surface to at least 1,200 ft (366 m); most longline catches at about 590 ft (180 m). Active and strong swimming, sometimes leaping out of the water.
Feeding Preys mostly on small schooling fishes, including mackerels, bluefishes, herrings and sardines, needlefishes, lancetfishes and lanternfishes; also feeds on squids, octopuses, and pelagic crustaceans, and - rarely - seabirds (one report of an individual killing and eating a loon off Ireland). Herds and stuns prey using long, whip-like caudal fin; often caught on longlines foul-hooked through the tail (apparently while striking at bait or hooked fish with caudal fin). Apparently hunts in small, loose groups, with the size of the group related to abundance of prey.
Reproduction Ovoviviparous, developing embryos oophagous; number of pups 2-7 (usually 2); pups are born during summer off Sicily and in northern Adriatic Sea, March-June off southern California, and between January and May near the surface in the northwestern Indian Ocean. No data on gestation period.
Age & Growth Regionally variable; off Australia, males mature at length of about 8.5-11 ft (2.6-3.4 m), females at about 11.5-13 ft (3.5-4 m); in equatorial Indian Ocean, 80% females mature at a length of 8.9-10.8 ft (2.7-3.3 m) and a weight of 134-22 lb (61-100 kg); off southern California, males mature at a length of about 11 ft (3.3 m) and at age 3-7 yrs, females at a length of 8.5-10.3 ft (2.6-3.2 m) but no data on age at maturity of females. Maximum longevity about 15 yrs.
Danger to Humans Probably minimal, but large size and powerful tail should invite respect; SAF lists 4 attacks on boats and 1 provoked attack attributed to generic Alopias, possibly this species; unconfirmed report of a fisherman on the North Atlantic coast of US decapitated by a tailstroke from a thresher, and a dubious report of an attack on a spearfisherman off New Zealand. Divers have occasionally encountered small individuals near rocky reefs and kelp beds; shy and difficult to approach underwater, typically swimming near the surface or bottom, circling at the limit of visibility without showing overt aggression.
Utilization Meat highly prized, used fresh, smoked and salted; fins valuable for sharkfin soup, leather and liver oil also used. Caught in oceanic longline fisheries operated by Russia and Japan, especially in the northwestern Indian Ocean and the central Pacific; taken as bycatch of Japanese tuna longline fishery off southern Australia, also taken with anchored bottom and surface gillnets, floating gillnets, and sportfishing gear; until recently, the object of an important pelagic gillnet fishery off southern California, but the species has not responded well to increased fishing pressure. Regarded as a gamefish, putting up an energetic fight when taken on rod and reel; IGFA all-tackle record is 767 lb 3 oz (348 kg), caught off Cape Brett, Bay of Islands, New Zealand, in February 1983.
Remarks Existence of a cryptic sibling species - closely resembling A. vulpinus - was revealed through gel electrophoresis in a 1995 study by Blaise Eitner; this as-yet un-named species occurs off southern California, nothing its known of its biology.

Sporting a resplendent, scythe-like caudal fin - as long or longer than its body - the Common Thresher is among the most instantly-recognizable of sharks. Small wonder that astonished descriptions and fanciful speculations about this remarkable fish date back to antiquity. One of the earliest of these speculators was the Greek philosopher Aristotle (384-322 B.C.). Aristotle although he made observations of the natural world (he was probably the first to note that sharks and rays, unlike other fishes, copulate; he also devised the term "clasper" to describe the organs with which male elasmobranchs held the females during sexual union) was not really a scientist, as he conducted no experiments, relying instead on pure reason to enable him to figure out how the world 'worked'.* In his encyclopedic Historia Animalia, Aristotle wrote of a clever trick allegedly demonstrated by hooked Threshers, in which the shark swims up the fisherman's line and bites it asunder, thereby freeing itself. Aristotle also wrote of Threshers' supposed habit of protecting their young by swallowing them (a behavior he also attributed to catsharks and electric rays). No wonder that the Thresher Shark was known to the ancient Greeks as alopex, which means "fox", in reference to its supposed cleverness. Neither of these improbable behaviors is supported by scientific evidence, but Aristotle's formidable reputation made him an unquestionable authority for nearly 2 000 years. Remarkably, neither of the fantastic scenarios reported by Aristotle has anything to do with the Thresher Shark's unusually long tail.

Perhaps the most dramatic and unlikely story about Threshers concerns their supposed partnership with Swordfish (Xiphias gladius) in attacking and subduing whales. One such co-operative battle is recounted in Hakluytus Posthumus or Purchas His Pilgrimes, a continuation of Hakluyt's Voyages compiled by Samuel Purchas and published in 1625. According to Purchas' account, the Thresher circles a hapless whale and uses its whip-like caudal fin to beat the ocean surface to a froth, thereby confusing the victim. With the whale thus distracted, the Swordfish swims beneath it and uses its blade-like rostrum (snout) to pierce the defenseless whale in a vital spot, thus dispatching it with ruthless efficiency. These two long-appendaged partners the Swordfish augmented in the fore and the Thresher in the aft are then free to feast on cetacean meat. Unfortunately for this nifty scenario, both the Thresher and the Swordfish lack the dental equipment necessary to feed on a whale and neither is known to regularly include whale meat in its diet.

How could such an improbable story become established as maritime lore? Since pods of Killer Whales (Orcinus orca) are well known to attack, kill and feed upon even very large cetaceans, it has been suggested that the tall, scimitar-shaped dorsal fin of males of this species, splashing above the surface, may be mistaken for the caudal fin of a Thresher Shark. Swordfish have long been known to puncture ocean-going vessels (ramming through even copper-plated hulls, often resulting in the rostrum breaking off) and to glance off the steel hulls of deep-sea submersibles (one individual attacked the Woods Hole Oceanographic Institution's submersible Alvin at a depth of 1,985 feet [605 metres] and its sword became wedged in a seam in the hull such that it could not break free). Commercial whalers and marine mammal scientists have reported finding Swordfish rostra imbedded in Blue (Balaenoptera musculus) and Fin (Balaenoptera physalus) whales. It is improbable that Swordfish attack whales and submersibles in order to feed upon them or our of sheer aggression; instead, it seems more likely that these inedible objects were simply in the wrong place at the wrong time (a Swordfish at speed - as when chasing down schooling prey - would be about as agile or maneuverable as a bullet in full flight, and therefore unable to avoid large objects in its path). Blue and Fin Whales are, however, known to be preyed upon by Killer Whales. Both Swordfish and certain toothed and baleen whales feed on small schooling fishes and squids that, in turn, feed on still smaller fishes, krill and plankton. These puny packages of protein - in turn, yet again - provide food for certain baleen whales, such as the Blue and Fin Whale. It is therefore not surprising that Swordfish and whales are occasionally found together in patches of ocean that offer particularly rich feeding. It is easy to imagine how whalemen or others who spend significant time at sea could mistake a Killer Whale's dorsal fin for a Thresher's tail and - recalling seeing or hearing tell of Swordfish rostra imbedded in whales - put the pieces together in a way that may have seemed plausible at the time, but which we now believe to be in error.

If Thresher Sharks do not use their whip-like caudal fins to distract or confuse whales, for what purpose do they use them? Dramatic features, such as the caudal fin of thresher sharks, often seem to cry out for dramatic explanations. So it is perhaps unsurprising that reports of Threshers using their tails to hit or stun prey are persistent in the literature. Irish ichthyologist Harry Blake-Knox, who had long decried "textbook ichthyologists" who never went to sea or studied actual specimens, is responsible for one of the most astounding reports of Thresher predatory behavior. According to Blake-Knox, in winter 1865 he actually saw a Common Thresher rise up to the surface of Dublin Bay, use the tip of its caudal fin to swat a wounded diver (probably Gavia immer, the Great Northern Diver, known in North America as the Common Loon), and then swallow it. Despite Blake-Knox's otherwise solid reputation and well-known stand on the need to back up ichthyological theory with observation in the wild, some authorities doubt Blake-Knox's report. In 1927, prominent American zoologists John Treadwell Nichols (who in 1915 founded Copeia, one of the most venerated journals of ichthyology and herpetology) and Charles M. Breder, Jr. (who made many fundamental ichthyological discoveries) stated that the Thresher Shark's tail is not sufficiently rigid or muscular to strike an efficient blow.

Fetus of Common Thresher Shark

Misgivings about Blake-Knox's report notwithstanding, it has long been assumed that the Common Thresher uses the upper lobe of its long, slender caudal fin to herd its prey. Known prey of the Thresher consists mostly of small schooling fishes, such as mackerels (family Scombridae), Bluefish (Pomatomus saltatrix), herrings (Clupeidae), needlefishes (Belonidae), and lanternfishes (Myctophidae), but it also takes large, solitary fishes such as lancetfishes (Alepisauridae) as well as squids, pelagic octopuses and crustaceans. In the vastness of the open ocean, small schooling fishes and squids would certainly be easier to catch if concentrated somehow. But French shark biologist Paul Budker has seriously questioned the traditional assumption that the Thresher uses its tail as a food-gathering device. In his 1971 book, The Life of Sharks, Budker points out that, when faced with a predator, the natural reaction of most schooling or herding animals is to bunch closer together; thus, a Thresher Shark could do just as well concentrating prey with an ordinary caudal fin and has no need to act like an aquatic sheepdog. Ironically, this is precisely the sort of armchair ichthyology for which Blake-Knox had no patience.

There are too many reports of Thresher Sharks herding and using their caudal fins to strike prey to discount them all. Some of these reports are buttressed by compelling circumstantial evidence. In a 1972 paper, Russian ichthyologist Ye. P. Gubanov reported that 97% of all Common Threshers caught on pelagic longlines in the northwestern Indian Ocean were foul-hooked through the upper lobe of the caudal fin, suggesting that the sharks were snagged as they tried to stun hooked fishes. There are also field observations of Thresher Sharks using their hyperextended caudal fins to capture fish prey, reported in the scientific literature by credible witnesses. In the same 1915 paper in which he described apparently co-operative feeding in Sandtiger Sharks (Carcharias taurus), shark-watcher Russell J. Coles reported a most remarkable observation. Late in July of 1914, in shallow waters of the bight of Cape Lookout, North Carolina, Coles actually saw a Common Thresher using its caudal fin to toss fish into its mouth; one fish the shark had failed to seize in its mouth was seen to be thrown a "considerable distance". In July 1923 - some four years before Nichols and Breder expressed their doubts that a Thresher's tail was capable of such feats -there appeared in the prime American journal, Science, a note by oceanographer W.E. Allen that seems an unimpeachable report of a Thresher Shark using its caudal fin to stun prey.

While collecting plankton at about 7:25 in the morning of 14 April 1923 off the pier at Scripps Institution of Oceanography (near La Jolla, California), Allen heard a nearby splash. At a distance of about 100 feet (30 metres), saw a swirl in the water like that made by a sea lion. A moment later, a three-foot- (one-metre-) long, slender, flattened tail flashed above the surface and lashed about "like a coachwhip". At about 7:45, roughly 50 feet (15 metres) from the end of the pier, Allen glimpsed a shark coming toward the surface, swimming rapidly. Almost immediately, Allen saw a small fish -which he thought might have been a California Smelt (Atherinopsis californiensis) - swimming frantically just in front of the shark. A moment later, the pursuing shark - now clearly identifiable as a six-foot (2-metre) Thresher Shark - passed partly (about half-way) ahead of its prey, turned quickly, and gave the same coach-whip lash with its caudal fin that Allen had seen earlier. The whip-stroke was repeated immediately, "with very confusing speed" and it became evident that the victim was seriously injured. At this point, however, the fish was almost under the drip from Allen's net, which apparently frightened the Thresher into darting away. The injured fish sank, swimming feebly, then came to the surface and lay on its side for a while, until eventually it sank out of sight. Although Allen did not actually see the Thresher Shark eat the fish it had injured, the speed and skill with which the Thresher struck its prey and the accuracy demonstrated in its lashes at a single, fast-moving target greatly impressed Allen. Anyone who has ever tried catching goldfish bare-handed or picking up objects with his toes cannot help but be impressed also.

Dorsal (top) and lateral (side) view of the
chondrocranium (skull) of a Common Thresher.

If the Thresher's caudal fin is capable of at least some of the amazing feats with which it has been credited, it must add considerably to the shark's overall propulsive power. Although when hooked it does not perform repeated acrobatic leaps like the Shortfin Mako (Isurus oxyrinchus), the Common Thresher ranked as a game fish by the International Game Fish Association (IGFA), putting up a strong, determined fight and even leaping on occasion. Some of these leaps are quite spectacular. One specimen was recently photographed leaping completely out of the water by a full body-length, a spectacular image captured by Bernie Tershy and Craig Strong against a backdrop of hazy, purple mountains (Sea of Cortez?). Legendary shark fisherman Frank Mundus (who served as a real-life model for the character Quint in the fictional novel and movie JAWS) has characterized the Common Thresher as "exceedingly stubborn" and, comparing this species with the Shortfin Mako, states that it is "pound for pound, a harder fish to whip".

Wielding such lethal, high-flying power raises the question of whether the Common Thresher Shark is dangerous to human beings. There are only five attacks by thresher sharks (species unidentified) against humans recorded in the International Shark Attack File (SAF), one provoked and four on boats. Of course, most wild animals will defend themselves if provoked. And since threshers are fished commercially and for sport, so-called 'attacks' on boats by these sharks could be considered provoked as well - they were probably due to accidental slamming into the vessel during desperate escape attempts rather than unalloyed aggression. There is, however, one - completely unverified - report of a person killed by a Common Thresher.

According to this third-hand report, a hapless longline fisherman off the Carolinas hooked something large. When he leaned over the side to see what he had caught, the caudal fin of a huge Thresher Shark (estimated to be 16 feet or nearly 5 metres long) whipped up and decapitated him with the dispatch of a guillotine. The man's body slumped back into the cockpit. His head fell into the water, where it could not be recovered. This story is recounted in Mundus' book, Sportfishing for Sharks, which he allegedly heard from an un-named New Jersey school chum whose father suffered the accident. While the tail of the Thresher Shark is exceedingly powerful and commands respect (it could easily shatter a limb or break ribs), this story seems highly improbable.

But, then again, the Common Thresher is a highly improbable shark.

Ecology of the Common Thresher


* Despite this methodological shortcoming, Aristotle was astonishingly successful in his time, having been a student of Plato, tutoring Prince Alexander of Macedon (who himself went on to become Alexander the Great, conquering most of the known world by the time he was 22, thereby demonstrating the power of a good education coupled with fortuitous parentage: his father was King Philip of Macedon, whom he succeeded in 336 B.C.), contributing to and unifying the whole field of human knowledge, at it was at that time from history to mathematics and logic to ethics and establishing in Athens his famous school, the Lycaeum. [Return to text]

 

ReefQuest Centre for Shark Research
Text and illustrations R. Aidan Martin
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