Thresher Sharks, It’s All In The Tail

It comes out of the murky dark waters below, it huge and as it turns away you know what it is, you have spotted the shy thresher shark. The most recognized sharks after the great white and the whale shark has to be the thresher shark. Their swept-up caudal fin is about half their total body length, which makes them easy to identify.

Thresher Shark Family

There are three different species within the family: the Common Thresher (Alopias Vulpinus), The Big Eye Thresher (Alopias Superciliosus) and the Pelagic Thresher (Alopias Pelagicus). They are all on the IUCN Red List as vulnerable.

According to their website:

“All members of genus Alopias, the thresher sharks, are listed as Vulnerable globally because of their declining populations. These downward trends are the result of a combination of slow life history characteristics, hence low capacity to recover from moderate levels of exploitation, and high levels of largely unmanaged and unreported mortality in target and bycatch fisheries.”

The Common Thresher is the most commonly encountered and are the largest ranging from 6.5 feet at maturity to 16 feet (2 to 5 meters) for an adult male. An average weight for an adult male would be around 500 pounds or 230 kilograms. Larger and heavier Common Threshers have been recorded, while females are slightly smaller. The common thresher is wide spread, found in the warm and temperate waters around the world. They live in deep waters off the coast and are known to come to shallow waters to feed. These are a solitary fish and seldom seen in the company of other sharks. Sightings of thresher sharks are infrequent. Not only are they seldom in the same waters as divers, they are shy and seem to be frightened off by the air bubbles of divers. The thresher sharks pose little danger to humans, first they will run away and if they do not, their mouths are relatively small with smaller teeth than most shark families.

The Big Eye Thresher is about the same size as the Common Thresher, the most noticeable difference is the eyes. The Big Eye Thresher has a much larger eye socket, it about the size of a softball.

The third member of the family is the Pelagic Thresher. It is smaller than the Common Thresher, with an average length of only 10 feet (3 meters). When sighted it is often misidentified as a Common Thresher as there is little difference in the appearances and what is different is difficult to see from a distance or from a moving shark. They spend more of their time in deeper water commonly found living at depths to over 500 feet (150 meters). The few studies that have been done on Pelagic Threshers show that they generally isolate themselves from others, selecting “territories” separated by distance and depth. They are found in the Indo-Pacific including the Red Sea. Pelagic threshers are known to breach and able to perform multiple breaches.

It is the Tail

For centuries, the whip-like tail of the thresher shark has awed people. Aristotle had mentioned the tail of the thresher in some of his writings. He gave fanciful stories on how the shark could use that tail. As far back as the early 1600’s it was suggested that the tail was used as a whip. In the late 1800s into the late 1900s, observations where the shark seem to be using its tail to stun fish were discounted. However, in the late 1990s, photographic and video evidence started to prove that the early observations may not have been far off. It is now known that the thresher shark can use its tail to stun fish, and in a manner not really considered before. Thresher sharks are fast but they will slowly approach a bait ball. A bait ball is a school of fish that are packed close together for protection. Sardines and similar fish do this. They will circle the fish bringing them in closer and then suddenly rush into the bait ball. They will then use their tail against the fish. However, they tuck their head down and bring the tail up and over their body. The speed that the tail moves exceeds 50 miles an hour. While the tail might strike a few fish, the energy release is such that a cavitation wave is produced. The force of the wave can stun many more fish than the tail touches. The shark then eats the fish before they recover. Studies of this method have shown the energy released is great enough to break water molecules into gas. The National Geographic website has an interesting article “Thresher Sharks Hunt with Huge Weaponised Tails” that expands on the thresher shark use of its tail as a weapon.

Malapascua Philippines Thresher Shark Central

While the thresher shark is elusive and seldom seen together, there is one place where they are commonly found. Malapascua island is a small island located a few miles off the coast of the Philippine Island of Cebu and is a part of the Province of Cebu. Located a 30-minute boat ride from Malapascua island is the Monad Shoal. The shoal is a seamount that comes from deep water to about 70 feet from the surface. It is here that much of the scientific studies on the threshers sharks have been done and it is the only location that recreational divers have a reasonably good chance to see a thresher shark close up.

In 1996, the local fisherman told a couple of visiting divers about breaching sharks. They came back the next year to find out for themselves and then open the first dive center. They found that the shoal was acting as a cleaning station for Pelagic Threshers. On average there are five different cleaning stations that the thresher sharks, as well as other pelagics, visit daily. The most active time is around sun rise, however, studies have found the stations are active all day.

Divers now come to this laid back and relaxing location to dive with the thresher sharks. To control the interaction and minimize any negative impact, the local dive operators have created a schedule to limit the number of divers on the shoal and each station at a time. Divers are limited in movements to selected viewing points. From here the diver can observe the sharks as they come up from the depths and watch as they are approached by the cleaning fish.

If you are a shark lover, this must be on your bucket list.