Featured this month is the Common Thresher Shark (Alopias vulpinus). With its distinctively large pectoral fins, dark blue-gray upper body and white belly and the unmistakable tail fin with its elongated upper lobe, that can grow up to the length of the body itself, this species is a truly captivating sight.
There are three species of thresher sharks in the world:
- Long-tailed or Common Thresher Shark,
- Bigeye Thresher Shark (Alopias superciliosus),
- Pelagic Thresher Shark (Alopias pelagicus).
The Common and Bigeye Threshers are thought to occur in all our Dutch Caribbean waters and are confirmed in the Windward Islands. Both species are also considered marine flagship species for the island of Saba. The Pelagic Thresher is strictly an Indo-Pacific species. All three species are globally listed on the IUCN Red List of Threatened Species as “Vulnerable” because of their declining populations.
The Common Thresher Shark is the largest of its kind and can grow up to 6.1 metres (20 feet) and weigh up to 500 kilogrammes (1,100 pounds). They eat pelagic schooling fish such as sardines, mackerel or juvenile tuna. Like many shark species, they are ovoviviparous (or live-bearing), which means that fertilisation of the eggs and embryonic development occur internally, resulting in small litters of two to four large, well-developed offspring, up to 150 centimetres (5 feet) at birth. When the young fish exhaust their yolk-sacs while they are still inside the mother, they begin feeding on the mother’s unfertilised eggs; this is known as oophagy.
Thresher sharks are slow to mature, only reaching sexual maturity between 8 and 14 years of age. They may live for 20 years or more. These slow life history traits mean a low capacity to recover from the high levels of largely unmanaged target and bycatch fisheries.
This beautiful, but elusive creature holds many mysteries. In July 2013 the mystery of why it has such a long tail was resolved when an article was published by the Thresher Shark Research and Conservation Project of the University of Liverpool in which they finally found confirmation that Thresher Sharks hunt with their tail. It was expected that they use their tail to stun, or even kill their prey, but since Thresher Sharks hunt in the open ocean, and usually at night, solid proof, and the exact method of hunting, had never been documented before.
This is how it goes: Instead of lashing out sideways with its tail, what was believed to be case, the Thresher Shark accelerates towards a ball of fish and brakes sharply by using its large pectoral fins. Next, it tilts its head down, throws its whole body forward, and flexes the base of its tail, catapulting the tail tip over its head with speeds that can go over 100 kilometres per hour. When the tail hits its target, the prey fish are often mortally wounded and all the shark has to do is swim around and swallow the pieces at their leisure. However, these attacks only seem to be successful once in every three attempts.
(Click HERE to view a hunting Thresher Shark in action).
Thresher Sharks are a vulnerable species and populations have fallen around 75% in the past decade alone after being targeted by fisheries for their meat, or being taken as bycatch by sardine fisheries. Understanding how thresher sharks feed and how they use their habitat will help manage and protect them more effectively.Sources: The Guardian – [Thresher sharks use their tails like bullwhips to kill or stun prey]. National Geographic – [Thresher Sharks Hunt with Huge Weaponised Tails] IUCN Red List of Threatened Species Reference: Oliver, S.P.; Turner, J.R.; Gann, K.; Silvosa, M. & D’Urban Jackson, T. (2013) Thresher Sharks Use Tail-Slaps as a Hunting Strategy. PLOS ONE. 8(7): e67380. http://dx.doi.org/10.1371/journal.pone.0067380 Related stories: ST. MAARTEN – Protection for Sharks