A unique shark in an unusual place: on the 20th of May 2020 a male pelagic thresher shark (Alopias pelagicus) washed up on the beach of Sodwana Bay.
As its name infers the pelagic thresher shark usually resides in the open ocean (pelagic zone) occurring to depths of approximately 150 meters. So how did this shark find itself washed up on the relatively shallow shore of Sodwana Bay?
Sodwana Bay is known for its bustling shallow coral reefs which extend for a few kilometres from the shore, but Sodwana also has a narrow continental shelf and near shore canyons reaching hundreds of metres in depth. Pelagic thresher sharks are most likely feeding on schooling fish in these canyons and are known to occasionally follow their prey into shallower waters.
In the Philippines pelagic threshers regularly visit cleaning stations on coral reefs where cleaner wrasses remove parasites and irritations from the sharks’ skin. Maybe this could explain the occurrence of this shark in the shallow waters off Sodwana Bay. Unfortunately the cause of death of this individual is unknown.
Along with other pelagic species such as the great white and mako sharks, thresher sharks belong to the order of Mackerel sharks (Lamniformes). There are three existing species of thresher sharks namely the; pelagic, common and bigeye thresher, all of which share these distinct characteristics:
- An extraordinarily long whip-like tail which is used to stun prey.
- Slender bodies with small dorsal fins and very large pectoral fins.
- Short conical snouts and large eyes.
Compared to the other thresher sharks, the pelagic thresher is the smallest of the three species typically reaching 3m in length; it also has the longest upper tail (caudal) fin lobe relative to its body size. For more differences between the thresher shark species see the FAO pelagic shark guide below.
The IUCN assessed the pelagic thresher shark as endangered in 2019, with this species frequently being hunted for the meat, liver, skin and fins (Seitz, 2008). A. pelagicus is also a highly valued sport fish where the shark is often caught by its tail, which it uses to try and stun the bait that fishermen have cast out.
Video of thresher using its tail:
Due to the species slow growth and low reproductive rate of only two pups per breeding cycle, populations of the pelagic thresher sharks are declining. Although it is tragic to see such a special creature stranded on the Sodwana Bay shoreline it is nice to know that they are still around.
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References:
Compagno, L. J. V. (2002). Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date. 2. Rome: Food and Agricultural Organization.
Oiver, S. (2005). The behaviour of pelagic thresher sharks (Alopias pelagicus) in relation to cleaning fish (Labroides dimidiatus & Thalasoma lunare) on Monad shoal, Malapascua Island, Cebu, Philippines. MSc Thesis, University of Wales, Bangor.
Seitz, J. C. (2008). Pelagic Thresher. Florida Museum of Natural History.
Author: Jamie Ayliffe - Sharklife Researcher
