Creature Feature: Thresher Shark

Most prey species in the sea are concerned about avoiding a predator’s mouth. However, if the predator is a thresher shark, its tail is their biggest threat. Thresher sharks are in the order Lamniformes, along with mako sharks, basking sharks, and even great white sharks (the species that has taken the spotlight in representing sharks worldwide after the release of the classic movie Jaws in 1975). While the white shark may be the most iconic, there is great diversity in shark appearance and predation behavior. For example, although white sharks and thresher sharks are in the same order, it is very easy to distinguish between the two even at a glance.

Thresher sharks use their long, specialized caudal fin to hunt prey, whipping at shoals of fish with lightning-quick accuracy. [Taken by: Rafn Ingi Finnsson, Source]

The upper lobe of the caudal (or tail) fin in thresher sharks is very long.  The purpose of this tail design is to assist in hunting. They whip their long upper lobe to produce a thrust, or slap, which stuns their prey (Aalbers 2010). The thresher will then come back around to gulp up any of its immobile victims. Sometimes, the pressure from a tail slap can be so strong that it can even produce bubbles, as dissolved gases diffuse out of the water column (Oliver 2013).


A recording of a thresher shark using its tail to successfully hunt three sardines; notice the ease with which it initiates each successful strike. [Taken by: Jan Acosta/Oliver SP et al., Source]

There are three different species of thresher shark: the common thresher (Alopias vulpinus), the bigeye thresher (Alopias superciliosus), and the pelagic thresher shark (Alopias pelagicus). All  occur in the Pacific and Indian oceans, while the common and bigeye threshers also occur in the Atlantic and Mediterranean (Smith 2008). Although they all use the same creative hunting method, there are some subtle differences between thresher shark species. The common thresher shark is noted to be the fastest to grow and the largest among the three, while the pelagic thresher is the smallest. Specifically, the common thresher can reach total lengths of about 6 m, while the pelagic thresher only reaches lengths of about 3.3 m. To give a better sense of their size, 6 m is also about the height of a giraffe, and 3.3 m is about one-and-three-fifths as long as a queen-sized bed (The Measure of Things). The common thresher also has the longest estimated life span, ranging from 19-50 years. In contrast, bigeye threshers are thought to live for about 20-21 years and pelagic threshers are thought to live up to 29 years. These species also differ slightly in their distribution along the water column. Bigeye threshers can occupy much deeper waters than the common and pelagic threshers. Despite some slight differences in diet between bigeye and common threshers, they mostly feed on the same prey species, specifically small species of schooling fish and squid. Currently, the diet of the pelagic thresher, in particular, is largely unknown, but is assumed to be similar to that of the other two (Smith 2008).

A common thresher that was caught, tagged, and released alive during the 2009 Juvenile Shark Study off of the Channel Islands. The hose in the shark’s mouth pumps seawater across the gills to allow the shark to breathe while out of the water. [Source]

Though we know some basic facts about these species, including their famous hunting strategy, there is still a lot of research yet to be conducted, especially on the pelagic thresher. Globally, IUCN has ranked each of these species as vulnerable, but the common and bigeye threshers have also been ranked as endangered in the Mediterranean specifically. This is due to the fact that in the early 20th century, many Mediterranean fishers targeted sharks specifically because they were viewed as pests and as an interference to the growing number of fisheries. Thresher sharks in this area were noted to have declined over the years, but the significance of the decline is still relatively unknown (Ferretti 2008). Even knowledge about the impact of harvesting is not fully known because many fisheries do not keep detailed landing reports. Threshers are harvested for their fins and meat, and are even sought after by recreational fishers because of their ability to put up a fight and the high value of their meat. Along the Atlantic Coast, thresher sharks are under a fishery management plan, where possession of bigeye threshers is prohibited, and a bag and size limit of any Atlantic pelagic shark has been imposed (Smith 2008).

Thresher sharks are threatened by fisheries and often get tangled up in fishnets as bycatch. They are also targeted by avid recreational fishers due to their rarity as well as for their meat for a dish called shark fin soup. However, concerted conservation and fishery management plans can help protect these vulnerable species. [Taken by: Klaus Stiefel, Source]

Overall, sharks all around the globe are in danger due to the demand for their fins in order to make a dish known as ‘shark fin soup’. This soup was originally very popular in China, but that popularity started declining in 2011. However, the rate of consumption has now increased in other places such as Vietnam, Thailand, Indonesia, and Macau (Denyer 2018). Shark fisheries should be managed to help protect these targeted species, as they are very important to our oceans and are considered the top predators of the sea (The Ocean Portal Team 2018).

I hope that after reading this, you will see that there are more to sharks than what movies portray, and you will have a greater appreciation for their beauty and diversity. The thresher sharks are only three out of hundreds of different species of shark in the world, and they should not be feared, but respected for the crucial role they play in maintaining the balance of many marine ecosystems.


[Written by Sarah Neff, an undergraduate student majoring in Wildlife, Fish and Conservation Biology at UC Davis. She is very interested in shark ecology and social behavior and is currently assisting PhD candidate Alexandra McInturf on her meta-analysis of shark social behavior.]


References:

Aalbers, S. A., Bernal, D., & Sepulveda, C. A. (2010). The functional role of the caudal fin in the feeding ecology of the common thresher shark Alopias vulpinus. Journal of Fish Biology, 76(7), 1863-1868.

Denyer, S. (2018). Even as China turns away from shark fin soup, the prestige dish is gaining popularity elsewhere in Asia. Retrieved from The Washington Post, February 15, 2018.

Ferretti, F., Myers, R. A., Serena, F., & Lotze, H. K. (2008). Loss of large predatory sharks from the Mediterranean Sea. Conservation Biology, 22(4), 952-964.

The Measure of Things. Bluebulb Projects. Retrieved from https://www.bluebulbprojects.com/MeasureOfThings/default.php

Oliver, S. P., Turner, J. R., Gann, K., Silvosa, M., & Jackson, T. D. U. (2013). Thresher sharks use tail-slaps as a hunting strategy. PLoS One, 8(7), e67380.

The Ocean Portal Team. (2018, December 20). Sharks. Retrieved from https://ocean.si.edu/ocean-life/sharks-rays/sharks

Smith, S. E., Rasmussen, R. C., Ramon, D. A., & Cailliet, G. M. (2008). The biology and ecology of thresher sharks (Alopiidae). Sharks of the open ocean: biology, fisheries and conservation, 60-68.

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