Introduction
The thresher shark is an all-encompassing term referring to three surviving species from the family Alopiidae: the pelagic thresher (Alopias pelagicus), the bigeye thresher (Alopias superciliosus), and the common thresher (Alopias vulpinus).
Morphology
Thresher jaws are small. They have clearly not evolved to attack prey like tuna and seals; if anything, they look like they should be eating small crabs or scavenging chunks of floating debris that someone else took the time to kill. They would seemingly struggle to catch a live fish because the long snout overhangs their tiny mouth so much. That’s where the notorious tail comes in. The thresher tail is designed to be used like a whip to strike and immobilise prey [1], [2]. Such force occurs with a single tail slap that gas is diffused out of the seawater and rises to the surface in bubbles [1]. Typically, pelagic sharks hunt and capture one fish at a time; this strategy enables the shark to catch on average 3 fish, and sometimes more, in one go.
The easiest way to differentiate between the three species is the colouration. Common threshers are likely to be more grey and they lack any colouration above their pectoral fins; sometimes, they have white dots on the tips of their fins. Pelagic threshers have distinct colouration above their pectoral fins. The bigeye’s eye is visible from the top of the head and has characteristic groves above the eyes and gills.
Distribution
Threshers are pelagic, meaning they live in the deep, open ocean. The only thresher known to inhabit New Zealand waters is the largest of the trio, the common thresher [3]. The bigeye may also be found in NZ, but due to its habit of staying hundreds of metres below the ocean surface during the day, it is unlikely to be encountered if it is there at all [4].
Threats
The common and bigeye thresher are ‘Vulnerable’ to extinction under the IUCN Red List [3], [5], and the pelagic thresher is further threatened and classified as ‘Endangered’ [6]. Estimated global population reductions are as follows:
- Common thresher: 30–49% over 3 generations (76.5 years)
- Bigeye thresher: 30–49% over 3 generations (55.5 years)
- Pelagic thresher: 50–79% over 3 generations (55.5 years)
All three species are globally targeted for their meat, fins, skin, and liver oil [7], the latter commonly used in modern cosmetics and supplements. All species are usually retained if accidentally caught on commercial vessels [8]–[11]. In one study [15] conducted in the Indian Ocean, 13 out of 19 bigeye threshers caught on a commercial vessel’s longline were dead upon retrieval. In the same study, in the Atlantic Ocean, 412 out of 849 bigeye threshers caught on longlines were dead. On the same ships were deaths of stingrays and manta rays as well as blue, shortfin mako, silky, and smooth hammerhead sharks. One of the largest hubs for shark fin trading globally is Hong Kong [9], and threshers accounted for up to 3% of the total fin mass in the past [8].
Threshers are also popular among recreational, big game anglers. Although a tag and release method is more commonly practised nowadays, there is also a risk of post-release mortality. With threshers likely to be hooked from their tail due to their hunting style, a study of common threshers found 78% of tail-hooked sharks died after release [12].
The New Zealand commercial fishery is not exempt from thresher shark landings. A recent report by Fisheries New Zealand [13] saw that in 5 years from the 2014/15 to 2018/19 commercial fishing seasons, 149,916 tonnes of common thresher shark were caught by the core deep-water fleet. With the average 5-metre common thresher shark usually weighing in at 230 kg, we can infer that in the 5 years, the deep-water fleet landed approximately 651 individual thresher sharks.
Thresher vs swordfish
In April 2020, a dead, 4.5 metre, female bigeye thresher was found beached on a Libyan coast [14]. A 30 cm swordfish (Xiphias gladius) rostrum was found embedded in its head. It is understood that the rostrum severely injured some of the shark’s nerves, arteries, and gill arches. It was concluded that the impalement was what led to the ultimate death of the shark.
[1] S. P. Oliver, J. R. Turner, K. Gann, M. Silvosa, and T. D’Urban Jackson, “Thresher Sharks Use Tail-Slaps as a Hunting Strategy,” PLoS One, vol. 8, no. 7, 2013, doi: 10.1371/journal.pone.0067380.
[2] S. A. Aalbers, D. Bernal, and C. A. Sepulveda, “The functional role of the caudal fin in the feeding ecology of the common thresher shark Alopias vulpinus,” J. Fish Biol., vol. 76, no. 7, pp. 1863–1868, 2010, doi: 10.1111/j.1095-8649.2010.02616.x.
[3] C. L. Rigby et al., “Alopias vulpinus,” IUCN Red List Threat. Species 2019, vol. e.T39339A2, 2019.
[4] H. Nakano, H. Matsunaga, H. Okamoto, and M. Okazaki, “Acoustic tracking of bigeye thresher shark Alopias superciliosus in the eastern Pacific Ocean,” Mar. Ecol. Prog. Ser., vol. 265, pp. 255–261, 2003, doi: 10.3354/meps265255.
[5] C. L. Rigby et al., “Alopias superciliosus,” IUCN Red List Threat. Species 2019, vol. e.T161696A, 2019.
[6] C. L. Rigby et al., “Alopias pelagicus,” IUCN Red List Threat. Species 2019, vol. e.T161597A, 2019, doi: https://dx.doi.org/10.2305/IUCN.UK.2019-3.RLTS.T161597A68607857.en.
[7] R. W. Jabado et al., “The trade in sharks and their products in the United Arab Emirates,” Biol. Conserv., vol. 181, pp. 190–198, 2015, doi: 10.1016/j.biocon.2014.10.032.
[8] S. C. Clarke, J. E. Magnussen, D. L. Abercrombie, M. K. McAllister, and M. S. Shivji, “Identification of shark species composition and proportion in the Hong Kong shark fin market based on molecular genetics and trade records,” Conserv. Biol., vol. 20, no. 1, pp. 201–211, 2006, doi: 10.1111/j.1523-1739.2005.00247.x.
[9] A. T. Fields et al., “Species composition of the international shark fin trade assessed through a retail-market survey in Hong Kong,” Conserv. Biol., vol. 32, no. 2, pp. 376–389, 2018, doi: 10.1111/cobi.13043.
[10] S. C. Clarke et al., “Global estimates of shark catches using trade records from commercial markets,” Ecol. Lett., vol. 9, no. 10, pp. 1115–1126, 2006, doi: 10.1111/j.1461-0248.2006.00968.x.
[11] F. Dent and S. Clarke, “State of the global market for shark products,” Rome, Italy, 2015.
[12] C. A. Sepulveda et al., “Post-release survivorship studies on common thresher sharks (Alopias vulpinus) captured in the southern California recreational fishery,” Fish. Res., vol. 161, pp. 102–108, 2015, doi: 10.1016/j.fishres.2014.06.014.
[13] Fisheries New Zealand, “Annual Review Report for Deepwater Fisheries for 2018/19,” 2020.
[14] P. L. Jambura, J. Türtscher, J. Kriwet, and S. A. A. Al Mabruk, “Deadly interaction between a swordfish Xiphias gladius and a bigeye thresher shark Alopias superciliosus,” Ichthyol. Res., vol. 68, no. 2, pp. 317–321, 2021, doi: 10.1007/s10228-020-00787-x.
[15] R. Coelho, J. Fernandez-Carvalho, P. G. Lino, and M. N. Santos, “An overview of the hooking mortality of elasmobranchs caught in a swordfish pelagic longline fishery in the Atlantic Ocean,” Aquat. Living Resour., vol. 25, no. 4, pp. 311–319, 2012, doi: 10.1051/alr/2012030.
The Ocean: Explained 
