Pelagic Thresher, Smalltooth Thresher, Fox Shark.
Proportionately small head. Snout narrow, with a rounded tip. Labial furrows absent. Teeth very small. First dorsal fin relatively small. First dorsal origin posterior to pectoral fin free rear tip. Second dorsal fin very small. Second dorsal origin slightly posterior to pelvic fin free rear tip. Upper caudal lobe extremely long; almost as long as body.
Dorsal coloration metallic blue. Silvery on the flanks near the gills. Ventral coloration white.
Maximum length 365cm. Size at birth 130-160cm.
Oceanic in tropical/sub-tropical seas and inshore. Surface to 300m.
Resident throughout the warmer latitudes of the Pacific and Indian Oceans. Absent from the Atlantic.
The pelagic thresher shark has a low fecundity (average two pups per litter) and a very low annual rate of population increase of 0.033. The species is caught as target and bycatch in pelagic and coastal commercial and small-scale longline, purse seine, and gillnet fisheries. The species is often retained for its meat and fins unless domestic or international regulations prohibit retention. However, high at-vessel mortality remains a threat even where retention is prohibited. The Pelagic Thresher is especially susceptible to fisheries exploitation because its epipelagic habitat occurs within the range of many largely unregulated and under-reported, small-scale and artisanal gillnet and longline fisheries, in which it is readily caught. The species is estimated to be declining in both the Pacific and Indian Oceans. Across its Indo-Pacific distribution, the Pelagic Thresher is estimated to have reduced by 50–79% over the last three generations (55.5 years), based on abundance data and levels of exploitation, and therefore the species is assessed as Endangered
Citations and References
Rigby, C.L., Barreto, R., Carlson, J., Fernando, D., Fordham, S., Francis, M.P., Herman, K., Jabado, R.W., Liu, K.M., Marshall, A., Pacoureau, N., Romanov, E., Sherley, R.B. & Winker, H. 2019. Alopias pelagicus. The IUCN Red List of Threatened Species 2019: e.T161597A68607857. https://dx.doi.org/10.2305/IUCN.UK.2019-3.RLTS.T161597A68607857.en. Downloaded on 06 December 2020.
An aplacental viviparous species with a very low reproductive ability i.e. 1-2 pups per litter. Developing embryos are oophagous; feeding on unfertilized eggs.
Poorly known. In the Sea of Cortez, pelagic threshers predate heavily on humboldt squid and clupeid fishes (small schooling bait fishes) which it catches by thrashing it’s long tail while swimming through schools.
Migratory but its large-scale movements are poorly understood. Visits cleaning stations at offshore seamounts early in the morning but retreats before light levels increase. Known to breach, possibly to rid itself of parasites.
Reaction to divers
Very shy but will inadvertently approach divers hunkered down near cleaning stations if they stay quiet. Retreats quickly if camera flashes or lights are used. Will not respond to chum.
Random sightings occasionally occur at offshore dive sites, but there are only two well known locations where pelagic thresher sharks are regularly seen.
By far the most reliable and productive place to see pelagic thresher sharks is at Monad Shoal near Malapascua in The Philippines. Monad Shoal is a deep, rocky/coral ledge a few kms in length that is dotted with cleaning stations. Just after dawn (and perhaps through the night) pelagic threshers swim up to the cleaning stations to rid themselves of parasites. Many dive boats drop divers at the cleaning stations to watch the sharks being cleaned. After an hour or two, the encounters dwindle. It is suspected this is because the threshers’ large eyes cannot tolerate high levels of sunlight.
To avoid spooking the light sensitive sharks, there is a ban on strobes and video lights at Monad Shoal.
The second spot for thresher shark sightings is at Daedalus, Brothers, and Elphinstone Reefs, off the coast of Egypt in the Red Sea. Week-long liveaboard dive trips visit these areas all year long but thresher sightings are hit and miss, and tend to be quite distant compared to the close up encounters that are possible in The Philippines.