Common name(s)
Basking Shark.
Identification
Enormous size. Unmistakable cavernous mouth (when open). Snout tapers to a rounded tip. Eyes small. Gill slits very long. First dorsal fin large with a mildly convex posterior margin. Long pectoral fins with convex posterior margins. First dorsal fin origin level posterior to free rear tips of pectoral fins. Second dorsal fin small; same size as anal fin. Prominent caudal keels support a large, powerful caudal fin. Well developed lower caudal lobe approx half as long as upper lobe. Upper caudal lobe has a small subterminal notch close to tip.
Dorsal coloration slate grey, greyish-brown, or almost black, usually with two irregular lighter flank stripes. Ventral surface usually paler, sometimes with irregular light blotches.
Size
The basking shark is the second largest fish in the world, exceeded only by the whale shark. Maximum length at least 10.4m. Size at birth 150-170cm.
Habitat
Cold to warm-temperate seas. Usually seen from close inshore to the edge of the continental shelf but known to migrate at great depth across oceans. Surface to at least 1264m.
Distribution
Found throughout the Pacific and Atlantic Oceans but largely absent from the Indian Ocean except off Western Australia.
Conservation Status
ENDANGERED
Currently there are no target fisheries for the Basking Shark, however it was targeted for several centuries by harpoon and net fisheries for meat, fins, skin, cartilage, and liver oil (Rose 1996, Dewar et al. 2018, SARA 2019). Surface fisheries have primarily caught large, recently-mated females (CITES 2002). The large fins are extremely valuable in the fin trade being worth up to US$57,000 for a single, large fin (Clarke et al. 2004, Magnussen et al. 2007). Targeted fisheries were banned in many countries during the 2000s (ICES 2016, Dewar et al. 2018). The species is still taken as bycatch by trawl, trammel nets, and set-net fisheries, and becomes entangled in pot lines (Fowler 2005, Mancusi et al. 2005, COSEWIC 2009, Francis and Smith 2010, Francis and Sutton 2012, Francis and Lyon 2012, Murua et al. 2013, ICES 2018). Many range states across its distribution now have regulations that require release of live individuals (see Conservation section). Strikes from recreational and commercial shipping are a threat due to the species’ habit of spending time at the surface (Pirotta et al. 2019). Collisions from recreational boats are relatively frequent in United Kingdom waters (OSPAR 2009).
Between 1955-1969 in western Canada, the Department of Fisheries and Oceans implemented an ill-conceived eradication program to stop basking sharks from getting caught up in commercial fishing nets. To acheive this, fitted blades onto the bows of large boats to cut through the spines of shallow feeding basking sharks. In little more than a decade, the northeastern Pacific population of basking sharks was reduced to virtual extinction.
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., Romanov, E. & Kyne, P.M. 2019. Cetorhinus maximus (errata version published in 2020). The IUCN Red List of Threatened Species 2019: e.T4292A166822294. https://dx.doi.org/10.2305/IUCN.UK.2019-3.RLTS.T4292A166822294.en. Downloaded on 10 December 2020.
Reproduction
Despite thousands of basking sharks being caught in directed fisheries in the 20th century, almost nothing is known about the basking shark’s reproductive cycle because no gravid females have been examined. Matthews (1950) demonstrated that the basking shark has a lamnid type ovary. Lamnids are largely oophagous (embryos live on unfertilized eggs) but this would mean a dramatic dietary shift to a plankton based diet immediately upon birth.
Diet
Basking sharks are ram feeders, consuming plankton (mostly copepods and other small crustaceans) which collect in their modified gill slits as they move forward. Periodically, the sharks close their mouths and swallow the build up of plankton. They are occasionally seen skimming the surface with mouths agape, possibly feeding on positively buoyant fish eggs.
Behavior
Basking sharks are known to undergo vast oceanic migrations. During the summer months, they are often seen close to the coast, probably feeding on shallow plankton blooms that are created by the summer sun.
Reaction to divers
Easily approached when encountered by snorkelers at the surface. Basking sharks probably avoid the noise associated with scuba. When directly approached by by swimmers, basking sharks generally change direction but do not bolt.
Diving logistics
Up until the 1950’s, basking sharks were abundant off the south coast of the UK and west coast of Scotland. After decades of overfishing, their numbers have been severely reduced and sightings are now unpredictable at best.
The most reliable place to find basking sharks is around the small isles of Coll and Tiree in the Inner Hebredes, on the west coast of Scotland. Sightings are possible from June through August but the most reliable time seems to be mid-July. Operators in Mull and Oban offer single and multi-day trips. Sightings are far from guaranteed.
On the south coast of Cornwall, basking sharks are even more unpredictable but they generally appear earlier in the summer than they do in Scotland. This makes sense as the water in the south warms sooner and brings more plankton for the sharks.
Southern Island and Isle of Man are also historical spots where basking sharks can be seen.
In the northwestern Atlantic, sightings of basking sharks off of Massachusetts and elsewhere in New England seem to be increasing each year. Hopefully this implies an increase in overall numbers.
In the eastern Pacific, random sightings still occur but basking shark numbers in the Pacific are significantly less common.
Similar species
Great White Shark Although white sharks are only superficially similar to basking sharks, they are often confused by boaters and ocean watchers on the coast of the UK. When swimming at the surface, a basking shark’s dorsal fin and caudal fin are visible. A white shark’s tail sits below the water line when at the surface.
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