The basking shark is the second-largest fish alive today, surpassed only by the whale shark, yet it is among the most harmless of giants — a slow-cruising planktivore that sieves vast volumes of seawater for tiny copepods. Its size once made it a target for fisheries supplying liver oil, meat, and fins, and the resulting catches collapsed regional populations within a few decades [Sims 2008; Rigby et al. 2021]. This profile examines the species' biology, the pressures that left it depleted across much of its range, and the international measures now working toward its slow recovery.
Biology and Identification
The basking shark (Cetorhinus maximus, Gunnerus 1765) reaches lengths of up to roughly 12 m and weights approaching 4 tonnes, making it the largest fish in temperate seas and the second-largest extant fish overall [Gore et al. 2023; Sims 2008]. The body is robust and conical-snouted, grey-brown to slate above and paler below, with very large gill slits that nearly encircle the head and a crescent-shaped caudal fin. The enormous mouth gapes up to a metre across when feeding and is supported internally by gill rakers that strain plankton from the water column.
Unlike the megamouth and whale sharks, which combine suction and ram filtration, the basking shark is an obligate ram filter-feeder: it swims forward with its mouth open, passing water across its gill rakers to capture zooplankton, especially calanoid copepods [Sims 2008; Gore et al. 2023]. The species derives its common name from its habit of cruising slowly at the surface in calm, sunlit conditions where plankton concentrate, appearing to "bask."
The basking shark has an exceptionally slow, low-output life history: maturity is reached late, gestation is thought to be prolonged, and fecundity is low, leaving the species with limited capacity to rebound from population losses [Rigby et al. 2021; Sims 2008]. Genetic study of individuals sampled across the Atlantic, Mediterranean, Indian, and Pacific basins found strikingly low worldwide diversity and a small effective population size for so widely distributed an animal, consistent with a historical bottleneck [Hoelzel et al. 2006].
Habitat and Range
The basking shark is distributed through temperate coastal and oceanic waters of both hemispheres, occurring across the North Atlantic and North Pacific, the Mediterranean, and temperate parts of the Southern Hemisphere [Rigby et al. 2021; Gore et al. 2023]. Sharks move into higher-latitude shelf waters in the warmer months, when loose surface-feeding aggregations form at productive coastal sites, and disperse more widely at other times of year.
Long-term satellite tracking has overturned the old assumption that the species stays within discrete coastal populations. Tagged sharks have been shown to undertake long migrations, including a transatlantic crossing of nearly 10,000 km and dives beyond 1,200 m at the shelf edge [Gore et al. 2008], while individuals tagged in the western North Atlantic have travelled south across the equator to waters off South America [Skomal et al. 2009]. Other tracking has revealed varied seasonal strategies among individuals in the north-east Atlantic, with some remaining in shelf seas and others ranging far offshore [Doherty et al. 2017]. The species also breaches clear of the water, a behaviour documented in detail through high-resolution biologging [Hawkes et al. 2021].
In accordance with NRWL sensitive-species policy, specific site locations, corridor routes, and seasonal movement details are not disclosed in this article.
Conservation Status
The basking shark is listed as Endangered on the IUCN Red List, the category assigned when the species was reassessed globally in 2019 (assessment published in an amended version in 2021) [Rigby et al. 2021]. This global reassessment represented an uplisting from the previous worldwide category of Vulnerable, which had applied since the 1990s; regional populations in the north-east Atlantic and North Pacific had already been recognised as more severely depleted in earlier evaluations [Rigby et al. 2021; Sims 2008]. The assessment cites severe historical declines driven by targeted fisheries, combined with the species' low biological productivity and limited ability to recover quickly.
The basking shark is included on CITES Appendix II, requiring that international trade in its parts and products be regulated and sustainable; the listing was upgraded from Appendix III to Appendix II in 2002 following a proposal led by the United Kingdom [CITES 2023]. The species is also listed on both Appendix I and Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), reflecting its migratory nature and the need for coordinated cross-border protection, and is covered under the CMS Memorandum of Understanding on the Conservation of Migratory Sharks [CMS 2023].
Threats
Historical target fisheries were the principal cause of the species' depletion. For more than two centuries, basking sharks were hunted in the north-east Atlantic and elsewhere for liver oil — rich in squalene and once used for lighting, lubricants, and cosmetics — as well as for meat and fins. More than 100,000 individuals were taken in the eastern North Atlantic between 1946 and 1997 alone, and catches typically collapsed within a decade of a fishery opening, followed by long-term local depletion with little sign of replenishment from elsewhere [Rigby et al. 2021; Sims 2008]. The species' large fins were among the most valuable in international trade, an incentive that sustained exploitation even after directed liver-oil fisheries declined [Rigby et al. 2021].
Bycatch in fishing gear remains a concern across the species' range, as surface-feeding, slow-moving sharks are vulnerable to entanglement in nets and other gear [Rigby et al. 2021].
Vessel strikes pose a risk where shark aggregations overlap with shipping and recreational boat traffic, given the species' habit of feeding and breaching at the surface [Gore et al. 2023].
Slow recovery is itself a threat: because maturity is late and reproductive output low, depleted populations rebuild over decades rather than years, leaving them exposed to renewed pressure long after fishing has ceased [Hoelzel et al. 2006; Sims 2008].
What Is Being Done
Legal protection. Directed fishing for and retention of basking sharks is now prohibited in the waters of many jurisdictions, including the European Union and parts of North America, removing the targeted exploitation that drove the historical collapse [Rigby et al. 2021; Sims 2008]. The CITES Appendix II listing regulates international trade in fins and other products, while the CMS listings provide a framework for cooperative conservation across migratory ranges [CITES 2023; CMS 2023].
Satellite tracking and biologging. Electronic tagging has transformed understanding of where basking sharks go and how they behave, revealing transoceanic migrations, deep offshore diving, varied seasonal strategies, and surface breaching [Gore et al. 2008; Skomal et al. 2009; Doherty et al. 2017; Hawkes et al. 2021]. These data underpin the identification of important habitats and inform spatial-management measures.
Protected areas and monitoring. Seasonal aggregation sites have been recognised through marine protected-area designations and long-term photo-identification and survey programmes that track abundance, site fidelity, and population trends [Gore et al. 2023]. Comprehensive scientific reviews consolidate decades of research on the species' biology and status, providing the evidence base for IUCN assessment and management decisions [Sims 2008; Gore et al. 2023].
How Readers Can Help
Citizen science. Report basking shark sightings to recognised recording schemes and log observations through platforms such as iNaturalist. Verified records contribute directly to range-mapping, abundance monitoring, and conservation assessment.
Responsible boating and wildlife watching. When near surface-feeding sharks, maintain distance, reduce speed, and follow established marine-wildlife guidelines to minimise disturbance and the risk of vessel strikes.
Informed consumer choices. Avoid products derived from sharks, including fins and shark-liver-oil (squalene) cosmetics, and support the trade controls established under CITES.
Policy engagement and education. Support marine protected areas and fisheries-bycatch reduction measures, and share accurate, science-based information about the basking shark's role as a harmless filter-feeding giant with schools, community groups, and social networks.
References
[CITES 2023] CITES. (2023). Appendices I, II and III. Convention on International Trade in Endangered Species of Wild Fauna and Flora. https://cites.org/eng/app/appendices.php
[CMS 2023] CMS. (2023). Cetorhinus maximus (Basking shark) — listed on CMS Appendix I and II. Convention on the Conservation of Migratory Species of Wild Animals. https://www.cms.int/en/species/cetorhinus-maximus
[Doherty et al. 2017] Doherty, P.D., Baxter, J.M., Gell, F.R., Godley, B.J., Graham, R.T., Hall, G., Hall, J., Hawkes, L.A., Henderson, S.M., Johnson, L., Speedie, C. & Witt, M.J. (2017). Long-term satellite tracking reveals variable seasonal migration strategies of basking sharks in the north-east Atlantic. Scientific Reports, 7, 42837. https://doi.org/10.1038/srep42837
[Gore et al. 2008] Gore, M.A., Rowat, D., Hall, J., Gell, F.R. & Ormond, R.F. (2008). Transatlantic migration and deep mid-ocean diving by basking shark. Biology Letters, 4(4), 395–398. https://doi.org/10.1098/rsbl.2008.0147
[Gore et al. 2023] Gore, M.A., Camplisson, E. & Ormond, R.F. (2023). The biology and ecology of the basking shark: A review. Advances in Marine Biology, 95, 113–257. https://doi.org/10.1016/bs.amb.2023.08.005
[Hawkes et al. 2021] Hawkes, L.A., Exeter, O., Henderson, S.M., Kerry, C., Kukulya, A., Rudd, J., Whelan, S., Yoder, N. & Witt, M.J. (2021). High resolution biologging of breaching by the world's second largest shark species. Scientific Reports, 11, 5236. https://doi.org/10.1038/s41598-021-84670-3
[Hoelzel et al. 2006] Hoelzel, A.R., Shivji, M.S., Magnussen, J. & Francis, M.P. (2006). Low worldwide genetic diversity in the basking shark (Cetorhinus maximus). Biology Letters, 2(4), 639–642. https://doi.org/10.1098/rsbl.2006.0513
[Rigby et al. 2021] 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. (2021). Cetorhinus maximus (amended version of 2019 assessment). The IUCN Red List of Threatened Species 2021: e.T4292A194720078. https://dx.doi.org/10.2305/IUCN.UK.2021-1.RLTS.T4292A194720078.en
[Sims 2008] Sims, D.W. (2008). Sieving a living: a review of the biology, ecology and conservation status of the plankton-feeding basking shark Cetorhinus maximus. Advances in Marine Biology, 54, 171–220. https://doi.org/10.1016/S0065-2881(08)00003-5
[Skomal et al. 2009] Skomal, G.B., Zeeman, S.I., Chisholm, J.H., Summers, E.L., Walsh, H.J., McMahon, K.W. & Thorrold, S.R. (2009). Transequatorial migrations by basking sharks in the western Atlantic Ocean. Current Biology, 19(12), 1019–1022. https://doi.org/10.1016/j.cub.2009.04.019