November 3, 2025. by David McGuire
International Basking Shark Day Recognizes the Second Largest Fish, and Works to Protect Them
The basking shark (Cetorhinus maximus) is the second-largest living shark. It is one of three plankton-eating shark species, along with the whale shark and megamouth shark.
The basking shark is a coastal-pelagic shark found worldwide from cold boreal waters to warm-temperate seas. It occurs around the continental shelf and occasionally enters brackish waters. It is found from the surface down to at least 2,990 ft. Typically, basking sharks reach 26 feet in length, but some individuals have been known to grow more than 30 feet long. Basking sharks are usually solitary, but during summer months in particular, they will aggregate in dense patches of zooplankton.
It is a migratory species found in all the world’s temperate oceans. A slow-moving filter feeder, its common name derives from its habit of feeding at the surface, appearing to be basking in the warmer water there. It has specialized adaptations for filter-feeding, such as a greatly enlarged mouth and highly developed gill rakers to filter out zooplankton and fish larvae. They are slow-moving sharks, seen feeding at the surface at about 2 knots (2.3 miles per hour).
Basking sharks migrate seasonally along the California coast. They are typically found off California between October and April and move northward to Washington and British Columbia during the summer, feeding on plankton in coastal temperate waters. However, recent research shows their movements can also include large-scale migrations, with some individuals traveling as far as Hawaii or Baja California, and that their seasonality and distribution may be influenced by environmental factors like sea surface temperature and prey availability.
From 2010 to 2011, four sharks were tagged with pop-off satellite archival tags with deployments ranging from 9 to 240 days. The tags provided both transmitted and archived data on habitat use and geographic movement patterns. Nearshore, sharks tended to move north in the summer and prefer shelf and slope habitat around San Diego, Point Conception and Monterey Bay. The two sharks with 180 and 240 days deployments left the coast in the summer and fall. Offshore their paths diverged and by January one shark had moved to near the tip of the Baja Peninsula, Mexico and the other to the waters near Hawaii, USA.
Since the 1980s, there has been a dramatic decline in basking shark sightings off California. During a peak in 1965, a maximum of 4,000 basking sharks were sighted, but after the 1990s, sightings dropped sharply. Fishing, bycatch in a now banned gill-net fishery and possibly prey and shifts in sea conditions have all contributed to their decline.
Environmental variables like sea surface temperature, chlorophyll-a concentration, and climate oscillations influence their presence. Movement patterns and vertical habitat use can change depending on location, linking movements to prey and oceanography. Their seasonality has shifted, with sightings occurring more in the summer in recent years compared to the fall and spring seen in earlier surveys, possibly as a response to sea surface temperatures and climate change. While historically common, they are now considered rare in the California Current Ecosystem.
Basking sharks are considered endangered globally by the International Union for Conservation of Nature (IUCN), with specific populations also listed as vulnerable or endangered. Their status is due to a long history of overfishing, a slow reproductive rate, and other threats like boat strikes and bycatch.
Sources
“Cetorhinus maximus”. Florida Natural History Museum.
Compagno, Leonard J. V. (1984). “CETORHINIDAE – Basking sharks” (PDF). Sharks of the World: An annotated and illustrated catalogue of shark species known to date. Food and Agriculture Organization of the United Nations
C. Knickle; L. Billingsley & K. DiVittorio. “Biological Profiles basking shark”. Florida Museum of Natural History.
Basking Shark (Cetorhinus maximus) Movements in the Eastern North Pacific Determined Using Satellite Telemetry
Dewar Heidi , Wilson Steven G. , Hyde John R. , Snodgrass Owyn E. , Leising Andrew , Lam Chi H. , Domokos Réka , Wraith James A. , Bograd Steven J. , Van Sommeran Sean R. , Kohin Suzanne Frontiers in Marine Science Volume 5 –2018, https://www.frontiersin.org/journals/marine-science