Taxonomy

Rarity

The basking shark is the second-largest fish in the ocean and it is Endangered.

The International Union for Conservation of Nature assessed the species in 2018 and lists it as Endangered, meaning a very high risk of extinction in the wild. It sits on Appendix II of the international wildlife trade treaty. The reason given in the scientific record is blunt: the species is at risk of extinction by overfishing because of low to very low productivity, and it is threatened by being caught as bycatch in fisheries aimed at something else.

Take is prohibited off California, and the mechanism is worth getting right because it differs from the white shark's. The white shark has its own California regulation banning take. The basking shark's protection here is federal: it is a prohibited species in West Coast highly migratory species fisheries, and retention is not allowed. A basking shark caught incidentally may only be sold or donated to a recognized scientific or educational organization. California's own sport fishing regulations do not mention the species at all.

Seasonality

Location

Basking sharks are cosmopolitan, common in cold to warm temperate water and rare near the equator. They prefer water between 46 and 61 degrees Fahrenheit. In summer they come to the surface in cool and temperate areas to feed, and in winter they go deep, with records to about 6,560 feet.

The part that matters for this site is how close in they come. FishBase describes basking sharks as found on continental and island shelves, offshore and often close to land, just off the surf zone, and entering enclosed bays. This is a 30 foot animal that turns up in swimming water.

They may be alone, in pairs, in groups of 3 or more, or in very large schools, with one report of about 100 animals together. They also make some of the longest migrations recorded for any shark, including a journey from the British Isles to Newfoundland and movements between the northern and southern hemispheres through deep tropical water. Those migrations have covered over 5,600 miles.

There is a gap in this entry worth naming. No California-specific abundance figure, sighting record or historical fishery number was found from a source read for this page. Basking sharks are widely said to have been far more common off this coast in the past than they are now, and this entry does not assert that, because it could not be verified here.

Deep Dive

A basking shark is a mackerel shark, the same order as the white shark and the mako, that gave up hunting entirely.

The head tells the story. The gill slits are enormous, practically encircling the head, and inside them are gill rakers made of the same tooth-like skin denticles that cover a shark's body. The mouth is huge and sits under a pointed snout, and the teeth in it are minute and hooked and do nothing. The whole apparatus is a strainer. A basking shark swims forward with its mouth open, water passes through the gill slits, and the rakers comb out the drifting animal plankton. Everything else about the body is standard fast-shark equipment, including the keels on the tail stock and the crescent tail, which is why an animal that eats plankton looks so much like an animal that eats seals.

The size figures are the ones people want. The scientific record puts the species at reportedly reaching 40 to 49.9 feet, and the largest recorded female was 32.2 feet. A typical adult runs about 23 feet. The maximum published weight is 4 metric tons, roughly 8,820 pounds. The species is thought to live up to 50 years and does not reach breeding size until about 29.4 feet.

Then there is the number that should stop anyone reading this page. Exactly 1 pregnant basking shark has ever been observed giving birth, and she had 6 pups. That single animal is the entire direct observational record of reproduction for the second-largest fish in the ocean, described by science in 1765. Everything else is estimate. Gestation is put at somewhere between 12 and 36 months, a range so wide it is really an admission. Females are thought to rest about a year between pregnancies, which would mean 2 to 4 years between litters. Pups are estimated to be born at 4.9 to 6.6 feet. Mating is thought to happen in early summer. Over a 5 year study off southwest England, researchers watched courtship-like behavior between May and July, always at the surface along thermal fronts where 2 bodies of water meet, and never once saw the animals actually mate, which is why they suspect it happens deeper.

What that courtship looks like, from an aerial observation, is oddly tender for a fish this size: parallel swimming, nudging, one animal following another nose to tail, approaching along the flank, and echelon swimming, where the sharks travel in a staggered line.

The final detail is a small one and it says something about scale. In the Bay of Fundy, parasitic lampreys have been found attached to the backs of basking sharks, feeding on their blood. An animal 30 feet long, straining the ocean for creatures too small to see, carrying passengers of its own.

Where to Find Them

Beaches whose habitats match this species

Matched by habitat: a beach is listed when its biome tags overlap this species' biomes.