Biology of the Basking Shark

(Cetorhinus maximus)

Field Marks Body enormous; snout pointed (in young juveniles, rostrum tip curiously sinusoidal resembling a wavy carrot); mouth huge and subterminal, with minute hooked teeth arranged in more than 150 rows; gill slits extremely long, nearly encircling the head, with dark, bristle-like gill rakers inside the gills; caudal peduncle with strong lateral keels; caudal fin lunate, with lower lobe almost as large as the upper. Color: greyish brown to slaty grey or black above, often with irregular lighter patches and streaks along the flanks; two albino specimens from the North Atlantic have been reported.
Size Pups probably between 5.0-5.6 ft (1.5-1.7 m) at birth, but this is uncertain; most individuals encountered are 22-29 ft (7-9 m), a 30-ft (9.2-m) specimen was reported to weigh 8,600 lbs (3,900 kg); largest measured 37.7 ft (11.5 m) in length and 9,900 lb (4,500 kg) in weight.
Range Amphitemperate in coastal regions, occasionally venturing into subtropical waters.
Habitat Coastal to pelagic in boreal to warm-temperate waters of continental and insular shelves, occurring well offshore and often very close to land, sometimes just off the surf zone; enters enclosed bays. May exhibit subtropical submergence, remaining in deeper waters in low latitudes; known depth range 0- 1,870 ft (0-570 m); usually encountered in surface waters, but may be primarily a midwater inhabitant, appearing in upper surface waters only when conditions of food and temperature are favorable; known to leap completely out of the water, possibly to dislodge parasites (particularly copepods and sea lampreys). Seasonally abundant in cool temperate waters at certain locations (such as off both coasts of North America), especially in coastal areas during summer plankton blooms at moderate to high latitudes; most common in New Zealand waters between Cook Strait and Dunedin during summer and fall, occurring either singly or in large groups. Apparently highly migratory in some regions, moving to higher latitudes in summer or autumn and disappearing in winter; appears to favor surface areas with temperatures 46-54F (8-12C), rare in areas with surface temperatures below 43F (6C) or above 57F (14C). In the western North Atlantic, often associated with Right Whales (Eubalaena glacialis).
Feeding Known prey consists entirely of planktonic organisms, especially copepods of the genus Calanus, but also known to take teleost eggs, chaetognaths, larval crustacea, and at least one species of deep-water oceanic shrimp (Sergestes similis), apparently taken at night at a depth of about 330 ft (100 m). An enormous filter-feeder, relying on bristle-like gill rakers to strain planktonic organisms from the seawater; feeding behavior features swimming slowly near the surface with the mouth held wide open - forming a huge, hoop-like structure; often observed feeding along current fronts which concentrate planktonic prey, apparently changing course to remain in patches of particularly rich feeding; sometimes three or more individuals observed to swim in tandem, nose-to-tail, either in a straight line or in circles. In northeastern Atlantic, gill rakers are shed during October-November and are not replaced until February (the only known case of an annual molt in fishes); whether these sharks hibernate or shift to different food during winter months has inspired much speculation and debate, but remain open mysteries.
Reproduction Ovoviviparous, probably featuring intrauterine oophagy; gestation period has been estimated as long as 3.5 yrs, but some workers have suggested 14 mo to be more likely; no data on litter size as no female with full-term pups has been collected, although six pups reported born (spontaneously aborted?) from a harpooned individual in Norwegian waters; pupping seasons and nursery grounds not known, but off the British Isles courtship and mating apparently occurs in late spring to summer; in late June 1998, possible mating behavior was filmed from a helicopter off Nova Scotia, Canada. Strong sex biases (18-40 females for every male) in commercial catches off British Isles suggest segregation by sex.
Age & Growth Males mature at a length of about 15-20 ft (4.6-6.1 m) and at an age of 12-16 yrs; no data on size or age at maturity for females. No data on longevity for either sex.
Danger to Humans Ordinarily harmless and inoffensive, but potentially dangerous if startled or attacked (particularly when harpooned); usually quite tolerant of approach by boats (much to this species' detriment when harpoon fisheries for it have been mounted), but has been known to flail dangerously and sound forcefully when startled by close approach of sea kayaks; allows divers to swim around and photograph them, and sometimes approaches divers quite closely, possibly out of curiosity; divers should avoid contact with skin of this shark, as its large dermal denticles point in all directions and can cause nasty abrasions and lacerations; the immense size and power of this shark should invite caution and respect.
Utilization Has been the object of numerous small-scale harpoon fisheries from small boats off Norway, Ireland, Scotland, Iceland, California, and Ecuador; often fished sporadically due to periodic depletion of stocks; during 19th Century, were also harpooned by whaling vessels; currently heavily fished off China and Japan by harpoon; has also been taken in nets (including bottom gillnets), and even bottom and pelagic trawls; sometimes a problem to salmon and ground fishermen in the western North Atlantic by becoming fouled in gillnets and fish traps, and to salmon gillnetters in the eastern North Pacific by fouling the nets, occasionally resulting in governmental intervention. The meat is used for human consumption fresh or dried salted, the fins for shark-fin soup, the liver (very large and rich in oil, formerly used as a vitamin A source, for tanning leather, and for lamp oil) is extracted for its high squalene content, the hide for leather, and the rest of the carcass for fishmeal.
Remarks The world's second-largest living fish (after the Whale Shark, Rhincodon typus, which grows to lengths of 50 ft [15 m] or more); individual Baskers 40-45 ft (12.2-13.7 m) long have been reported, unfortunately without supporting evidence; due to habit of swimming in groups nose-to-tail and plesiosaur-like form characteristic of partially decomposed specimens, believed responsible for many 'sea monster' reports. Due to long history of commercial exploitation, its natural history is better known than most lamnoids. Highly vulnerable to directed fishing pressure, this species is now sporadic to rare in many areas where it was once common. Declared a protected species in the western North Atlantic by US National Marine Fisheries Service (NMFS 50 CFR, Pt. 678), April 1997; IUCN Red List status: VU A1ad + 2d.

One's first clear and entire view of a free-swimming Basking Shark is a momentous occasion. Even if one recognizes the great fish as a harmless planktivore, the experience is also more than a little unnerving. Picture yourself in a small boat, floating gently along just off shore . . .

Suddenly an enormous, broadly triangular dorsal fin punctures the surface a short distance away. Sheathed in a rough, armor-like skin, the fin resembles the dark sail of a ghost ship more than part of an animal. Beneath it you can discern a dark shadow, hinting for the first time at the sheer dimensions of this real-life 'sea monster'. The creature is almost unbelievably huge. It is almost impossible to think of what you are seeing as a fish. As long as a city bus and nearly as wide, it seems more a living mountain of muscle and sinew. Through the distorted glass of the surface, you can see that the animal's dark flanks are patterned with irregular pale streaks and blotches. Moving silently below the vast bulk of this behemoth, you can just make out the spectral shadows and disembodied markings of others. Suddenly, you feel very small and alone out here on the ocean

The Basking Shark is the largest of the lamnoids and a true titan among sharks. Individuals up to 30 feet (9 metres) long are encountered frequently. The largest Basker recorded in the scientific literature measured some 37.7 feet (11.5 metres) long and weighed roughly 5 tons (4.5 tonnes). Some sources claim that this species attains lengths of more than 50 feet (15.2 metres), rivaling the orectoloboid Whale Shark (Rhincodon typus) for the title of World's Biggest Living Fish.

Regardless of it's enormous size, the Basking Shark is generally not aggressive toward people. An enormous filter-feeder in cool temperate and boreal seas, the Basker has enormous gill slits which virtually encircle its head, making the shark look nearly decapitated. Its modus operandi is laid-back and far from the razor-toothed predatory image of most sharks. The Basking Shark swims slowly close to the surface (hence its name) with its huge jaws agape. Its low-density liver enables the shark to cruise at a languid pace of about 2.3 miles (3.7 kilometres) per hour without sinking. When feeding, the Basking Shark's normally streamlined head changes dramatically: its jaws expand to resemble a circular-mouthed butterfly net and its gill pouches billow spinnaker-like. Every 30 to 60 seconds or so, each Basker closes its mouth, flutters its gills briefly, and swallows the planktonic creatures that had accumulated on its filtering mechanism. Over 330,000 gallons (1.3 million litres) of water - enough to fill two Olympic-sized swimming pools - pass over its bristle-like gill rakers an hour.

A veritable forest of these gill-rakers strains from the sea the tiny planktonic organisms on which Basking Sharks feed. The rakers which are elongate, single-shafted structures derived from dermal denticles are about 3 inches (8 centimetres) long in a 30-foot (9-metre) specimen, occurring at a frequency of about 8 per inch (20 per centimetre) of gill bar. The total number of gill-rakers in an individual Basker might therefore be something in the neighborhood of 27,000. Despite the apparent simplicity of its filter-feeding mechanism, the Basking Shark may be a highly selective feeder studies in the North Atlantic show that its diet consists almost entirely of copepods of the genus Calanus. In winter, the Basking Shark apparently sheds its gill rakers and disappears (it has been speculated that it may hibernate or sink to the bottom and shift to other food), re-growing its gill rakers in the spring. This is the only known example of an annual molt in fishes.

The life cycle of the Basking Shark is suitably bizarre. Like other sharks, the Basker practices internal fertilization; the males are equipped with paired intromittant organs called claspers, which develop along the inner margin of the pelvic fins. Basking Sharks have tiny teeth that do not seem to function in feeding as an adult, but they may be useful before birth as this species seems to feed on unfertilized eggs in utero. Gestation period in the Basking Shark is believed to be about 3.5 years, by far the longest gestation of any vertebrate. Born at a length of about 5 feet (1.7 metres), the Basker is larger at birth than many other sharks are full-grown. Juvenile Basking Sharks have a peculiar, elongated snout which resembles a short trunk. It has been suggested that this 'trunk' might be a developmental remnant, serving to channel the yolky soup that sustains late-term Basker pups in the womb. By six months of age, Basking Sharks have grown to a length of about 7.5 feet (2.3 metres). Males mature at 12 to 16 years of age at a length of about 13 to 16 feet (4 to 5 metres), females at about 24.5 to 29 feet (7.5 to 9 metres). No one knows how old female baskers are when mature, and they're not telling.

Basking Sharks are apparently quite social, at least during certain times of the year at locations offering rich feeding. Studies by ichthyologist David Sims off Plymouth, England, suggest that during summer months Basking Sharks tend to congregated along current fronts, places where water masses of differing properties meet and often concentrate planktonic organisms. Groups of 3 to 50 Baskers are often reported from April to September in North Atlantic and eastern North Pacific waters. During May 1998, fishermen reported an enormous aggregation of some 500 Baskers off the coast of Cornwall, England. In patches of the western North Atlantic with particularly thick concentrations of plankton, Basking Sharks are often associated with another giant filter-feeder, the Right Whale (Eubalaena glacialis).

Although the Basking Shark is not aggressive toward people, the same cannot be said for the reverse relationship. This species has long been hunted commercially, primarily for its liver oil. To its misfortune, the Basking Shark has an elongated body cavity is filled with an enormous, oily liver which may comprise some 20% of its total weight. Up to 590 gallons (2,270 litres) of oil has been obtained from the liver of a single individual, a 29-foot (8.8-metre), 6.5-ton (5.9 tonne) specimen. This oil was formerly used as an extremely rich source of vitamin A, for tanning leather, as a 'smokeless' lamp oil, and as a remarkably resilient lubricant for machine parts.

Although most of these uses of liver oil have been replaced by more cost-effective synthetic substitutes, Basking Sharks are still hunted commercially at certain locations. Off Japan, for example, Baskers are killed by hand-thrown or gun-fired harpoon and harvested for food and liver oil; the meat is sold for human consumption and livestock feed while the liver oil is used in traditional medicine. In the 1950's, a small Basking Shark fishery was conducted off California, where spotter planes are used to locate the sharks which are then dispatched by explosive-tipped harpoon; the sharks' first dorsal and pectoral fins and the lower lobe of caudal fins are sold to Asian markets, where they are highly prized as a base for soup, and the livers are processed to remove the oils, which are used as a base for cosmetics such as lipstick (With apologies to Dooley Wilson, a kiss is never 'just' a kiss.)

Even the Baskers scales have found a human use. The dermal denticles studding the skin of the Basker are unique among sharks. Instead of having denticles with cusps that point uniformly tailward, those of the Basking Shark seem to be set into the skin at random, pointing every which way. Seafaring Norwegians, who have harvested this species for centuries, have developed a clever use for this peculiarity: they paste a strip of Basking Shark skin to the soles of their boots, making them slip-resistant on wet, rolling decks.

Occasionally, Basking Sharks slaughtered even when they are not being converted to salable products. These sharks sometimes become so numerous that they are regarded as a menace to commercial fishing gear set for other species. The huge, lumbering sharks frequently foul themselves in cod or salmon nets, causing large-scale damage and economic loss in the process (due to net repair costs and lost fishing time). During the 1950's off British Columbia, Canada, Basking Sharks were responsible for so much economic loss due to entanglement in fishing gear, fishermen pleaded with the federal Department of Fisheries and Oceans [DFO] to help. When harpoons and bullets failed to drive away the 'marauding' sharks, the DFO hit upon a novel idea: attach a pointed, sharp-edged steel ram to the bow of the fisheries protection vessel, Comox Post. When rammed at full speed, basking sharks were more-or-less cut in two by the blade - as many as 18 sharks were killed in this way each day. Basking Sharks have been relatively scarce off British Columbia ever since.

Basking Shark (Cetorhinus maximus)
 Jeremy Stafford-Deitsch

Resolving like a specter out of the planktonic murk, a Basking Shark (Cetorhinus maximus) shown here cruising off the Isle of Man, United Kingdom, with its mouth agape and its throat expanded filters calanoid copepods from the sea.

Photo Jeremy Stafford-Deitsch; used with the gracious permission of the photographer, who asks that you support the Shark Trust.

Sometimes Basking Sharks cause a fuss by being mistaken for something else. Dead Baskers often wash ashore, perhaps owing to the buoyancy afforded by the large, oily liver combined with the gases liberated by decomposition of the gastrointestinal tract. This tendency renders Basking Sharks particularly notorious among cryptozoologists, as their rotting carcasses have frequently been reported as 'sea serpents'. Such reports are due largely to the Basking Shark's characteristic pattern of decay: after the skeletal elements supporting the gills slough off, what remains is a tiny skull on the end of a long, plesiosaur-like 'neck' (remnants of the gills may further disguise the piscine origin of the carcass by giving the impression of a 'mane'); a deep 'chest' with large pectoral flippers; adding to the carcass' overall strangeness, the claspers of a male Basker may appear like a sixth pair of flippers or legs; and (because the backbone extends into the upper lobe of the caudal fin) the lower lobe may rot away, leaving a single-lobed, snake-like tail. The end result of all this post mortem transformation is a carcass that looks remarkably like the popular conception of a sea monster. In fact, one of these washed-up Baskers the so-called Stronsa Beast of 1808 was thought to be a whole new kind of creature and even given a formal scientific name, Halsydurus maximus ("great sea snake") before the blunder was discovered. As recently as April 1977, it nearly happened again when a plesiosaur-like creature was hauled up in the nets of a Japanese commercial fishing vessel off New Zealand; dubbed the New Zealand Monster, examination of its cartilaginous skull and vertebrae quickly established its identity as our old friend, the Basking Shark.

Basking Sharks resemble and are closely related to the mackerel sharks (family Lamnidae). Baskers are superficially most similar to, but far more commonly seen at the surface than, the infamous White Shark (Carcharodon carcharias). To those who have seen both species in the wild, they are easy to distinguish by behaviour alone: a White Shark generally swims in a more 'purposeful' manner and is much more likely to alter its course to investigate a boat than a disinterested, gently grazing Basker. Most people, of course, have never seen a White Shark in its natural habitat, but most have at least heard of this near-legendary predator. Given their quite similar appearance when seen from above (as from the vantage of a boat deck), it is hardly surprising that free-swimming Basking Sharks are often reported as White Sharks During the summer of 1978, for example, there appeared off Long Island what looked for all the world like a giant White Shark, more than 30 feet (9 metres) long, which was quickly dubbed the 'Montauk Monster'. Aerial photographs published in local newspapers clearly revealed that it was actually a medium-sized Basker.

Due to all this commercial and cryptozoological attention, the basic biology of the Basking Shark is better known than that of most lamnoids. But many mysteries remain about this enormous creature. Despite its lethargic lifestyle, the Basking Shark is capable of rapid bursts of speed and has been known to leap completely out of the water no mean feat for a fish whose weight is calculated in tons (tonnes) perhaps to dislodge ectoparasites such as lampreys. Off the coast of the United Kingdom, adult females Baskers outnumber males by as much as 40 to 1 (possibly indicating sexual segregation). In addition, groups of Basking Sharks often swim lined up nose-to-tail, like a submerged herd of circus elephants. No one knows why they do this (although, as will be reveal later in the book, I have some ideas about this). And no one knows where the Baskers go in winter or why. Who says a shark has to be a super-predator in order to be interesting?

In the final analysis, the true value of the Basking Shark cannot be measured in mere dollars and cents. They are an enormous, enigmatic form of marine wildlife that adds greatly to the richness of our shared natural heritage. It is encouraging that Basking Sharks are protected in British waters and in a few other select locations, such as Clayoquot Sound in British Columbia. Let us hope that conservation of these peaceful giants becomes ever more widespread. Our experience of the ocean would be very much diminished if chance encounters like the one that began this essay were no longer possible.

Ecology of the Basking Shark


ReefQuest Centre for Shark Research
Text and illustrations R. Aidan Martin
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