Author: (Gunnerus, 1765)
Field Marks:
The great size, enormous gill slits that virtually incircle the head, dermal denticle gillrakers, pointed snout, huge, subterminal mouth with minute hooked teeth, caudal peduncle with strong lateral keels, and lunate caudal fin distinguish this shark from all others.
Diagnostic Features:
Trunk fusiform and moderately stout. Head moderately long but much shorter than trunk; snout moderately long, pointed and conical, not depressed, flattened and bladelike; eyes small; mouth large and arcuate, ventral on head, gill openings extremely large, extending onto dorsal and ventral surfaces of head, all anterior to pectoral fin bases; gillrakers present on internal gill slits, in the form of hairlike modified dermal denticles with extremely elongated crowns; teeth very small, hooklike, not blade-shaped, and in over 200 rows in either jaws; several rows of small anterior teeth in upper jaw, separated from the laterals by a broad gap. First dorsal fin large, high, erect and angular; second dorsal and anal fins moderately large but less than half size of first dorsal, with broad, non-pivotable bases; pectoral fins long and moderately broad, much shorter than head in adults; pelvic fins smaller than first dorsal fin but larger than second; caudal fin lunate, upper lobe moderately long but less than one-third length of rest of shark, lower lobe nearly as long as upper lobe. Precaudal pits present, caudal peduncle depressed and with strong lateral keels.
Geographical Distribution:
Coastal and amphitemperate. Western Atlantic: Newfoundland to Florida; southern Brazil to Argentina. Eastern Atlantic: Iceland, Norway and western Barents Sea to Mediterranean and Senegal; western Cape Province, South Africa. Western Indian Ocean: Eastern Cape Province, South Africa. Western Pacific: Japan, the Koreas, China; Australia (New South Wales, Victoria, Tasmania, South and Western Australia), New Zealand. Eastern Pacific: Gulf of Alaska to Gulf of California; Ecuador, Peru and Chile, ?Galapagos Islands.
Habitat and Biology:
A coastal pelagic shark found in boreal to warm temperate waters of the continental and insular shelves, occurring well offshore and often very close to land, just off the surf zone; enters enclosed bays. This huge, impressive, conspicuous shark is often seen at or near the surface, singly, in pairs, triads or in schools up to a hundred or more individuals, basking with dorsal fins out of the water or even with bellies upward, or moving slowly forward or in short arcs with their mouths open like hoops while feeding. Surface basking in this shark is thought to be correlated with surface concentrations of food plankton and also with courtship and mating. Two, three or more individuals may swim in tandem, in a straight line or in circles, which suggests to some writers that a row of these sharks swimming together may have been mistaken for a single huge 'sea serpent' in the past. Dead basking sharks are often stranded on a beach, in a state of advanced decay and damaged and partly dismembered from rolling in the surf; several times such carcasses have been misidentified as 'sea serpents' or other fabulous monsters.
Basking sharks have been reported as jumping out of the water, and it has been suggested that they do so to dislodge parasites or comensals like remoras. In addition to the ectoparasitic copepods found on other sharks, basking sharks often have sea lampreys attached to their skin, and although lampreys apparently are unable to cut through the formidable denticle-armored skin of the shark, they may be enough of an irritant to evoke a reaction like jumping or rubbing on objects or the bottom to dislodge them. However, some recent writers doubt that the basking shark can jump, due to its ordinarily slow cruising-speed of some 2 knots, and its reaction to harpooning by speeding up to only about 4 mph without jumping; according to this view, records of jumping in basking sharks result from mistaking leaping dolphins and thresher sharks for them.
The massive liver of the basking shark, contained in a long body cavity, apparently serves at a 'hepatic float' to adjust it to approximately neutral buoyancy.
Basking sharks are highly migratory, and noteworthy for their seasonal appearance in given localities and subsequent disappearance. Off the Atlantic seaboard of North America they appear in the southern part of their range in spring (North Carolina to New York), apparently shift northward in summer (New England, and Canada), and disappear in autumn and winter. Off the eastern North Pacific basking sharks occur in greatest numbers during autumn and winter in the southernmost part of their range there (California), but shift at least in part to more northern latitudes in spring and summer (up Washington and British Columbia). Off the British Isles the bulk of the population there appears in the springtime and disappears by autumn, but individuals may be present at all seasons of the year. Research is currently in progress in the eastern North Atlantic using long range radio tags attached to basking sharks and satellite tracking of the tags to resolve some of the problems of migration in this species. Individuals found off the British Isles in summertime are apparently engaged in courtship activity and copulation, as indicated by behavioural observations and courtship and mating scars found on captured individuals.
Pronounced spatial and seasonal populational segregation may be a characteristic of this species, as suggested by fisheries catches off the British Isles. Most individuals caught there in the summer were subadult, or nonpregnant adult females, outnumbering the males by 40:1, but in the winter the few individuals caught were mostly males. Pregnant females are almost entirely unknown for the species, suggesting that such females are spatially and bathymetrically separated from those members of the population that are regularly seen basking at the surface. Juveniles below 3 m long are also extremely rare, with a single record of a free-living individual about 1.7 m long reported from the British Isles.
Adult, nonpregnant female basking sharks have immense numbers of small eggs in their ovaries. Presumably this shark is ovoviviparous and has uterine cannibalism like other lamnoids, with embryos feeding on the small eggs and possibly smaller siblings, but this remains to be seen. An unconfirmed record of a fetus about 1.7 m long and the above mentioned free-living individual suggests that size at birth may be about 1.7 m, and hence greater than any other known ovoviviparous or viviparous shark.
Age of this shark has been estimated by counting vertebral rings and attempting to correlate them with supposed changes in size of individuals within a population. It has been suggested that birth occurs after 3 and 1/2 year gestation period, and that two calcified rings per year are laid down until maturity at between 6 or 7 years for males. The biannually calibration of the rings is uncertain and controversial; a yearly rate of ring deposition has been suggested, with possible age at maturity for males doubled to 12 to 16 or more years. Whatever the case, the basking shark has proved to be extremely vulnerable to overfishing, perhaps more so than most sharks, and this can be ascribed to its slow growth rate, lengthy maturation time, long gestation period, probably low fecundity, and probable small size of existing populations (belied by the immense size of individuals in their small schools).
The basking shark is one of the three types of huge, filter-feeding sharks, the other two being the megamouth and whale sharks. The basking shark may be unique in relying entirely on the passive flow of water through its pharynx generated by swimming for filtration; the other two giant filter-feeders may assist the process of food ingestion by actively pumping or gulping water and food organisms into their pharynxes. The basking shark feeds exclusively on small planktonic organisms trapped on its unique gillrakers, apparently with the help of mucous secreted in its pharynx. Food items include small copepods, barnacles, decapod larvae, and fish eggs. On the average, a half ton of material may be present in the stomach of these sharks. While feeding the basking shark cruises with mouth widely open and gills distended, occasionally closing its mouth to ingest its prey. An average adult has been estimated to be capable of filtering over 2000 tons of water per hour assuming a constant cruising speed of about 2 knots.
The facts that the basking shark periodically sheds its gillrakers and that plankton densities seasonally fall below levels thought essential to maintain ordinary swimming and metabolic activity in this shark have spawned a controversy over whether or not the species remains active when deprived of gillrakers and high plankton densities. lt has been suggested that the basking shark may 'hybernate' on the bottom, perhaps at the edge of continental shelves, until its rakers are replaced and plankton blooms reoccur. Proof of hybernation has never been forthcoming, and an alternate hypothesis has been suggested that the basking shark may turn to benthic feeding when it loses its gillrakers. A possible additional factor is that the massive oil-filled liver of this species may serve as a metabolic store to supply energy to support a reduced rate of activity (slower swimming in colder, deep water) while gillrakers and plankton supplies regenerate. Estimates have been proposed that, in north European waters, the basking shark drops its gillrakers in early winter and takes about 4 or 5 months to fully replace them.
The basking shark is usually quite tolerant of boats approaching closely to it (to its detriment when harpoon fisheries for it have been mounted), and divers have been able to swim right up to individuals and photograph them without invoking an extreme fright reaction. Basking sharks may approach divers quite closely, possibly out of curiosity, and swim around them. This species is regarded as ordinarily harmless and inoffensive but potentially dangerous if attacked (particularly when harpooned). The immense size and power of the basking shark should invite respect, however. Divers should avoid contact with the skin of this shark, which has large dermal denticles with sharp, hooked crowns that point forward and sideways as well as backward; while involved in the dissection of a large basking shark the writer has experienced first hand the lacerations that can result from contact with its skin.
Size:
Basking sharks have been credited as reaching a maximum total length of 12.2 to 15.2 m, but even if this is correct most specimens do not exceed about 9.8 m. Males mature at about 4 or 5 m and reach about 9 m, females are mature at 8.1 to 9.8 m. Size at birth unknown; the smallest known free-living individual was 165 cm long. The basking shark is the second largest shark, fish-like vertebrate, and elasmobranch after the whale shark (Rhiniodon typus).
Interest to Ficheries:
The basking shark has been the object of smallscale harpoon fisheries from small boats off the Norwegian coast, Ireland and Scotland, Iceland, California, Peru and Ecuador, often sporadically fished due to periodic depletion of basking shark stocks; during the last century they were also harpooned by whaling vessels. Currently they are being heavily fished off China and Japan by harpoon. The basking shark has also been taken in nets, including bottom gillnets and even bottom and pelagic trawls, and sometimes is a problem to salmon gillnetters in the Pacific northwest of North America by fouling gillnets. The basking shark meat is used for human consumption fresh or dried salted; its fins are used for shark-fin soup; its liver, rich in oil and very large, is extracted for its high squalene content but the liver oil was formerly used for tanning leather for lamp oil; the hide for leather; and the carcass for fishmeal.
Remarks:
Siccardi (1960, 1961) suggested that there are four species of Cetorhinus, two from the North Atlantic and Mediterranean (C. maximus and C. rostratus), one from southern Australia (C. maccoyi) and one from the South Atlantic (C. normani). Pending further work, I prefer to follow Springer and Gilbert's (1976) reasoning that there is insufficient evidence at present to separate these species.
Type material:
Holotype: Apparently none. Type Locality: Trondhjem, Norway.