Basking shark – The gentle giant

Story and Illustration by Richard Martin

richardmartin_baskingsharkCommon Name: Basking Shark
Scientific Name: Cetorhinus maximus
Adult Size: Up to 7-11.5m
Habitat: Coastal to pelagic in boreal to warm-temperate waters, occurring well offshore and often very close to land, sometimes just off the surf zone; enters enclosed bays.
Occurrence: Common
Food Source: Plankton

The Basking Shark is the largest of the lamnoids and a true titan among sharks. Individuals up to 9m long are encountered frequently. The largest Basker recorded in the scientific literature measured some 11.5m long and weighed roughly 4.5 tonnes.

Regardless of its enormous size, the Basking Shark is generally not aggressive towards people. An enormous filter-feeder in cool temperate and boreal seas, the Baskers has enormous gill slits which virtually encircle its head, making the shark look nearly decapitated.

Feeding style
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 3.7km/h 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 1.3 million litres of water – enough to fill two Olympic-sized swimming pools – pass over its bristle-like gill rakers each hour.

A bizarre life cycle
Like other sharks, the Basker practices internal fertilisation. 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 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. By six months of age, Basking Sharks have grown to a length of about 2.3 metres. Males mature at 12 to 16 years of age at a length of about 4 to 5 metres, females at about 7.5 to 9 metres. No one knows how old female baskers are when mature.

Basking Sharks are apparently quite social, at least during certain times of the year at locations offering rich feeding. Groups of 3 to 50 Baskers are often reported from April to September in North Atlantic and eastern North Pacific waters.

Human threat
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 that is filled with an enormous, oily liver, which may comprise some 20% of its total weight. Up to 2,270 litres of oil has been obtained from the liver of a single individual, a 8.8-metre, 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. A small Basking Shark fishery also persists 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.

Occasionally, Basking Sharks are slaughtered even when they are not being converted to sellable 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).

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 tonnes. 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.

Richard Martin is a marine biologist who heads the ReefQuest Centre for Shark Research.