Creature Feature by Dr Gerald Goeden: The Unbelievable Basking Shark by Dr. Gerald Goeden

While the Whale Shark is the largest living fish, the Basking Shark, Cetorhinus maximus, takes a close second place. Basking Sharks often grow to 6 metres and sometimes to over 8 metres. Before heavy commercial fishing, some Basking Sharks reached 9-10 metres. The largest Basking Shark ever measured was caught off the coast of Canada in 1851. Its total length was 12.3 metres, and it weighed an estimated 17-18 tonnes.

Unlike bony fish with air-bladders, all sharks have a problem staying ‘afloat’. Most solve this problem by constantly swimming and having large, light livers inside them. The Basking Shark's liver, makes up about 25% of its body weight, and takes up much of the abdominal cavity.

Basking Sharks are found in most of the cool to temperate waters of the world. It is a coastal surface dwelling shark common to the continental shelves of most continents. It gets its name from slowly cruising in the sunny surface waters but will descend to about 1000 m in search of food.

The Basking Shark is a passive filter feeder, relying only on the water that is pushed through its gills by swimming. It filters out zooplankton and small fish from up to 2,000 tonnes of water per hour. Unlike the Whale Shark, the Basking Shark does not actively search for its food. Ichthyologists (people who study fish) have found that they may have an acute sense of smell that can detect plankton up-current of them.

After its size, the next thing you notice when swimming with a Basking Shark is its huge mouth. Preferred feeding seems to be at the surface where the slow-moving sharks (about 4 km/h) open their mouths and spread out the gill rakers that filter the food from the water. Even though they look like weak swimmers there are reports of bursts of speed and leaps completely out of the water (breaching).

The sight of an approaching Basking Shark with its mouth open is awesome but they are harmless to humans. Unfortunately, humans are not harmless to Basking Sharks. It is fished mainly for its fins (for shark fin soup). The skeleton is sometimes used in traditional Chinese medicine and as an aphrodisiac in Japan.

Because of increasing demand for shark fin the giant Basking Shark is disappearing fast. Only a few countries (UK, Malta, New Zealand, and US Gulf and Atlantic waters) have protected the species and stopped trade in its products.

Many thanks to Wiki-commons for the excellent photos.

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