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Shark of the Month - Basking Shark
Tuesday, 08 October 2013

It’s time for shark of the month by Shark Guardian again! This month we chose a shark is more common along the UK coastline, the Basking Shark! It is the second largest living fish, after the whale shark, and the second of three plankton-eating sharks, the other two being the whale shark and Megamouth shark. It is a migratory species, found in all the world’s temperate oceans. It is a slow moving and generally harmless filter feeder as it has a greatly enlarged mouth and highly developed gill rakers.

The basking shark is usually greyish-brown in colour and often seems to have a mottled appearance. The teeth of the basking shark are very small and numerous and often number one hundred per row. Basking sharks are a migrating species and are believed to overwinter in deep waters. They may occur in either small schools or alone. Small schools in the Bay of Fundy have been seen swimming nose to tail in circles in what may be a form of mating behavior. Basking sharks are not aggressive and are generally harmless to people. It has long been a commercially important fish, as a source of food, shark fin, animal feed, and shark liver oil. Over exploitation has reduced its populations to the point that some have apparently disappeared and others need protection.

The basking shark is a coastal-pelagic shark found worldwide warm-temperate waters around the continental shelves. It prefers 8 to 14.5 °C (46 to 58 °F) temperatures, but recently has been confirmed to cross the much-warmer waters at the equator. It is often seen close to land, including bays with narrow openings. The shark follows plankton concentrations in the water column and is therefore often visible at the surface. It characteristically migrates with the seasons. The basking shark is found from the surface down to at least 910 metres. Satellite tagging confirmed basking sharks move thousands of kilometres during the winter months, seeking plankton blooms. It also found they shed and renew their gill rakers in an ongoing process, rather than over one short period. A 2009 study tagged 25 sharks off the coast of Cape Cod, Massachusetts, and indicated at least some individuals migrate south in the winter. Remaining at depths between 200 metres (660 ft) and 1,000 metres (3,300 ft) for many weeks, the tagged sharks crossed the equator to reach Brazil. One individual spent a month near the mouth of the Amazon River. It is unknown why they undertake this journey.

Lead author Gregory Skomal of the Massachusetts Division of Marine Fisheries, suspects it may be related to reproduction. They are slow-moving sharks (feeding at about 2 knots (3.7 km/h; 2.3 mph)) and do not evade approaching boats (unlike great white sharks). They are harmless to humans if left alone and are not attracted to chum. Even though the basking shark is large and slow, it can breach, jumping entirely out of the water. This behaviour could be an attempt to dislodge parasites or commensals. Such interpretations are speculative, however, and difficult to verify; breaching in large marine organisms such as whales and sharks might equally well be intraspecific threat displays of size and strength.

Anatomy and appearance The largest accurately-measured specimen was trapped in a herring net in the Bay of Fundy, Canada in 1851. Its total length was 12.27 metres (40.3 ft), and it weighed an estimated 19 tonnes (19 long tons; 21 short tons). Dubious reports from Norway mention three basking sharks over 12 metres (39 ft), the largest at 13.7 metres (45 ft), dubious because few anywhere near that size have been caught in the area since. On average, the adult basking shark reaches a length of 6–8 metres (20–26 ft) and weighs about 5.2 tonnes (5.1 long tons; 5.7 short tons). Some specimens still surpass 9–10 metres (30–33 ft), but after years of large-scale fishing, specimens of this size have become rare.

 

 

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