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Basking sharks

We are lucky to have the second largest fish in the world cruising Scottish waters each summer - an exciting sight! The basking shark grows up to 10m (33ft) long, and a few places in Scotland are particular hotspots for seeing them. They are fish of open waters, but move closer to shores in summer, when we can see them 'basking' at the surface, feeding with their huge mouths wide open. These gentle giants have no teeth, and their massive bodies are nourished entirely by plankton soup!

Why worry about basking sharks?

Because basking sharks swim at the surface, these magnificent fish are easily harmed, either deliberately or accidentally. Basking sharks were once fished commercially on a small scale around Scotland for their huge livers, which contain oils formerly used in various industries. The peak recorded kill was almost 250 sharks in 1947, but, in response to concerns over dwindling numbers, the basking shark has had full legal protection since 1998 under the Wildlife & Countryside Act 1981 (as amended by the Nature Conservation (Scotland) Act 2004).

Currently, potential threats include bycatch in fishing nets, and disturbance or impact by jet-skis, speedboats and other vessels. The Scottish Wildlife Watching Code provides best guidance for wildlife watching operators, and continued measures to raise awareness of the code and of the location of Scottish hotspots are invaluable in conservation of this extraordinary fish. There is a UK Biodiversity Action Plan for the basking shark, now taken forward by the Scottish Government as part of the Scottish Biodiversity Strategy external site . Globally, its conservation status is assessed as vulnerable.

How to recognise them

No other marine creature behaves quite like a basking shark, cruising slowly along at the surface. It can be mistaken for two sharks following each other, because the prominent dorsal fin and the top of its tail fin often show above the surface at the same time. When feeding, the rounded tip of its snout may also show above the water, and no other fish has such a huge mouth! The fish are dark slaty grey to black, with paler belly.

Where do they live?

Basking sharks are found in temperate seas across the world. Although spending much time in the open ocean, they are attracted inshore to places with regular, reliable tidal fronts (places where different water masses meet), where the tiny plankton it eats is concentrated. In Scottish waters, basking sharks are seen most commonly off western coasts, and especially around the outer Firth of Clyde. Recent studies funded by SNH , collating data collected by the Wildlife Trusts, have confirmed two other hotspots for basking sharks: in Gunna Sound, between Coll and Tiree, and around the rocky islet of Hyskeir, southwest of Canna. Considerable numbers of sharks have been consistently recorded here up to four times the numbers seen elsewhere in the UK.

The sharks are highly migratory, but long-distance tracking of individuals only began recently, and it is still unknown whether they migrate between lower and higher latitudes or between deep and shallower water. Their livers contain a large proportion of oil typical of deepwater sharks, which may indicate that they spend some time in deep water.

How do they live?

Basking sharks are usually solitary, but occasionally gather in aggregations of 100 or more where feeding is especially rich. In an hour, a feeding basker can filter 1.5 million litres (330,000 gallons) of water through its gills, extracting tiny crustaceans that resemble pink, swimming grains of rice. Sharks in Scottish hotspots can often be seen in large groups, and showing courtship behaviour, though otherwise we know little about their breeding. One captured basking shark gave birth to six live young, 1.5 to 2m long. The fish probably matures late and reproduces slowly, making it particularly vulnerable to overfishing, especially as fisheries catch many more females than males.

Last updated on Wednesday 6th January 2016 at 13:50 PM. Click here to comment on this page