Scottish seas reveal basking shark secrets

Basking sharks can weigh as much as 7 tonnes. Credit Colin Speedie/Swiss Shark Foundation.

In Gunna Sound between the islands of Coll and Tiree, and the sea around the islands of Canna and Hyskeir, four times as many basking sharks have been recorded per hour of survey than anywhere else in the UK.

Canna & Coll
The report identified the hotspots from the consistently high numbers of basking sharks seen at the surface, often in large groups. On one occasion in July 2006 83 sharks were recorded round Canna, with the highest daily tally round Coll in August 2005, with 94 sightings. Frequent displays of courtship-like behaviour, including breaching - where the shark leaps clear of the water - suggests that both areas are important for sharks looking for a mate.

The report interprets data collected along the west coast of Scotland between
2002 and 2006 by The Wildlife Trusts' Basking Shark Project. Set up because so little was known about basking sharks, the project has carried out surveys along the western seaboard of the UK over the last nine years.

Colin Speedie
Colin Speedie, the shark expert who carried out the research for SNH said:
"Basking sharks are fascinating but we've still got much to learn about them. They are huge - the length of a double decker bus - but they feed entirely on plankton, tiny animals that drift through the water. These minute creatures pass through their enormous gaping mouth and are filtered out by their comb-like gills. In one hour an adult shark filters enough water to fill a 50m Olympic sized swimming pool.

"Basking sharks are most often seen in coastal areas in the summer and autumn when plankton are abundant at the surface and this is how they get their name - from apparently ‘basking' at the surface in calm, sunny weather."

Basking sharks are huge but harmless to humans

11m long & 7 tonnes
Basking sharks can grow up to 11m in length and 7 tonnes in weight. They are also long lived, with some surviving as long as 50 years. Because they are slow moving, slow to mature and long lived, they are very vulnerable to human disturbance and impacts. For generations they were hunted for the high oil content of their large livers. More recently they were hunted in European waters for their colossal fins which reportedly sell in East Asian markets for an average price of US $50,000.

No significant threats to the sharks were identified at either of the hotspots.

Boating guidelines
However they are particularly at risk during courtship, when two or more sharks swim along nose to tail or in contact with each other, in a trance-like state.
To help reduce the risk of accidental collisions and highlight the hotspots, SNH is producing a leaflet and poster for leisure and commercial boat users. These will be sent out to boat clubs and training centres in the spring.

The report will also help consider what action could be taken to look after basking sharks in the waters of the west of Scotland in the future, if climate change, offshore developments, fisheries or marine tourism became an issue. This is particularly timely as the Scottish Marine Bill, currently making its way through Parliament, looks at the future management of our seas.

Dr Rebecca Boyd, Marine Policy Officer with the Scottish Wildlife Trust (SWT) added: "It is excellent news that these giant sharks seem to be finally making a comeback, and that Scotland has clearly become so important to them for feeding and mating. We must give them every chance to thrive safe from accidents with boat traffic and fishing gear. SWT welcomes the opportunity that the Marine (Scotland) Bill offers to consider the protection required for basking shark 'hotspots' to ensure the sharks get the space and time they need to reproduce successfully."

The report was published by Scottish Natural Heritage (SNH),