Basking Sharks in the Hebrides

The Hebrides is often described as romantic so it is no surprise to find that this may be one of the reasons why one of our most fascinating and rare species visits these waters too!

Seas between the islands of Skye and Mull on Scotland’s west coast are highly important for basking sharks, according to a report published today by Scottish Natural Heritage (SNH). This new research also suggests that basking sharks may be coming to the area to find a mate.

Each year large numbers of basking sharks are seen in an area of the Sea of the Hebrides which is currently being assessed as part of the Scottish Marine Protected Areas Project.

The report sets out findings from the first two years of a project which aims to reveal some of the mysteries surrounding the world’s second largest fish. The project is a joint venture between SNH and the University of Exeter, and is the first known to use satellite tagging technology to track the near real-time movements of basking sharks.

Twenty sharks were tagged in summer 2012 and a further 31 were tagged a year later. In both years the tagged sharks spent most of July, August and September in waters around the islands of Coll and Tiree and the Hyskeir lighthouse. In these months more than 80% of the satellite transmissions received from tagged sharks came from within the Sea of the Hebrides. This is seen as further evidence that the area is a special place for these sharks.

Scientists at SNH and UoE believe the sharks return each year to feed in the area’s plankton-rich seas but the sharks’ behaviour suggests they might come for other reasons too.

Dr Suzanne Henderson from SNH, who is managing the project said:

“As well as cruising around and feeding at the surface they can be seen showing courtship-like behaviours, such as jumping clear  of the water, known as breaching and swimming around nose-to-tail. These social behaviours suggest that the sharks return to the area not just to feed on the plankton bloom but for other reasons too, perhaps even to find a mate.”

Information received from the tags also shows that the sharks spend these summer months at different depths, moving up and down in the water on a daily basis. A large proportion of their time is spent in shallow water less than five metres deep, but they also spend time in deeper water down to 250 metres. There doesn’t seem to be a single pattern to this daily vertical migration, and it appears that the sharks adapt their behaviour to local conditions.

Dr Matthew Witt, of the Environment and Sustainability Institute at the University of Exeter’s Penryn Campus in Cornwall said:

“We know quite a lot about basking shark biology and distribution but relatively little about their seasonal movements, although we have already learnt a lot from the first two years of this project. We now know that as autumn approaches the sharks start to spend more time in deeper water, with less activity at the surface. Two of the sharks reached depths of over 1000 metres, indicating that they were off the continental shelf.

“After September the sharks head away from Scotland and evidence suggests that the Celtic and Irish Seas are an important migration route for them.”

Find out more about the basking shark tagging project >. The basking shark image was captured aboard one of our cruises by Chris Gomersall