Summers spent basking in the Sea of the Hebrides

Within a few weeks of the first satellite tags being attached to basking sharks  thousands of visitors were hooked on a website which followed the sharks as their movements were displayed online in close-to-real-time, as Dominic Shann reports.

Twenty sharks were tagged in 2012, in a ‘hotspot’ for these gentle giants around the islands of Coll and Tiree off Scotland’s west coast. Each summer large numbers of basking sharks are seen here, cruising around feeding at the surface, making it an exciting place for wildlife watchers.

Shark head

While sharks are seen in many places around Scotland, displays of social and courtship behaviour, like breaching and following each other nose-to-tail, have only been observed in these areas, suggesting they are important for key stages in the life cycle of the sharks. It’s thought that, as well as coming to feed on the area’s rich plankton soup, the sharks might also come to find a mate.

Learning some of the secrets of the world’s second largest fish caught the public imagination and the tracking website received 42,000 hits in the first three weeks, causing the site to crash over the first weekend.

Two sharks

Dr Suzanne Henderson from SNH, who is managing the basking shark satellite tagging project, said: “It was fascinating to see where the sharks were going in those first weeks, and that most stayed around the Inner Hebrides. One shark made its way to Jura and another two headed west beyond the Outer Hebrides, but all three returned to the waters where they were tagged. It was also great to learn that the public shared our enthusiasm and curiosity about these majestic animals.”

Over the next two summers 41 more sharks were tagged in the project, a partnership between Scottish Natural Heritage and the University of Exeter, and the first known to track the near real-time movements of basking sharks.

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Tags are attached to the sharks using a pole.

“We were particularly intrigued to see where the sharks went during the winter”, observed Suzanne. “From autumn onwards the tagged sharks dispersed widely, leaving the shallow coastal waters for deep sea. Some went south as far as the Iberian Peninsula and North Africa, some headed west of Ireland and others remained relatively close to Scotland throughout the winter.

“One of the highlights of the project for me was seeing the first of the tagged sharks making its way back towards the Sea of the Hebrides, and then to Coll and Tiree where it was tagged the previous year. It was really exciting to learn that the same individual basking sharks return in consecutive years to use Scottish waters. It’s something we thought happened but we now have the first proof that this occurs. It really does emphasise that the Sea of the Hebrides is highly important for this migrating species.”

Sharks return 2013

Sharks were seen to disperse over the winter before returning to the Sea of the Hebrides.

Protecting highly mobile species such as sharks and whales is difficult due to the large areas they cover. So identifying and managing areas where the animals gather to feed, or for important life-cycle events, such as courtship, can play an important role in their conservation.

As part of the Scottish Marine Protected Areas Programme, SNH has recommended that an area of the Sea of the Hebrides from Skye to Mull be designated to protect the basking sharks, and also minke whales. Scottish Ministers are currently considering the proposal.

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As one person attaches the tag, another takes a genetic swab. (C) Rachel Graham, University of Exeter.

Basking sharks can grow to about the length of a double-decker bus (11 metres) and their huge bodies are nourished entirely by plankton. In an hour, a feeding shark can filter 1.5 million litres of water through its gills, extracting these tiny crustaceans.

The sharks can survive as long as 50 years but being slow moving, slow to mature and long lived, they are very vulnerable to human activities. For generations they were hunted widely around Scotland for the high oil content of their large livers but basking sharks have been a protected species here since 1998.

Basking shark

The tags were attached to sharks at the base of the dorsal fin with a titanium metal dart, using an extendable darting pole. Some tags provided information on the shark’s position each time it neared the surface, allowing it to be tracked online. Other tags collected data on depth, temperature and light levels over several months before detaching from the shark. These tags then float to the surface and transmit gathered information to satellites passing overhead. However, if they are physically retrieved then much more data can be collected.

Suzanne certainly feels the sense of excitement around this project : “We are extremely grateful to the public whose remarkable beach-combing abilities saw more than a third of the tags returned to us. But there are still tags out there to be found, with secrets yet to be discovered, and there’s a £100 reward for each one returned to us. So when you’re out walking on the west coast, keep an eye open and you could help us to learn even more about these truly wonderful fish.”

More information on the Basking shark tagging project can be found on our website.

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