Mystery Sharks

The basking shark (Cetorhinus maximus) is one of Earth’s gentle giants. They are the second largest shark, after the whale shark, and are one of three passive sharks that feed by filtering plankton from the water. Basking sharks used to be abundant in the oceans, but their numbers have declined for decades and are continuing to decrease. Basking sharks were hunted voraciously for their large fins for shark fin soup, as well as for their livers and meat. Despite their historically large population, scientists today know very little about this shark species.

On average, an adult basking shark can grow up to 26 feet (8 meters) in length, with the largest recorded shark captured off the coast of Canada in 1851 measuring 40 feet (12 meters). Due to their size, these sharks are often mistaken for great white sharks, however the most distinguishing feature of basking sharks is their large jaws – exceeding 3 feet – to filter water for feeding. It is estimated that a single shark is capable of filtering nearly an Olympic-size swimming pool’s water every hour.

Basking Shark Protection

Basking sharks are predominantly threatened by human activities. Overfishing remains the main threat to these sharks. For many years, the government of Canada hunted basking sharks, since the sharks were frustrating salmon fishermen by coming up as bycatch and damaging equipment. Since they feed near the surface, they are also prone to get hit by vessels.

The basking shark is considered a vulnerable species by the International Union of Conservation of Nature (IUCN Red List) world-wide, but listed as endangered in the north-east Atlantic. In Scotland, basking sharks are a protected species, with full legal protection since 1998.

Tagging Basking Sharks

Human activity within the north-east Atlantic Ocean is increasing, in terms of commercial fishing activities, as well as movement of vessels. Improved knowledge of the movements of basking sharks is thus important for the conservation of the species.

In the summer months, large groups of basking sharks congregate in the plankton-rich waters of the Sea of Hebrides, off the north-west coast of Scotland. Deep, cool waters full of nutrients mix with warm surface waters, creating areas of high productivity – ideal feeding grounds for basking sharks. The question remains, where do these sharks go in winter?

The University of Exeter and Scottish Natural Heritage set about trying to find the answer in the summer months of 2012, 2013 and 2013. The researchers were able to tag 61 basking sharks using GPS equipment. This involved attaching a satellite tag to the base of each shark’s dorsal fin.

Satellite tags are programmed by the scientist, and once the tag breaks the surface, it transmits information to a satellite array that is orbiting Earth. This satellite array beams the data to a central computer, and the scientists who tagged the shark are able to view the data transmitted.

Each time a shark surfaced to feed, the tag would transmit the location of the shark via satellite, allowing the team to remotely track the movements of the shark. The tagging of basking sharks continued in 2016 and 2018, and included sharks from Gunna Sound. The sharks tagged in 2018 in the Sea of Hebrides were also tagged with camera tags. The footage captured by these cameras revealed behaviors never seen before, including sharks potentially forming social groups and aggregations near the seabed.

Where Do The Sharks Go?

Despite this, and other similar studies, the population estimates, the age of sexual maturity, and when and where the sharks reproduce are not known for certain.

The tagging study revealed some interesting results. During winter, the researchers discovered that the sharks feeding in the waters off the coast of Scotland migrate to deeper waters, some staying relatively close to the Scottish coast. Others, however, swam further and sharks were observed within the Exclusive Economic Zones (EEZs) of Iceland, the Faroe Islands, UK, Ireland, France, Spain, Portugal, Madeira, Morocco and the High Seas. One shark even traveled as far as the Canary Islands. The areas deemed of the highest importance in the winter months are the west coast of Scotland, and the Celtic and Irish Seas. The general pattern of movement during winter (from October onward) followed a steady transition to more southern locations. However, the majority of the tagged sharks returned to the Sea of Hebridean in summer to feed. Tagged sharks demonstrated high fidelity to waters around the Isles of Coll and Tiree within the Sea of Hebrides during summer. This confirmed the citizen science sightings collected by the Hebridean Whale and Dolphin Trust.

The study further confirmed the importance of the Sea of Hebrides as one of Scotland’s most important marine wildlife habitats, and assisted with formally declaring the area as a marine protected area in December 2020. The locations transmitted by the tags are most recently being used to discover whether the sharks are breeding within the Scottish waters. Despite the use of towed camera-tags in 2018 and 2019, breeding has not been observed.

Why Is It Important To Track Basking Sharks?

The ability to record the movement of large marine animals is becoming more and more possible and affordable, and provides important information on migratory species. Little continues to be known about basking shark habitat and site preference during winter, due to them swimming at depths averaging between 50 m and 200 m. Studies of the sharks’ anatomy previously suggested the sharks hibernate in deep waters during winter, however in recent years it was discovered that hibernation seems less likely due to information provided by the satellite tags. The tags prove that basking sharks do not lie dormant during winter, but that they make frequent trips to the surface, providing evidence that they are likely not hibernating. It is possible that not all basking sharks adopt the same migration strategy during winter, but that individuals perform their own movements.

The reasons behind the winter-summer movements of basking sharks remain unclear, but it is hypothesized that they search for food, thermoregulate by moving to areas and depth of preferred temperature, move to mating grounds or move to areas where they were hatched.