The Independent newspaper has reported that a Cornwall-based wildlife tour operator made 19 Basking Shark sightings in just one weekend earlier this month. Captain Keith Leeves, a skipper with AK Wildlife Cruises, said: “To see so many this early has been an absolute honour… This has been one of the best starts to a shark season in living memory.” A total of 266 Basking Shark sightings were reported to the Shark Trust for the whole of 2013 – with numbers of these graceful giants now rising once more, the Trust is hopeful of even more sightings in 2014. As the Basking Shark becomes more ‘visible’, we have a great opportunity to learn more about this mysterious summer visitor to our shores, including how best to protect it.
Reaching up to 11 metres in length and 7 tonnes in weight, the Basking Shark (Cetorhinus maximus) is Britain's largest fish. Distributed in regional ‘populations’ across the globe, the sharks were once numerous in UK waters; sadly, for many years they were hunted here for their liver oil, with specialist shark fisheries established on the coasts of Scotland and Ireland. As a result, the Basking Shark population declined to such an extent that they are now considered to be endangered in UK waters. Sustained campaigning by concerned individuals and groups, such as the Marine Conservation Society (MCS), finally led to Basking Sharks gaining legislative protection under Schedule 5 of the Wildlife and Countryside Act 1981, the Countryside Rights of Way Act 2000, the Wildlife (Northern Ireland) Order 1985 and the Nature Conservation (Scotland) Act 2004. These Acts make it illegal to intentionally kill, injure or recklessly disturb or harass Basking Sharks in British waters.
In the UK, Basking Sharks are often seen near the surface of the sea in summer, apparently enjoying the sun (or ‘basking’, hence their name). The massive grey body moves slowly along, with the blunt snout, dorsal fin and top part of the tail fin breaking the surface. When feeding, the shark swims with an open mouth – this can look fearsome, but in fact the shark is simply drawing in vast quantities of sea-water, which pass out through its gills while plankton is simultaneously filtered by a sieve-like structure formed by interlocking gill-rakes (rather like the teeth of a comb). Despite its huge size, the Basking Shark feeds entirely on plankton, migrating thousands of kilometres to find food. The sharks seem to prefer ocean fronts where different masses of water meet and where plankton can flourish; these areas include headlands, islands and bays with strong tidal flow. Basking Shark summer-feeding ‘hotspots’ are now being identified around the UK coastline, specifically in the south-west, Wales, Isle of Man and western Scotland. The animals typically stay in British regions until around October.
Basking Sharks are seen only rarely during the winter. For years it was believed that they hibernated during colder months, a theory supported in part by findings showing that the liver weights of fishery-caught Basking Sharks were much lower in spring than in autumn, indicating a cessation of feeding – this in its turn was backed up by reports that the sharks shed their gill-rakes during winter (and so were presumably incapable of feeding). However, recent studies have disproved the hibernation theory, instead showing that Basking Sharks are very active throughout the winter. Careful satellite tagging has shown that in the winter months, rather than spending time near the surface, Basking Sharks spend more time in deeper water, feeding on the plankton communities there. The research also showed Basking Sharks continuing to move thousands of kilometres during the winter months, actively tracking plankton blooms in order to feed. In addition, rather than losing their gill-rakes in one moult, it was found that the sharks shed and re-grow new gill-rakes as an on-going process during the winter months.
Overall, satellite tagging is showing that Basking Sharks are considerably more wide-ranging than previously thought. In 2008 a Basking Shark was tagged off the Isle of Man; it promptly headed straight for Newfoundland, the first recorded trans-Atlantic migration by a Basking Shark. The same shark also reached a record depth (for Basking Sharks) of 1264 metres. In 2009 a Basking Shark was tagged off New England and went on to provide the first proven trans-equatorial migration for the species, crossing the equator into Southern Hemisphere waters, where it spent time at depths of between 800m and 1000m. The majority of recorded sightings from the United Kingdom, Japan and Newfoundland are from water 8–14°C in temperature, although most records from New England are in water from 11–24°C, with most of these over 16°C.
The evidence of Basking Shark migration highlights the need for global, not just regional, protection for this species. The Shark Trust, MCS and others continue to campaign for better international legal protection, and the MCS Basking Shark Watch programme has also generated the largest Basking Shark sightings database in the world. There is still much to learn about these elusive giants; it is not known, for example, where the breeding-grounds are for the Basking Sharks found in British waters. As the population recovers, the sharks still face threats of accidental entanglement, disturbance at their surface feeding sites and illegal fishing for their valuable fins. Again, it is not known how many sharks are accidentally caught in fishing gear or hit by boats, and how significant these threats may be. We do not know the significance of disturbance at basking shark hotspots. Only by continuing to learn more about these mysterious animals will we build up a better idea of how best to protect them
Sources
www.sharktrust.co.uk
www.wildlifetrusts.org
www.mcsuk.org
www.independent.co.uk
Seawatching by Tony Soper and Noel Cusa (Dinosaur Publications Ltd, 1978)
This blog post was commissioned by Quba Sails.
