Just How Many Baskers Are There Down There?

We should have updated our basking shark blog before now, but the late summer was beset with various difficulties and delays, not least of which was the poor weather. The calm periods most productive for fieldwork were fewer in number than usual, and inclined to be brief, necessitating sudden returns to port. Nevertheless, we did in the end get some good spells, and eventually the season has produced some exciting results.

To recap on the background, for those unfamiliar with the story, the basking shark is the world’s second largest shark (up to 12 metres long). Like the largest species, the whale shark, it is a plankton eater, but whereas the whale shark is a tropical animal, the basking shark is restricted to cooler waters, occurring in both northern and southern hemispheres. The North Atlantic population was for two or three centuries exploited for its liver oil, and as a result is currently listed by IUCN as Endangered. Over the past 10-20 years however the species has been given increasing protection under both national legislation, notably within the UK and Isle of Man, and under international conventions, such as CITES and CMS. Thus a key question for biologists is whether, given these measures, the North-East Atlantic population remains in decline, or may perhaps by now be recovering?

Since basking sharks only occasionally come to the surface to feed, and do not like other sharks respond to bait, you can’t just count or catch them to see how many there are. However our early work, also generously funded by SOSF, led us to suggest that we could perhaps use the varying features of the dorsal fin, visible above water when the sharks are surface feeding, to recognise as individuals at least a portion of them. Then, in theory, by comparing the numbers of previously unrecorded to previously recorded individuals (and using the data in what are known as mark-recapture models), we would be able to come up with at least a reasonable estimate of the populations using either a particular area, or eventually, a wider region.

Consequently in April 2010 we launched the present Joint Photo-ID Project, planning to use quality photographs of the fins to recognise different individuals. To record basking sharks around the whole of the west coast of the British Isles we have collaborated with three other field teams – Colin Speedie and colleagues based in Cornwall, the Manx Basking Shark Watch (Isle of Man) led by Jackie Hall and Fiona Gell, and the Irish Basking Shark Study Group led by Simon Berrow. In addition it was arranged that we should support the Shark Trust in Plymouth, and its scientific director Ali Hood and basking shark project officer Cat Gordon, in establishing a common basking shark photo-ID database to which the different observer groups could add their data. Now work on the project is bearing fruit. The Shark Trust has its North-East Atlantic Basking Shark photo-identification database operational (members of the public will be able to access it through a dedicated website www.baskingsharks.org). And the Trust has published an impressive glossy booklet (Basking Shark: photo-identification) that both provides background information on the species, and explains to anyone interested can become involved, by taking photographs of any sharks they see, and contributing them to the database.
Now we ourselves, with great assistance from two MSc students, Peter Frey at Heriot-Watt University (near us in Edinburgh), and Gabi Gilkes, based at the Eden project in Cornwall, have completed analysis of the identification photographs that we took in our West of Scotland study area over this last two summers. This approximately 50 x 50 km area covers the west coast of the Island of Mull together with the isles of Coll and Tiree, and so incorporates one of the hotspots in which basking sharks, because of local oceanographic conditions, regularly aggregate to surface feed during summer months. There in 2010 we photographed 242 sharks, and during this past summer a further 210. However on detailed comparison of the photographs we have found that they were of just 154 and 138 individuals respectively – because sometimes the same shark was re-sighted on the same or a subsequent day. But the number actually seen is never the total number present – some will be feeding in areas away from the survey boat, while others will be below the surface. This is where the mark-recapture models come in – by calculating the ratio of new sharks to ones that we have seen already we can estimate the total number likely to be present. The numbers we have come up with are about 1700 for 2010 and approaching 500 for 2011, the lower numbers for this past summer being most likely due to the unfavourable weather.

These numbers it should be stressed are just the approximate numbers making use of our local west coast study area over the period that we were able to carry out survey work – typically a month or so. The total population using the whole west coasts of the Britain and Ireland throughout the summer will be much larger. But this is where working over several years and collaborating with other field teams comes in. We expect to be able to get an increasingly good idea of the size of the overall population by looking for matches, first between sharks recorded in different years, and second between sharks catalogued by the different groups, based as they are in widely separated locations. To date we ourselves have recorded 6 sharks that we had previously photographed in a previous year. The Irish Basking Shark Study group has likewise had re-sightings of sharks that they have tagged in previous years, with coloured plastic tags. The work of looking for matches between years and between areas is ongoing, and we are anticipating the first fruits of this effort with some excitement.

Apart from pursuing research important for an animal’s protection, promoting public awareness of the species and its conservation needs is also vital, if indeed it is to remain protected. With this in mind we have also been supporting plans for a Basking Shark Information Centre on the Isle of Coll, at Breachacha Castle, owned by local fisherman Innes Henderson and his wife Caroline. Innes has been one of the several local people who have provided us with valuable guidance during our fieldwork in the area, and in August this year he and Caroline hosted a BBC film crew which spent 10 days based at the castle, filming local basking shark and recording the work of the project. Their material will be featured in a series about the wildlife of the Hebrides scheduled for broadcasting in about 18 months time, and we hope used to produce a short video for visitors to the exhibition.

Rupert Ormond & Mauvis Gore