Abstract
Following centuries of exploitation, basking sharks (Cetorhinus maximus) are considered by IUCN as Endangered in the Northeast Atlantic, where they have now been substantially protected for over two decades. However, the present size of this population remains unknown. We investigated the use of photo-identification of individuals’ dorsal fins, combined with mark-recapture methodology, to investigate the size of populations of basking shark within the west coast of Scotland. From a total of 921 encounters photographed between 2004 and 2011, 710 sharks were found to be individually identifiable based on dorsal fin damage and natural features. Of these, only 41 individuals were re-sighted, most commonly both within days of, and close to the site of, the initial encounter. A smaller number were re-sighted after longer periods of up to two years. A comparison of the distinguishing features of individuals on first recording and subsequent re-sighting showed that in almost all cases these features remained little changed, suggesting the low re-sighting rate was not due to a loss of distinguishing features. Because of the low number of re-sighting we were not able to produce reliable estimates for the long-term regional population. However, for one 50 km diameter study area between the islands of Mull, Coll and Tiree, we were able to generate closed-population estimates for 6–9 day periods in 2010 of 985 (95% CI = 494–1683), and in 2011 of 201 (95% CI = 143–340). For the same 2011 period an open-population model generated a similar estimate of 213 (95% CI = 111–317). Otherwise the low rate and temporal patterning of re-sightings support the view that such local basking shark populations are temporary, dynamic groupings of individuals drawn from a much larger regional population than previously supposed. The study demonstrated the feasibility and limitations of photo-identification as a non-invasive technique for identifying individual basking sharks.
Highlights
With sharks gaining recognition as a global conservation priority, a wider range of methods for assessing population levels is urgently required to underpin management and protection of the most vulnerable species
In the event relatively few sharks were encountered in the Firth of Clyde, and population estimates have focused on the Inner Hebrides study area (Isles of Mull, Coll and Tiree)
This study demonstrates the feasibility of using photo-identification to recognise individual basking sharks. 83% of photographed sharks bore distinctive dorsal fin features that would allow them to be recognised over a period of time
Summary
With sharks gaining recognition as a global conservation priority, a wider range of methods for assessing population levels is urgently required to underpin management and protection of the most vulnerable species. Basking sharks (Cetorhinus maximus) are the second largest species of fish globally and have been subject to commercial exploitation in the North Atlantic for over 200 years [1] [2] [3] Following concern over their apparent decline, they were given full protection in British territorial waters in the 1980s, and subsequently in waters of some other countries bordering the North Atlantic and Mediterranean [3]. In the IUCN Red List they are assessed as globally Vulnerable, but both North Pacific and Northeast Atlantic stocks are considered Endangered [3] Despite this conservation concern, no scientific estimate of basking shark population size has yet been possible for any region within their circumglobal distribution [4]. Historic fisheries records for basking shark, as well as other large shark species, are of limited value for estimating present-day abundance, not least because of the problem of obtaining present-day catch data for stocks that are scarce or protected [11]
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