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Population abundance and apparent survival of the Vulnerable whale shark Rhincodon typus in the Seychelles aggregation

David Rowata1 c1, Conrad W. Speeda2 p1, Mark G. Meekana2, Mauvis A. Gorea3 and Corey J.A. Bradshawa4

a1 Marine Conservation Society Seychelles, PO Box 384, Victoria, Mahe, Seychelles and University Marine Biological Station, Millport, KA28 0EG, UK.

a2 Australian Institute of Marine Science, Perth, Australia.

a3 Marine Conservation International, Edinburgh, UK.

a4 The Environment Institute and School of Earth and Environmental Sciences, University of Adelaide, Adelaide, Australia and South Australian Research and Development Institute, Henley Beach, Australia.

Abstract

Identifying individuals through time can provide information on population size, composition, survival and growth rates. Identification using photographs of distinctive physical characteristics has been used in many species to replace conventional marker tagging. We evaluated photographic records over 7 years of Vulnerable whale sharks Rhincodon typus, at an aggregation in the Seychelles, for estimation of population size and structure. We collected 11,681 photographs of which only 1,149 were suitable for comparison using semi-automated matching software (I3S) of individual spot patterns behind the gills. Photo-identification showed that there was considerable loss of marker tags and enabled an estimation of the rate of tag loss. The combination of photo-identification with marker tagging identified a total of 512 individual sharks over 2001–2007. Of these, there were 115 resightings in subsequent years with two sharks identified in 2001 resighted 5 years later in 2006 and another shark sighted in 2001 resighted in 2007. Estimates of abundance using conventional open mark–recapture models for 2004–2007 were 348–488 sharks (95% confidence interval), with a high level of entry into the population by itinerants. Annual apparent survival probability was 0.343–0.781 over 2004–2007, with an average annual recapture probability of 0.201. These results are the first to suggest a highly transient population of whale sharks around the Seychelles, indicating that international or at least regional-scale conservation approaches are required.

(Received May 20 2008)

(Reviewed September 15 2008)

(Accepted October 02 2008)

Correspondence:

c1 Marine Conservation Society Seychelles, PO Box 384, Victoria, Mahe, Seychelles and University Marine Biological Station, Millport, KA28 0EG, UK. E-mail david@mcss.sc

p1 Also at School for Environmental Research, Charles Darwin University, Darwin, Australia.

Footnotes

This paper contains supplementary material that can be found online at http://journals.cambridge.org

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