Seeing Spots: Photo-identification as a Regional Tool for Whale Shark Identification

Abstract

The identification of individual animals over temporal and spatial scales can provide robust estimates of population size and distribution. While marker tagging can provide an option to achieve this, it can be problematic both in terms of tag loss and the associated difficulties and effects of attaching the tags. Photo-identification of distinctive characteristics which remain stable over time has replaced tagging in some species but usage at regional scales has been hampered by a lack of standardisation of matching methods. We describe the use of a semi-automated computer program (I3S) for matching the spot patterns of whale sharks, Rhincodon typus, in the Seychelles aggregation and compare this to images captured from other areas in the Western Indian Ocean. Sharks totalling 443 individuals were uniquely identified in the Seychelles from 2001 – 2009, 109 of which were seen in multiple years. Conventional open mark-recapture models for 2004 – 2009 gave an abundance estimate of 469 to 557 sharks (95% C.I.). I3S digital fingerprints were shared with researchers in Djibouti, Mozambique, and Tanzania and, while no matches were found between locations, the ease with which regional comparisons were made will help to define whether the shark populations in these areas are distinct, enabling long-term and broad-scale regional comparisons.

Keywords: Whale shark, Rhincodon typus, Indian Ocean, photo-identification, mark-recapture, population estimation.