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Photographic identification is widely used to census individuals in studies of cetacean and pinniped populations. However, it has seldom been implemented in studies of white sharks, with identification relying primarily on conventional tags. However, we have found that tags were easily discoloured and became fouled with algae and seaweed within a very short period of time, and were also sometimes displaced and/or rejected by the sharks’ tissue. (See related article on tag fouling)These events caused confusion as to the exact identification of individuals and the number of sharks in the study area. Therefore, to prevent uncertainty over shark identification, a photographic database was compiled for the white sharks observed at the Neptune Islands.
Photo identification is a well-known and used technique in studies many organisms, including dolphin, some whale species, killer whales, pinnipeds, seadragons and cheetahs. It has also been used previously in white shark studies at the Farallon Islands, California. It is non-invasive and allows animals to be identified without direct interaction and therefore is ideally suited for studying a protected species such as the white shark. The identification process is also much more successful than the tagging technique as the sharks can be identified from a much greater distance.
It is widely thought that the trailing edge of the dorsal fin of white sharks is specific to each shark. Such patterns are analogous to human fingerprints and are unique to each individual and hence can be used for identification. This method of identification is also being conducted by fellow researcher Michael Scholl in South Africa, with very successful results. Photo identification represents a dynamic identification system: notches will appear, and evolve in shape and size with time, hence photographic identification is a reliable tool in an ongoing project that allows tracking of these evolving changes and adapting the database. The dorsal fin of white sharks has proven to be very useful for long-term identification, while scars and patches are useful for short-term (day to day) recognition.
Even by eye, distinct differences in the dorsal fin profiles can be seen, such as distinguishing notches and degree of smoothness, and, in some cases, sharks can be identified by sight alone in this way. For harder cases, a comparison with the dorsal fin database easily determines if the individual is a newcomer to the islands or is a past visitor by matching it to the fin profiles already obtained. Scarring is most often observed in larger individuals, whether it be as a result of injuries inflicted by other sharks during mating or conflict, or, more unfortunately, as a result of human interaction.
The catalogue of fin profiles created in South Africa was compared to the photographs accumulated by the FSRF thus far, but no positive matches were made. However, as both databases expand, positive matches may become more likely, and hence the collaboration of researchers on this issue may generate positive results in the future.