Saturday, May 16, 2009

1240th Indian Ocean whale shark is 400th for Seychelles

On Friday the 15th of May Divemaster intern Kerstin Gillray and instructor Theirry Vandamme from the Underwater Centre / Dive Seychelles captured the identity of the 400th whale shark identified by photo ID in Seychelles... regular readers might wonder about the 400 figure as we have previously reported more sharks identified than this, but read on and all will become clear!

Photo ID has revolutionised the study of whale sharks globally and in Seychelles it has become the main tool for monitoring the population of whale sharks visiting these islands.
Being able to identify individuals allows scientists to establish the make up of the population in terms of numbers of males, females and their age or maturity; this information allows estimates of population size and trends or changes.

For many years researchers had identified individuals
either by tagging them with numbered marker tags or by using prominent features or scars; however, tags do come out and scars do change over time.

The first real attempt at whale shark photo ID was in the early 1980's by Geoff Taylor a medical doctor working at Exmouth near Ningaloo reef in Western Australia. Geoff was able to identify the sharks using the patterns of spots and matching the photos by eye and for many years this was the only way to do this.

A variety of areas were used by different groups, some used dorsal fins, others body markings, but all matched by-eye. After a while it became apparent that the spot pattern behind the gills was like a fingerprint and appeared to remain stable over many years.

Since then two computer based matching programs have been developed, one based on the astronomical pattern matching algorithm used in the space telescope to match star-fields which
was adapted for the EcOcean project. The other was developed by researchers working on ragged tooth sharks called IRIS which was further refined as I3S and made available free of charge to those interested. Using these tools researchers were able to quickly and accurately match whale shark photos.

The I3S computer matching program and the 'target' area for photo identification of whale sharks

So why the discrepancy in the numbers of sharks identified in Seychelles? The key is that just like a human fingerprint, the spot pattern from the right is not the same as on the left. In the early days in Seychelles we used marker tags and only with the advent of affordable digital undwerwater cameras in 2004 did we move into photo ID en masse....

As such, at the end of 2007 we had a total of 512 sharks identified by marker tag and photo ID combined, of which 360 were photo IDs. In 2008 although we added 38 new photo IDs, we also matched together several sharks that were originally identified by just a left or a right hand image...

And so it is that we have now achieved 400 photo IDs from Seychelles with Kerstin's new shark, which was in fact her very first whale shark!

This is also the 1240th photo identified whale shark in the Indian Ocean regional database shared between Seychelles, Western Australia, Djibouti, Mozambique, Madagascar and the Red Sea!

But Kerstin didn't care... it was Number One for her!!!

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