10/09/2019

How do we know that the turtle in the photo is unmistakably “Agisilaos”?

No doubt you have noticed the ease, with which sometimes we refer to specific sea turtles. It seems natural to give the turtles names in the Rescue Centre, but what about who is who at sea? The answer lies with the “tagging” processes that are in use in ARCHELON projects and also worldwide. Below you will find more on info on the this subject.

Giving an identity to wild animals is a fundamental way to study their populations. This has been the case for sea turtle populations since the very start of their conservation back in the 50s. In fact one has to think hard to find sea turtle studies where the identification of individuals through some "tagging" process does not play any role in one way or another. For instance by tagging nesting females one can follow their reproductive history for as long as these females are encountered at the nesting beaches, as well as to record their whereabouts in other areas and seasons. Growth rates of individuals can also be monitored at both nesting and foraging grounds, in the latter case via the so-called mark-recapture studies, where individuals are caught, tagged and released in a few minutes.

Which are the different ways that one can "tag" a turtle? The most popular method that has been used for more than half a century is via metal or plastic tags that are placed to the turtles' flippers. These tags bear a unique identification number that can be easily recorded by anyone who encounters the turtle. The method is cost effective, does not require any previous experience or equipment to "read" the tag but on the other hand "tag loss" that is, the fact that these tags often do not remain in place for ever, remains a challenge.

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On the other hand, PIT tags (Passive Integrated Transponder) provide a more reliable solution. These are small circuits that are inside a glass capsule that has the size of a rice grain. They are inserted into the turtle's arm with a sterile injection. A special scanner is then able to read the tag which provides a unique ID code. PIT tags are considered to be more permanent than traditional flipper tags. However one drawback is the increased cost per tag as well as the need for a specific scanning equipment necessary to read it.

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Another possibility is photo identification, where one exploits the fact that the "scales" (scutes) of the head form a pattern unique for every turtle, like the human fingerprints. This technique has specially come to rise after the advances in digital photography. This form of tagging is permanent as is the scute pattern throughout the sea turtle's life. It is cost effective and requires no handling of the animal since a simple photograph of good quality suffices. However, the matching process, that is examining whether a newly photographed individual exists in a given database, is often time consuming. Nevertheless, automated matching algorithms are constantly being developed and improved, often reducing a lot the time required for the matching process.

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Finally, genetic tagging has been recently used successfully, with a prominent example that of the loggerhead population of the south east United States. For example there, an egg from every clutch is collected (or skin sample in other projects), and the genetic profile of the mother loggerhead is extracted and stored in a database. Hence in that case, it is not even necessary to encounter the animal in order to tag it. This has provided some unique insights into the nesting dynamics of the studied populations. In certain cases, mother-daughter or sibling relationships are identified opening new horizons in answering some important demographic questions, like for example age at sexual maturity. However, the increased cost as well as the requirement for highly specialized personnel limits these methods to only a few parts of the world.

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ARCHELON has been tagging nesting females at the most important nesting sites of Greece since the beginning of the 80's with thousands of individuals tagged so far. Also ARCHELON has been PIT-tagging nesting females in Kyparissia Bay since 2005 and in Zakynthos. Furthermore, males as well as immature turtles are tagged in the mark-recapture project of Amvrakikos bay. For that, metal and PIT tags are typically used and occasionally photo identification as well.

(text and photos by Kostas Papafitsoros)

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