Gili Air turtles have names: inside turtle photo-ID

So how do you recognise a turtle you met last year? The short answer: by the scales on its cheek. Every sea turtle has its own pattern there, and a clear side photo is enough to match it against a catalogue. That is turtle photo-ID, and around the Gili Islands it has been quietly building a list of individual animals for years.

A face you can recognise

Turtles do not carry name tags. They carry something very close to one. The scales on the side of a sea turtle's head form a pattern that is different in every individual, a little like a fingerprint.

The local team around the Gilis works with the cheek for two good reasons. As they explain, these facial scales are not likely to change during the turtle's life, unlike fins, shell or tail, which can get scratched or bitten. And the cheek is flat, so it photographs well.

From one photo to a name

The method is simple. A diver takes a clear photo from the side. Pattern-matching software called I3S compares it with the photos already in the catalogue.

If there is a match, a known turtle has been seen again. If not, a new profile is created and the animal gets a code, G for green or H for hawksbill. Some get a name too. The local report gives green turtle G149 "Marnie" and hawksbill H50 "Femke" as examples.

Sharks get the same care. For whitetip reef sharks, the team uses a full side view of the body and the pattern of its blotches.

What the Gili catalogue has found

These numbers come from a technical report by Gili Shark Conservation, an NGO based on Gili Air. It has not been peer-reviewed, so read it as careful field data from a local team, not as a published population estimate.

In the year the report covers, the team logged 342 underwater surveys. It reported 181 green turtles, 51 hawksbill turtles and 18 whitetip reef sharks identified through photo-ID, some of them from photos shared by other dive professionals.

A few patterns stand out. Green turtles made up 86.2% of turtle sightings in the team's roving surveys, and Turtle City off Gili Meno gave the most turtle sightings of any site. The photo-ID record also suggested that green turtles and whitetip reef sharks stay on one reef for long periods. The authors read that as a sign the marine park is working as a protection tool.

Whitetips were also the most reported shark or ray in a citizen-science scheme where 12 dive centres on Gili Air logged 1,106 sightings. Of those, 799, or 72%, were whitetip reef sharks. The work continues today under Gili Matra Bersama, whose research includes photo identification of foraging green and hawksbill turtles and a project on juvenile whitetip reef sharks.

Why does knowing one turtle matter?

Counting sightings tells you how often turtles are seen. Counting individuals tells you who is actually there, for how long, and whether the same animals return. That is a very different question, and often the more useful one.

Studies elsewhere show what it can reveal. On the rocky reefs of Arraial do Cabo in Brazil, photos from social media, citizen scientists and researchers identified 174 green turtles and 32 hawksbills, and showed that some individuals had used the area for at least six years. In Taiwan, a citizen-science project used photos of the left side of the face to identify 762 turtles from 3,515 sighting records.

The bigger picture is hopeful, but not simple. The IUCN moved the green turtle from Endangered to Least Concern after the global population grew by about 28% over roughly five decades. It also warned that take, bycatch and coastal development are still serious threats, and that some subpopulations are still in trouble.

The hawksbill is in a harder place. It is listed on CITES Appendix I, and its shell is still taken for the illegal tortoiseshell trade. Knowing individuals is how a local team can tell whether the turtles of one small marine park are holding steady.

A good photo does not need to be close

Photo-ID is only gentle if the photographer is gentle. A widely used benchmark from NOAA Fisheries, in its viewing guidelines for Hawaiʻi, is to stay at least 3 metres (10 feet) from sea turtles, in water and on land, and never to touch, chase or feed them.

A turtle that is feeding or resting should not have to move because of you. One that is rising to breathe needs a clear path to the surface.

What helps science is a steady, side-on shot of the cheek, taken as the turtle turns on its own. No chase, no flash in its face. I keep my small action camera ready before I get in, so I am not fiddling with buttons next to an animal. And if the turtle swims away, that photo was simply not meant to be.

Afterwards, note the date, time and site, and ask your dive centre whether it shares photos with local turtle research. Good operators brief guests before the first entry, keep groups small around any one turtle, and leave whitetip reef sharks resting under ledges alone. Juvenile sharks and resting animals are best watched briefly, from a distance, and then left in peace.

It is the same spirit we look for in the local teams behind our Gili Trawangan referral trip: let the turtles set the distance, and let the photo come, or not.

Questions people ask

Wildlife encounters are never guaranteed and never forced. Ocean Calling works only with vetted, ethical local operators, and every animal is free to approach, or not, entirely on its own terms.