When turtles come onshore to nest, a transmitter is fitted to their shells in a painless process using fibreglass cloth and resin. The transmitter has a smart switch that is sensitive to sea water. When the turtle comes up to the surface to breathe, the transmitter switches on and, using a global system of satellites, it sends a signal up to an orbiting satellite which keeps constant track of it. The information is relayed to a receiving station in France, then passed on to scientists and conservationists throughout the world via email or a dedicated website.

To save battery life, the transmitter automatically switches off when under water and it can be programmed so that it is only in action for certain amounts of time. Its duration will obviously depend on how active the turtle is, but normally it will last around one year. At some point after it has outlived its usefulness, the adhesives that have clamped it to the shell will crack and the unit will simply drop off. At no time will the turtle have felt any pain or discomfort; it will likely not even be aware that the transmitter had been attached to it.

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How do we receive and analyse satellite tracking data?

The small, low wattage transmitters are controlled by a microprocessor which is programmed before they are attached to the turtle. The programme tells the microprocessor how to store information and when to transmit the information to the satellites.

There are four polar orbiting satellites currently used for tracking animals; they are in fact the same satellites that are used to monitor global weather patterns. Attached to these satellites are special instruments, operated by French company ARGOS CLS, which constantly listen for the transmitters and determine where those transmitters are located.

While such a task may seem simple, it is not. Each satellite circles the earth every 101 minutes, so it can only receive transmissions from any one place on the planet for about 10 minutes. At the equator, this means that the satellites make six to eight passes per day over a particular spot for about 10 minutes each.

It takes anything up to five minutes for the satellite to pinpoint exactly where a transmission is coming from, and the transmitter must be on the surface to be detected - yet turtles rarely remain on the surface for that long. So, all in all, we do not always receive a location from a turtle every day.

One problem with getting a ‘fix’ on a turtle is how long the satellite ‘sees’ it as it passes overhead. The longer the contact, the better the fix. Shorter contacts of a few seconds allow a fix, but not as accurate as a longer contact of a few minutes. However, computer mapping programmes are able to work with the data they receive and make logical, credible assumptions on the basis of it. They can plot courses that show where the turtles are, the route they are taking and how fast they are swimming.

So, when viewing the migration maps, we need to bear in mind that the plotted turtle movements are based on the best data available, and they may not be 100% exact at any given moment. In any event, this limitation in no way detracts from the proven value of the research. While a particular location point may actually be a few miles off a given turtle's actual location, the accumulation of data stills tells us where the turtles are generally moving and where their primary foraging areas are located. Using this information, we can focus conservation efforts where they are most needed.

Why is there a variation in the average transmission period?

The overall transmission period depends on the battery life of each transmitter, as well as the behaviour of each turtle.

We all know that even the batteries we use in our daily lives do not last exactly the same length of time; some will burn out faster, and others will last for months. In a way, the same thing happens with satellite transmitter batteries: while technology improves daily and the unit manufacturers include circuitry to minimise variability, we just can’t get away from the fact that some units will outlast others. So this is one reason why some of the units might have gone off the air earlier than others.

Also, not all turtles behave the same way; if the turtle spends a lot of time on the surface using up battery power, the transmitter will not last as long as that on an animal which lies deep and only comes up occasionally. The transmitters have a salt water switch which turns them on as the turtle surface, and turns them back off when the turtle goes back under water. So if a turtle spends more time on the surface than another, it will send more signals than the other, and drain the battery faster.

What happens when a transmitter stops sending signals?

We often get asked if the turtle had suffered any major trauma to justify the end of the signals. When a turtle has reached its foraging ground, there is little reason to believe that

a turtle which had been sending signals for months without problems would have suddenly suffered some major mishap. Far more likely is that the battery finally failed, or that the algae and other fouling organisms that settle on the transmitter over time had fouled the salt-water switch.

However, it is worth pointing out that these turtles are classified as Critically Endangered globally by the International Union for the Conservation of Nature (IUCN), and this is for a reason. This species has suffered (worldwide) a reduction in numbers of over 80% over the last 100 years, and only now are discrete populations starting to recover – with the implementation of long-term dedicated conservation programmes.

But they still face a number of threats, and this is one of the reasons we are running this project; so we can identify where the turtles go, and see if there are overlaps between turtle habitats and these key threats. Turtles can get entangled in fishing nets, and even swim into fish traps from which they can’t escape.

They are also impacted by habitat loss, and as more and more beaches get developed, less and less area is available for nesting. Oil spills which wash up on beaches pose a hazard to emerging hatchlings, and boat strikes from the ever-increasing maritime traffic also take their toll. Topping all of this is climate change, and the Gulf’s cold winters leave dozens, if not hundreds, of turtles cold-stunned. When these turtles sit on the bottom to conserve energy (they are cold blooded and cannot really control their inner body temperature), they get covered in barnacles to the point they can barely swim or feed.