SnapperGPS on sea turtles

[amanda-matthes]

We have been receiving lots of questions from people interested in using SnapperGPS for sea turtle tracking. In summer 2021 we deployed a set of SnapperGPS receivers on nesting loggerhead sea turtles with the Maio Biodiversity Foundation in Cape Verde. So I thought I'd share our experience and talk about what you may have to consider when using SnapperGPS for sea turtle tracking. I hope I can answer the most commonly asked questions! Some of this information might also be useful for other species, especially other marine animals.

This write-up includes excerpts from the abstract for our talk at the 40th International Sea Turtle Symposium (ISTS40).

Advantages of SnapperGPS for marine wildlife tracking

Location tracking with global navigation satellite systems (GNSS), such as GPS, is a great tool for ecologists. Unfortunately, GNSS signals are radio waves that do not travel underwater. This makes tracking marine animals difficult because they often only surface briefly and sparsely.

Traditional GNSS receivers need to regularly be at the surface for at least 30 consecutive seconds to decode satellite orbit data from satellite signals. Assisted GNSS can reduce this time, but typically still requires several seconds. In contrast, snapshot GNSS is an approach that only needs to receive a few milliseconds of the signal as it does not decode any satellite data. Instead, the satellite data is obtained some other way. That means that a snapshot receiver is able to capture even very short surfacing events.

SnapperGPS is such a snapshot GNSS receiver. It records 12 ms of satellite data for each so-called snapshot. No further processing happens on the device. Once the receiver is retrieved, you can upload the recorded snapshots to the SnapperGPS processing service where the location tack is finally calculated. (Or you can host the code yourself if you don’t want to wait for our servers. It’s all open-source!)

Enclosure and attachment

When you deploy any electronics in the field, you need to protect it from environmental damage. In the case of turtles, the tag needs to be waterproof enough to withstand deep dives. It also needs to be quite tough to withstand heavy scratching.

We placed our SnapperGPS receivers into a custom enclosure. The base is milled out of a sheet of aluminium and the top is milled from a thermoplastic (polyoxymethylene). We tested the tags to be waterproof to 10 bar (that corresponds to around 100 m depth). The design is based on a previous turtle tag by Arribada. I uploaded a version of our enclosure in our SnapperGPS housings repo if you are interested in adapting it for yourself. Check the readme for some more details.

The tags were attached to the carapace using fibreglass matting, two-part epoxy and quick-drying metal epoxy putty. Make sure to find glues that dry quickly enough and check that they can handle sea water.

We glued the tags on while the turtles were laying their eggs. This meant that we had to work quickly, but we found that we usually had just enough time. Here’s a photo from the attachment process (left to right: Alasdair Davies, Amanda Matthes, Jonas Beuchert).

Make sure to think carefully about what size and weight limits you need to consider with your species. Test any quick-drying epoxy ahead of time. Some get very hot when drying and could damage the carapace.

Snapshot scheduling

Before deploying your SnapperGPS receiver you need to configure how frequently it should take a fix. The current version of SnapperGPS can record up to 10901 snapshots. That means it could attempt a fix every minute for a week or every hour for one year.

But the receiver will record a snapshot whether the tag is at the surface or not. That means that if your tagged sea turtle spends 2 min at the surface for every 20 min underwater, you would only get 1000 location data points. This is still enough to record interesting information, though. Here is one of the tracks we recorded in Cape Verde over two weeks:

We have been experimenting with a surfacing detection sensor, but it’s not reliable enough to share right now. Stay tuned for updates!

(If you are ready to make modifications to the firmware: The SnapperGPS tag features three GPIO pins, so if you already have a surfacing detection sensor, you could connect it to your SnapperGPS board and use it to trigger snapshots.)

Retrieval

SnapperGPS is an archival tag. That means that you can only access the data it collects by retrieving the receiver.

For our 2021 deployments, we tried to only tag nesting females during their first clutch. This made it likely that they would return to the same beach about two weeks later. We still lost a few tags. But this approach can work if the population you are considering is faithful to their beach and you have enough field workers to cover the area.

Consider deploying as many tags as possible in one night, rather than doing gradual deployments. That way you can focus your recovery efforts on a few nights.

If you want to tag turtles in the water, you could combine SnapperGPS with a drop-off enclosure and a VHF beacon.

If you have experience with retrieval of turtle tags, please share your story below!

Resources

Abstract for our talk at the ISTS40
Preprint of the SnapperGPS hardware paper
Paper on SnapperGPS algorithms
SnapperGPS website
SnapperGPS twitter

Please feel free to use this thread to share your own experience with tracking sea turtles! If you have a more general question about SnapperGPS, consider opening a new thread in our discussions page.