GPS and satellite tags as surveillance devices to detect animal deaths – including the illegal killing of protected species

Fabrizio Sergio explains how the development of biologging tools can help protect some of our most vulnerable species. 

satellite-tagged kite (Photo credit F. Sergio)
A satellite-tagged kite by F. Sergio

Detecting animal casualties is often a major goal for wildlife biologists and managers. Conservationists may need to know which mortality agents are driving the decline of an endangered species, while game managers may want to estimate mortality causes as precisely as possible in order to plan sustainable harvest levels by hunters. In these contexts, researchers and managers literally act as [ecological] detectives, searching for unbiased proof, hints and clues of what led to the disappearance of individual animals. And, just as in popular CSI fiction, technology is empowering ecologists and managers with an arsenal of new tools. Among these, the marking of animals with satellite and GPS tags, which record the location and activity (i.e. speed) of the tag at pre-defined intervals (e.g. every few minutes) and allow the constant monitoring of the movements of the tagged individual, thus also making it possible to identify when and where an animal dies and stops moving. However, the interruption of the tag dataflow may also be caused by factors other than death, such as the sudden malfunctioning of the GPS device, which has challenged the use of these tags as mortality-surveillance devices.

To resolve this issue, we exploited an extensive GPS-tagging programme integrated within an intensive demographic study of various raptor species. We discovered that engineering and diagnostic data routinely collected by the tags could reliably pinpoint cases of real death (when the raptors were found dead by the researchers) and separate them from cases of radio-malfunctioning (when the raptors were observed alive after the tag had stopped functioning). For example, deaths were typically identified by an individual that remained stationary for too long a time, but also by the sudden disappearance of the dataflow from a very well-functioning tag, such as when a raptor was hit by a car, which destroyed the device. In contrast, dataflow interruption signaled malfunctioning when this was preceded by symptoms of poor tag performance (e.g. a tag that had already recorded few of the supposed GPS-locations). Overall, a simple combination of a few tag-parameter diagnostics enabled the correct classification of 100% of the cases of death or malfunctioning, opening promising avenues for future projects.

The progressive miniaturization and lowering costs of GPS and satellite tracking devices are increasing their use in research and applied projects, especially related to threatened and harvestable species. This study shows that modern telemetry devices can be used as remote surveillance tools that reliably detect animal casualties. Such capability will bring an added value to future tagging programmes and to the many existing GPS tracking datasets – over 4200 datasets on more than 700 taxa are already stored in public repositories such as Movebank.

Examples of projects that may benefit from such mortality-detection potential abound. First, managers may need to know whether individuals of a target population are disproportionately dying from human threats that may be difficult to detect because they are illegal and thus unreported (e.g. persecution), politically controversial (e.g. collisions at wind-energy facilities), or of difficult documentation because they occur in remote areas or inaccessible private properties. For example, a recent study of Scottish Golden eagles showed a suspicious disappearance of satellite-tagged eagles within hunting estates of difficult access and known to perpetrate illegal raptor persecution.

Second, scientists and managers may need to assess the feasibility, impact and efficacy of certain management procedures, such as mortality as a result of supposedly non-lethal ‘catch and release’ angling methods; the willingness of hunters to follow legal prohibitions to harvest certain species; or the efficacy of legal predator-control plans in actually increasing predator mortality (e.g. of alien predators that cause declines of threatened native species).

In conclusion, as this research shows, biologging is rapidly increasing the mortality surveillance toolkit available to managers and conservationists.

Read the full article, Reliable methods for identifying animal deaths in GPS‐ and satellite‐tracking data: Review, testing, and calibration in Journal of Applied Ecology.

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