The basic concept of pop-up satellite biotelemetry is that an electronic tag is externally attached to the study animal collecting depth, temperature and light intensity data for a pre-set time interval. After reaching the programmed pop-up date (usually weeks to several months), the tag will detach from the animal (via a corrosion mechanism), float to the surface and start downloading the archived data via the Argos satellite system. From the time of pop-up until battery life ceases, the tag will transmit its position and continue to record and report the respective parameters.
The major advantage of pop-up tags is that one does not need to resight the animal in order to get hold of the tag with the archived data. This makes it a suitable tool to study free-ranging animals such as sharks which are often only sighted once and then disappear in the vast ocean realm. Between tag attachment and pop-up, the tag collects data but does not "talk" to the satellites even if the animal comes to the surface (there are other satellite tag models which do so). Only after the tag popped to the surface it will transmit the stored data together with its position.
What the scientist gets back are depth, temperature and light intensity data. Based on light intensity data, the approximate track of the animal can be reconstructed. This is not an easy task and the errors can be substantial. Because errors (especially for latitude) can be huge, pop-up satellite tags are best attached to animals that move around great distances. Small-scale movement patterns cannot be detected with this technology. However, even in the case where geolocation is not possible, pop-up tags produce a wealth of data such as dive and temperature profiles which can tell a lot about the behaviour of the study animal and the environment it lives in.
Especially if pop-up tags are programmed to stay on the animal for longer time periods (months), many of them detach prematurely for unknown reasons. Tags might not be properly anchored or the animal carrying the tag gets rid of it somehow. I’ve seen a few shark species (blacktip sharks, bull sharks) rubbing against reef structures in order to get rid of tags (or sharksuckers!).
In the next blog, I will give you an example of how data collected with a pop-up tag look and what they tell about the behaviour of the study animal.