Silent pop-up tags

The basic concept of pop-up satellite tagging is straightforward. However, many things can happen to a tag attached to a free-ranging animal. Often, pop-up tags detach prematurely. Most tags are equipped with a so-called emergency and/or constant pressure release mechanism. For example, if the tagged animal dives below a certain depth (e.g. 1800 m), the tag will automatically detach and float to the surface. This makes sense because otherwise the expensive device might get crushed. Another reason for premature release is that the tagged fish does not change depth plus/minus a certain depth, for example 5 m for several days. The tag then ‘assumes’ that the animal is dead and it will pop-up and start transmitting data. Such features make a lot of sense if indeed tha animal goes very deep and/or dies during tag attachment. However, in the majority of cases the reason for premature release is unknown.

Looking at the depth data from the pop-up tags I attached to bull sharks in the Bahamas, it is obvious that premature release occured because the sharks stayed very shallow and did not change depth much. The tags I attached to bull sharks in Fiji popped up for unknown reasons. Some of them after being on the shark for only one day, but others stayed on for as long as 50+ days. Despite popping up early, they still transmitted a good amount of data except for one tag which never ever transmitted. Some pop-up tags might be malfunctioning or washed up on a beach from where it’s unlikely to get good transmissions. Anyway, so far I was lucky with not hearing back at all from only one tag (out of 22 attached to different shark species).

Unfortunately, this number has recently increased to three with the tags I attached in February being completely silent. Both tags were attached to female bull sharks in Fiji. One was programmed to be on the shark for 20 days and the other should have collected data for three months. I did re-sight both animals with the tags attached a few days after I tagged them. Then, both sharks turned up without their pop-up tags attached. One animal (pictured above) is a regular visitor to the site and we can easily identify her looking at her first dorsal fin. She also has an acoustic tag on her right side just below the dorsal fin. The suspicious white dot you can see below the dorsal fin is where the pop-up tag was attached. This healed within a few days. Knowning that both shark lost their tags I was expecting to hear from them within a few days. But they didn’t. No signal, nothing.

Until today, I have no clue what happened to the tags. Losing them prematurely is one thing but not getting back any data at all is bad. And a waste of money!