Save Our Seas Blogs

I told you about the two pop-up tags I attached to bull sharks in Fiji earlier this year and that were prematurely lost by both sharks, but did not uplink to the satellites for unknown reasons. Well, at least one of them started transmitting after being “lost” for almost two months! I have been receiving messages starting a few days back and the data keep coming in. Of course, this was a big (very positive!) surprise and of course I was curious to learn what had happened to the tag. Looking at the depth data, the reason for the tag being lost and then silent for two months is the following: apparently the tag had been trapped at the bottom, but due to the very narrow constant depth band the “noise” in the pressure sensor and the tidal movements were enough to prevent initiation of release due to constant depth. The data show that the tag was at 170 m until somehow got out of the “trap” (whatever that was), floated to the surface and started to transmit. In other words, this indicates that the bull shark went down to 170 m and probably rubbed itself against the substrate. Sharks do that … I will see what else the data tell me once I have the full archived data set at hand.

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.

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