One of the main goals of the Bull Shark Tagging Programme has been to locate the nursery grounds of the bull sharks encountered at the Shark Reef Marine Reserve in Fiji. From visual observations made over the years it became obvious that the sharks leave Shark Reef in September/October each year and start returning back to the site in December. Interestingly, female bull sharks that are clearly pregnant before they leave return non-pregnant and we also see quite a few females with fresh mating scars when they turn up in December/January at the site. So we are confident that reproduction takes place when they cannot be encountered in the Shark Reef Marine Reserve. Which also means that we should try to find their nursery grounds and movement corridors that link the protected area with habitats where they reproduce. In order to achieve this, I have been pop-up satellite tagging adult bull sharks at Shark Reef just before they leave the site in September. Unfortunately, there’s a lot of issues with externally attached pop-up satellite tags and this approach most likely will not tell us where the Fijian nursery grounds are.
3 September 2009
23 March 2009
Not so silent anymore!
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.
17 March 2009
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.
