In principle, there are two ways to equip sharks with acoustic tags: you can either attach the tags externally or internally. For both methods there are good examples to be found in the scientific literature. If done internally, you would usually catch the fish for surgical tag implantation. An alternative method can be feeding acoustic transmitters to sharks. Stomach tagging can be an interesting option and it allows you to, for example, monitor the stomach temperature of the animal in addition to obtaining presence-absence data. However, the indigestible tag will eventually be regurgitated, most likely via stomach eversion, and therefore tracking time is limited. In order to feed a tag to a shark, all you need is the tag wrapped in bait and a site where you know you can attract the sharks close enough to a feeder. There’s no better place than the Shark Reef Marine Reserve in Fiji to test the feasibility of stomach tagging and to estimate tag retention time. And so we did. The superb photograph on the right (taken by Klaus Jost) gives you an idea of how it looks when you hand-feed a tag to a good-sized bull shark in Fiji.
A paper that is published in Marine and Freshwater Behaviour and Physiology reports the results of our attempts over the past years and shows that fed acoustic transmitters or sensors are viable tools to collect presence-absence data or physiological parameters of free-ranging sharks. What I found is that the minimum tag retention time ranged from less than 24 hours (bull shark) to 34 days (tiger shark), and bull sharks could be tracked for a minimum mean duration of 6.8 days. This is not a long time, but good enough to address some of the questions we try to answer. And as you can read in the paper, some of the bull sharks were double tagged with a stomach tag and an externally attached acoustic tag. We also attach acoustic tags externally to get longer tracks which helps to get information on presence-absence of individual bull sharks over a time scale of months or even years. I hope I can tell you more about these results soon. Keep watching this space!