In part 2 of the “History of Tagging” segment we look at the development and use of electronic tags.
The development of electronic tags opened up a wide range of new possibilities and allowed more detailed behavioural studies on short-term movements, migrations, diving behaviour and habitat use. Sharks in particular were identified as strong candidates for electronic tagging programmes due to their size, robustness and the toughness of their skin allowing external tags to be firmly anchored. To date electronic tags have been used to monitor the behaviour of many species of fish, sea bird, turtle and marine mammal.
Early electronic tagging projects aimed to provide basic information on shark movements and were carried out by fitting a transmitter to a shark and tracking it manually. Many projects now use static hydrophone receivers which are moored in areas of interest and continuously record the movements of any tagged sharks in the vicinity. One example of this is the common skate acoustic tagging project currently ongoing in the Sound of Jura.
This project used both acoustic pinger tags and data storage tags to monitor the movement of common skate in the Sound of Jura using an array of hydrophone receivers. The image below shows a common skate from the Sound of Jura fitted with a white data storage tag.
Data storage or archival tags have been used to identify changes in behaviour in response to seasonal or daily variations. Blue, shortfin mako and bigeye thresher sharks have shown pronounced variations in diving behaviour on a diel (24 hour) scale; sharks were found to make their deepest dives during daylight hours and spend the vast majority of the night in surface waters. This behaviour has also been observed in swordfish, bluefin tuna and black marlin. It is thought that periodic vertical movements in these predatory species are likely to be closely linked to the diel movements of prey species (which is in turn influenced by the vertical migration of zooplankton!)
Electronic tags have also been fitted internally by surgical implanting the tag in the body cavity of anaesthetized sharks or by hiding the tag in a piece of bait and feeding it to free swimming sharks. This image shows a spurdog from Loch Etive, Argyll being fitted with an acoustic tag as part of an ongoing tagging project in the area.
Since the first electronic tagging project in 1965 at least 26 species of shark have been tagged and their movements tracked. During this time electronic tags have become more sophisticated and can now perform a wide range of functions including monitoring body temperature, swimming speed, depth, salinity, light intensity and water temperature.
Some species of shark, particularly those that spend a significant amount of time at the surface, have been tracked using satellite tags. Satellite tags are generally attached to the shark by a tether and internal anchor, this allows the tag to float on the surface and emit radio waves that can be picked up by satellites allowing accurate positioning and tracking. Blue sharks, whale sharks and basking sharks have been tracked using satellite tags.
Some forms of satellite-tracked data storage tags can be set to “pop off” after a certain period of time, this allows the tag to detach from the shark and float to the surface where it can be located using a GPS and retrieved.
Data recorded using electronic tags can provide a valuable insight into the natural behaviour of sharks. Movements can generally be attributed to sexual activity, habitat characteristics and prey type and as such the data retrieved is often integral to developing sound fisheries management or conservation measures.
Tomorrows "History of Tagging" segment will look specifically at the growth of shark tagging projects around the World.