You have probably heard about scientists tagging sharks before. We do this so that we can track a particular animal, to see where it travels. This can tell us a lot about where shark crucial habitats are and what routes they take when migrating. As tagging technologies become increasingly advanced, what tags are able to record and what this can teach us about sharks is becoming more and more exciting. Now there is a wide array of different types of tags, which can be used in many different contexts. So what are sharks tags? How do they work? And why are there so many different kinds?
Early Shark Tagging Methods
The most basic kinds of shark tags are used in "Mark Recapture studies". This means that the animal is marked in some way, so that when it is caught in the future, that individual can be recognised. Kind of like putting rings around the legs of birds. This can be done by marking an individual with a coloured clip if the researchers are monitoring them in a small space for a short period of time. On a larger, longer scale, scientists might attach a more permanent tag with an identification code, which can be recorded if the shark is caught at another location (Stevens, 1999; Kohler & Turner, 2001).
This method can be problematic because tags can detach by accident and be lost. To avoid this small tags can be placed internally; under the skin of a shark. However, this is an invasive procedure and this relies upon whoever catches the shark being able to report it to the scientists, so results can be very patchy (Stevens, 1999; Kohler & Turner, 2001).
Modern Electronic Tags
More advanced are electrical tags. These also include many onboard sensors, which can measure multiple variables. For example, certain tags also record data about the environment, including temperature, salinity, light levels and pressure (to determine depth). Whatsmore, many tags are capable of recording their orientation in the water, meaning that the pitch, roll and heading of the animal can be visualised (Kohler & Turner, 2001).
One example of electronic tagging is "acoustic telemetry". This involves attaching an "acoustic tag", which is equipped with a transmitter, onto the shark. These tags are capable of reporting their location to an array of receivers which have been deployed in a study area. Each time a tagged shark passes a receiver, the tag transmits its unique ID. This allows scientists to study the sharks movement throughout the receiver array and can give us great detail about their short-range patterns. However, this method is limited by where the receivers are positioned and nothing can be gleaned about a shark's movements if it swims beyond the array (Stevens, 1999; Kohler & Turner, 2001).
Even more technological are tags which are capable of transmitting information via satellite; known (very creatively) as "satellite tags". There are a few different kinds, that fall into two main categories: archival and transmitting (Stevens, 1999; Kohler & Turner, 2001).
Archival tags, such as "Pop-Up Archival Satellite Tags (PSAT or PAT tags)" have large storage drives onboard, which allow them to save the data recorded into digital memory. They can record large amounts of fine-grained information about location and environmental conditions. When the tag detaches, the data can then be downloaded from the tag, either directly or via satellite transmission. As these tags are quite large and buoyant, they can be too cumbersome for some smaller shark species (Stevens, 1999; Kohler & Turner, 2001).
Transmission tags, however, send their data via sound or radio waves whilst still attached to the shark. For example, "Smart Position or Temperature Transmitting Tag (SPOT tags)" are able to transmit their recorded data via satellite as soon as they are above the surface of the water. They can even have enough battery life to monitor a shark for as long as five years! This means we can track movements over extended time-frames and long distances (Stevens, 1999; Kohler & Turner, 2001).
However, they are not very useful for deep-dwelling species which rarely come near to the surface (Kohler & Turner, 2001).
As these tags are attached to a freely ranging shark, without the limits associated with a receiver array, they can tell us a lot about long-range movements. With their associated onboard seniors, they can also help us to understand the environmental conditions that the shark experiences. However, they can still be troublesome. As they must be attached externally (usually to the first dorsal fin), tags can easily be shed before they have recorded any interesting data (Kohler & Turner, 2001).
The Future of Shark Tagging
As technology continues to advance, there will undoubtedly be new tags developed which are more efficient, smaller and record even more data. There are many teams around the world working on this right now.
In fact, one of the more recent developments is to include a camera onboard the tag, so we can literally get a sharks-eye-view! This technology allows us to get right into the centre of the action and learn a lot about a sharks natural behaviour in the wild (Jewell et al, 2019). Who knows... maybe in the not too distant future, we will literally be able to see the world how a shark sees it!
If you are interested in shark tagging, you can follow @OCEARCH, who tags sharks and track their movements with real-time updates on Twitter. There are even individual celebrity sharks equipped with tags that you can follow.
References
Chapple TK, Gleiss AC, Jewell OJD, Wikelski M & Block BA (2015). Tracking sharks without teeth: a non-invasive rigid tag attachment for large predatory sharks. Animal Biotelemetry, 3:14. Access online.
Howey-Jordan LA, Brooks EJ, Abercrombie DL, Jordan LKB, Brooks A, Williams S, Gospodarczyk E & Chapman DD (2013). Complex movements, philopatry and expanded depth range of a severely threatened pelagic shark, the oceanic whitetip (Carcharhinus longimanus) in the western north Atlantic. PLoS One, 8:2, e56588 Access online.
Jewell OJD, Gleiss AC, Jorgensen SJ, Andrzejaczek S, Moxley JH, Beatty SJ, Wikelski M, Block BA & Chapple TK (2019). Cryptic habitat use of white sharks in kelp forest revealed by animal-borne video. Biology Letters, 15: 20190085. Access online.
Kohler NE & Turner PA (2001). Shark tagging: a review of conventional methods and studies. Environmental Biology of Fishes, 60, 191-223. Access online.
Munroe SEM & Huveneers C (2018). Monitoring the presence and residency of sharks at key locations off Victor Harbour (Encounter Bay Marine Park). Report to the Department of Environment, Water and Natural Resources. Access online.
Stevens J (1999). Shark tagging: A brief history of methods. Fish Movement and Migration. Access online.
By Sophie A. Maycock for SharkSpeak
