Animal biologging is a technique that’s quickly becoming popular in many cross-disciplinary fields. The main aim of the method is to record aspects of an animal’s behaviour and movement, alongside the bio-physical conditions they encounter, by attaching miniaturised devices to it. In marine ecosystems, the information from these devices can be used not only to learn how we can protect animals, many of whom are particularly vulnerable to disturbance (e.g. large fish, marine mammals, seabirds and turtles), but also more about the environments they inhabit.
Challenges when Tracking Marine Animals
Many marine animals have incredibly large ranges, travelling 1000s of kilometres. A huge advantage of biologging technologies is the ability to track an individual remotely throughout its range. For animals that dive, information on sub-surface behaviour can be obtained too. This information can then be retrieved when an animal returns to a set location. If this isn’t possible (e.g. individuals that make trips that are too long or die at sea), carefully constructed summaries can be relayed via satellite. This option provides information in real time, which can be very useful for researchers.
Tracks of juvenile southern elephant seals. Red tracks are individuals that returned to their natal colony. Grey are those individuals whose information would have been lost had it not been transmitted via the Argos satellite system.
Nowadays animal telemetry tags for air-breathing divers come in all shapes and sizes. In four short decades tags for diving animals have gone from prototypes like the one built by Jerry Kooyman for deployment on Weddell seals – which consisted of a kitchen timer and a roll of graph paper – to a multitude of sophisticated electronic devices, fit for just about any animal or purpose you can think of.
All this progress has meant we can collect more information than ever before and do so remotely. Nevertheless, the lives of most divers remain a well-kept secret. For tags that transmit what they collect (as opposed to those that store data until they’re retrieved), the transmission stage is usually the bottleneck. This has driven the development of energy and time efficient software and data processing.
For a tag like the conductivity-temperature-depth Satellite Relay Data Logger (CTD-SRDL) built by the Sea Mammal Research UnitInstrumentation Group at the University of St Andrews – which was designed to spend months at sea – the problem boils down to one thing. Data are collected at a high resolution on-board the tag amounting to 100kB daily, but only 1kB of this information (at best) can be transmitted to the ground station. Therefore in preparation for transmission, the data need to be chosen carefully, compacted and fitted into several satellite messages of fixed size to ensure that enough useful information is received. Each satellite message can hold up to 248bits of information. To give an idea of how limiting this is, consider that this sentence would (without compaction) take up 896bits! Continue reading →