In order to meet the objectives outlined in Chapter 1, it was necessary to employ remote telemetry techniques to track the movements of the target shark species. Two main types of transmitter, or tag, were used to track sharks across the different chapters: acoustic tags and position-only satellite tags. The former relies on the receipt of sound waves at acoustic receivers, while the latter works via satellite-mediated communication. For reasons that will be outlined in this chapter, satellite telemetry is better suited to larger species of shark that are expected to move away from the study site. As such satellite telemetry was used in Chapters 3 and 4 to investigate the migratory behaviour of large sharks in the Atlantic Ocean. In contrast, acoustic telemetry is better suited to the study of fine-scale movements of animals that can be tracked across an array of acoustic receivers. Consequently, acoustic telemetry was employed
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in Chapters 5, 6, 7 and 8, which aimed to characterise reef shark spatial dynamics at a remote atoll in the Indian Ocean.
All field work was approved by, and conducted with the knowledge of, the appropriate authority for each location: the Marine Resources Section of the Bermuda Department of Environmental Protection, and the Ministry of Environment, Energy, and Climate Change, Seychelles. All animal handling and tagging methods were performed in accordance with the approved guidelines of the University of Plymouth, UK.
2.2.1 Satellite Telemetry
In essence satellite positioning tags are relatively simple, consisting of a radio transmitter that continually transmits to Argos satellites while at the surface, and switches off to conserve battery power while submerged by means of saline sensitive conductivity circuits (Eckert &
Stewart 2001). Historically this method is most reliably applied to marine mammals that have an obligation to surface and so transmit at regular intervals (Eckert & Stewart 2001).
Consequently for elasmobranch research position-only satellite tags are best used on sharks that spend significant amounts of time at or near the surface or can at least be relied on to return to it periodically. Several methods have been developed to try and maximise the likelihood of transmission, including the application of tethers several metres long so the shark only has to be relatively near the surface for the tag to break it (e.g. (Gifford et al. 2007)), or by clamping the tag to the apex of the dorsal fin such that a semi-rigid transmission aerial protrudes above it (Weng et al. 2005).
Although satellite tags transmit continuously while at the surface, the accuracy of the location estimates obtained, referred to as the location class, is limited by the time spent at the surface and surveillance coverage of the Argos satellite system (Eckert & Stewart 2001). Data
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collection relies on surfacing while a satellite is available; there are only two Argos satellites that orbit every 101 minutes, which can detect signals between 6 and 28 times per day depending on the latitude (Eckert & Stewart 2001). The location of the transmitter is calculated via the Doppler shift of successive transmissions during a single orbit, with timing and number of transmissions within that orbit determining the quality of the location class (Eckert & Stewart 2001). The location classes available are 3, 2, 1, A and B, with 3 providing the highest accuracy (to within 250 m of the individual’s real position) and B the worst (within 10 km; (Hays et al. 2001; Hazel 2009)). There is another location class, Z, where no position can be calculated, but the general area (within thousands of kilometres) of the tag can be determined by the time at which it was detected and knowing which satellite made the detection (Heithaus et al. 2007). Unfortunately in many cases less accurate data have to be used for large fish species as they tend not to surface too often or for long periods, making satellite telemetry better suited for animals likely to move at scales larger than the location class errors - insights can still be obtained from rare uplinks if they occur over large distances (Heithaus et al. 2007). Feasibility of displacement estimates is often assessed by comparison with known movement rates, although issues arise from long periods without up-linking as the intervening time may have contained large movements that go undetected (Heithaus et al. 2007).
2.2.2 Acoustic Telemetry
As with the aforementioned satellite telemetry, remote sensing is traditionally performed using radio waves for communication between emitter and receiver, however a number of problems make this inappropriate for use in aquatic environments. Electromagnetic energy is rapidly absorbed and scattered as it passes through water, eliminating it as a suitable transmission medium for sub-surface tags on non-surfacing elasmobranchs (Voegeli et al.
2001). In addition to being attenuated, radio signals are also reflected from the sea surface (Wilson et al. 2006). Consequently acoustic signals in the ultrasonic range of 30–100 kHz tend
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to be used for localised telemetry as these are above most animal auditory ranges and transmit with low energy loss through seawater (Nelson 1976). Originally individual sharks were identified according to the frequency of the transmitter or ping interval, but now codes based on the ping interval, such as random repeat infrequent codes (RCODE) are used for more reliable identification (Voegeli et al. 2001). RCODE transmitters are exceptionally useful, as the random interval minimises detection collisions on monitors that identify tags based on ping interval, as if two tags collide on one run of their code they won’t on the next (Voegeli et al. 2001). Depending on the type of transmitter and battery used, useful life of acoustic manner, arrays of permanent listening stations detect the presence/absence of tagged sharks within the detection radius, whereby multiple receivers, located by GPS, can be used to reconstruct movements retrospectively (Voegeli et al. 2001). Fully submerged bottom monitors are effective as they are listening at all times and in all weather conditions, but are more difficult to access than surface moored buoys for battery replacement and data retrieval (Voegeli et al. 2001). The present study moored VR2W acoustic receivers (Vemco Ltd, Bedford, Canada) to concrete blocks using steel chain and line, attached to a float approximately 5 m above the bottom (Figure 4). The acoustic receiver would be cable tied to the line, sitting 1–2 m off the bottom. Also attached to each receiver mooring was a temperature logger, providing a temperature reading every 10 minutes (HOBO Water Temperature Pro v2 Data Logger, Onset, Bourne, USA). Each receiver had to be retrieved on SCUBA to have its detection record downloaded to a laptop every few months, as well as its battery replaced annually.
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Temperature loggers were also downloaded at the same time using an underwater shuttle. To ensure no gaps in the detection records, receivers were swapped out underwater, with the time and date of the swap carefully recorded so that detection data were assigned to the correct location.
Figure 4: Image of an acoustic receiver mooring with VR2W and temperature logger in situ.
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