Transmissions of acoustic signals between transmitter/tag and receiver used by the Animal Tracking Facility operate on 69 and 180kHz. Additionally to the temporal and spatial location of a tagged animal, receiver arrays collate biological, physical and/or physiological data that provide a view into the behaviour of animals. Generally, the Animal Tracking Facility uses two different setups, shallow and deep water units.
The shallow water units consist either of a tyre filled with concrete including a central star picket (Fig. 1) or just a single star picket that is directly fixed to the substrate. The acoustic receiver is attached to the star picket by cable ties and typically a metal bolt. These units are characteristically used in depths of up to 30m and deployed and recovered by a diver.
In order to successfully recover acoustic telemetry data from the ocean in areas too deep for standard SCUBA diving and free diving techniques, the Animal Tracking Facility relies on a variety of acoustic releases integrated into the mooring design. These releases are activated through the water by hydrophones/acoustic modems so that the upper part of the mooring is able to float to the surface. Moorings are anchored to the sea floor and suspended in the water column but vary in size depending on water depth and other environmental features (Fig. 2). Some receiver models can also upload data remotely. This has the advantage that the service intervals of moorings can be extended. Deep water units allow scientists to design receiver arrays capturing animal movements in previously non-accessible environments.
Acoustic telemetry is evolving constantly and new, advanced technologies are developed together with researchers continuously. In the past, the Animal Tracking Facility used VR3 receivers in some of the arrays. Although having an average battery life of approx. 18 months (slightly longer than most of the receivers used currently), they had the disadvantage of being relatively bulky. Most of the current arrays use a mixture of VR2W, VR2-AR, and VR4 receivers. VR2W receivers are easy to handle and are considered ‘all-rounders’ that fit most applications with an average battery life of approx. 12 months. The next generation of those, the VR2-AR receivers, are very similar but have an acoustic release integrated that makes them a valuable alternative in some of the arrays. VR4 receivers are very different from other receiver models as they have unique features such as an extended battery life of approx. 9 years, the option of detecting 69 and 180kHz tags simultaneously, and the possibility to upload data remotely via a hydrophone. For more detailed information on these products visit the Vemco website.