Time depth recorders (TDR)
Time-depth recorders (TDRs) are miniaturized sensors recording the depth of the tagged animal as a function of time. They are used in diving species (particularly marine mammals and birds) to study their diving behaviour. Most models can also provide other information such as water temperature, swimming speed or even stomach temperatures. Indeed, stomach temperature is a useful parameter to monitor: mammals are endotherms (warm-blooded animals) feeding on ectothermic (cold-blooded) prey. Ingestion of prey is therefore associated with a sudden decrease in stomach temperature. The timing and, to some extent, the amount of ingested material can therefore be inferred from stomach temperature curves. Linked with swimming speed and depth information, stomach temperature telemetry can provide valuable information on feeding.
As opposed to satellite-linked transmitters, the data collected by TDRs is not transmitted, but is instead stored in an internal memory. The advantage with TDRs is that often more detailed information can be gathered than what is possible via satellite tags. However, a major disadvantage is that the instrument must be recovered. To facilitate relocation of equipped animals, a radio (VFH) transmitter may be glued to the animal's head or embedded into the TDR device. Once the animal is located it must either be captured, or the instrument must be released via a remote or incorporated release system.
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