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.

A beluga whale equipped with a TDR/VHF package attached to its back using a suction cup. The TDR is deployed from a distance using a crossbow, and the animal tracked from a distance using radio-telemetry. Floatation material around the instruments and a release system incorporated to the tag (a magnesium cap which allows the intrusion of water inside the suction cup once corroded by salt water) allows the recovery of the instrument once it fell off the animal.

A beluga whale equipped with a TDR/VHF package attached to its back using a suction cup. The TDR is deployed from a distance using a crossbow, and the animal tracked from a distance using radio-telemetry. Floatation material around the instruments and a release system incorporated to the tag (a magnesium cap which allows the intrusion of water inside the suction cup once corroded by salt water) allows the recovery of the instrument once it fell off the animal.

The three panels show dive depth (top), swimming velocity (middle) and changes in stomach temperature (bottom) in a wild harbour seal equipped with a time-depth recorder and stomach temperature transmitter.

The three panels show dive depth (top), swimming velocity (middle) and changes in stomach temperature (bottom) in a wild harbour seal equipped with a time-depth recorder and stomach temperature transmitter.

Dive and swim speed profile of a blue whale in the St. Lawrence Estuary. The dive profile shows the animal repeatedly diving to depths of approximately 120 metres. The velocity profile of this dive shows that the swim speed drops to nearly zero each time the whale initiate an ascent while at depth. These drops in swimming velocity are interpreted as an indication of moments when the mouth of the whale fills with water, causing the animal to drastically slow down.

Dive and swim speed profile of a blue whale in the St. Lawrence Estuary. The dive profile shows the animal repeatedly diving to depths of approximately 120 metres. The velocity profile of this dive shows that the swim speed drops to nearly zero each time the whale initiate an ascent while at depth. These drops in swimming velocity are interpreted as an indication of moments when the mouth of the whale fills with water, causing the animal to drastically slow down.

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