The Thelma Biotel mortality sensor transmitter measures mortality events in tagged animals using orientation technology. The sensor triggers as the animal loses the ability to maintain proper orientation.


  • Available in all transmitter sizes 6, 7, 9, 13 and 16
  • Operational lifetime ~ 3 months -> 5 years
  • Length down to 17 mm

The mortality sensor transmitter records mortality events using an embedded accelerometer which tracks the orientation, thus tilt values over time.

Thelma Biotel offers the smallest mortality sensor acoustic transmitters on the market. It can be delivered down to 6.3 mm in combination with other sensors, such as depth and acceleration, with several months of operational lifetime.



The versatile mortality sensor contributes to largely reducing the uncertainty of whether movement data belongs to the tagged animal or a predator. Therefore, the mortality sensor is highly applicable in studying migratory animals, such as salmon smolts, migrating downriver towards the sea. With the additional temperature and depth sensors, the cause of mortality can be assessed in great detail.


The mortality sensor is based on orientation technology and records the tilt values randomly at every interval to create an average baseline orientation, which is constantly updated. Larger deviations from the baseline over a certain time period results in the triggering of the mortality sensor and a change in the status (or ID) from alive to dead. To avoid sudden orientation changes at a few time points resulting in the triggering of the sensor, the average tilt value from the past hour is compared to the baseline. This means that the sensor spends some time comparing the baseline to the average before triggering the ID switch. For example, tests have shown that a 180° sudden change in orientation will switch the mortality trigger after around 21 minutes. The sensor also accounts for larger deviations in orientation during surgery with a delayed reset. The reset functionality assures that the tag does not switch during surgery resulting in biased data.