Acoustic (Ultrasonic)
Acoustic tags utilize ultrasonic wavelengths to transmit signals to a submerged receiver (hydrophone). Acoustic tags require an internal battery to power a transmitter so the size of the battery dictates the size of the tag. The life of the tag depends on the battery size, transmitter size, range and rate of the signals. Tag attachment may be internal or external depending on the tag design, fish species, and size. Detection of tag data is typically gathered with hydrophones for acoustic signal is greatly reduced through air. As with radio telemetry there are a limited number of distinct codes that can be transmitted per frequency.
Advantages:
- Individual can be detected repeatedly without handling
- Preferred in saltwater and estuarine environments where radio telemetry is not applicable
Disadvantages:
- The receiver (hydrophone) must be in water for detection
- Noise may reduce signal strength
- Signal is bent at thermoclines