Acoustic Telemetry and Transmitters

ERDC TN-DOER-E18

February 2005

highly stable radio transmitter with a long operational life relative to its size. Weight in air (other

measurements given were in water, which is considerably less than the weight in air) is less than

3 grams for all models except the largest (4.5 g). Typical operational life ranges from 3 to 314

days. The smallest tag is 14.5 (L) x 6.3 mm (W) x 4.5 mm (H), whereas the largest tag is 309 mm

in length and 9.1 mm in diameter. One model (NTC-6 Nanotag radio transmitter, size 9 mm x

29 mm) with an operational life of 299 days at a 5-second burst rate costs slightly less than $250

each.

Acoustic Telemetry and Transmitters. Acoustic (ultrasonic) telemetry is the preferred

technology where radio telemetry is not practical, such as in deep or highly conductive (saline)

environments. Ultrasonic transmitters work well in brackish or marine environments because

sound is transmitted over long distances in salt water, whereas radio waves undergo rapid

attenuation. Frequencies for acoustic tags range from 30 to 300 kHz. Acoustic signals can be

detected manually with a small field crew, boat, and hydrophone or the process can be automated

using a fixed hydrophone connected to a datalogging system. Fixed hydrophones can be either

hard-wired directly to the receiver datalogger or they can be wireless, transmitting data via the

hydrophone to a shore-based Yagi receiving antenna connected to the datalogger.

There is a general relationship between transmitter weight (grams), operational life of the tag

(days or months), and minimum acceptable fish size (kilograms). These factors combined will

determine the effective signal detection range. Depending on environmental conditions, the

detection range can vary but is typically 1 km. Since the signal can only be received in water,

surveys involving aircraft-mounted antennas are not possible. Acoustic signals can be degraded

by a number of factors including: thermoclines, vegetation, engine noise produced by passing

boats, and high levels of suspended sediment.

Similar to radio tags, signal transmission can be either pulsed or coded. Several species of fish,

tagged with acoustic pulsed transmitters, have been tracked in the open sea using a directional

hydrophone, portable receiver and headphones (Holland et al. 1985). This technique is suitable

when knowledge of the absolute position of the target is unnecessary. If an accurate position of

the target is needed, this can be achieved by triangulation using multiple hydrophones, although

this will require a greater upfront expense for additional equipment. Coded acoustic tags

transmit unique numerical codes that differentiate individual tags from one another. One

manufacturer (Lotek Wireless) allows for tracking up to 212 unique codes on a single frequency,

while retaining the ability to identify individual fish targets. This capability has two advantages

over conventional pulsed systems: fewer frequencies are needed to track large numbers of fish

and total scan time is reduced, as fewer frequencies are used. These advantages greatly enhance

both spatial and temporal resolution of collected data.

Most manufacturers offer as many as 12 models of coded acoustic transmitters of various sizes.

Cost ($200-300) per transmitter varies by manufacturer, operational life, and additional options

such as depth or temperature sensors. Dimensions range from 32 to 101 mm in length, with a

diameter between 8.5 and 32 mm. In water, the smallest tags weigh as little as 2 grams, while the

largest tags weigh nearly 38 grams. A maximum detection range of 1000 m is reported for

acoustic transmitters by all manufacturers, with the exception of miniature tags (500 m+).

Environmental conditions can affect overall maximum detection range. Repeated testing, as part