Trials of an acoustic transponding fish tag compass.

The instantaneous orientation of a fish can be determined by attaching a magnetic compass sensor to it and then by acoustically telemetering the information to a research vessel's electronic sector scanning sonar. The sensor is housed in a cylindrical capsule with an acoustic transponder to which it is electronically interconnected. Its size and weight are such that it can be fitted to fish of about 40 cm length and above. An operational life of 52 h is obtained from the tag when it is interrogated by the sonar at a consistent rate of 2 pulses s-1. If the rate is reduced by half, the life can be extended to about 68-70 h. Directional accuracy is limited to +/- 22.5 degrees by the physical size of components used in the sensor. Trials of the prototype are described.