||A coded acoustic transmitter used to measure the activity of free-ranging marine fauna, most commonly fish. The transmitter measures the 3D acceleration with depth of fauna moving within a reciever array as a vector quantity, i.e. it measures acceleration on 3 axes X, Y and Z, in metres per second squared. Other applications may include determination of swimming speed via tail beat acceleration, activity responses to changing oxygen, temperature and salinity, and detecting mortality through predation, seismic blasting, toxic spills, feeding events, spawning activity, nocturnal/diurnal activity or wave action. One of two modes of operation can be specified; an activity alogorithm or tailbeat algorithm. Coded transmitters send acoustic pings at 69 kHz that are infrequent and random about an average delay. This ping train includes an ID number which permits identification of the specific tag and sensor data. For dual sensor tags, there are two tag IDs.