Detection efficiency of acoustic biotelemetry sensors on Wave Gliders

Abstract

Background: Detecting tagged animals in coastal environments is often limited to stationary arrays of acoustic receivers that can decode transmissions from tags on animals. However, mobile autonomous platforms are becoming important tools that support the science of understanding biophysical relationships because they can concurrently detect tagged individuals and measure properties of their ocean habitat. To assess the effectiveness of these emerging platforms, proper validation and range detection studies are necessary. Here, we report on the deployment of a wave powered unmanned surface vessel, the Liquid Robotics Wave Glider SV3, equipped with a forward- and backward-facing acoustic receiver (VR2W) and transceiver (VR2Tx) at 4m depth. Surveys were conducted around two stationary moorings equipped with receivers, transceivers or tags emitting signals with different power outputs. Results: During our study, the sea state was mild with low wind speeds (<10 kts) and small wave heights (<0.8m). We determined the influence of environmental and vehicle factors on the detection range of transmitters with various power outputs. Overall, the highest detection efficiencies (~50-95%) were at distances<0.1km, lower detection efficiencies (0.1-50%) were>0.5km and the maximum range was 0.5-1.2km. The forward-facing receiver had almost half the detection efficiency of the backward-facing transceiver, suggesting a backward configuration is optimal to reduce the influence of the moving platform. The higher power output transmitters had a 20% detection efficiency to ranges of~0.5km (153dB) and~0.8km (160dB). Distance between the receiver and transmitter was the main factor affecting detection probability, with background noise, receiver heading, angle between transmitter and receiver and wave height also being important. Wind speed, water temperature, mooring line tilt angle and vehicle dynamics were found not to be as important over the limited range of conditions over which our study was conducted. Conclusions: Wave Gliders equipped with receivers can provide useful data and can be an effective biotelemetry asset that could supplement stationary arrays of acoustic receivers or act as an exploratory technology to search for biologically important areas.