Going with the flow: tidal and tag influences upon the performance of acoustic telemetry systems

Abstract Background Acoustic telemetry is a widely used tool for studying the behaviour of aquatic species. Underpinning acoustic telemetry research is an understanding of parameters influencing the ability of receivers to detect tags, facilitating accurate study design and interpretation of the data. Tide is a regular predictable phenomenon that may affect detection probabilities, for example via signal loss and/or distortion due to water movement. Here, we examined the performance of acoustic receivers in the Bristol Channel, UK (an area with one of the largest tidal ranges in the world), investigating the influence of tidal phase, tidal height and other covariates such as receiver orientation and depth on the detection efficiency of acoustic tags. Results Tidal phase had a strong influence on detection efficiency, with reduced detection efficiency during the mid-tide period when water movement was greatest. Detection efficiency was further reduced during spring tides, where tidal flow is increased, and with larger surface waves. Moreover, surface-deployed receivers experienced stronger tidal effects compared to receivers deployed on the seabed. Detection range varied with tide, falling during mid-tide periods. The distance at which 50% of expected pings were detected fell by 44% for low power test tags from high water to mid-tide. Detection ranges also varied with tag model and power, with low-power tags having smaller detection ranges compared to high power, and test tags having smaller detection ranges compared to receiver sync tags. Conclusions Detection efficiency and range can strongly vary throughout the tidal cycle. Neglecting the tidal cycle when analysing acoustic telemetry data may result in erroneous conclusions regarding animal behaviour in response to tide (e.g. incorrectly assuming animal absence is due to tide) or poor study design for future studies (e.g. fine-scale arrays with receiver spacing too wide for positioning during mid-tide periods). Given the regular nature of tide, we highlight the need for acoustic telemetry users to quantify and understand tidal influence on their study systems with the same tag models as to be used by animals and adjust study design and data analysis appropriately. Graphical abstract