Potential effects of animal-borne 69 kHz acoustic telemetry on pinniped behaviour and survival in the wild

Abstract Acoustic telemetry has become a widely used method to remotely monitor aquatic animal behaviour, and survival because of its relative affordability, miniature size, and extended battery life. However, its use in pinnipeds remains limited, partly due to ethical concerns of potential auditory disturbance, although limited scientific evidence exists on this aspect. Our study aims to address this knowledge gap by investigating whether animal-borne acoustic transmitters, using the common 69 kHz frequency, induce lethal or sub-lethal effects in seals, carrying the transmitter, during their most vulnerable life-stage: young-of-the-year. Between 2023 and 2025, we equipped a total of 130 weaned Atlantic grey seal pups ( Halichoerus grypus Fabricius, 1791) with satellite transmitters to monitor their behaviour and survival during their first year at sea. Approximately half—68 grey seals—were also fitted with a 69 kHz acoustic transmitter (producing a 3–5 s low power signal, source level 147 dB re 1 µPa @ 1 m in water, on average every 120 s). Compared to the control group, acoustically-tagged seals did not exhibit a higher haul out probability, associated with avoidance behaviour of acoustic signals that were only audible to them underwater. While at sea, acoustically-tagged seals allocated a higher proportion of time to widespread movement, compared to the control group, suggesting a prolonged exploration phase. However, this difference began diminishing by the third week at sea. Although the latter behavioural difference may support the presence of sub-lethal effects, overall survival did not differ between groups, during this sensitive life-stage. Collectively, these findings indicate that the effects of 69 kHz acoustic telemetry may be transient, acting primarily through early behavioural pathways; however, the underlying mechanism leading to the attenuation of the effect of the acoustic pings on seal behaviour remains unknown. The study suggests that animal-borne 69 kHz acoustic telemetry could be used as to study grey seal ecology over long time periods, while emphasizing that its use, like any telemetry method, requires weighing scientific benefits against potential consequences.