Acoustic deterrence as a mitigation tool for seal-human conflicts in the Helgoland Dune swimming zone.

Successful species conservation and rising tourism in coastal ecosystems lead to more human-wildlife conflicts. Non-invasive conflict mitigation measures are needed to ensure human safety and wildlife conservation. The Helgoland Dune, a small German offshore island, is home to Germany's largest grey seal (Halichoerus grypus) colony alongside popular tourist beaches, leading to frequent human-seal encounters. Previous physical incidents in the shared swimming zone have raised safety concerns. This study evaluated an acoustic deterrent system (ADS) using Targeted Acoustic Startle Technology (TAST) to reduce such interactions. During two field trials, the ADS emitted pulsed signals (2 s, 700-1500 Hz, 180 dB re 1 µPa) while a camera tracked seal movements. Analysing 22,086 positions via a cumulative link mixed model (CLMM) revealed seals were 3.57 times more likely (odds ratio, CI: 2.38-5.37, p-value < 0.001) to occupy the two farther distance categories (10-25 m, > 25 m) during signal emission, compared to the closest distance zone (< 10 m). Individual variability accounted for 94% (ICC) of behavioural differences. Results provide the first empirical evidence of the ADS's effectiveness in a recreational context as a non-invasive deterrent tool, while maintaining seal access to surrounding habitat crucial to them.