Vessel noise exposures of harbour seals from the Wadden Sea

The North Sea faces intense ship traffic owing to increasing human
activities at sea. As harbour seals (Phoca vitulina) are abundant top
predators in the North Sea, it is hypothesised that they experience
repeated, high-amplitude vessel exposures. Here, we test this hypothesis
by quantifying vessel noise exposures from deployments of long-term sound
and movement tags (DTAGs) on nine harbour seals from the Wadden Sea. An
automated tool was developed to detect intervals of elevated noise in the
sound recordings. An assessment by multiple raters was performed to
classify the source as either vessels or other sounds. A total of 133
vessel passes were identified with received levels >97 dB re 1µPa
RMS in the 2 kHz decidecade band and with ambient noise >6 dB below
this detection threshold. Tagged seals spent most of their time within
Marine Protected Areas (89 ± 13%, mean ± SD) and were exposed to
high-amplitude vessel passes 4.3 ± 1.6 times per day. Only 32% of vessel
passes were plausibly associated with an AIS-registered vessel. We
conclude that seals in industrialized waters are exposed repeatedly to
vessel noise, even in areas designated as protected, and that exposures
are poorly predicted by AIS data.