Underwater noise monitoring in the semi-pristine waters off El Hierro, Canary Islands.

Marina Puebla Aparicio

Shipping noise is a potential threat for marine fauna and especially for cetaceans increasing worldwide. In this study, we provide a reference framework on baseline levels of ambient noise in the semi-pristine waters off El Hierro (Canary Islands), and test whether received levels at shipping indicator frequencies have changed over a 5-year period in relation with environmental and cetacean presence variables. For this, we deployed drifting passive acoustic sensors suspended at 200m depth to continuous audio record and concomitant data from vessel and cetacean presence in the area was gathered. A total of 422 hours of acoustic data were collected from 2016, 2017, 2020 and 2021 and third-octave band levels (TOLs) in the shipping indicator bands 63Hz, 125Hz, 250Hz, 500Hz, and 1kHz were estimated. We demonstrate that noise levels have increased within each TOL, which might be explained by both the global increment on shipping and rise in coastal traffic of small boats. Although current noise levels are not expected to threat resident cetacean populations, management plans should consider acoustic monitoring and noise emission guidelines within this area, to mitigate potential adverse effects of the rise of ambient noise due to human activities.