Sound Exposure in Harbour Seals During the Installation of an Offshore Wind Farm: Predictions of Auditory Damage

Abstract

1. With ambitious renewable energy targets, pile driving associated with offshore wind farm construction will become widespread in the marine environment. Many proposed wind farms overlap with the distribution of seals, and sound from pile driving has the potential to cause auditory damage.
2. We report on a behavioural study during the construction of a wind farm using data from GPS/GSM tags on 24 harbour seals Phoca vitulina L. Pile driving data and acoustic propagation models, together with seal movement and dive data, allowed the prediction of auditory damage in each seal.
3. Growth and recovery functions for auditory damage were combined to predict temporary auditory threshold shifts in each seal. Further, M-weighted cumulative sound exposure levels [cSELs(M_pw)] were calculated and compared to permanent auditory threshold shift exposure criteria for pinnipeds in water exposed to pulsed sounds.
4. The closest distance of each seal to pile driving varied from 4·7 to 40·5 km, and predicted maximum cSELs(M_pw) ranged from 170·7 to 195·3 dB re 1μPa²-s for individual seals. Comparison to exposure criteria suggests that half of the seals exceeded estimated permanent auditory damage thresholds.
5. Prediction of auditory damage in marine mammals is a rapidly evolving field and has a number of key uncertainties associated with it. These include how sound propagates in shallow water environments and the effects of pulsed sounds on seal hearing; as such, our predictions should be viewed in this context.
6. Policy implications. We predicted that half of the tagged seals received sound levels from pile driving that exceeded auditory damage thresholds for pinnipeds. These results have implications for offshore industry and will be important for policymakers developing guidance for pile driving. Developing engineering solutions to reduce sound levels at source or methods to deter animals from damage risk zones, or changing temporal patterns of piling could potentially reduce auditory damage risk. Future work should focus on validating these predictions by collecting auditory threshold information pre- and post-exposure to pile driving. Ultimately, information on population-level impacts of exposure to pile driving is required to ensure that offshore industry is developed in an environmentally sustainable manner.