Abstract
Underwater noise generated during the construction of offshore wind farms can impact marine mammals, especially in the case of intense sound generation activities like impact pile driving. Two offshore wind farms off the east coast of Scotland used alternative pile installation techniques for wind turbine generator foundations such as drilling (Neart na Gaoithe Offshore Wind Farm, NnG) and suction caissons (Seagreen Offshore Wind Farm), while only the Offshore Substation Platform at the Seagreen Offshore Wind Farm was installed used impact pile driving.
A regional passive acoustic monitoring array was designed to investigate the variations in harbour porpoise and delphinid acoustic presence and underwater noise levels around the Forth and Tay areas, East coast of Scotland of the North Sea in relation to baseline and construction activities around these two offshore wind farms. The aim of this study was to relate any observed changes to a range of anthropogenic and non-anthropogenic factors and determine whether there were any significant changes in cetacean detections within the wind farm areas and adjacent waters pre, during and post construction. This monitoring was a direct requirement as outlined in Project Environmental Monitoring Plan (PEMP) documents, which were prepared to address the specific requirements of the relevant conditions attached to the Section 36 (S36) consent and Marine Licences issued to NnG and Seagreen.
The PAM array consisted of 21 monitoring locations spread between Eyemouth and Stonehaven, off the east coast of Scotland. Passive acoustic data were collected between March 2019 and December 2024 using a combination of broadband recorders and echolocation click logging devices. Covariate data were collected for the monitored period to contextualize and interpret trends and patterns in cetacean presence observed in the Forth and Tay study area, and these were split into the following categories: spatial, temporal, underwater noise, physical, oceanographic, tidal variables, vessel traffic and those related to the construction of the NnG and Seagreen foundations. Response variables (cetacean acoustic presence) and covariate data were checked for missing values and prepared for model fitting. Separate statistical models were fitted for dolphins and porpoises using a range of Generalised Additive Models (GAM) and Generalised Additive Mixed Models (GAMM).
The results of this study indicated that both dolphins and porpoises were recorded at each monitoring site, with cyclical temporal patterns in acoustic presence particularly noted for porpoises. Underwater noise levels recorded during drilling and suction bucket caisson installation hours generally exhibited a 3-5 dB increase in the 0.1 – 1 kHz frequency range compared to non-construction days, whereas piling exhibited an 18 dB increase in the same frequency band. The models fitted for porpoises and dolphins highlighted that anthropogenic covariates (related to construction and vessel traffic) explained the lowest proportions of deviance in all the models and for both species groups, indicating that patterns in dolphin and porpoise presence and detections are primarily driven by natural and environmental variation at the spatial and temporal scales evaluated in this study. Construction activities at the two offshore wind farms did not have a discernible impact on dolphin and porpoise detections.
This study offers a rare, long-term, and spatially extensive analysis of the effects of offshore wind farm construction on marine mammals in the Forth and Tay region. Through six years of passive acoustic monitoring across 21 stations, we evaluated not only traditional piling activities, but also the comparatively under-studied drilling and suction bucket caisson installation methods. The inclusion of pre-, during-, and post-construction phases across two concurrently constructed wind farms allows for a more holistic understanding of potential impacts than most existing studies, which typically focus on single construction types in isolation over a shorter period of time. Monitoring at the regional scale identified that the effects of construction on both species were benign. Whilst many past studies on the effects of noise focus on short term, localised effects, this study highlights the importance of data collection over longer time and wider space and consideration of broader scale impacts.