Offshore Oil and Gas (O&G) exploration and production has been cycling from cradle to grave for over 100 years, spanning many generations of marine fauna. Despite global occurrence of offshore infrastructure, implications of their presence for apex predators, including cetaceans, is under-studied. We analyzed data from autonomous underwater passive-echolocation-click detectors (C-PODs) deployed at an offshore O&G production platform and at control locations to investigate acoustic activity of harbor porpoises(Phocoena phocoena) from 2015 to 2020, before, during, and after platform construction. Despite a statistically significant decrease in porpoise detections following platform construction and initial drilling operations (2015–2016),detections returned to baseline levels within five months.We detected no long-term effects of platform presence on porpoise detection rates. While additional study is required to further understand activity of porpoise at O&G platforms during continued operation through to decommissioning, our findings, nonetheless, have important implications forEnvironmental Impact Assessments (EIAs), which do not currently consider effects of platform presence on marine mammals in any context other than short-term noise mitigation. While we know porpoise forage at ageing platforms, this is the first study to demonstrate porpoise also utilize space around new platforms, which must be accommodated in long-term EIA processes.

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