Impacts from Offshore Wind Farms on Marine Mammals and Fish – A review of the current knowledge

This report addresses the need for a robust overview of the science, in order to understand current knowledge of the impacts of offshore wind development, as the sector develops in Ireland. As such, the report summarises an extensive literature review of the known impacts from construction and operation of offshore wind farms (OWFs) on marine mammals and fish/shellfish. In addition, an overview of the currently available mitigation methods has been provided. The chief potential stressors of marine life from offshore wind farms have been identified as 1) noise, 2) built structures, 3) vessel activity, and 4) cables, comprising both power cables and mooring lines.

Noise is a known stressor of both marine mammals and fish and is currently the most significant stressor from OWFs. Impacts include acoustic trauma, hearing impairments, masking of biologically important acoustic signals, behavioural changes, and physiological stress. How these impacts manifest varies between fish and marine mammals, will often vary between species within these groups, and between size/age of individuals within a single species. Marine mammals detect noise through sound pressure alone, crustaceans likely detect noise solely through particle motion, fish may use either one or both. Recently revised thresholds for marine mammals in the literature may influence changes in Irish guidelines for permitted anthropogenic noise emission levels. Guidelines for thresholds for fish could also be adopted from the available literature.

Man-made structures have the potential to negatively impact marine life through habitat displacement, pollution, and behavioural disturbances. However, they have also proven to benefit ecosystems through the addition of complex hard substrate which increases biodiversity and may provide greater foraging opportunities, as well as shelter from predators, in addition to shelter from fishing activities and shipping noise. While localised increases in productivity and abundance of fish have been recorded around turbines, there is not yet evidence of increased productivity at a regional scale from the expansion of OWFs in European waters.

Vessels, whether commercial, shipping, or recreational, may impact both marine mammals and fish through noise, localised chemical pollution from leaks and spillages, possibility of ship-strikes, and as vectors for invasive species. Impacts include disruption to functional behaviours such as resting, foraging, or communication. In general, marine mammal numbers decrease in response to increased vessel numbers, and vessels travelling at speed have greatest impact. Fish display lower anti-predator responses, or spend more time guarding nests than feeding in the presence of increased vessel activity, and few reports exist of fish being struck by vessels.

The potential impacts from cables include possible entanglement, localised changes in the electro-magnetic field (likely negligible impact for marine mammals, local moderate impact for elasmobranchs and possibly some crustaceans), sediment suspension, noise/vibration, heat, and reef/reserve effects. However, this stressor is poorly studied, and more research would provide greater clarity on several of these issues.

Impacts will be best mitigated by appropriate site selection and assessment of potential windfarm sites, using the best available data on species spatial and temporal distributions or conducting additional surveys where necessary. Additional mitigation measures including time-area restrictions, the controlled use of acoustic deterrence devices, and other noise mitigation devices should be implemented to reduce impacts from noise – most importantly pile driving, but also vessel and operations noise. An update of Irish guidelines for permitted anthropogenic noise emission levels would help to inform these measures. Where possible, if one option has been shown to have lower negative impact over another available option, the first should be selected. Future OWF developments should take the findings of this report in to consideration, and further research should be carried out to fill in the gaps in knowledge.