Offshore wind turbines

An Unseen Problem

It is fair to say that, when discussing the environmental impacts of offshore wind, birds and marine mammals have certainly been at the forefront of the collective thinking of regulators, consultants, academics and the industry in general. In our experience, it is very rare that we are asked to consider bats in our approach to pre-construction surveys or impact assessments. In attending last September’s Conference on Wind Energy & Wildlife Impacts in Portugal, we found that studies regarding bats formed a very small proportion of the overall representations of the assembled experts from around the world. However, the evidence that was presented was intriguing, and most certainly a clear warning that we are not doing all we should to consider the potential impacts on bats from these installations.

The problems are clear – bats are small, nocturnal animals and, if they are crossing our seas, the chances of detecting them on radar systems (designed to detect flocks of migrating birds), or observing casualties of such a tiny animal in a vast waterbody like the North Sea are minimal. We do know, however, that various bat species cross our seas, and perhaps even forage further away from the coast than we would think is feasible. Anecdotal evidence, and increasing numbers of localised studies, show bats of several species feeding up to 10km offshore, and occasionally even beyond that, as well as being observed resting/ roosting on offshore installations such as oil rigs, and even wind turbines themselves (Ahlen et al 2007). In the right conditions, large numbers of bats could be following significant aggregations of insects far out to sea, possibly placing them in harm’s way as more and more wind farms spring up around the Baltic and North Seas, as well as Europe’s Atlantic coastlines.

Regulation & Guidance

The lack of evidence, and the difficulty in collecting that evidence, has posed a problem for the organisations tasked with ensuring that the environmental impacts of offshore development are minimised and mitigated for appropriately. To date, only Germany has devised formal guidance for offshore bat study and impact assessment. Other nations propose an approach based on the EUROBATS publication ‘Guidelines for consideration of bats in windfarm projects’ (revised 2014). The primary function of the EUROBATS initiative is to conserve Europe’s bat populations, and it is recognised that we simply do not know enough about offshore bat activity to rule out significant problems for the populations of several species, most notably Nathusius’ pipistrelle Pipistrellus nathusii, soprano pipistrelle P. pygmaeus, and noctule Nyctalus noctula (Arnett et al 2015).

Broadly, the guidance recommends that any boat-based surveys be conducted during April and May (inclusive) and August and October (inclusive) to cover the vital migratory season. For installations closer to shore, or in narrower channels, land-based surveys conducted from headlands will supplement this work, with additional surveys in June and July to cover periods of higher foraging activity in calmer conditions.

A Bespoke Approach

In interpreting this guidance, and ensuring that any development complies fully with regulations and minimises any ecological impact, it is vital that the programme of survey work is designed to provide us with a strong, evidence-led basis for our assessment. All potential offshore turbine sites will present their own unique conditions and challenges. We would always aim to make the most efficient use of our time by combining other necessary work, such as boat-based bird survey, with the deployment and retrieval of bat detection equipment, and simultaneous nocturnal bird and bat activity surveys.

The use of appropriate technology will be crucial to any study of offshore bats. Automated bat detectors would be used on land (at potential crossing points) and, where feasible and necessary, at sea on platforms such as rigs, buoys or night operating ferries if they are in the vicinity of the proposed wind farm. As well as hand-held bat audio detection equipment on our boat-based transects, thermal imaging cameras will be utilised to visually monitor the area and add as much as possible to the dataset.

As technology advances, our work, and collaborative efforts with academic institutions, will assist our understanding of where, when and how bats are using the open sea. Studies involving radar and GPS tags (available now for even the smaller bat species) will be consulted to broaden our understanding of this phenomenon, and how we can combine this with our data to design meaningful mitigation for bats as part of the spectrum of ecological considerations at future offshore wind farms.


Bats in the Anthropocene: Conservation of Bats in a Changing World; Chapter 11 – Impacts of Wind Energy Development on Bats: A Global Perspective, DOI 10.1007/978-3-319-25220-9_11. E.B. Arnett at el (2015).

EUROBATS Publication Series No. 6: Guidelines for Consideration of Bats in Wind Farm Projects – Revision 2014. L. Rodrigues et al (2014).

Bats and Offshore Wind Turbines Studies in Southern Scandinavia (Swedish Environmental Protection Agency). Ahlen et al (2007).