A really interesting study from our colleagues at the Sea Mammal Research Unit (SMRU) has explored how tagged seals react to offshore wind farms. This follows on from another study by the same group, published last year, exploring how seal hearing might be affected by pile driving.
This study, led by Dr. Debbie Russell, analysed data from special GPS tags to understand how harbour seals move through the environment off the east coast of England – a site of offshore wind farm development. The tagging effort was part of SMRU’s extensive work attaching telemetry devices to seals to explore how they use their environment (for information on SMRU Instrumentation‘s awesome tags and the cool stuff they’ve done elsewhere check out their site!).The study was carried out using historical tagging data (24 ARGOS tags) and the deployment of 22 GPS tags in 2012, during which period the Lincs offshore wind farm was being constructed (and part of the Sheringham Shoal site was operational). The Lincs wind farm developer (Centrica plc) provided piling data from the construction period. During the 2012 tagging period, 27 (of the total 77) piles were installed using a range of hammer energies (i.e. how hard you need to strike the pile) between 100 – 2000 kJ (5.2 metre diameter monopiles were installed).
The study found:
Seal usage (abundance) was significantly reduced up to 25 km from the piling activity; within 25 km of the centre of the wind farm, there was a 19 to 83% (95% confidence intervals) decrease in usage compared to during breaks in piling […]. This amounts to significant displacement starting from predicted received levels of between 166 and 178 dB re 1 μPa(p-p). Displacement was limited to piling activity; within 2 h of cessation of pile driving, seals were distributed as per the non-piling scenario.
There are a couple of really interesting findings in this new paper. Firstly, that seal occurrence was reduced up to 25 km from the centre of the wind farm (the exact range to the piling source is not known but the wind farm covered an area of around 39 square km (stretching more north-south). It’s important to consider that usage was not reduced to zero within this range – i.e. animals were not completely displaced out to the range of 25 km. It’s likely that the likelihood of animals being disturbed decreases as you move further away from the piling activity. It is noteworthy that the observed displacement range is larger than was predicted in the Environmental Statement – which predicted temporary displacement out to 9 km from the piling locations. This might be because the seals appeared to respond to lower received levels than those modelled in the impact assessment stage.
Secondly, it is interesting that seals appeared to ‘recover’ (i.e. return to non-piling distribution) within 2 hours of piling. Crucially, the Wash population (to which the tagged seals likely belong) is doing very well and is increasing – and there has not been an observed effect on the population. The authors note:
However, some proposed wind farm sites (e.g. off the east coast of Scotland) are in areas of decreasing harbour seal populations (Duck, Morris & Thompson 2014), where any energetic impacts of avoidance during piling bouts may impose an additional stress on already compromised populations (Thompson et al.2013b). Furthermore, the area encompassing the Lincs wind farm was not an area of high use, rather seals pass near it as they transit to and from foraging areas. The energetic costs of displacement from a key foraging area may be greater; it could result in reduced foraging opportunities or increased foraging competition in some areas.
It will be interesting to understand how these relationships vary across different environments (e.g. water depth and sediment type), in what might be considered key foraging areas/transit areas and in response to larger piles and higher hammer energies being used to install piles (and considering multiple piling sites). In addition, it is crucial to understand how the longer term effects of exposure to noise could impact on individuals and populations.
This paper, and the work by Hastie et al, provide an excellent study on disturbance and hearing damage at this specific site and raise interesting questions of exploring the trade off between these two impact pathways. It’s unclear which pathway is more likely to result in an impact at a population level. This is the kind of cool study that provides a really useful jigsaw piece in the big picture and brings us closer to answers.
A number of other interesting points, assumptions and caveats are presented in full in the paper and it makes for an informative and interesting read! You can (and should!) read the full paper here:
Russell, D. J.F., Hastie, G. D., Thompson, D., Janik, V. M., Hammond, P. S., Scott-Hayward, L. A.S., Matthiopoulos, J., Jones, E. L. and McConnell, B. J. (2016), Avoidance of wind farms by harbour seals is limited to pile driving activities. J Appl Ecol. doi:10.1111/1365-2664.12678.