Advisory note prepared by Natural England (NE) and JNCC on the iPCoD English Wind Farm Cumulative Assessment report – June 2017




Following the development of the interim PCoD (Population Consequences of Disturbance) model (in February 2013), the Statutory Nature Conservation Bodies (SNCBs) produced some key messages to sit alongside any use of the model, two of which were:


To date, due to uncertainties about the impact of disturbance on marine mammal individuals and populations, current consenting decisions have drawn upon expert opinion, but not always in a transparent, comparable and auditable way. Whilst interim PCoD relies on some strong assumptions and on expert opinion, its strengths include being transparent, auditable and quantitative.

One of the main strengths of the interim PCoD may be at assessing the cumulative impact of several developments and SNCBs advise that this is better achieved at the strategic level (e.g. SEA, and/or as a result of a joint effort between regulators, their advisors and developers (e.g. regional monitoring groups).  We will be working to encourage this approach in the future.


Since this time, NE and JNCC have funded SMRU Consulting to develop scenarios with iPCoD to allow this cumulative assessment to take place. For the time being, the resulting cumulative assessment only incorporates wind farms within the English North Sea.


The assessment in the final report predicts that at the end of the construction period the additional risk of a 1% annual decline in the harbour porpoise population, as a consequence of the planned construction works, is between 1 in 16 and 1 in 333. Overall, the risk to the North Sea harbour porpoise population of a 1% or greater annual decline is forecast by iPCoD to be low, but heavily dependent on a range of assumptions and estimated parameters. The interpretation of the results in this report should, therefore, be made in the context of the uncertainty surrounding those assumptions and estimates, some of the key ones being described in the sections below and detailed in the report itself.


Uncertainty regarding the installation schedule


While the most realistic installation schedule possible was developed in conjunction with developers, some related input parameters to which the model is sensitive remain uncertain:


  • The number of wind farms to be built
  • The number of foundations to require piling
  • The time frame for piling
  • The maximum hammer energy required for each foundation


However, the construction schedule used in this analysis constitutes a refinement from the one presented in previous similar iPCoD assessments[1], given that the developers were approached individually for the most up-to-date information on piling schedules. This has resulted in a considerable reduction in the number of piling days planned and, as a result, the potential numbers disturbed and ensuing population consequences were also predicted to be considerably lower than in the previous studies.


Uncertainty regarding the extent of disturbance and the numbers of animals affected


There is considerable uncertainty regarding the spatial and temporal extent of disturbance/displacement and its sources of variability, in particular:

  • The residual disturbance to harbour porpoise in terms of hours of displacement (key literature used within the project measured responses on isolated wind farms using smaller diameter piles and significantly lower hammer energies).
  • The maximum footprint of disturbance, which has been estimated by developers through various noise models and differing assumptions.
  • The dose response curve chosen, which will have a strong bearing on the estimated number of porpoise disturbed. Unlike other similar iPCoD analyses1 which assumed a worst-case scenario where all animals within the disturbance footprint are disturbed, this analysis used a more realistic scenario based on field observations2, in which the effects of pile driving on porpoise decrease with distance from the noise source. Whilst the dose response curve for each piling event is likely to vary according to, for example, the size of the pile and hammer energy used, sound propagation, and even the behavioural context of the porpoises present, its use allows for a better approximation of the proportion of animals affected within the displacement footprint.
  • The porpoise abundance estimate. Mean abundance estimates calculated for the affected areas did not account for the uncertainty around those estimates.


Uncertainty regarding the effects of disturbance on porpoise vital rates


Another source of uncertainty comes from the fact that the magnitude of the effects of disturbance on harbour porpoise survival and fertility is unknown. In this analysis, iPCoD used estimates of these effects obtained through an expert elicitation process carried out in 2013, where experts were asked for their opinion3. For example, most experts felt that disturbance lasting more than 50-100 days may result in reduced foraging efficiency, which could cause a maximum 50% reduction in fertility. The estimates are not based on empirical data, and more research is needed to increase our understanding of cause and effect.


In addition, for this study it was assumed that the risk of auditory injury (Permanent Threshold Shift, PTS onset) from pile-driving could be considered negligible given the routine application of mitigation (e.g. Marine Mammal Observers and Passive Acoustic Monitoring). Noise modelling carried out for several of the wind farms EIAs, had so far indicated that porpoises would be at risk of injury only in the first few tens to hundred meters from the source. However, NOAA have recently published revised thresholds for injury (NOAA 2016), which indicate that porpoise may have lower thresholds of PTS onset than previously assumed and therefore, in order to avoid PTS onset, porpoises would have to be further away from the source than the mitigation guarantees, which could result in a residual risk unaccounted for in this analysis.


This project also did not take account of other noisy activities occurring in the region, both inside and outside of English waters (e.g. other wind farms, seismic operations, UXO explosions), which could affect the harbour porpoise population.




The results presented in this report suggest that the risk of decline to the North Sea harbour porpoise population as a result of pile driving in the English North Sea is relatively low. However, given the uncertainty surrounding several of the assumptions and parameters used, NE and JNCC advise that some form of noise management, particularly in important areas for harbour porpoise (e.g. the Southern North Sea pSAC), should be considered to reduce the magnitude of disturbance.



[1] Verfuss, U.K., Plunkett, R., Booth, C.G. & Harwood, J.: Assessing the benefit of noise reduction measures during offshore wind farm construction on harbour porpoises. Report number SMRUc-WWF-2016-008 provided to WWF UK, June, 2016.

Heinis, F, de Jong, CAF, & Rijkdwaterstaat Underwater Sound Working Group. (2015). Cumulative effects of impulsive underwater sound on marine mammals. TNO 2015 R10335-A.


[2] Brandt, MJ., Diederichs, A., Betke, K. & Nehls, G. (2011). Responses of harbour porpoises to pile driving at the Horns Rev II offshore wind farm in the Danish North Sea. Marine Ecology Progress

Series 421, 205-216.

Dähne, M., Gilles, A., Lucke, K., Peschko, V., Adler, S., Krügel, K., Sundermeyer, J. & Seibert, U. (2013)

Effects of pile-driving on harbour porpoises (Phocoena phocoena) at the first offshore wind farm in Germany. Environmental Research Letters, 8(2), 025002.


[3] Harwood, J., King, S., Schick, R., Donovan, C. & Booth, C. (2014) A protocol for implementing the interim population consequences of disturbance (PCoD) approach: quantifying and assessing the effects of UK offshore renewable energy developments on marine mammal populations. Report number SMRU-TCE-2013-014. Scottish Marine and Freshwater Science, 5(2).