We wanted to highlight to everyone the release of an exciting new paper which presents the predicted impacts on the harbour porpoise population as a result of the construction of 65 offshore wind farms in the North Sea, using the ‘Disturbance Effect on the Harbour Porpoise in the North Sea’ (DEPONS) model.
Assessment of population consequences of disturbance as a result of exposure to anthropogenic noise is an area of increasing concern for policy makers and a requirement of most environmental impact assessments. DEPONS is a spatially-explicit, process-based modelling framework for the North Sea harbour porpoise population. It assesses population consequences of disturbance based on changes to animal movement, foraging and energetics in response to exposure to (for example) pile driving noise.
Using impact of offshore wind farm construction noise on the North Sea harbour porpoise population as a case study, a group of researchers at Aarhus University, led by Jacob Nabe-Nielsen demonstrate the model for assessing population level effects of anthropogenic disturbances. They used harbour porpoise response data collected at the Gemini Offshore Wind Farm in the Netherlands, where data showed that porpoise responded to piling noise out to 8.9 km and and that porpoise densities returned to normal 2-6 hours after piling.
Assuming that porpoise responded to piling in the same way as they did at Gemini, their model showed that: “the North Sea porpoise population was not affected by construction of 65 wind farms as required to meet the EU renewable energy target“.
At the North Sea scale, population dynamics were indistinguishable from those in the noise-free baseline scenario when porpoises reacted to noise up to 8.9 km from the construction sites. Wind farm construction noise only influenced population dynamics when animals were assumed to respond at distances >20–50 km from the wind farm. Most PAM studies have demonstrated impact ranges up to approximately 20 km (Brandt et al., 2011; Dähne et al., 2013; Rumes et al., 2017).
Wind farm construction schedules and the length of the breaks between individual piling events influenced the population effects of noise. The modelling showed that when the best foraging grounds in the western North Sea were continuously exposed to piling noise for several years, the effect of disturbance was larger and lasted longer when the wind farms were constructed under the “ordered scenario”, than when they were constructed in a random order (see figure below). The ‘fast’ construction scenarios which involved near continuous pile driving, also had a larger population level effect than the ‘slow’ construction scenario where animals had more time to recover between consecutive pilings. However, these effects were only apparent when using a 200 km response radius which is far beyond anything that has been recorded to date (see references above).
The model has usefully helped to identify species sensitivities to pile driving noise and requirements for further work. For example, harbour porpoise population dynamics were most sensitive to the distance at which animals responded to pile driving noise. As such collection of data from other wind farm developments will be important to measure response distances and validate if the distances recorded at Gemini are representative. In addition, what happens to individuals when they are disturbed is poorly understood. So we also need to better understand what disturbance means, if animals stop foraging, if animals can compensate following lost feeding opportunities etc. The paper highlighted that population dynamics were also sensitive to energetic parameters and further research is recommended on animal energetics and the dynamics of their food and the distances at which they respond to noise. As with PCoD, the differing levels of effect shown under the differing piling scenarios demonstrates how the modelling framework can be used for spatial planning to help mitigate population effects of disturbances.
Here at SMRU Consulting we are also investigating the nuances of disturbance effects on populations using the iPCoD framework. One key unknown when modelling population level consequences of disturbance is how disturbance can impact on vital rates such as survival and fecundity. As such, we are co-organising the upcoming INPAS symposium – exploring the latest research on the effects of impulsive noise on and harbour porpoise and seals. This will include a presentation from Jacob Nabe-Nielsen on this new paper as well as presentations from a host of other experts working on these issues. You can see the abstracts for the talks here. Following the symposium Cormac will be coordinating and facilitating a closed expert elicitation to update the transfer functions in the iPCoD model (linking disturbance levels to survival and fecundity rates) based on the latest research in the field.