1. Introduction

1.1. Overview

  1. This Interim Population Consequences of Disturbance (iPCoD) Technical Report provides a description of the methods used for modelling of marine mammal populations in the presence of construction piling for the Ossian Array which is the subject of this application (hereafter referred to as “the Array”). It also reports and interprets the results of the iPCoD modelling which models the potential impacts on populations from elevated underwater noise from piling at the Array, as well as with other projects that have the potential to contribute a cumulative effect upon marine mammal populations.
  2. An Environmental Impact Assessment (EIA) has been carried out to determine the potential effects of the Array, on sensitive marine mammal receptors from a range of impacts. A key impact assessed is the potential for an elevation in underwater noise during piling to lead to injury and behavioural/or disturbance to individual marine mammals.
  3. Underwater noise modelling was undertaken (see volume 3, appendix 10.1) to predict the potential spatial scale of the impact of piling associated with the installation of seabed anchors for up to 265 wind turbines and jacket foundations for up to 15 Offshore Substation Platforms (OSPs).
  4. Two piling scenarios are modelled across the Array: a single vessel piling sequentially, and concurrent piling where two vessels pile at the same time (i.e. up to two piles installed simultaneously). The impact of single-vessel piling is expected to occur over the greatest temporal scale, and the impact of concurrent piling is expected to occur over the greatest spatial scale. The piling scenarios are hereafter referred to as the ‘maximum temporal scenario’ and the ‘maximum spatial scenario’, respectively. For both scenarios a maximum of 1,806 piles are expected to be installed: 1,590 piles for wind turbine anchors, and 216 piles for OSP foundations (section 2.1).
  5. Population modelling was carried out to determine the potential for a short to medium term exposure to piling, which could occur intermittently within a 72-month piling period, during the eight-year offshore construction timeframe (expected to occur between 2031 and 2038, inclusive), to result in long term population level effects on any marine mammal species for which population modelling is possible within the iPCoD framework. In this context short term refers to the duration of individual piling operations (i.e. days), medium term refers to the duration of the piling phase (i.e. up to eight years) and long term refers to the period of time over which iPCoD models are able to robustly predict population trajectories (i.e. up to 25 years).
  6. The iPCoD model (developed by the Sea Mammal Research Unit (SMRU) with a team of researchers at the University of St Andrews), was adopted to simulate the potential changes in the population over time and is described within this appendix. This approach has been agreed in consultation with Marine Directorate - Licensing Operations Team (MD-LOT) and NatureScot during the initial scoping process (MD-LOT, 2023, Ossian OWFL, 2023).

1.2. Background

  1. The iPCoD model simulates the potential changes in a population of marine mammals over time, for both a “disturbed” and an “undisturbed population”. This provides a comparison of the type of changes in a population that may result from natural environmental variation, demographic stochasticity (i.e. variability in population growth rates) and anthropogenic disturbance (Harwood et al., 2014, King et al., 2015). This approach has been widely used in previous offshore wind applications, and consented projects in the UK (e.g. Berwick Bank Offshore Wind Farm (SSE Renewables, 2022a), Awel y Môr Offshore Wind Farm (RWE Renewables UK, 2021), Hornsea Four Offshore Wind Project (Ørsted, 2021) and Hornsea Project Three Offshore Wind Farm (Ørsted, 2018a)).
  2. The iPCoD model is based on expert elicitation, a widely accepted process in conservation science whereby the opinions of many experts are combined when there is an urgent need for decisions to be made but a lack of empirical data with which to inform them (Donovan et al., 2016). The marine mammal experts, detailed in Sinclair et al. (2020), were asked for their opinion on how changes in hearing resulting from Permanent Threshold Shift (PTS) and behavioural disturbance (equivalent to a score of 5* or higher on the ‘behavioural severity scale’ described by Southall et al. (2007)) associated with offshore renewable energy developments and how they affect calf and juvenile survival, and the probability of giving birth (Harwood et al., 2014). Experts were asked to estimate values for two parameters which determine the shape of the relationships between the number of days of disturbance experienced by an individual and its vital rates, thus providing parameter values for functions that form part of the iPCoD model (Harwood et al., 2014).
  3. The relationship between disturbance and survival/reproduction assumes that individual animals would have a limited ability to alter their activity budget to compensate for a reduction in time spent feeding (King et al., 2015, Houston et al., 2012). The individual's ability to provision/care for young, evade predation or resist disease would likely be affected, and it is expected that effects would be reflected in changes to vital rates. Note, however, that this relationship is highly simplified (Harwood et al., 2014), and an individual’s response to disturbance will depend on factors including the context of the disturbance, the individual's existing condition and its exposure history (Ellison et al., 2012). The iPCoD framework applies simulated changes in vital rates to infer the number of animals that may be affected by disturbance to iteratively project the size of the population.
  4. Following the initial development of the iPCoD model, a study was undertaken to update the transfer functions on the effects of PTS and disturbance on the probability of survival and giving birth to viable young for harbour porpoise, harbour seal and grey seal (again via expert elicitation) (Booth and Heinis, 2018, Booth et al., 2019). The iPCoD model has been updated in light of additional work undertaken since it was originally launched in February 2014 (version 1) and iPCoD version 5.2 was used in the modelling for this report (Harwood et al., 2014, Sinclair et al., 2019).
  5. A potential limitation of the iPCoD model is that no form of density dependence has been incorporated into the model due to the uncertainties as to how to estimate carrying capacity or how to model the mechanism of density dependence. As discussed in Harwood et al. (2014), the concept of density dependence is fundamental to understanding how animal populations respond to a reduction in population size. Density-dependent factors, such as resource availability or competition for space, can limit population growth. If the population declines, these factors no longer become limiting and therefore, for the remaining individuals in a population, there is likely to be an increase in survival rate and reproduction. This then allows the population to expand back to previous levels at which density-dependent factors become limiting again (i.e. population remains at carrying capacity).
  6. The limitations for assuming a simple linear ratio between the maximum net productivity level and carrying capacity of a population have been highlighted by Taylor and DeMaster (1993), as simple models demonstrate that density dependence is likely to involve several biological parameters which themselves have biological limits (e.g. fecundity and survival). For UK populations of harbour porpoise (and other marine mammal species) however, there is no published evidence for density dependence and, therefore, density dependence assumptions are not currently included within the iPCoD protocol.
  7. The iPCoD model v5.2 (Harwood et al., 2014) was set up using the program R v4.3.1 (R Core Team, 2023) with RStudio v 2023.12.0+369 (Posit team, 2023) as the user interface. To enable the iPCoD model to be run, the following data were provided:
  • reference population size (section 2.3) and demographic parameters (section 2.4) for the key species;
  • user-specified input parameters:

–           vulnerable subpopulations (where appropriate); and

–           residual days of disturbance (section 2.5).

  • number of animals predicted to experience PTS and/or disturbance during piling (section 2.6); and
  • estimated piling schedule during the proposed construction programme (section 2.7).