4. Summary

  1. This Technical Report presents the results of iPCoD modelling for harbour porpoise, bottlenose dolphin, minke whale and grey seal populations as a result of the Array alone and cumulatively with other projects. The numbers of animals potentially experiencing disturbance were based on two maximum design scenarios. The maximum temporal scenario of a combination of 3,000 kJ and 4,400 kJ maximum hammer energies (depending on the foundation type) across 602 days, and the maximum spatial scenario of the same maximum hammer energies, incorporating concurrent piling at 3,000 kJ and 4,400 kJ, and single piling at 4,400 kJ, across a total of up to 306 days. However, these would not be the maximum hammer energies for all locations (with maximum hammer energies only anticipated to be required at locations where there are challenging ground conditions), and the realistic hammer energy would likely be lower, affecting fewer animals. Therefore the iPCoD modelling presented represents a conservative precautionary assessment, which uses maximum hammer energies at all piling locations as the MDS.
  2. The results suggest that, even with the conservative assumptions made, there would be no population-level effects either in the short-term or long term for any of the species investigated, as a result of piling at the Array alone, or when considered alongside cumulative projects.
  3. Results of the cumulative models should be interpreted with caution as limited information on the actual piling schedules was available to inform the iPCoD models presented here. However, the parameters used to develop the models for the cumulative projects have been informed by the respective Environmental Statements, and are considered to represent a conservative and accurate depiction of these projects.
  4. The assessment adopted a precautionary approach throughout, considering the maximum design scenario for the Array, precautionary demographic parameters for each species, conservative assumptions in the noise modelling and conservative estimates for the densities of key species to apply to the quantitative assessment. For all species there was negligible difference predicted in the trajectory of the impacted population compared to the un-impacted populations. The predicted differences in the numbers of animals would be expected to fall within the natural inter-annual variation of the population. Such variation is exemplified in the differences in abundance estimates from successive survey campaigns in the Small Cetacean Abundance in the European Atlantic and North Sea (SCANS) monitoring programme. This is particularly evident in the results of SCANS-III (Hammond et al., 2021) and SCANS-IV (Gilles et al., 2023), which employed survey blocks that largely coincide spatially between surveys.
  5. Variation in demographic rates between years may exist as a result of changes in environmental conditions, as a result of random processes or chance events which impact vital rates (e.g. survival, fertility, etc.), or other sources of heterogeneity. In two, otherwise identical populations that experience exactly the same sequence of environmental conditions, demographic stochasticity will mean populations could follow slightly different trajectories over time.
  6. These models assume that the effects of environmental variation on survival and fertility are adequately reflected by the range of values obtained from the expert elicitation (and shown in the spread of simulated trajectories around the means). In addition, the models assume that survival and fertility rates are not density-dependent and are therefore not affected by population size.
  7. Whilst it is understood that iPCoD is a relatively simple population model (simulating the link between days of disturbance and changes in individual vital rates), the most obvious sources of uncertainty are considered to have been adequately captured in the development of these models. In addition, the precautionary approach applied throughout the marine mammal assessment has been adopted to buffer the uncertainties with respect to how animals respond to repeated piling over time.

5. References

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[1] Since no animals of any species were predicted from modelling of the SPLpk metric to experience PTS, the values presented in Table 2.6   Open ▸ and Table 2.7   Open ▸ are those from modelling of SELcum only

[2] Shading Indicates Construction Phases for Respective Projects. ‘P’ Indicates Years in Which Piling is Expected

[3] Figure 3.12 illustrates a potential increase in grey seal population trajectory for the cumulative iPCoD modelling scenario. This is likely to be due to different conditions (e.g. piling parameters and number of animals affected) at the cumulative projects at the outset of this model, so the simulated trajectory for this model also reflects this difference.