4. Summary

  1. This annex provides a summary of estimated marine mammal distribution recorded during the 24-month DAS campaign across the Array marine mammal study area, encompassing the site boundary plus 8 km buffer.
  2. A mean coverage of 10% of the aerial survey area was processed by HiDef across the DAS campaign, and this target was met in 16 out of 24 surveys ( Table 3.1   Open ▸ ). The mean area processed for design based analyses was 10.01% (SE: ± 0.04%), and the mean area included in model-based analysis (after removal of erroneous transects) was 10.00% (SE: ± 0.01%). The removed transects contained no observations of marine mammals, there was no reduction in the number of individuals included in the model-based analyses. The small shortfalls in survey coverage (maximum 0.074% deviation) were not considered to reduce the accuracy of the analyses, and in the event that all surveys had achieved the 10% target, the subsequent estimates would be expected to fall within the inherent range of uncertainty accounted for in the reported 95% CLs.
  3. The division of the year into two bio-seasons for harbour porpoise, based upon bimodal patterns of spatial distribution (“winter” and “summer”) is an approach intended to address the difficulties in implementing criteria for designating Special Areas of Conservation (Heinänen and Skov, 2015). Similarly for grey seal, broad-scale seasonal patterns of density have been determined based upon potential changes in distribution between the breeding season (defined as September to December for this region (Marine Scotland, 2020; SCOS, 2020)) and the non-breeding season (January to August). For white-beaked dolphin, meteorological season was used as a means to determine broader-scale temporal patterns in density. This approach has also been taken forward to the EIA.
  4. Harbour porpoise accounted for the highest number of individuals identified to species level (n = 825, based on raw count data) across the aerial survey area, and were recorded in all survey months except for January 2022, October 2022 and January 2023.
  5. White-beaked dolphin was the second most abundant species (n = 30) and were sighted in only seven months across the 24 month survey period. Eighteen grey seal were identified across nine months, and 47 unidentified seal species were observed across 12 survey months.
  6. Twelve minke whale, three common dolphin and two harbour seal were also identified, and no bottlenose dolphin were sighted across all 24 surveys.
  7. For harbour porpoise, white-beaked dolphin and grey seal, relative density estimates were corrected for availability bias to give absolute densities, calculated with correction factors derived from studies of these species’ diving behaviour. These correction factors give an indication of the average proportion of time that individuals of a species may be on, or near, the surface and available for detection.
  8. The limitations of using availability bias estimates from published studies are recognised (e.g. potentially subject to geographic, seasonal, diurnal, and individual animal variation) and therefore absolute densities are considered to be approximations only but have been employed in an effort to obtain more precautionary estimates of marine mammal abundance.
  9. Peaks in harbour porpoise abundance were estimated to occur in the Summer (n = 2,038) and the ‘summer’ bio-season (n = 1,475). In the latter, for which modelling was possible to undertake, an estimated density of 0.651 animals/km2 from the design-based approach, and a predicted density of 0.648 animals/km2 from the model-based approach ( Table 4.1   Open ▸ ).

 

Table 4.1:
Summary Table of Mean Estimated Absolute Abundance and Density (Corrected for Availability Bias) for Annual and Seasonal Divisions for Design-Based Estimates. For Harbour Porpoise, Model-Based Estimates are Shown in Parentheses

Table 4.1: Summary Table of Mean Estimated Absolute Abundance and Density (Corrected for Availability Bias) for Annual and Seasonal Divisions for Design-Based Estimates. For Harbour Porpoise, Model-Based Estimates are Shown in Parentheses

 

  1. There was no clear spatial pattern in distribution for any of the species, although seasonal concentrations of harbour porpoise were predicted by the MRSea analysis in the western portion of the aerial survey area ( Figure 3.15   Open ▸ ).
  2. These data suggest seasonality in the occurrence of marine mammals within the aerial survey area. However, interpretation of seasonal differences should be treated with caution due to potential confounding effects of environmental variables during the aerial surveys and the limitations of the ‘snap-shot’ nature of aerial data.


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