3. Results

3.1. Summary Data

3.1.1. Survey Effort

  1. A summary of monthly survey effort, as calculated from the clipped data set described in section 2.2, is presented in Table 3.1   Open ▸ , which also provides information for surveys in which minimum target coverage of the aerial survey area was not met.
  2. For the design-based analysis, the 10% target coverage was met or exceeded in 16 out of 24 surveys. The minimum coverage obtained in any survey was 9.93% on 08 November 2022. For the model-based analysis, one transect was removed in two surveys: 18 March 2022 and 03 May 2022 (see paragraph 27), meaning that although the minimum 10% coverage was achieved during these surveys, the corresponding subset of data used in the analysis was below the 10% target by 0.47% and 0.58%, respectively. For model-based analyses, the 10% target was therefore met or exceeded in 14 out of 24 surveys. Critically, there was no reduction in the number of individuals included in the model-based analyses, since the two removed transects contained no observations of marine mammals.
  3. Since the size of deviations from the 10% target were minimal, and the overall coverage across the 24-month DAS campaign exceeded the 10% target, the small shortfall in some surveys was not considered to affect the outcome of the analyses. Moreover, 95% CLs for marine mammal densities have been reported in all cases (refer to section 3.5) to account for inherent uncertainty in the calculation of summarised density estimates. The use of 95% CLs also reduces the sensitivity of calculated density estimates to the small deviations from the 10% target coverage which occurred in some surveys. Given the small size of these deviations, if slightly greater survey coverage had been achieved in these surveys (bringing survey coverage to the 10% target), then subsequent density estimates would still be expected to fall within the reported 95% confidence intervals.

3.1.2. Species Counts

  1. Harbour porpoise accounted for the highest number of sightings identified to species level (based on raw count data) across the aerial survey area and was recorded in all but three survey months: January 2022, October 2022 and January 2023 ( Table 3.2   Open ▸ ). White-beaked dolphin accounted for the second highest number of sightings and was recorded in seven months over the 24 month survey period. For other sightings identified to species level – grey seal, minke whale, common dolphin and harbour seal – both the number and frequency of sightings was small ( Table 3.2   Open ▸ ). No bottlenose dolphin Tursiops truncatus were observed during any survey months across the DAS campaign.
  2. There were also four cetaceans observed that could not be assigned to species level, and sightings classified as ‘seal species’ (due to the difficulty of identifying phocids to species level from aerial survey data) occurred in 16 of the 24 months surveyed.

 

Table 3.1:
Monthly Survey Effort Across the Array Marine Mammal Study Area. Raw Survey Data Refer to those Data Used in the Design-Based Analysis and Processed Survey Data Refer to those Data Used in the Model-Based Analysis

Table 3.1: Monthly Survey Effort Across the Array Marine Mammal Study Area. Raw Survey Data Refer to those Data Used in the Design-Based Analysis and Processed Survey Data Refer to those Data Used in the Model-Based Analysis

 

Table 3.2:
Monthly Raw Sightings Data (Number of Animals) (Uncorrected for Effort) Across the Aerial Survey Area. To Improve Clarity and Aid Readability, Zero Counts are Indicated as Dashes

Table 3.2: Monthly Raw Sightings Data (Number of Animals) (Uncorrected for Effort) Across the Aerial Survey Area. To Improve Clarity and Aid Readability, Zero Counts are Indicated as Dashes

Table 3.3:
Seasonal Raw Sightings Data (Number of Animals, Uncorrected for Effort) Across the Aerial Survey Area. To Improve Clarity and Aid Readability, Zero Counts are Indicated as Dashes

Table 3.3: Seasonal Raw Sightings Data (Number of Animals, Uncorrected for Effort) Across the Aerial Survey Area. To Improve Clarity and Aid Readability, Zero Counts are Indicated as Dashes

 

  1. Seasonal marine mammal sightings across the aerial survey area are summarised in Figure 3.1   Open ▸ .

Figure 3.1:
Seasonal Percentage of Marine Mammal Sightings in the Aerial Survey Area

Figure 3.1: Seasonal Percentage of Marine Mammal Sightings in the Aerial Survey Area

 

3.1.3. Surfacing Categories

  1. There were no clear temporal patterns in surfacing behaviour across the 24 month survey period. In four survey months all sighted marine mammals were observed to be surfacing, and during eight of the months surveyed a clear majority of observations were of submerged individuals. For seven surveys there was a broadly equal split between individuals submerged or surfacing, and in three months this was split between surfacing animals and those for which a behavioural category could not be assigned ( Figure 3.2   Open ▸ ). No surfacing categories were assigned to any individuals in the January 2023 survey.

Figure 3.2:
Summary of Surfacing Categories by Month, Combined Across Species

Figure 3.2: Summary of Surfacing Categories by Month, Combined Across Species

 

  1. There were also inter-species differences noted in the surfacing categories for the more abundant species. Harbour porpoise, minke whale and white-beaked dolphin were recorded as ‘submerged’ in at least 65% of observations, whilst grey seal and ‘seal species’ were most often recorded as they surfaced (‘surfacing’ plus ‘surfacing at red line’) ( Figure 3.3   Open ▸ ). This highlights the potential differences in availability bias between species.

Figure 3.3:
Summary of Surfacing Categories by Species, Combined Across Months

Figure 3.3: Summary of Surfacing Categories by Species, Combined Across Months

 

3.1.4. Confidence Assessment

  1. Confidence in identification varied by species/species group ( Figure 3.4   Open ▸ ). Where an animal was identified to species level there was typically a high level of confidence in the identification and subsequently most identifications were classified as ‘definite’, with one species (common dolphin) split 1:2 between ‘Definite’ and ‘Probable’.
  2. Seals appear to have been the hardest group to identify to a high level of confidence, given that 40% of identified grey seal (n = 18) and 50% of identified harbour seal (n=2) were classified as ‘definite’ identifications. A total of seven individuals were identified as ‘Definite’ grey seals whilst a total of 48 animals were identified as ‘seal species’ (i.e. could not be assigned to either grey seal or harbour seal). Of the seal species, 36 animals were ‘Definite’ identifications, seven were ‘Probable’ identifications and five were ‘Possible’ identifications. Three sightings could be identified only as ‘seal/small cetacean species’, all of which were classified as ‘Probable’.

Figure 3.4:
Proportion of Marine Mammal Sightings Classified as ‘Definite’, ‘Probable’ or ‘Possible’. Numbers within Bars Indicate Numbers of Sightings

Figure 3.4: Proportion of Marine Mammal Sightings Classified as ‘Definite’, ‘Probable’ or ‘Possible’. Numbers within Bars Indicate Numbers of Sightings

 

3.2. Distribution of Sightings

  1. Sightings of marine mammals were spatially distributed throughout the aerial survey area. Figure 3.5   Open ▸ indicates the distribution of harbour porpoise sightings, Figure 3.6   Open ▸ indicates the distribution of all cetacean sightings, excluding harbour porpoise for clarity, and Figure 3.7   Open ▸ indicates the distribution of seal species, within the aerial survey area.
  2. No clear spatial patterns were observed in the distribution of any species.

Figure 3.5:
Distribution of Harbour Porpoise Sightings within the Array Marine Mammal Study Area (Solid Line), with Site Boundary Illustrated by Dashed Line

Figure 3.5: Distribution of Harbour Porpoise Sightings within the Array Marine Mammal Study Area (Solid Line), with Site Boundary Illustrated by Dashed Line


Figure 3.6:
Distribution of Cetacean Sightings within the Array Marine Mammal Study Area (Solid Line), with Site Boundary Illustrated by Dashed Line. Harbour Porpoise Sightings have been Removed for Clarity

Figure 3.6: Distribution of Cetacean Sightings within the Array Marine Mammal Study Area (Solid Line), with Site Boundary Illustrated by Dashed Line. Harbour Porpoise Sightings have been Removed for Clarity

Figure 3.7:
Distribution of Seal Observations within the Array Marine Mammal Study Area (Solid Line), with Site Boundary Illustrated by Dashed Line

Figure 3.7: Distribution of Seal Observations within the Array Marine Mammal Study Area (Solid Line), with Site Boundary Illustrated by Dashed Line