3.3. Encounter Rate

  1. Encounter rate varied across and within species throughout the 24 month DAS period. The highest encounter rate for a given species or species group was for harbour porpoise, which was encountered in every survey except three and for which a mean of 0.041 animals per km (95% CI = 0.018, 0.064) was estimated. Monthly harbour porpoise encounter rate varied across months with the encounter rate for July 2021, April 2022, June 2022 and July 2022 estimated to be considerably higher compared to all other months of the year ( Figure 3.8   Open ▸ ).
  2. White-beaked dolphin were found to have the second highest mean encounter rate (0.005 animals per km, 95% CI = 0.002, 0.008) and were encountered in consecutive surveys between April 2021 and May 2021, and between June 2022 and July 2022, with numbers peaking in July 2021 and February 2022. However, subsequent encounters were much less frequent, and this species was only encountered during one other survey: September 2021 (seven surveys in total) ( Figure 3.9   Open ▸ ).
  3. Minke whale were mostly encountered during the spring and summer months (with the encounter rate peaking in July 2022 at 0.006 animals/km. Although observations in this case are sparse, this seasonality corroborates observations from previous surveys undertaken in waters off the north-east coast of Scotland with minke whales showing seasonal peaks during summer months (e.g. MacLeod et al., 2007; Weir et al., 2007).
  4. Grey seal were encountered in nine surveys and had a mean encounter rate of 0.002 animals per km (95% CI = 0.001, 0.003) in June 2021 and February 2022. ‘Seal species’ (0.003 animals per km, 95% CI = 0.002, 0.004) and common dolphin (0.003 animals per km, no 95% CI available as n = 1) were the third highest. A total of two harbour seal were sighted across two surveys (0.001 animals per km (no robust 95% CI available as n = 2)) and no bottlenose dolphin were observed in the study area during the 24 month DAS period.
Table 3.4:
Monthly Encounter Rate (Number of Animals per km of Track Line) for Marine Mammals within the Aerial Survey Area

Table 3.4: Monthly Encounter Rate (Number of Animals per km of Track Line) for Marine Mammals within the Aerial Survey Area

* Surveys for May 2021 and February 2022 were undertaken in June 2021 and March 2022, respectively (see section 2.5.2)

Figure 3.8:
Monthly Encounter Rate of Harbour Porpoise Across the Aerial Survey Area. Error Bars Indicate 95% CIs

Figure 3.8: Monthly Encounter Rate of Harbour Porpoise Across the Aerial Survey Area. Error Bars Indicate 95% CIs

Figure 3.9:
Monthly Encounter Rate for Marine Mammals Identified to Species Level (Excluding Harbour Porpoise). Error Bars for 95% CIs have been Omitted to Retain Clarity

Figure 3.9: Monthly Encounter Rate for Marine Mammals Identified to Species Level (Excluding Harbour Porpoise). Error Bars for 95% CIs have been Omitted to Retain Clarity

 

3.4. Group Size

  1. Marine mammals were considered to occur as a group on any occasion when more than one individual of a species was identified within the same survey image. Group size varied by species and across the months. The largest group sizes of four individuals were recorded for harbour porpoise and white-beaked dolphin, with an average group size of 2.15 animals (95% CI ±0.08) and 2.56 animals (95% CI ±0.58) respectively, across all sightings over the 24 months of survey ( Table 3.5   Open ▸ ). Figure 3.10   Open ▸ shows the monthly variation in the mean and maximum group size for harbour porpoise and Figure 3.11   Open ▸ presents the monthly variation in mean and maximum group size for white-beaked dolphin. The higher count of harbour porpoise in spring and summer months ( Table 3.2   Open ▸ ) coincided with larger groups of animals sighted within the aerial survey area. For example, in June a maximum group size of four animals was recorded whilst the overall mean for this month was 2.29 animals.
  2. In the 24 months of survey, most sightings of common dolphin were of single animals, with only a single instance in which two animals were recorded for either species, both in the month of July ( Table 3.5   Open ▸ ). For minke whale and grey seal, all sightings were of single animals, so it was not possible to calculate group size for these species.

 

Table 3.5:
Monthly Mean and Maximum Group Sizes for Species Sightings Across the Aerial Survey Area

Table 3.5: Monthly Mean and Maximum Group Sizes for Species Sightings Across the Aerial Survey Area

Figure 3.10:
Monthly Mean and Maximum Group Sizes (i.e. ≥2 Individuals Sighted in the Same Survey Image) for Harbour Porpoise. Error Bars Indicate 95% CIs

Figure 3.10: Monthly Mean and Maximum Group Sizes (i.e. ≥2 Individuals Sighted in the Same Survey Image) for Harbour Porpoise. Error Bars Indicate 95% CIs

Figure 3.11:
Monthly Mean and Maximum Group Sizes (i.e. ≥2 Individuals Sighted in the Same Survey Image) for White-Beaked Dolphin. Error Bars Indicate 95% CIs

Figure 3.11: Monthly Mean and Maximum Group Sizes (i.e. ≥2 Individuals Sighted in the Same Survey Image) for White-Beaked Dolphin. Error Bars Indicate 95% CIs

 

3.5. Density Estimates

  1. Mean densities of marine mammals were produced from the DAS count data, averaged at the monthly, seasonal and (where relevant) bio-season scale, and an overall mean was also estimated across the survey period. Coefficient of variation (CV) has also been calculated to present the variability in the raw data. Density estimates were calculated only for species that occurred at sufficient frequency to allow patterns of occurrence to be inferred: harbour porpoise, white-beaked dolphin and grey seal.

3.5.1. Harbour Porpoise

                        Design-based approach: relative densities

  1. Mean estimates of relative density for harbour porpoise were estimated across monthly, seasonal, bio-season and annual scales ( Table 3.6   Open ▸ ), with 95% CLs obtained via bootstrapping (1,000 simulations) (Wessa, 2019). Figure 3.12   Open ▸ illustrates the simulated mean relative densities resulting from the bootstrapping process.

 

Table 3.6:
Summary of Mean Relative Density of Harbour Porpoise, Aggregated Across Monthly, Seasonal, Bio-Season and Annual Scales

Table 3.6: Summary of Mean Relative Density of Harbour Porpoise, Aggregated Across Monthly, Seasonal, Bio-Season and Annual Scales

Figure 3.12:
Bootstrapped (n = 1,000) Simulation of Mean Relative Density Estimates of Harbour Porpoise

Figure 3.12: Bootstrapped (n = 1,000) Simulation of Mean Relative Density Estimates of Harbour Porpoise

 

  1. Peaks in density were estimated for the months of July 2021, June 2022 and July 2022, with a maximal relative density of 0.617 (95% CLs = 0.348, 0.898) animals per km2 in July 2021. Due to the large variability in relative density across surveys, with very low densities in some months and higher densities in others (range = 0.000, 0.617; CV = 1.367), monthly trends in density are not easy to interpret visually on a linear scale ( Figure 3.13   Open ▸ ). Data were therefore also plotted on a log10 scale ( Figure 3.14   Open ▸ ), where seasonality is more apparent and suggests that densities are broadly higher in spring and summer months (March to August) with lower values in late autumn and winter (September to February).

Figure 3.13:
Estimated Relative Density of Harbour Porpoise Over the Aerial Survey Area (Solid Line) for Each Survey Month, with Bootstrapped 95% CLs (Dotted Lines)

Figure 3.13: Estimated Relative Density of Harbour Porpoise Over the Aerial Survey Area (Solid Line) for Each Survey Month, with Bootstrapped 95% CLs (Dotted Lines)

Figure 3.14:
Estimated Relative Density of Harbour Porpoise (log10 scale) in the Aerial Survey Area (Solid Line) for Each Survey Month, with Bootstrapped 95% CLs (Dotted Lines)

Figure 3.14: Estimated Relative Density of Harbour Porpoise (log10 scale) in the Aerial Survey Area (Solid Line) for Each Survey Month, with Bootstrapped 95% CLs (Dotted Lines)

 

  1. The overall mean relative density of harbour porpoise, estimated from data pooled across all transects and all months for the 24 month survey period, with bootstrapping, was 0.152 animals per km2 (95% CLs = 0.084, 0.224; CV = 1.367). The maximal mean relative density estimates for respective temporal divisions were:
  • monthly: 0.577 animals/km2 (95% CLs = 0.320, 0.854; CV = 0.098) (July);
  • seasonal: 0.382 animals/km2 (95% CLs = 0.212, 0.566; CV = 0.602) (summer); and
  • bio-season: 0.277 animals/km2 (95% CLs = 0.154, 0.410; CV = 0.848) (‘summer’).

                        Design based approach: absolute densities

  1. Relative density estimates of harbour porpoise can be corrected for availability bias using published correction factors based on the proportion of time individuals are likely to be at or near the surface and available for detection. For example, availability bias was estimated based on a tagging study in the Baltic/North Sea which looked at the proportion of time that harbour porpoise spent surfacing or in the top 0 m to 2 m (Teilmann et al., 2013). Notably, in this study Teilmann et al. (2013) found no significant difference in diving behaviour between geographic areas or in relation to the size of the animals, although there was a significant seasonal difference in diving behaviour. The correction factor which gave the lowest estimate of availability (i.e. most conservative) was 42.5%, based on winter months, when surfacing time was found to be lower than in other seasons (Teilmann et al., 2013).
  2. Similarly, fine scale movements of harbour porpoise in the Danish North Sea were investigated by van Beest et al. (2018). GPS and dive recorders (V-tags) were used to record the diving behaviour of tagged individuals and the study estimated a mean dive duration of 53 s (min = 10.1 s, max = 250.0 s) and a mean surfacing time of 39 s (min = 2 s, max = 309 s). Using the mean values, the availability bias was calculated as 42.4% (mean surfacing time as a proportion of the mean surfacing time plus mean dive time) which is almost identical to the value estimated by Teilmann et al. (2013).
  3. Using the most conservative correction factor (0.425), the mean absolute density estimate across all transects and all monthly surveys for the 24 month survey period, with bootstrapping, was estimated as 0.357 animals per km2 (95% CLs = 0.200, 0.510; CV = 1.367). The maximal absolute density estimate in respective temporal divisions was:
  • monthly: 1.357 animals/km2 (95% CLs = 0.761, 1.939; CV = 0.098) (July);
  • seasonal: 0.900 animals/km2 (95% CLs = 0.505, 1.286; CV = 0.602) (summer); and
  • bio-season: 0.651 animals/km2 (95% CLs = 0.365, 0.931; CV = 0.848) (‘summer’).
  1. Temporal patterns in monthly harbour porpoise density are identical to those presented in paragraphs 65 to 67 for relative densities, so to avoid duplication these have not been plotted here. Mean estimates of absolute density for harbour porpoise across monthly, seasonal, bio-season and annual scales are summarised in Table 3.7   Open ▸ .

 

Table 3.7:
Summary of Mean Absolute Density of Harbour Porpoise, Aggregated Across Monthly, Seasonal, Bio-Season and Annual Scales

Table 3.7: Summary of Mean Absolute Density of Harbour Porpoise, Aggregated Across Monthly, Seasonal, Bio-Season and Annual Scales

 

                        Model based approach

  1. Harbour porpoise were only present in sufficient numbers for robust modelling when divided by bio-season, and when considered across the whole year. Abundance varied across bio-seasons, with higher densities in the aerial survey area observed during the ‘summer’ bio-season (April to September, inclusive). Estimates of relative density (plus 95% CLs) are presented in Table 3.8   Open ▸ , and estimates of absolute density are presented in Table 3.9   Open ▸ .

 

Table 3.8:
Modelled Relative Density of Harbour Porpoise, with 95% CLs and CV. Relative Abundance is Calculated as Mean Density Scaled up to the Total Aerial Survey Area

Table 3.8: Modelled Relative Density of Harbour Porpoise, with 95% CLs and CV. Relative Abundance is Calculated as Mean Density Scaled up to the Total Aerial Survey Area

 

Table 3.9:
Modelled Absolute Density of Harbour Porpoise, Corrected for Availability Bias, with 95% CLs and CV. Abundance is Calculated as Density Scaled up to the Total Aerial Survey Area

Table 3.9: Modelled Absolute Density of Harbour Porpoise, Corrected for Availability Bias, with 95% CLs and CV. Abundance is Calculated as Density Scaled up to the Total Aerial Survey Area

 

  1. Seasonal relative density maps for harbour porpoise distribution are shown in Figure 3.15   Open ▸ , illustrating estimates of mean predicted densities for each bio-season alongside estimates of the lower and upper 95% CLs. The mean modelled relative density across the 24-month survey period was 0.151 (95% CLs = 0.107, 0.205; CV = 0.362) and the corrected (absolute) density for this same period was 0.355 (95% CLs = 0.252, 0.482; CV = 0.362).
  2. Spatial distribution during the ‘Winter’ bio-season appears to be concentrated in two locations in the south and west of the Array marine mammal study area, and density is broadly lower than during the ‘summer’ bio-season. Spatial distribution is also less concentrated during the ‘summer’ bio-season, and spread more evenly across the Array marine mammal study area. When considered across the whole 24 month DAS campaign, spatial distribution is greater at the east and north of the Array marine mammal study area.

Figure 3.15:
 Predicted Mean Relative Density of Harbour Porpoise, with 95% CLs for the ‘Winter’ (Top) and ‘Summer’ (Bottom) Bio-Seasons. Note that Colour Scales for the Two Bio-Seasons are Different to Accommodate the Differences in Seasonal Abundance

Figure 3.15:  Predicted Mean Relative Density of Harbour Porpoise, with 95% CLs for the ‘Winter’ (Top) and ‘Summer’ (Bottom) Bio-Seasons. Note that Colour Scales for the Two Bio-Seasons are Different to Accommodate the Differences in Seasonal Abundance

Figure 3.16:
Overall Predicted Mean Relative Density of Harbour Porpoise, with 95% CLs, Across the Aerial Survey Area for the Full 24-Month Survey Period

Figure 3.16: Overall Predicted Mean Relative Density of Harbour Porpoise, with 95% CLs, Across the Aerial Survey Area for the Full 24-Month Survey Period

 

3.5.2. White-beaked Dolphin

                        Design based approach: relative densities

  1. Alongside estimates of monthly, seasonal and annual means calculated directly from observations, bootstrapped estimates of mean relative density for white-beaked dolphin were obtained and used to calculate 95% CLs across these temporal scales. Table 3.10   Open ▸ presents a summary of the aggregated mean relative densities and Figure 3.17   Open ▸ illustrates the simulated mean relative densities resulting from the bootstrapping process.

 

Table 3.10:
Summary of Mean Relative Density of White-Beaked Dolphin, Aggregated Across Monthly, Seasonal and Annual Scales. Note that Missing Monthly Values are a Result of ‘Zero Counts’ in Respective Months, Across the 24-Month Survey Period

Table 3.10: Summary of Mean Relative Density of White-Beaked Dolphin, Aggregated Across Monthly, Seasonal and Annual Scales. Note that Missing Monthly Values are a Result of ‘Zero Counts’ in Respective Months, Across the 24-Month Survey Period

Figure 3.17:
Bootstrapped (n = 1,000) Simulation of Mean Relative Density Estimates of White-Beaked Dolphin

Figure 3.17: Bootstrapped (n = 1,000) Simulation of Mean Relative Density Estimates of White-Beaked Dolphin

 

  1. White-beaked dolphin were sighted during seven of the 24 surveys, which included just two surveys in the second year. An overall mean of 0.006 animals per km2 (95% CLs = 0.002, 0.010; CV = 2.216) was estimated. Peak densities were recorded during July 2021 when 12 animals were sighted (in groups of two to six), equivalent to a relative density of 0.053 (95% CLs = 0.016, 0.095) animals per km2 ( Figure 3.18   Open ▸ ).

Figure 3.18:
Estimated Relative Density of White-Beaked Dolphin Over the Aerial Survey Area (Solid Line) for Each Survey Month with Bootstrapped 95% CLs (Dotted Lines)

Figure 3.18: Estimated Relative Density of White-Beaked Dolphin Over the Aerial Survey Area (Solid Line) for Each Survey Month with Bootstrapped 95% CLs (Dotted Lines)

 

  1. The maximal relative density estimates for respective temporal divisions were:
  • monthly: 0.026 animals/km2 (95% CLs = 0.008, 0.047; CV = 1.414) (July); and
  • seasonal: 0.010 animals/km2 (95% CLs = 0.003, 0.018; CV = 2.057) (summer).
  1. Given the inconsistent temporal distribution of white-beaked dolphin within the aerial survey area, and the occurrence of the species in only two surveys during the second year of the 24 month survey period, the relative densities presented above should be interpreted with caution.

                        Design based approach: absolute densities

  1. There is limited information on diving and surfacing times of white-beaked dolphin and consequently many studies report relative density estimates only (refer to Paxton et al., 2016). A bio-logging study of two individual free-ranging white-beaked dolphins in Iceland found that, on average, animals spent 18% of time close to the surface (0 m to 2 m depth) and 82% of the time diving (Rasmussen et al., 2013). Therefore, based on these data, the correction factor to account for availability bias would be 0.18.
  2. The mean absolute density estimate across all transects and all monthly surveys for the 24 month survey period, with bootstrapping, was estimated as 0.036 animals per km2 (95% CLs = 0.011, 0.064; CV = 2.251). The maximal absolute density estimate in respective temporal divisions was:
  • monthly: 0.147 animals/km2 (95% CLs = 0.054, 0.260; CV = 1.414) (July); and
  • seasonal: 0.057 animals/km2 (95% CLs = 0.021, 0.101; CV = 2.057) (summer).
  1. Temporal patterns in monthly white-beaked dolphin absolute density are identical to those presented in paragraphs 75 to 78 for relative densities, and to avoid duplication these have not been plotted here. Mean estimates of absolute density for white-beaked dolphin across monthly, seasonal and annual scales are summarised in Table 3.11   Open ▸ .
Table 3.11:
Summary of Mean Absolute Density of White-Beaked Dolphin, Aggregated Across Monthly, Seasonal and Annual Scales, and Corrected for Availability Bias. Note that Missing Values are a Result of ‘Zero Counts’ in Respective Months, Across the 24-Month Survey Period

Table 3.11: Summary of Mean Absolute Density of White-Beaked Dolphin, Aggregated Across Monthly, Seasonal and Annual Scales, and Corrected for Availability Bias. Note that Missing Values are a Result of ‘Zero Counts’ in Respective Months, Across the 24-Month Survey Period

 

                        Model based approach

  1. White-beaked dolphin were not observed in sufficient numbers for robust modelling, and as such their density in the aerial survey area can only be estimated via design based methods.

3.5.3. Grey Seal

                        Design-based approach: relative densities

  1. Mean estimates of relative density for grey seal were estimated across monthly, seasonal, bio-season and annual scales ( Table 3.12   Open ▸ ), with 95% CLs obtained via bootstrapping (1,000 simulations) (Wessa, 2019). Figure 3.19   Open ▸ illustrates the bootstrapped mean relative densities.

 

Table 3.12:
Summary of Mean Relative Density of Grey Seal, Aggregated Across Monthly, Seasonal, Bio-Season and Annual Scales. Note that Missing Monthly Values are a Result of ‘Zero Counts’ in Respective Months, Across the 24-Month Survey Period

Table 3.12: Summary of Mean Relative Density of Grey Seal, Aggregated Across Monthly, Seasonal, Bio-Season and Annual Scales. Note that Missing Monthly Values are a Result of ‘Zero Counts’ in Respective Months, Across the 24-Month Survey Period

Figure 3.19:
Bootstrapped (n = 1,000) Simulation of Mean Relative Density Estimates of Grey Seal

Figure 3.19: Bootstrapped (n = 1,000) Simulation of Mean Relative Density Estimates of Grey Seal

 

  1. Grey seal were sighted within the aerial survey area during nine months of the 24 month survey period, and of these, three months (February, April and May) contained sightings in both survey years. Peaks in presence occurred in June 2021 and February 2022 (relative density = 0.018; 95% CLs = 0.009, 0.029) when four individuals were sighted, with an equivalent abundance across the Array marine mammal study area of approximately 40 animals when corrected for survey effort.
  2. The overall mean relative density of grey seal, estimated from data pooled across all transects and all months for the 24 month survey period, with bootstrapping, was 0.003 animals per km2 (95% CLs = 0.002, 0.005; CV = 1.632). The maximal relative density estimates for respective temporal divisions were:
  • monthly: 0.011 animals/km2 (95% CLs = 0.005, 0.018; CV = 0.844) (February);
  • seasonal: 0.005 animals/km2 (95% CLs = 0.003, 0.008; CV = 1.371) (winter); and
  • bio-season: 0.005 animals/km2 (95% CLs = 0.003, 0.009; CV = 1.172) (‘non-breeding’).
  1. Temporal variability in grey seal observations in the aerial survey area is illustrated in Figure 3.20   Open ▸ , although given the scarcity of sightings, plotting these data on a log10 scale did not provide further clarity, and this has not been presented here.

Figure 3.20:
 Estimated Relative Density of Grey Seal Across the Aerial Survey Area (Solid Line) for Each Survey Month, with Bootstrapped 95% CLs (Dotted Lines)

Figure 3.20:  Estimated Relative Density of Grey Seal Across the Aerial Survey Area (Solid Line) for Each Survey Month, with Bootstrapped 95% CLs (Dotted Lines)

 

                        Design based approach: absolute densities

  1. The densities shown in Figure 3.20   Open ▸ are relative values and do not account for availability bias during aerial surveys. A tracking study of three male grey seals in the Farne Islands (north-east England) found that the average proportion of time animals were submerged as they travelled was 84.3%, which was slightly lower during short duration trips (83.4%) (Thompson et al., 1991). It therefore follows that the average proportion of time a travelling grey seal would be available for detection ranges between 15.7% and 16.6%.
  2. Similarly, telemetry data from tags deployed by the Sea Mammal Research Unit (SMRU) on grey seals in the North Sea recorded 1,551 grey seal dives. These data were analysed for the Hornsea Three Offshore Wind Farm (to estimate detection probability) and showed that 60% of surfacing periods were between 15 s and 45 s, with an average of 40 s (Ørsted, 2018). Recorded grey seal dive durations varied between 20 s and 496 s with an average of 216 s (Ørsted, 2018). The average values reported from the telemetry data were used to estimate the proportion of time that grey seals were surfacing compared to diving to give an indication of the availability bias for the site-specific aerial surveys. The estimated availability was calculated as 15.6% (Ørsted, 2018) and was therefore similar to the figures cited by Thompson et al. (1991).
  3. As with harbour porpoise, it was assumed that all animals on (or near) the surface were available for detection during the aerial surveys (i.e. no perception bias) (section 2.5.3). The correction factor for availability bias, based on the telemetry studies described in paragraph 88, was 15.6% as the most conservative estimate. Thus, estimates for absolute density for grey seal across the aerial survey area ( Table 3.13   Open ▸ ) ranged between 0.028 and 0.113 animals per km2 and mean corrected density across all transects and all seasons was 0.021 animals per km2 (95% CLs = 0.009, 0.034; CV = 1.632).
  4. The maximal absolute density estimate in respective temporal divisions was:
  • monthly: 0.071 animals/km2 (95% CLs = 0.031, 0.115; CV = 0.844) (February);
  • seasonal: 0.033 animals/km2 (95% CLs = 0.014, 0.054; CV = 1.371) (winter); and
  • bio-season: 0.034 animals/km2 (95% CLs = 0.015, 0.056; CV = 1.172) (‘non-breeding’).

 

Table 3.13:
Summary of Mean Absolute Density of Grey Seal, Aggregated Across Monthly, Seasonal, Bio-Season and Annual Scales, and Corrected for Availability Bias. Note that Missing Values Result from ‘Zero Counts’ in Respective Months, Across the 24-Month Survey Period

Table 3.13: Summary of Mean Absolute Density of Grey Seal, Aggregated Across Monthly, Seasonal, Bio-Season and Annual Scales, and Corrected for Availability Bias. Note that Missing Values Result from ‘Zero Counts’ in Respective Months, Across the 24-Month Survey Period

 

                        Model based approach

  1. Grey seal were not observed in sufficient numbers for robust modelling to be undertaken, and as such their density in the aerial survey area can only be estimated via design based methods.