4. Baseline Environment

4.1. Marine Mammal Presence Within Study Areas

4.1.1. Regional Marine Mammal Study Area

  1. The northern portion of the North Sea (delineated by Weir (2001) approximately between 57°N and 60°N) supports several species of cetaceans, with both numbers and species diversity decreasing towards the southern North Sea (Weir, 2001). There are 11 species of cetaceans and two species of pinnipeds that are regularly encountered within the regional marine mammal study area (Hammond et al., 2021, Weir, 2001, Hammond et al., 2013, NMPi, 2023). Weir (2001) suggested that due to the presence of pelagic species entering the waters of the northern North Sea in areas adjacent to the deep Atlantic and along the continental shelf edge, these areas attract a higher number of cetaceans (individuals and species) compared to the southern part of the northern North Sea. Given that the distribution of marine mammals is strongly influenced by the distribution of their prey, the distribution of marine mammals in the North Sea is variable. Nevertheless, some areas consistently support a higher number of species.
  2. The east coast of Scotland and north-east of England support multiple haul out sites for grey seal and harbour seal (SCOS, 2023, Weir, 2001). Although densities of these species might be expected to be higher in the vicinity of haul outs at certain times of the year, Carter et al. (2022) reported that links between grey and harbour seal distribution at sea and use of land-based sites have not been researched in detail. One of the reasons for this is that grey seal exhibit partial migration and may move between regions for breeding and foraging, e.g. seals that breed in a given SAC do not necessarily forage nearby (Carter et al., 2022).
  3. Within the waters of the east of Scotland, the more commonly recorded cetaceans include harbour porpoise, bottlenose dolphin, white-beaked dolphin and minke whale. Other species of cetacean have been recorded as occasional or rare visitors to this region ( Table 4.1   Open ▸ ).

 

Table 4.1:
Summary of Cetacean Species Found in the Regional Marine Mammal Study Area. Sources: Weir (2001), Hammond et al. (2013), Hammond et al. (2021), NMPI (2023) and Citizen Projects (pers. comm.)

Table 4.1: Summary of Cetacean Species Found in the Regional Marine Mammal Study Area. Sources: Weir (2001), Hammond et al. (2013), Hammond et al. (2021), NMPI (2023) and Citizen Projects (pers. comm.)

 

  1. Data from historic surveys conducted within the Firth of Forth ( Table 3.3   Open ▸ ) demonstrated that several marine mammal species occur regularly in inshore and offshore waters. Harbour porpoise was the most frequently recorded cetacean during all historic surveys and was recorded in every month of the year. Species recorded during the historic surveys also included minke whale and white-beaked dolphin (both with seasonal occurrence during spring/summer months), grey seal (year-round) and harbour seal (with only a few sightings recorded to species level during the TCE and Berwick Bank aerial surveys).
  2. Bottlenose dolphins were positively identified in historic TCE aerial surveys in inshore (within 12 nm) and offshore (beyond 12 nm) waters with only one encounter of one individual outside 12 nm (Grellier and Lacey, 2011). No bottlenose dolphins were encountered during Seagreen boat-based surveys between 2010 and 2011 (Sparling, 2012). The Berwick Bank aerial surveys also observed a relatively small number of bottlenose dolphin with two sightings and a total of seven individuals across two years of surveys. No bottlenose dolphin were observed during DAS.
  3. Although there are no records of humpback whales in any of the historic surveys conducted in the Firth of Forth, data from citizen science projects[1],[2] suggest a recent increase in sightings of this species on the east coast of Scotland (section 5.2.2).

4.1.2. Array Marine Mammal Study Area

  1. Given that the extent of the Array marine mammal study area was covered by the DAS campaign, data from DAS is considered the most accurate to inform marine mammal presence and distribution in the vicinity of the Array.
  2. During DAS, harbour porpoise was the most frequently recorded cetacean, followed by white-beaked dolphin. Minke whale were mostly encountered during the spring and summer months, but the number of animals encountered was small (12 individuals over 24-month survey period).
  3. Only three individual sightings of short-beaked common dolphin were recorded, all during a single survey in July 2021. Studies predicted a significant northwards range expansion of short-beaked common dolphin within the next few decades (Lambert et al., 2011, van Weelden et al., 2021). There are accounts that show that this species has already expanded its range northward in UK waters (van Weelden et al., 2021, MacLeod et al., 2005), however, these studies are focussed on the south-west of Britain and north-west Scotland. Given the short-beaked common dolphin preference for tropical and warm temperate seas, it is still regarded as an occasional visitor on the east coast of Scotland and within the northern North Sea.
  4. There were no records of bottlenose dolphins across the Array marine mammal study area over 24 months of DAS. Considering that the site boundary is located approximately 80 km south-east of Aberdeen, it lies outside of the main distributional range of the coastal east Scotland bottlenose dolphin population (refer to paragraph 121) and therefore lack of sightings offshore was anticipated.
  5. Grey seal were the most recorded pinnipeds during monthly site-specific DAS, however only 18 sightings were recorded. There were only two records of harbour seal over 24 months of DAS. There is no evidence of hotspots or regular foraging areas for grey and harbour seal in the vicinity of the site boundary and therefore low numbers of both species within the Array marine mammal study area were to be expected.
  6. More details about species accounts within the Array marine mammal study area is provided in section 4.2.

4.2. DAS Results

  1. This section provides an overview of the DAS campaign for the Array marine mammal study area. The DAS campaign commenced in March 2021 and survey flights were undertaken monthly, with a total of 24 months of data collected up until February 2023.

4.2.1. Marine Mammal Counts

  1. A summary of marine mammal counts observed in each calendar month during the DAS campaign (i.e. number of marine mammals identified in DAS imagery, uncorrected for effort and availability bias) is presented in Table 4.2   Open ▸ Based on raw count data, harbour porpoise accounted for the highest number of sightings identified to species level across the Array marine mammal study area and was recorded in all but three survey months ( Table 4.2   Open ▸ ). White-beaked dolphin accounted for the second highest number of sightings and was recorded in seven months over the 24-month survey period, followed by grey seal with 18 individuals recorded over nine months. For both minke whale and harbour seal, the number and frequency of sightings, was very low ( Table 4.2   Open ▸ ). No bottlenose dolphin were observed during any of the surveys over 24 months.
  2. There were also a number of cetacean sightings that could not be assigned to species level, although the numbers of these sightings were also low. Similarly, sightings classified as ‘seal species’ (due to the difficulty of identifying to species level from aerial survey data) occurred in 16 months.

 

Table 4.2:
Monthly Raw Sightings Data (Number of Animals, Uncorrected for Effort) Across the Array Marine Mammal Study Area

Table 4.2: Monthly Raw Sightings Data (Number of Animals, Uncorrected for Effort) Across the Array Marine Mammal Study Area

1The May 2021 survey was replaced by a supplementary flight on 09 June 2021.

2 The survey for February 2022 was conducted on 06 March 2022.

 

4.2.2. Density Estimates

  1. For those species where there were sightings in a sufficient number of surveys to allow for temporal trends in observations to be estimated (namely harbour porpoise, white-beaked dolphin, and grey seal), relative densities were calculated from the DAS count data ( Table 4.3   Open ▸ ). To provide estimates of relative density and associated variance, the data were analysed using a non-parametric bootstrap approach, with replacement (Buckland et al., 2001). Bootstrapping allows more robust estimates (e.g. mean or confidence limits (CL)) to be made from a data set when sample sizes are smaller than the optimum. This is achieved by repeatedly analysing random subsets of the data, and when bootstrapping is undertaken ‘with replacement’ this simply means that each data point may be included more than once when subsets of the data are generated. Non-parametric bootstrapping makes no assumptions about the data, in contrast to parametric bootstrapping which assumes that data follow a specific distribution. Density estimates with bootstrapping were undertaken for harbour porpoise, white-beaked dolphin, and grey seal on a monthly and seasonal basis. Other species observed in the DAS campaign did not occur in sufficient numbers to allow monthly or seasonal density estimates to be produced.
  2. Research into temporal patterns of harbour porpoise density has identified two broad divisions in distribution, termed by Heinänen and Skov (2015) as ‘Summer’ (April to September) and ‘Winter’ (October to March). 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). Pooling data into two bio-seasons allows the robustness of analyses to be improved where sample sizes in seasonal or monthly divisions may be small, while retaining greater resolution than pooling data by year or across the whole DAS campaign. As well as monthly and seasonal estimates, spatial density estimates for harbour porpoise and grey seal were also produced on a bio-seasonal basis.
  3. It was not possible to produce model-based density estimates for other marine mammal species within the Array marine mammal study area due to low numbers of sightings. Model-based analysis was undertaken using the MRSea package (Scott-Hayward et al., 2013) in R statistical software (R Core Team, 2023) with environmental covariates used to predict species distributions.
  4. Estimates of relative density and abundance were subsequently corrected for availability bias to provide an approximation of absolute density and abundance. Correction factors were derived from studies on dive behaviour of marine mammals and their availability at the surface (further detail of correction factors is provided in section 3.5 of volume 3, appendix 10.2, annex A).

Table 4.3:
Estimated Densities Based on the DAS Data (March 2021 to February 2023). For Harbour Porpoise, Model-based Estimates are Shown in Parentheses

Table 4.3: Estimated Densities Based on the DAS Data (March 2021 to February 2023). For Harbour Porpoise, Model-based Estimates are Shown in Parentheses

1 Bio-seasons for harbour porpoise are defined as winter (October to March) and summer (April to September).

2 Meteorological seasons were applied in modelling for white-beaked dolphin as winter (December to February) and summer (June to August).

3 Bio-seasons for grey seal are defined as breeding (August to December) and non-breeding (January to July).