1. Introduction

  1. This Marine Mammal Technical Report presents a detailed baseline characterisation of the marine mammal ecology to inform the assessment of impact for the construction, operation and maintenance and decommissioning phases of the Array. The Ossian Array includes the offshore components of the offshore wind farm, e.g. wind turbines, Offshore Substation Platforms (OSPs) and inter-array/interconnector cabling and is hereafter referred to as the ‘Array’ (for more details about the Project Description see volume 1, chapter 3). Data were collated through a detailed desktop study of the existing resources available for marine mammals within the region, incorporating data from third party organisations as well as data from a 24-month programme of site-specific Digital Aerial Surveys (DAS) (March 2021 to February 2023). Additionally, Sea Mammal Research Unit (SMRU) provided telemetry maps and haul out counts for harbour seal Phoca vitulina and grey seal Halichoerus grypus which have also been used to inform baseline characterisation (Stevens, 2023).
  2. The aim of this Technical Report is to provide a robust baseline characterisation of the marine mammals likely to be present within the marine mammal study areas (please see section 2), against which the potential impacts resulting from the development of the Array can be assessed.

2. Study Area

  1. Marine mammals are highly mobile, wide-ranging species, with varying behaviour and ecology between species. To account for this and to provide wider geographic context, the marine mammal study area proposed for the purpose of baseline characterisation has been defined at two spatial scales ( Figure 2.1   Open ▸ ):
  • Array marine mammal study area: an area encompassing the site boundary (within which the Array will be located) plus 8 km buffer ( Figure 3.1   Open ▸ ). The Array marine mammal study area corresponds with the aerial survey area, in which 24 months of DAS were conducted (see section 3.2.1 for more details).
  • Regional marine mammal study area: an area encompassing the wider northern North Sea to account for the highly mobile nature of marine mammals. The boundaries of the northern North Sea are closely aligned with those of Marine Natural Areas (Wildlife Trusts, 2021). The regional marine mammal study area has informed the screening of internationally designated sites and has been used to identify projects included in the cumulative effects assessment.
  1. The Array marine mammal study area and the regional marine mammal study area were presented in the Array Scoping Report (Ossian OWFL, 2023). Subsequently in the Ossian Array Scoping Opinion (MD-LOT, 2023), stakeholders recognised both study areas as appropriate to take forward to the Array EIA Report.
  2. To assess population level impacts for a given species, species-specific Management Units (MUs) are presented in this Technical Report. The Special Committee on Seals (SCOS) and Inter-Agency Marine Mammal Working Group (IAMMWG) reports were used to define MUs for cetaceans and seals, respectively (SCOS, 2023, IAMMWG, 2022). Where the MU for a given species extends over a very large scale (e.g. the Celtic and Greater North Sea MU for minke whale Balaenoptera acutorostrata: Figure 5.12   Open ▸ ) the appropriate Small Cetaceans in European Atlantic waters and the North Sea survey (SCANS) IV block which overlaps with the Array marine mammal study area is provided for context.

Figure 2.1:
Marine Mammal Study Areas

Figure 2.1: Marine Mammal Study Areas


3. Methodology

3.1. Desktop Study

3.1.1. Regional Data Sources

  1. Information on marine mammals within the regional marine mammal study area was collected through a detailed desktop review of existing studies and datasets, listed in Table 3.1   Open ▸ .

 

Table 3.1:
Key Sources of Information for the Marine Mammal Baseline Characterisation

Table 3.1: Key Sources of Information for the Marine Mammal Baseline Characterisation

 

3.2. Surveys

3.2.1. Site-Specific Surveys

  1. A summary of the site-specific surveys undertaken to inform the marine mammal assessment for the Array are outlined in Table 3.2   Open ▸ and presented in more detail in volume 3, appendix 10.2, annex A.
  2. The DAS campaign for the Array commenced in March 2021 and continued monthly up to and including February 2023 to allow 24 months of data collection, including any additional surveys to account for delayed survey flights (see paragraph 22 for more details). The study area for the DAS campaign was delineated as the site boundary plus an 8 km buffer. The site boundary plus 8 km buffer is hereafter referred to as the ‘Array marine mammal study area’. The extent of the DAS area was presented in the Scoping Report (Ossian OWFL, 2023) and subsequently received approval from NatureScot in the Ossian Array Scoping Opinion (MD-LOT, 2023).
  3. The surveys were conducted by HiDef Aerial Surveying Limited (hereafter ‘HiDef’). A total of 31 transects were spaced 2.5 km apart across the Array marine mammal study area, aligned in a broadly north-east to south-west orientation, perpendicular to the depth contours along the coast ( Figure 3.2   Open ▸ ). The total survey effort was approximately 5,439.85 km2, with a monthly mean of approximately 226.66 km2.

 

Table 3.2:
Summary of Surveys Undertaken Across Array Marine Mammal Study Area

Table 3.2: Summary of Surveys Undertaken Across Array Marine Mammal Study Area

 

  1. Marine mammal sightings from the DAS were analysed in line with the methodology consulted with the stakeholders via the Array Marine Mammal Methodology Note (volume 3, appendix 5.1, annex B) as well as Array Marine Mammal Consultation Note 1 (volume 3, appendix 5.1, annex D). The DAS results and data analysis methodology is presented in more detail in volume 3, appendix 10.2, annex A.

Figure 3.1:
Digital Aerial Survey Transects Flown Over the Array Marine Mammal Study Area

Figure 3.1: Digital Aerial Survey Transects Flown Over the Array Marine Mammal Study Area


3.2.2. Historic Surveys of the Firth of Forth and Tay

  1. Historically, surveys have been carried out within the wider Firth of Forth and Tay area in inshore and offshore waters that do not overlap with the site boundary ( Figure 3.2   Open ▸ ). As some species are known to have distinct inshore and offshore populations (for example bottlenose dolphins, see paragraph 112), no conclusions on marine mammal distribution within the Array marine mammal study area can be made based on this historic survey data as the Array is located at considerable distance from the shore (80 km south-east of Aberdeen). However, given that marine mammals are wide ranging species, findings of historic surveys are considered in section 4.1.1 and presented for each species in section 5 to provide context for their abundance and distribution within the regional marine mammal study area.

                        TCE aerial surveys

  1. A series of visual aerial surveys of offshore wind farm sites was commissioned between 2009
    and 2010 by TCE (hereinafter referred to as “TCE aerial surveys”). Surveys were carried out during 24 days between May and August 2009 (summer: nine survey days) and November 2009 and March 2010 (winter: 15 survey days) (Grellier and Lacey, 2011). The transect design of these visual aerial surveys of Scottish territorial waters was based on parallel flight tracks, equally spaced at approximately 5 km, in both inshore (up to 12 nm) and offshore (greater than 12 nm) areas (Grellier and Lacey, 2011, MacLeod and Sparling, 2011). Between five and 48 transects were flown in any one survey day and observed track length varied from 341 km to 1,116 km ( Figure 3.2   Open ▸ ).

                        Seagreen boat-based surveys

  1. Visual boat-based surveys for marine mammals and seabirds were commissioned by Seagreen Wind Energy Limited and were carried out by ECON Energy between May 2010 and November 2011 (hereinafter referred to as “Seagreen boat-based surveys”). The surveys encompassed the Firth of Forth Round 3 Zone, which is approximately 2,850 km2 and approximately 25 km offshore of the Firth of Forth ( Figure 3.2   Open ▸ ). The monthly surveys followed transect lines distributed 3.7 km apart across four different routes (east, west, north and south), spaced at 300 m from each other (Sparling, 2012). Over the 19 surveys, a total of 17,017 km of survey effort was undertaken (Sparling, 2012).

                        Seagreen ornithology surveys

  1. Incidental recordings of marine mammals were recorded during visual, boat-based ornithology surveys undertaken for what was previously known as the Seagreen Alpha/Bravo project area (and known since 2018 as ‘Seagreen’). Surveys were conducted in summer 2017 (May to August inclusive) with sea state ranging between one and four on the Douglas sea scale, resulting in excellent to average conditions for sighting marine mammals. A summary of the marine mammal incidental sightings was reported in the Seagreen Marine Mammal Baseline Technical Report (Seagreen Wind Energy Limited, 2018).

                        Berwick Bank aerial surveys

  1. DAS of seabirds and marine mammals were commissioned by Berwick Bank Wind Farm Limited (hereinafter referred to as “Berwick Bank aerial surveys”). Surveys encompassed the Berwick Bank array area plus 16 km buffer ( Figure 3.2   Open ▸ ), covering a total area of 4,980 km2 through 37 transects spaced 2 km apart. Transects were surveyed to cover a total length of approximately 2,490 km each month, and data from two cameras (0.25 km combined width) were subsampled to provide a minimum target of 10.0% coverage of the total survey area. Surveys commenced in March 2019 and continued monthly up to and including April 2021 with an additional survey undertaken in both May 2020 and April 2021.

Figure 3.2:
Surveys Conducted in the Wider Area of Firth of Forth and Tay

Figure 3.2: Surveys Conducted in the Wider Area of Firth of Forth and Tay


Table 3.3:
Summary of Surveys Undertaken in the Wider Firth of Forth and Tay Area

Table 3.3: Summary of Surveys Undertaken in the Wider Firth of Forth and Tay Area