Production of Seabird and Marine Mammal Distribution Models for the East of Scotland

4. Introduction

4.1 Context

Information on the abundance and distribution of seabirds and cetaceans at sea is required to assist in the management of internationally protected populations, for developing sectoral marine plans and their associated Strategic Environmental Assessments, and assessing sensitivity to human pressures including, for seabirds, oil spills via the Oil Sensitivity Index.

During the 1980s and 1990s, in response to concerns over possible impacts from developments by the oil and gas industry within the North Sea, European Seabirds at Sea (ESAS) surveys were undertaken by the UK Nature Conservancy Council (later becoming the Joint Nature Conservation Committee, JNCC) and other bodies (e.g. Ornis Consult from Denmark). Analyses of those data were published in a series of reports (Blake et al., 1984; Tasker et al. 1987, Carter et al. 1993, Stone et al. 1995, Pollock et al. 2000). Since then, there has been a review of at-sea surveys within the UK Exclusive Economic Zone to identify possible marine Special Protection Areas (SPAs) (Kober et al. 2010), and a wider collation of surveys (vessel based, aerial visual and aerial digital) analysed for both seabirds and cetaceans as part of the 5-year NERC-Defra funded Marine Ecosystems Research Programme (MERP) (Waggitt et al. 2020, Donovan et al. 2020). Three large-scale surveys of cetaceans (SCANS, SCANS-II, and SCANS-III) have been undertaken in the North Sea in 1994, 2005, and 2016, during the month of July (Hammond et al. 2002, 2013, 2021). In addition to the distribution maps produced by Waggitt et al. (2020), several reviews of the status and distribution of cetaceans in UK waters have been undertaken (Evans 1980, Evans et al. 1986, Evans 1992, Evans & Wang 2003, Evans et al. 2003, Reid et al. 2003, Paxton et al. 2016) as well as focused upon Scottish waters (Hague et al. 2020).

During the 1980s and 1990s, most surveys were vessel-based but over the last twenty years, aerial surveys have increasingly been used, with several now employing digital photography in place of visual recording (Thaxter & Burton 2009, Buckland et al. 2012). Aerial surveys have the advantage of providing more accurate representations of the perpendicular distance of detections to the track-line, and are less likely to be susceptible to responsive movement. Digital photography either by video or still images provides the opportunity for a permanent record of the strip covered by the survey. Although there have been few direct comparisons between aerial visual and aerial digital surveys, the latter appear to be better at estimating abundance for birds that form aggregations, for example common scoter, resulting in higher abundance estimates (Buckland et al. 2012, Zydelis et al. 2019).

On the other hand, it is more difficult to digitally detect and identify some taxa to species, and notable examples include auks such as razorbill and guillemot, and various gull species (Zydelis et al. 2019, Waggitt et al. 2020). The height at which the plane flies is another consideration. Visual aerial surveys tend to be undertaken at lower altitudes (e.g. 76m) than digital aerial surveys (e.g. 460+m) in order to better enable species identification (Zydelis et al. 2019). Flying at greater heights has the advantage of improved safety, less chance of disturbance of sensitive species, and a broader survey strip allowing larger sample sizes of detections, but species identification becomes more challenging. Improvements in camera technology have made this less critical but it remains an issue compared with vessel surveys where there is often much closer proximity to species. A comparison between digital aerial and vessel surveys found that the former could identify only 23% of birds to species level compared with 95% for the latter (Johnston et al. 2017). Vessel surveys also operate at much slower speeds than planes so there is more opportunity to detect animals at the surface. All forms of survey, however, require corrections to account for availability bias, particularly for cetaceans that spend a high proportion of their lives underwater, out of sight of observer or camera. Although less significant an issue, the same applies to diving seabirds such as auks, cormorants and shags, divers, grebes and sea ducks. The fact that digital aerial surveys can detect marine mammals just below the surface complicates corrections for availability bias and will be dependent upon the turbidity at the time.

Twenty-four seabird species regularly breed in Scottish waters, of which eleven were recorded in sufficient numbers within the study area to be considered for modelling: northern fulmar (Fulmarus glacialis), northern gannet (Morus bassanus), great skua (Stercorarius skua), common gull (Larus canus), lesser black-backed gull (Larus fuscus), herring gull (Larus argentatus), great black-backed gull (Larus marinus), black-legged kittiwake (Rissa tridactyla), common guillemot (Uria aalge), razorbill (Alca torda), and Atlantic puffin (Fratercula arctica). Other species of seabirds occurring in the region that were recorded but too few times during the surveys for their density distributions to be modelled, are listed in Table 1.

All 25 species of cetaceans recorded in Scottish territorial waters are classed as European protected species and are protected under the Conservation (Natural Habitats, &c.) Regulations 1994 (as amended) and the Offshore Marine Conservation (Natural Habitats, &c.) Regulations 2017. Knowledge of the spatial and temporal distribution and abundance of marine species is important in order for the Government to make evidence-based decisions regarding their status. There are four cetacean species of particular interest in this study due to their regular occurrence: minke whale (Balaenoptera acutorostrata), common dolphin (Delphinus delphis), white beaked dolphin (Lagenorhynchus albirostris) and harbour porpoise (Phocoena phocoena). Other species such as killer whale (Orcinus orca), long-finned pilot whale (Globicephala melas), Atlantic white-sided dolphin (Lagenorhynchus acutus), Risso’s dolphin (Grampus griseus), and humpback whale (Megaptera novaeangliae) do occur in the region on a regular basis but were identified too few times during the digital aerial surveys for their density distributions to be modelled. The number of individuals recorded within each species or species group per survey is given in Table 1.

4.2 Aims of the project

The aims of this project were 1) to produce seabird and marine mammal distribution models from which density surface and abundance estimates could be obtained, for offshore waters east of Scotland. These would be derived from data collected from strategic digital aerial surveys undertaken in 2020-21 in the northern North Sea beyond 12nm from the coast; 2) to summarise the modelling methods applied and the resulting distributions, species by species (for those species with sufficient sample sizes for modelling), and then relate these to previous studies of at-sea distributions; and 3) to identify any limitations in the methodology and make recommendations for how these could be addressed in the future.

Table 1. Species recorded during the eight digital aerial surveys, Feb 2020 – Mar 2021.