Digital aerial seabird and cetacean surveys off the east coast of Scotland

4. Discussion

4.1. Comparison of results to previous European Seabirds at Sea (ESAS) studies and other relevant literature

European Seabirds at Sea (ESAS) data have been used to inform the findings of the Marine Scotland (2020–2021) survey results presented in this report.

The ESAS database was established in 1991 as a collaboration between individuals and institutes who collected data on the distribution of seabirds and marine mammals in north-west European waters. The database, collated by the JNCC, contains over two million records collected over 25 years (Halpin et al. 2009, Dunn 2012). The data can be explored on an interactive map. The data were collected from boat-based surveys using standardised protocols. The data have been used to designate Special Protection Areas (SPAs) for birds and Special Areas of Conservation (SACs) for marine mammals.

The ESAS data was used to create "An atlas of seabird distribution in north-west European waters" (herein referred to as the Atlas) in the 1990s (Stone et al. 1995).

Here, we compare the bird species observations from this study with the densities (bird per km²) and abundances (bird per km) outlined in the Atlas for the relevant areas. Total numbers cannot be compared but patterns in observations have been compared, for example peak counts or densities in particular seasons. The area surveyed by this study covered sections of the areas defined in the atlas as Northern Isles, Western North Sea and Central and Northern North Sea.

Wader species

Wader species were not included in the Atlas but were observed during the summer surveys in 2020 and the winter 2020/2021. The wader species identified in the summer surveys included curlew (Numenius arquata) and redshank (Tringa tetanus) and, in the winter, knot (Calidris canutus) were identified.

Kittiwake

Kittiwake (Rissa tridactyla) were observed across the year in this study with high counts throughout the year. Peak counts of kittiwake were recorded in autumn with high counts in the summer and lower counts in the winter. This corresponds to the observations in the Atlas, higher numbers in summer and peak numbers in autumn.

Gulls and terns

The patterns of observations of gulls were the same for this study and the Atlas. Great black-backed gull (Larus marinus) had peak counts and densities in autumn with lows in the summer which corresponded between the studies. Lesser black-backed gull (Larus fuscus) had low numbers across the year in both studies. Herring gull (Larus argentatus) had high numbers in summer, autumn and winter, which was similar in the Atlas, although there were low densities recorded in summer.

The low numbers of common gulls corresponded between this study and the Atlas, although the Atlas had peak densities in winter and in this study the peak was in the autumn surveys.

In the Atlas, low numbers of Iceland and glaucous gulls were recorded, in this study only one Iceland gull (Larus glaucoides) and two glaucous gulls (Larus hyperboreus) was recorded in the winter of 2020/2021.

In this study, terns were only observed in the summer and autumn and this corresponds to the peak numbers in summer in the Atlas. Arctic (Sterna paradisaea) and common (Sterna hirundo) terns were positively identified in this study along with 'commic' terns (this group includes both common and Artic terns). Only 'commic' terns were identified in the Atlas and these had peak counts in summer with none observed between October to March.

Skuas

Great skua (Stercorarius skua) were observed in very low numbers in the spring, summer and autumn surveys in this study, and Arctic skua (Stercorarius parasiticus) was observed in summer, autumn and winter in very low numbers. This corresponds with reports in the Atlas, with no sightings between November and March and only low densities April to October.

Auks

In this study, peak counts of guillemots (Uria aalge) were observed in the autumn, with high counts in the summer and lower counts in the winter and spring surveys. Razorbill (Alca torda) had peak counts in the summer and autumn. This corresponds to the high counts for these species in the summer and autumn in the Atlas. This study and the Atlas included a guillemot/razorbill group, due to the difficultly in distinguishing between these species. In this study the observations of guillemot/razorbill were high across the seasons with peak counts in autumn, in the Atlas peak counts were seen in the summer and autumn. Differentiation between the two species is difficult between the survey techniques and this results in the variation in patterns of observations between the two studies. If all guillemot, razorbill and guillemot/razorbill observations are taken together, there are high numbers of individuals observed in the area across the year.

Black guillemot (Cepphus grylle) were observed in low numbers across the seasons in this study. This corresponds to the Atlas, where the species were observed throughout the year, although abundance was low, with peak abundance (0.06) in the autumn.

The pattern of puffin (Fratercula arctica) observations were similar between the two studies with observations throughout the year and peak counts/densities in the summer.

Divers

In the Atlas, the Moray Firth was described as locally important in winter for divers, even though divers were reported in low densities across the year. Red-throated diver (Gavia stellata) were observed in the winter in the Atlas in very low abundances (0.01) whereas this study observed them in the summer, autumn and winter surveys in very low numbers (one-six); great northern diver (Gavia immer) were observed between October to May in the Atlas in very low abundances (0.01-0.02) and were observed in the spring, summer and autumn surveys in this study (one).

Storm petrels

Storm petrels (Hydrobates pelagicus) were only observed in the summer in this study, this matches the densities reported in the Atlas.

Fulmar

Fulmar (Fulmarus glacialis) had high counts and densities throughout the year as reported in this study and the Atlas; in this study the peak numbers were counted in autumn with high counts in summer and winter surveys, the peak densities in the Atlas were May to November.

Shearwaters

Only Manx shearwaters (Puffinus puffinus) were observed in the study, in the summer with low counts and peak numbers in autumn. The numbers were low in the Atlas, with peak numbers in summer.

No other shearwater species were observed in this study, however sooty shearwater (Puffinus griseus) was reported in the Northern Isles and Western North Sea areas of the Atlas data between August and November. Also, one Mediterranean shearwater (Puffinus mauretanicus) was observed in the relevant areas in the Atlas between July and November.

Gannet

Gannet (Morus bassanus) were observed throughout the year for both the Atlas and this study. From the ESAS data the densities of gannet were similar throughout the year whereas in this study there was a high number in summer with the peak counts in the autumn.

Cormorant and Shag

In this study, cormorant (Phalacrocorax carbo) and shag (Phalacrocorax aristotelis) were observed in the summer and autumn in low numbers. This corresponds to the low densities of cormorants were reported in the Atlas, with observations between March and September, peak density of 0.03 in April. Although for shag the Atlas showed low densities all year round, with peak counts in autumn.

Cetaceans

The data from ESAS, plus other European datasets including Sea Watch and the Small Cetacean Abundance in the North Sea (SCANS) were used to produce the "Atlas of Cetacean Distribution in north-west European Waters" (Reid et al, 2003). This Atlas provides accounts of the distribution of 28 cetacean species known to have occurred in north west European waters since the 1980.

Five cetacean species were positively identified in the surveys from this study. The Ried et al (2003) Atlas provides maps of the sightings rates across the NE Atlantic and NW Europe region. Limited information on seasonal variation is provided. The text and maps were used to compare with the observations from this study.

• Common minke whale (Balaenoptera acutorostrata): observed across the seasons with peak counts in the summer. This is reflected in the Atlas where the whales are shown to have high sighting rates across the area, they are also observed throughout the year.
• Common dolphin (Delphinus delphis): observed in the winter, summer and autumn surveys, peak counts in summer. The Atlas shows they have high sightings around the coast of Scotland; but the survey effort was limited in the offshore areas outside of the Firth of Forth.
• White-beaked dolphin (Lagenorhynchus albirostris): observed in the winter and summer surveys of this study, peak counts in summer. The Atlas shows they have high sighting rates across the area for this study and are most frequently seen in the summer.
• Risso's dolphin (Grampus griseus): observed in a winter survey of this study. The Atlas shows they are uncommon in the study area and are seen most frequently in the winter.
• Harbour porpoise (Phocoena phocoena): observed across spring, summer and autumn, peak numbers were recorded in the summer surveys; in the Atlas the sighting rate is high and highest observations in the summer.

Data was not provided on the ESAS observations of grey seals or common seal which were observed in the surveys for this study. In addition, this study observed one unidentified shark species, one basking shark (Cetorhinus maximus) and one ocean sunfish (Mola mola).

4.2. Any significant changes and possible explanations of any changes

Environmental changes at sea will greatly influence the distribution and abundance of seabirds in offshore areas. The changes in seabirds seen between the ESAS data reported in "An atlas of seabird distribution in north-west European waters" (Stone et al. 1995) and the observations recorded during aerial surveys undertaken in 2019-2021 could be due to a large number of factors. A few of the factors that could have caused changes to seabird populations in the last 25 years include (JNCC, 2020):

• Changes to food discard by commercial fishing;
• Bycatch;
• Climate change;
• Protective legislation;
• Effects in wintering areas;
• Mink predation.

The cetaceans observed in the surveys for this study do not greatly differ from the observations reported in the "Atlas of Cetacean Distribution in north-west European Waters" (Reid et al. 2003). However, there are changes within the marine ecosystem that will impact on these species (Waggitt et al. 2019), such as:

• Bycatch;
• Habitat loss;
• Energy extraction;
• Noise disturbance;
• Prey reductions;
• Pollution;
• Vessel traffic.

The key differences between the ESAS bird data (Stone et al. 1995) and the observations from this study are: in the surveys for this study:

• wader species were observed;
• no little auks (Alle alle) were observed;
• no sooty shearwaters were observed; and
• fewer European shags were observed.

The differences between the ESAS bird data (Stone et al. 1995) and the surveys from this study could stem from differences in survey techniques. However, they can also suggest changes in the bird populations.

The wader observations in 2020 are unlikely to indicate an increase in the populations as the numbers were small and wader abundance in Scotland has decreased over the last 20 years (NatureScot, 2018). The observations could highlight differences in the survey technique with waders being disturbed from boat surveys whereas they are not disturbed by aerial digital survey aircraft and can be detected in aerial digital stills survey images.

Little auk and sooty shearwaters are passage migrants or winter visitors (Robinson, 2005) and would be expected to have been recorded in the winter survey (2019/2020). Although none were recorded in the 2019-2021 surveys, this may indicate the population wintering in waters around the UK is reducing. Sooty shearwaters are vulnerable to bycatch from demersal longline fisheries across their global distribution.

The reduction in European shag observations between the ESAS database and the surveys in 2020 is in line with the current population trends. The shag population in Scotland is expected to be decreasing with reductions in population in 2020 and a reduction in breeding abundance from 1992 to 2018 (JNCC, 2020). The low breeding abundance has been linked to winter "wrecks" of shags along the east coast from which numbers are slow to recover or do not recover at all.

4.3. Recommendations for improvements to the methodology and further possible research

Methodology limitations

A limitation of this study is that the data from the study are not directly comparable to the previous ESAS datasets. However, the surveys undertaken for this study (2019-2021) have benefited from being digital aerial surveys compared to visual aerial surveys or boat surveys which were previously used for the ESAS data collection.

Aerial digital surveys have been shown to provide higher numbers of bird sightings and identified species with higher spatial accuracy (Žydelis et al. 2019) than aerial visual surveys. In addition, aerial digital surveys have been shown to be more effective at detecting megafauna in the marine environment (Garcia-Garin et al. 2020). The technology on board survey aircraft means that each animal identified in the survey imagery can be accurately georeferenced and data does not need to be corrected for distance-related detection bias (Buckland et al. 2012; Coppack et al. 2017). In addition, the higher flight height for aerial digital surveys compared to visual aerial surveys has the benefit of no observed disturbance to the marine species being observed and prevents flushing for the seabirds (Thaxter & Burton, 2009; Buckland et al, 2012; Coppack et al, 2017) and this is of much greater benefit when compared to disturbance and attraction provided by boat surveys.

Although aerial digital surveys rely on good weather, the speed at which the area can be covered allows for these large-scale surveys to be undertaken. However, in winter particularly the weather windows are short lived and therefore the survey data has been collected across a number of days. Good weather is required for the safety of the aircraft as well as for optimal identification of species from the imagery.

In addition, aerial surveys are not able to reliably monitor species migration, with birds known to migrate at high altitudes and at night (MacArthur Green et al. 2015).

Recommendations for future research

APEM would recommend that the observations of species across the area are used to create new estimates of the offshore abundance and density of the observed species. This would be useful to compare to the density and abundances laid out in "An atlas of seabird distribution in north-west European waters" (Stone et al. 1995) and allow for greater interrogation of changes in species distributions or abundance. In addition, more complex analysis techniques could be used to produce density maps for seabirds and marine mammals across the surface area. This would have the advantage of being comparable to the density maps of seabirds and cetaceans across European waters created by Waggitt et al. (2020) from ESAS data compiled with other databases on seabird and cetacean distributions.

In addition, it would be beneficial to have another year of surveys to be able to understand inter-annual variation in species observations.

For future surveys in this area, closer lines would be recommended to cover more of the survey area, however, more transect lines would increase the reliance on good weather windows and would increase the risk of surveys covering days/weeks. As well as greatly increasing the cost of the surveys.