The use of citizen science to advance scientific understanding and enhance public participation is increasingly recognised (Evans, 1976, 1980; Evans et al., 2003; Silvertown, 2009; Merenlender et al., 2016; Gutiérrez-Muñoz et al., 2021) and valued by governments, policy makers and the scientific community (Hecker et al., 2018). Local volunteers and wildlife enthusiasts can contribute valuable data to improve the understanding of spatial and temporal patterns in local cetacean populations. This is particularly true where sightings can be made close to the shore by well-trained individuals using suitable, standardised equipment (Scullion et al., 2021; Beck et al., 2014). Increasingly, high quality professional digital cameras are widely available to the general public and citizen scientists, facilitating the collection of and contribution to photo-identification catalogues relevant to research projects. Social media platforms are also expanding in their popularity and the number of people posting their photographs of cetaceans (and other wildlife) to various marine-focused interest groups is growing.

Field research typically focuses on common or frequently seen species, such as harbour porpoise (Phocoena phocoena) or bottlenose dolphins (Tursiops truncatus). There are far fewer studies on less frequently observed species, such as Risso’s dolphin (Grampus griseus). However, Risso’s dolphins are highly distinctive, displaying a tall, well-marked dorsal fin and individual differences that make them good subjects for mark recapture studies. Natural markings include distinctive nicks and notches in the dorsal fin, body scarring from intra-specific interactions, and injuries from fishing gears and vessels which do not re-pigment as readily as in other delphinids (Hartmann, 2018). Photo-identification is therefore appropriate for use as a research tool for Risso’s dolphins (Atkinson et al., 1997; Hartmann et al., 2008; de Boer et al., 2013; Maglietta et al., 2018; Weir et al., 2019) to better understand the ecology of the species and identify potential threats (Hartmann et al., 2008; Urian et al., 2015; Weir et al., 2019).

Risso’s dolphins (Grampus griseus) typically inhabit deep, offshore waters (from 200 to 1,000 metres in depth) concentrated along the upper continental slope or around steep shelf-edge areas (Hartmann, 2018). However, research has shown that Risso’s dolphins can sometimes occur in shallower waters, namely those less than 200m in depth, around the northeast Atlantic shelf (Jefferson et al., 2013; Bloch et al., 2012) and mainly over slopes of 50 to 100 metres along the European continental shelf, although in several areas of the British Isles they regularly inhabit coastal waters of between 20 and 30 metres depth (Evans et al., 2003). Around the west coast of Scotland, the Northern Isles of Orkney and Shetland, and the northern Scottish mainland, Risso’s dolphins have frequently been encountered in shallow waters less than 20 metres deep and in close proximity to the shore (Robinson et al., 2007; Evans et al., 2011; Weir et al., 2019; Orkney Marine Mammal Research Initiative (OMMRI), 2021; OMMRI, unpublished data ¹ ; OWIARC, unpublished data ² ; SWF, unpublished data ³ ; WDC Shorewatch, unpublished data ⁴ ).

Large-scale cetacean abundance surveys were carried out in European Atlantic waters during the summers of 1994, 2005, 2016 and 2022 (SCANS I, II, III & IV; Hammond et al., 2021; Gilles et al., 2023). There were insufficient Risso’s dolphin sightings in 1994 and 2005 to allow for abundance estimates. In 2016 however, the survey estimated an abundance of 13,584 individuals in European shelf waters and 8,687 in UK waters (Hammond et al., 2021; IAMMWG, 2021) with the 2022 survey generating an abundance estimate of 13,854 animals for European shelf waters but no estimate for UK waters (Gilles et al., 2023). Whilst the species is considered widely distributed within Scottish waters, there is currently a lack of data to determine sub-populations and no separate regional abundance estimates have been made. Subsequently, in UK waters, all Risso’s dolphins are encompassed within a single Management Unit (MU) (IAMMWG, 2023).

Although there is limited understanding of the population structure of Risso’s dolphins in the northeast Atlantic, previous research indicates genetic differentiation between UK (N=18; of which 16 were Scottish samples) and Mediterranean populations (Gaspari et al., 2007). In addition, whilst Risso’s dolphins are believed to be fragmented in their distribution within Scotland, there is some evidence for site fidelity by known individuals (Weir et al., 2019).

To date, detailed research into the presence and distribution of the species in Scotland has focused primarily on the west coast, specifically around the Outer Hebrides (Atkinson et al., 1999; Hodgins et al., 2014; Weir et al., 2019) and subsequently, documented sightings of Risso’s dolphins are most common in northwest Scotland (Evans et al., 2003; WDC Shorewatch, unpublished data⁴) and the Outer Hebrides, specifically around the Isle of Lewis (Paxton et al., 2014; Weir et al., 2019) with the majority of sightings recorded during the summer and autumn months (Evans et al., 2003; Paxton et al., 2014; Hall et al., 2019; Weir et al., 2019). Seasonal dedicated boat-based surveys for Risso’s dolphins have been undertaken off northeast Lewis since 2010 (Hodgins et al., 2014; Weir et al., 2019) and the area has hosted a dedicated WDC Shorewatch site since 2011, facilitating the land-based collection of data year-round. Through long-term data collection, Weir et al. (2019) have demonstrated the year-round occurrence of Risso’s dolphins off Tiumpan Head in northeast Lewis. Risso’s dolphins have been recorded in the North Sea (central & northern) for many decades both from strandings (Natural History Museum, 2018) and sightings (Evans, 1976, 1980; Evans et al., 2003; Paxton et al., 2014) but sightings nevertheless are generally uncommon except around Caithness, Orkney (Orkney Marine Mammal Research Initiative (OMMRI), 2021; OMMRI, unpublished data¹; OWIARC, unpublished data²; SWF, unpublished data³; WDC Shorewatch, unpublished data⁴) and Shetland (Hall et al., 2019; SWF, unpublished data²). In recent years, more frequent records from further south (central North Sea) have been recorded (Evans and Waggitt, 2020a). This increase in the number of sightings recorded may reflect a change in the way that dolphins are using some coastal areas but also may result from an increase in observer presence, effort and species identification confidence.

The northeast coastline of Scotland comprises the coasts of Sutherland, Caithness, Morayshire and north Aberdeenshire. Orkney is an archipelago of 68 islands and smaller skerries in the Northern Isles of Scotland, situated approximately 6.2 miles off the northeast tip of mainland Scotland (Porter et al., 2020). The bathymetry of these waters’ ranges from depths of 10 to 50 m close inshore, with increasing depths (of up to 90 m) within the peripheral offshore areas. It comprises the deepwater channel of the Pentland Firth, Scapa Flow, the Atlantic Ocean and North Sea around the Orkney North Isles and Sula Skerry (Orkney Islands Council, 2020).

Shetland is a subarctic archipelago in the North Atlantic. The southern tip of mainland Shetland lies approximately 160 km from the northeast coast of mainland Scotland and approximately 135 km from mainland Orkney. As with Orkney, the islands separate the North Sea to the east, from the Atlantic Ocean to the west. Around Shetland, the bottom topography shelves steeply close inshore, with deep shelf edges of 150 to 250 m depth. Fair Isle is the southernmost island and lies 38 km south of mainland Shetland and 44 km from North Ronaldsay, Orkney’s most northerly isle ( Figure 1 ).

By undertaking two distinct analyses – (1) historical and current sightings data of Risso’s dolphins were gathered from northeast Scotland and the Northern Isles of Orkney and Shetland to examine the presence of this species in the region; and (2) all suitable photographs of Risso’s dolphin sighted in the region were collated to help create two regionally specific photo-ID catalogues – this paper seeks to demonstrate the power of collaborative citizen science to contribute to our understanding of Risso’s dolphin in Scottish waters. In recognising the limitations of simply analysing reported sightings and ad hoc photographs taken by citizen scientists, targeted ongoing research is recommended to enable data gaps to be filled leading to our further understanding of the presence and habitat use of individual Risso’s dolphins in Scottish waters.

Historical sightings data indicates the long-term presence of Risso’s dolphins in coastal waters throughout the northeast of Scotland, Orkney and Shetland, with records analysed in this study dating back to 1933 (OWIARC, unpublished data²), and descriptions from Orkney and Shetland of the hunting of Risso’s dolphins in the 1880s (Evans and Scanlan, 1988). Although there was a gap between the earliest reported sightings in the 1930s and more regular sightings reported from 1970 onwards, this is most likely a reflection of the lack of targeted effort to capture these data rather than an absence of the species.

Risso’s dolphins were found to be present in the region year-round, with a higher number of reported sightings in August, consistent with similar previous studies in the area (Evans et al., 2010), and with annual and seasonal trends seen in other regions such as the Outer Hebrides (Weir et al., 2019), Moray Firth (WDC Shorewatch, unpublished data⁴) and East Grampian coastline (Evans et al., 2003; Genesis Oil and Gas Consultants, 2011; Anderwald and Evans, 2020; Gutiérrez-Muñoz et al., 2021). Although with considerable spatiotemporal variation, initial photo-ID and reported sightings data suggests that Risso’s dolphins are consistently recorded inshore at specific locations within the study area. These projections will be somewhat biased as this study has utilised reported sightings rather than effort corrected monitoring and data collection has been impacted by observer behaviour, weather, and seasonal limitations. However, this observation does mirror spatial and temporal patterns seen in Risso’s dolphins in other regions, which could be a response to increased prey abundance and changing sea temperature (van der Kooij et al., 2016).

The results of this study suggest the following areas as potentially important habitat for Risso’s dolphins: Noss Sound, Sumburgh Head, Fetlar and Bluemull Sound (Shetland), the Churchill Barriers and Hoxa Head (Orkney), Lybster, Latheronwheel (Caithness), and Strathy Point (Sutherland). The majority of sightings data used in this analysis were collected opportunistically so sighting “hotspots” may be influenced by the locations of towns, popular lookout points, or regional data initiatives. Due to utilising only reported sightings, this study confirms Risso’s presence in the areas where reports have been submitted and does not indicate presence or absence in areas where there are less or no sightings reports. Nonetheless, in spite of the lack of effort data and the gaps in coverage, Risso’s dolphins have been regularly recorded throughout the study area. Individual Risso’s dolphins were found to demonstrate some level of site fidelity, with a third of catalogued Risso’s dolphins resighted over multiple years. A higher proportion were resighted within the northeast Scotland and Orkney catalogue than within the Shetland region. However, this may be due to the greater number of images received from northeast Scotland and Orkney, the variability of observer behaviour, and access to coastal vantage points within the two regions.

Across both regions, several individuals were resighted together multiple times between 2010 and 2021. This is consistent with Risso’s dolphin photo-ID studies in northeast Lewis (Weir et al., 2019; WDC Shorewatch, unpublished data⁴) and elsewhere in the world (Hartmann et al., 2008), where individuals are shown to return to the same habitat over multiple years and are often associating with the same individuals. Further research is required to assess habitat use at a finer scale and to increase our current knowledge of the movements and ecology of the species. However, given the apparent preference for some areas by Risso’s dolphins over multiple years, these may be considered important habitat and may be appropriate candidates for further research to inform future site protection. Given the documented long term and year-round presence of Risso’s dolphins in these waters, it is recommended that future developments in the northeast of Scotland, Orkney and Shetland, particularly by both the noise-producing (military, fossil fuel and renewable energy) sectors and wave and tidal power developments, consider robust mitigation for the species, employing the precautionary principle at all times.

In addition to finding that Risso’s dolphins show some level of site-fidelity, photo-ID data analysis showed that several individual dolphins also travelled long distances of over 100km, and occasionally over 200km. These individuals were not sufficiently recaptured to be able to conclude if this is a one-way or returning movement. Such long-distance movements should not preclude the potential for multiple Management Units (MUs) for Risso’s dolphins as exist for other species and similar delineation of protected areas and site-specific MUs for Risso’s dolphin are suggested to ensure management is at an appropriate scale. In Scotland, Risso’s dolphins are afforded some protection through the MPA designation in northeast Lewis and out with the MPA, through European Protected Species legislation. This study has the potential to help focus attention on important habitats for Risso’s dolphin in northeast Scotland and highlight the requirement for more detailed surveys to provide further evidence of habitat-use and determine appropriate additional spatial protection measures.

Data collected by citizen scientists have proven to be invaluable to marine conservation in providing supplementary evidence for policymakers. For example, where there had previously been a winter data gap, the WDC Shorewatch programme provided evidence of the year-round presence of Risso’s dolphins in the Outer Hebrides which was used to inform the 2020 designation of the North-east Lewis Marine Protected Area (Weir et al., 2019).

The collection of historical sightings originated from several different citizen science sources (Sea Watch Foundation, Orkney Wildlife Information & Records Centre (OWIARC), and members of the public). While accepting variation in the robustness of data verification between organisations, we still expect some error and/or bias due to observer variability, knowledge on cetacean identification and motivation for reporting sightings. A higher number of records over the summer months is likely to be a result of greater observer effort, better weather conditions and longer daylight hours. Additionally, at-sea dedicated research is predominantly undertaken in the summer months resulting in a gap in data collection at other times of the year. Analysis of the collated data, however, indicates a clear trend of Risso’s presence over time and throughout the year in the coastal waters around northeast Scotland, Orkney and Shetland. In 2020 and 2021, harbour porpoise and Risso’s dolphins were the two most commonly sighted cetacean species in Orkney waters (Orkney Marine Mammal Research Initiative (OMMRI), 2021; OMMRI, unpublished data¹; WDC Shorewatch, unpublished data⁴). Citizen scientists may be able to fill gaps over the winter as they can take advantage of weather windows and are less restricted compared to other traditional effort-based research methods. Supporting citizen scientists to maintain effort over the winter, through initiatives such as WDC Shorewatch’s Winter Watch Challenge each year promotes effort during the winter months.

The analysis further indicated an increase in reported sightings of this species in both regions from 2010. This could be because of increased Risso’s dolphin presence in the region, a result of increased effort and reporting through citizen science and social media networks, or, most likely, a combination of both. Between 2020-2021 alone, OMMRI launched in Orkney and saw sightings reports rise by at least three-fold in two years (OMMRI, unpublished data¹), WDC Shorewatch expanded into the Northern Isles, training 100 new volunteers in cetacean identification, and the “COVID-19 effect” influenced an increase in the overall number of citizen science observers. However, there is also previous evidence suggesting an upward trend in the occurrence of Risso’s dolphin around Scotland from effort-corrected monitoring (Gutiérrez-Muñoz et al., 2021). Changes in prey distribution due to climate change could also play a part (Hall et al., 2019; Evans and Waggitt, 2020b) and is worthy of further consideration, given the implications for future spatial protection. For example, the distribution and biomass of squid, a favoured prey species of Risso’s dolphins, have increased across the North Sea over a 35-year period as associated changes in sea temperature (van der Kooij et al., 2016).

Through citizen science, it is possible to inspire conservation-minded behaviour and encourage stewardship of the marine environment and species within (Merenlender et al., 2016). This is particularly relevant when the focus is a species or population that can be studied from land, as the opportunity to take photographs does not rely on access to a boat and is therefore open to more potential contributors. Collaborating with wildlife photographers and the public can result in the contribution of quality images. Nevertheless, one constraint with this approach can be the consistency and quality of data received (Newman et al., 2003; Embling et al., 2015; Gutiérrez-Muñoz et al., 2021). However, keeping volunteers engaged and providing guidance as to what is required for a valid photo-ID image can help to improve both of the above. An additional constraint to using photographs of Risso’s dolphins which have been taken “opportunistically” or “off-effort” is that this information provides a “snapshot” of their lives and habitat use. Nonetheless, as this study has shown, even opportunistic photo-ID data can be further analysed to improve understanding of social structure, behaviour and importantly, habitat use, which could support the identification of areas of critical importance for Risso’s dolphins.

This research demonstrates that Risso’s dolphins are regularly and repeatedly utilising coastal and inshore waters around northeast mainland Scotland and the Northern Isles. Further, regular sightings of mothers and their dependent calves indicate that areas of sheltered, shallow coastal waters (<200 m) are being used as nursery grounds. Understanding the habitat use of Risso’s dolphins on a broad spatial scale is expensive, time-consuming, and labour intensive. Although not a substitute for systematic, effort-based surveys and accepting the use of citizen science data has clear limitations, increased uptake and facilitation of citizen science, productive collaboration between organisations, and growing community awareness of Risso’s dolphins, have the potential to fill data gaps and contribute to understanding the presence of this species around the coast in northeast Scotland, Orkney and Shetland. The methods demonstrated in this study can be replicated for other areas (a similar study is currently underway on Risso’s in south-west England) and/or other cetacean species observed in coastal waters.

Ongoing consistent and effort related data collection, alongside the development and expansion of photo-ID catalogues will be vital in filling the gaps in understanding of Risso’s dolphin distribution and behaviour within Scottish waters, with the potential to support efforts to inform important processes such as defining population management units and future MPA designations to support the conservation of Risso’s dolphins and other species.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Ethical approval was not required for the study involving animals in accordance with the local legislation and institutional requirements because the study used sighting and photographic data only (predominantly taken from land) with no close contact between humans and animals.

NH: Conceptualization, Methodology, Validation, Visualization, Writing – original draft. ES: Formal analysis, Investigation, Validation, Writing – original draft. KD: Formal analysis, Investigation, Validation, Writing – original draft. AW: Data curation, Formal analysis, Writing – review & editing. SD: Writing – review & editing. KH: Data curation, Writing – review & editing. EN-W: Data curation, Writing – review & editing. PE: Data curation, Writing – review & editing. CB: Writing – review & editing. KR: Data curation, Writing – review & editing. EM: Writing – review & editing. RF: Writing – review & editing. HH: Writing – review & editing. AK: Writing – review & editing. KM: Writing – review & editing.