NatureScot Research Report 1364 - Hebridean Cetacean Research Programme, 2021-2024

Year of publication: 2024

Authors: Hartny-Mills, L., Hampson, J. and Lomax, A. (Hebridean Whale and Dolphin Trust)

Cite as: Hartny-Mills, L., Hampson, J. and Lomax, A. 2024. Hebridean Cetacean Research Programme, 2021-2024. NatureScot Research Report 1364.

Keywords

population monitoring; cetaceans; basking shark; harbour porpoise; minke whale; Risso’s dolphin; Scotland; underwater acoustics

Background

Twenty-four species of cetacean have been recorded in the waters off the west coast of Scotland. Of these, fifteen species are recorded annually, some are present year-round, and others migrate into the area. Long-term monitoring is essential to understand the distribution and abundance of highly mobile and long-lived animals, how they utilise their habitats and to detect or monitor any threats.

The Hebridean Whale and Dolphin Trust (HWDT) Cetacean Research Programme has been collecting data on marine megafauna (primarily cetaceans but also species of basking shark and seals) in the Hebrides from the research vessel, Silurian, since 2002. HWDT’s research programme provides a unique, long-term visual, acoustic, and photo-identification dataset from which assessments of cetacean, seal, and basking shark (Cetorhinus maximus) presence, distribution, habitat preference, site usage and behaviour can be made. Additionally, data are recorded on pressures that might impact these animals, including anthropogenic noise, fisheries, and marine litter.

HWDT’s Cetacean Research Programme has followed the same visual and acoustic line transect survey protocol for more than 20 years. Visual surveys are carried out by trained volunteers on a watch rota. Acoustic surveys utilise a hydrophone array towed 100 m behind the vessel. The hydrophone is calibrated and able to detect high frequency harbour porpoise (Phocoena phocoena) echolocation clicks and sounds at lower frequencies. HWDT have surveyed more than 100,000 km of Hebridean Seas (equivalent to 2.5 times around the world), trained 1,000 citizen scientists in on-board data collection protocols and provided the knowledge and evidence to identify and protect important areas for marine mammals and basking sharks in Scottish seas.

In 2019 Silurian began year-round monitoring, conducting surveys in winter (November – March) to assess seasonal distribution of marine megafauna species identified on Scottish list of priority marine features, in addition to the continued summer (April – October) surveys. These combined data provide a year-round assessment of cetacean habitat-preference, behaviour and distribution in the region. 

In this report we present our results from three years of surveys (April 2021 – March 2024) and highlight the importance of continued long-term monitoring.

Main findings

• Between April 2021 and March 2024, 22,645 km of dedicated visual and/or acoustic survey effort was conducted off the west coast of Scotland. This consisted of 16,596 km of visual survey effort and 20,601 km of acoustic survey effort covering the waters from Cape Wrath in the north to Islay and the Kintyre peninsula in the south and out to the Flannan Isles in the west. 
• Ten species of mammal and two species of fish were recorded in 3,089 sightings. The most frequently recorded species was the harbour porpoise (Phocoena phocoena) (n=801). 
• The sighting rates in the summers of 2022 and 2023 for minke whale (Balaenoptera acutorostrata) (2022=1.08 sightings per 100 km, 2023=1.57 sightings per 100 km) and common dolphin (Delphinus delphis) (2022=2.31 sightings per 100 km, 2023=2.86 sightings per 100 km) were the highest recorded in the 20 years the programme has been running. 
• In contrast, the sighting rates for basking shark (Cetorhinus maximus) in the summers of 2022 (0.068 sightings per 100 km) and 2023 (0.069 sightings per 100 km) were the lowest recorded in the 20 years the programme has been running. Most sightings of basking shark were within the Sea of the Hebrides Nature Conservation Marine Protected Area (NCMPA) for this species. 
• Acoustic surveys generated 1,766 hours of full bandwidth acoustic recordings, from which 2,777 acoustic detections of harbour porpoise were identified.  
• Acoustic Deterrent Devices (ADDs) were detected in real-time aural assessments of the marine soundscape in 3% of all listening stations in 2021. No Acoustic Deterrent Devices were detected between 2022 and 2024 indicating a cessation in the use of ADDs around salmon farms in the research area.
• 294 volunteers were trained in HWDT’s survey protocol and carried out data collection.

Acknowledgements

The Hebridean Whale and Dolphin Trust (HWDT) extends its thanks to the volunteers, staff, and crew, past and present, who have contributed to the long-term species monitoring and research programmes on Silurian. HWDT would also like to thank its members, donors and project funder NatureScot who have supported this project.

The authors would also like to extend our thanks to the staff at NatureScot in particular Fiona Manson and Rona Sinclair for supporting and managing the grant during this reporting period.

We are also grateful to the time taken by reviewers for their valuable comments, which have improved this report. 

Abbreviations

Acoustic Deterrent Device (ADD)

Hebridean Whale and Dolphin Trust (HWDT)

Marine Protected Area (MPA) 

Nature Conservation Marine Protected Area (NCMPA)

Special Area of Conservation (SAC)

Scottish Association for Marine Science (SAMS)

Passive Acoustic Monitoring (PAM)

Priority Marine Feature (PMF)

Introduction

The Hebridean Whale and Dolphin Trust’s (HWDT) Cetacean Research Programme has been conducting surveys on the west coast of Scotland since 2003 using a standardised survey methodology to ensure data collected are consistent and comparable between years. The visual, acoustic, and photo-identification data collected by HWDT from the research vessel (RV) Silurian provides a significant component of the baseline evidence available for cetaceans in the Hebrides. This can be used to determine continued presence, detect trends and changes, and relate these changes to possible causes.

Data collection covers a wide geographical area from Cape Wrath in the north, to Mull of Kintyre in the south, and as far west as St. Kilda. Line transect surveys are conducted by trained volunteers under the supervision of an experienced marine mammal observer and boat crew. Straight line transects are selected randomly, as weather and sea conditions allow, to maximise coverage across the research area.

The surveys cover a large geographical area and range of environmental conditions (i.e. depth, slope, and salinity) providing a wide scale assessment of cetacean distribution that can be used to assess population size, habitat preference, site usage and animal behaviour. Anthropogenic parameters, such as underwater noise, marine litter, location and type of fishing gear, shipping and military exercises are also monitored to identify emerging threats and understand the conservation risks they could pose. This long-term assessment of cetacean distribution provides a powerful and comparable data set that can be used to detect trends and changes in the marine environment.

The programme gathers long-term evidence to underpin the Scottish Government’s assessment, reporting and surveillance obligations under the Marine Scotland Act (2010), the draft UK Dolphin and Porpoise Conservation Strategy, the draft Scottish Biodiversity Strategy to 2045, and a number of other European and International agreements including the European Union Habitats Directive, The Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR) and The Bern Convention on the Conservation of European Wildlife and Natural Habitats.

The programme also supports the delivery of the 2017 Scottish MPA Monitoring Strategy (Scottish Government, 2017) by helping to identify important areas for marine mammals and basking sharks. This collaborative, citizen science programme contributes to the development of the Scottish mobile species MPA monitoring programme through participation in the Scottish West Coast Cetacean Monitoring Expert Group. 

This report presents a summary of the data collected during visual and acoustic surveys carried out from April 2021 to March 2024 on the west coast of Scotland by HWDT’s Cetacean Research Programme. Survey effort in 2021 was reduced due to COVID-19 restrictions. The full data collection programme resumed in March 2022. Data from this period contributes to the ongoing, long-term effort of more than 1,000 volunteer citizen scientists who have contributed to the data collection over the past 21 years. 

Methods

Data collection

Data were collected between April 2021 and March 2024 during dedicated visual and acoustic surveys conducted by the Hebridean Whale and Dolphin Trust using the 18.5 m research vessel, Silurian. Surveys took place throughout the coastal waters of western Scotland (Figure 1. The survey area ranges from 55.10°N to 58.70°N, and -4.50°W to -8.70°W. An extensive programme of surveys was carried out between April and October (hereafter ‘summer surveys’) and between November and March (hereafter 'winter surveys’). 

Due to COVID-19 restrictions, data collection in 2021 was limited to acoustic-only monitoring (hereafter referred to as 'COVID restrictions’). Dedicated visual effort was conducted wherever possible but was limited and most sightings were recorded opportunistically. Therefore, effort-based visual data are sparse for this period. HWDT’s normal protocol of dedicated visual and acoustic surveying resumed in March 2022 and has continued to run as normal since then (Table 1).

Table 1. A summary of effort-corrected data methods carried out seasonally and annually from April 2021 to March 2024.

Year 1: April 2021 – March 2022

Year 2: April 2022 – March 2023

Year 3: April 2023 – March 2024

Surveys were conducted under a NatureScot research licence using a standard line transect methodology for cetaceans (Buckland et al., 2001). A survey speed of as near as possible to 6 knots was maintained during surveys. To achieve this speed, most surveys were conducted under motor or a combination of motor and sailing. When there was sufficient and constant wind to maintain this speed, surveys were carried out under sail. 

Visual surveys

During visual surveys, two observers were positioned at the mast with an eye height of approximately 4 m. Observers scanned the forward 180 degrees with the naked eye and 7x50 mm binoculars. Observers recorded any sightings of marine wildlife (cetaceans, basking sharks and seals), as well as marine debris and static fishing gear (i.e. creel marker buoys) within 500 m and 1000 m from the survey transect line respectively. 

Data were collected by trained volunteers and overseen by an experienced marine mammal observer (MMO) and boat crew. Including all species determinations made by trained volunteers quality assured by the MMO, to ensure data confidence and integrity is maintained. Volunteers were trained to visually detect marine life and collect and record information including the species (to the lowest taxonomic grouping), bearing (animal location in relation to the vessel), distance (how far from the vessel) and heading (animal’s direction of travel relative to the vessel’s heading). Observers conducted visual assessments for no longer than an hour at a time with a minimum rest of one hour between watches to minimise fatigue. 

An automated data logging program, Logger 2010, developed by the International Fund for Animal Welfare (IFAW), was used to collect data during the survey. The vessel’s position, speed and course over ground were recorded automatically from the vessel’s NMEA feed every 10 seconds, and all other survey data (i.e. sightings, changes to search status) were recorded in real time, using standardised, time-stamped forms. Environmental conditions (sea state using the Beaufort scale, swell, predominant weather, glare, sightability and visibility) were recorded every 15 minutes.

Passive acoustic monitoring

A towed hydrophone array was deployed in all sea conditions during daylight hours in waters >20m depth. Passive Acoustic Monitoring (PAM) software, PAMGuard (Gillespie et al., 2009), made continuous broadband recordings (500 kHz sample rate), as well as binary files from the automated harbour porpoise click-detector. PAMGuard plotted and stored the vessel’s position via an external GPS receiver.  Volunteers also completed standardised one-minute assessments of the underwater soundscape every 15 minutes, identifying and scoring sound levels of natural and anthropogenic sounds. The engine was brought out of gear and the vessel slowed to reduce background noise during these assessments.

In 2022 HWDT implemented the first significant upgrade to Silurian’s survey equipment since the Cetacean Research Programme began. Long-term high-frequency harbour porpoise monitoring continued, with the addition of calibrated acoustic recordings of mid-frequency dolphin species, baleen whales and anthropogenic sounds. 

Acoustic equipment: 2021 (Year 1)

HWDT’s hydrophone array consisted of two high-frequency elements, each comprising a piezoelectric ceramic spherical element and a preamplifier (Magrec HP03) spaced 30 cm apart, with a near flat frequency-response between 2-140 kHz. Each pre-amplifier (broadband Magrec HP02) provided 29 dB of gain. These components were housed in a streamlined sensor section consisting of 5 m of 35 mm diameter polyurethane tube filled with ISOPAR-M oil, towed on 100 m of Kevlar-strengthened cable behind the vessel. A National Instruments sound card digitised the signal. PAMGuard operated continuously when the hydrophone was deployed to generate full bandwidth (up to 500 kHz sample rate) recordings on two channels, spectrograms to monitor the incoming sound and binaries from the automated harbour porpoise click detector.

Acoustic equipment: 2022 – 2024 (Years 2 and 3)

HWDT’s hydrophone array consisted of two high-frequency elements containing a piezoelectric ceramic and a preamplifier (Magrec HP03) spaced 30 cm apart, and two mid-frequency elements (Magrec HP03) spaced 3 m apart. The high-frequency elements had a sensitivity with the preamplifier of -159 dB re 1 V/µPA and incorporated a low-cut filter on the preamplifiers set at 2 kHz. The mid-frequency elements had a sensitivity of -159 dB re 1 V/µPA, and a low-cut filter set at 50 Hz. The units had a good frequency-response between 2 and 150 kHz. Each pre-amplifier (broadband Magrec HP02) provided 29 dB of gain. These components were housed in a streamlined sensor section consisting of 5 m of 35 mm diameter polyurethane tube filled with ISOPAR-M oil, towed on 100 m of Kevlar-strengthened cable behind the vessel. A St Andrews Instrumentation Ltd data acquisition device (SAILDAQ) was used to digitise the signal. PAMGuard operated continuously when the hydrophone was deployed and saved continuous broadband (500 kHz sample rate) 4 channel recordings. Spectrograms were generated in real time to monitor the incoming sound and an automated harbour porpoise click detector was operating on the high frequency channels. 

Harbour porpoise acoustic monitoring 

Sighting rates for inconspicuous species such as harbour porpoise drop very rapidly with rising sea state (Teilmann, 2003; Evans and Hammond, 2004). Acoustic monitoring provides an effective method of detection, as it can be conducted in all sea states, allowing for unbiased estimates of absolute abundance. 

Harbour porpoise vocalisations were collected and analysed using Passive Acoustic Monitoring software PAMGuard in conjunction with the relevant hardware configurations described above. 

The PAMGuard Click Detector module (Gillespie et al., 2009) was used to process and identify possible harbour porpoise clicks in real time. The module automatically classified clicks using several identification parameters including the peak frequency (100 to 150 kHz), click length (0.05 to 0.2 ms), and the energy ratio between the ‘porpoise band’ and the ‘control band’ (6 dB below maximum). The click classifier displayed likely porpoise clicks as red triangles, allowing them to be distinguished from unclassified transients. 

The characteristics of each click event was checked visually in PAMGuard Viewer, using the bearing, waveform, power spectrum and Wigner (time-frequency) plot in the Click Detector program window. Harbour porpoise acoustic detections were determined by the analyst from possible clicks identified by the PAMGuard click classifier module. Click events were classified based on specific criteria (Annex 1). 

Appropriate event tracks (Porpoise Certain Train, with clear directionality) were analysed by estimating perpendicular distances from the vessel track to the vocalising animal using Target Motion Analysis (TMA). Each encounter was linked to a GPS position from PAMGuard and plotted in QGIS. This extensive dataset was analysed by six trained data analysts (Becky Dudley, Jenny Hampson, Hannah Lightley, Sadie Gorvett, Craig Mackie and Emma Longden) who underwent the same training (provided by Susannah Calderan or Jenny Hampson).  The dataset was reviewed by the same analyst (Jenny Hampson) to minimise subjectivity and improve consistency across the entire dataset (2021 – 2024).

Photo-identification

Where possible during cetacean or basking shark encounters, the vessel left the transect line and paused effort-based surveying to initiate a careful approach of the animal/s to gain high quality photographs. Photo-identification images of any identifying features, including the left and right side of the dorsal fin, body, or tail fluke were collected using a Canon 7D Mark II with a 100 – 400 mm Canon L lens. Acoustic recordings continued throughout encounters, where possible. The aims of photo-identification were to gain photographs of both sides of the dorsal fin, perpendicular to the animal, and to photograph each animal in the group including as much of the body as possible. This was not always possible, but once satisfactory images had been gained, or the encounter ended, the vessel returned to the previous transect line or began a new transect. 

Photographs were stored categorised sequentially by year, encounter, and individual. The photographs from every encounter were compared against the catalogue of known individuals. If a match could be made, the best photographs showing each body part (head, dorsal fin, flank and tailstock) of both sides of the animal were added to the catalogue along with ancillary information about the encounter (date, location and image copyright). Since 2016, the photo-identification catalogues for minke whales, bottlenose dolphins, Risso’s dolphins and killer whales have been stored and maintained using the photographic data-management system Discovery (Gailey and Karczmarski, 2012). Animals in each catalogue were identified by a unique alpha-numeric code comprising the key distinguishing feature used to identify the animal and a sequential number denoting the number of animals of that species identified with that mark type. Photographs were analysed and the catalogues maintained by experienced photo-identification researchers. 

Data analysis

The ability to detect surfacing cetaceans is affected by sea state with detectability for some species decreasing rapidly in sea states greater than two (Teilmann 2003; Evans and Hammond, 2004). Therefore, any effort or sightings collected in sea states greater than three were excluded when calculating sightings rates to ensure comparable detectability.

The HWDT survey protocol was designed to achieve an un-biased and representative spatial coverage throughout the survey area.  Inevitably however, locations near to rendezvous locations (Tobermory, Mallaig and Ullapool) received greater survey coverage. Areas to the west of the Outer Hebrides were surveyed on fewer occasions due to the need for extended periods of settled weather to access these comparatively remote locations (Hebridean Whale and Dolphin Trust, 2018). The survey area was divided into a grid of 50 km² hexagonal cells. A hexagonal grid has been used to reduce sampling bias (Birch et al., 2007). The total kilometres surveyed during dedicated visual survey effort for each year was calculated for each cell. Cells where the total survey effort was less than 9 km (the length of the widest part of the cell) were excluded to reduce small sample effects. Cells that did not meet the minimum effort or sea state criteria were excluded. 

Data were collated for all years throughout the study period into summer and winter surveys. The number of sightings recorded during visual surveys in sea states ≤3 were calculated for each grid cell and corrected for search effort. Sighting rates were calculated across the study area for summer and winter as the number of sightings per kilometre of visual survey effort. 'Presence only' grids were used for species where there were too few sightings to meet these criteria and included sightings recorded in all sea states.

Annual summer sightings rates were also calculated for basking shark, minke whale and common dolphin between 2003 and 2023. This was calculated using the total number of sightings recorded between April and October each year divided by the total dedicated visual survey effort for that period. These rates were calculated as sightings per 100 km of survey effort. Acoustic detections were stored in a database of harbour porpoise events that were confirmed and classified by experienced analysts. All analysed acoustic detections of harbour porpoise were used to calculate a total for each grid cell. Detection rates were calculated as the number of harbour porpoise detection events per kilometre of acoustic survey effort. 

Data were mapped in QGIS, showing the vessel track, sightings and acoustic detection data. Results were broken down annually (April – March of each survey year) and seasonally (summer: 01/04-31/10 and winter: 01/11-31/03). The amount of effort within and out with the Sea of the Hebrides and North-east Lewis NCMPAs and the Inner Hebrides and Minches SAC was also calculated.

Results

Data collection

HWDT carried out 22,645 km of survey effort in the period April 2021 to March 2024 (Figure 1), including 14,553 km of visual and acoustic survey effort, 2,044 km of visual survey effort and 6,048 km of acoustic survey. The extent of the area surveyed in the period was 55.10°N to 58.70°N, and -4.50°W to -8.70°W. Effort varied seasonally with the length of daylight hours and variable weather conditions. Survey effort in 2021 was lower than other years due to COVID-19 restrictions. Surveys from March 2022 onwards were delivered at full capacity and followed the full programme protocol of both visual and acoustic monitoring. 294 trained volunteers collected data from April 2021 to March 2024

In Year 1 (April 2021 - March 2022), HWDT conducted nine research surveys; six summer surveys and three winter surveys which took place in November 2021 and March 2022. 

In Year 2 (April 2022 - March 2023), HWDT conducted 21 research surveys, including 18 surveys in summer (April – October 2022) and three surveys in winter (January, February and March 2023). 

In Year 3 (April 2023 – March 2024), HWDT conducted 19 research surveys, including 17 in the summer (April – October 2023) and two surveys in the winter (February and March 2024). 

Survey effort was not uniformly distributed, with more effort concentrated in the areas close to the rendezvous points of Tobermory, Ullapool and Mallaig. Areas around the north of the Isle of Mull, Small Isles and Summer Isles received more coverage (Figure 2). 

There was more acoustic effort than visual effort conducted in the period due to COVID-19 restrictions in 2021, and more effort in summer than winter. There was 12,107 km of visual survey effort recorded in sea state 3 or less in summer and 1,332 km in winter, and 18,582 km of acoustic effort in summer and 2,018 km in winter (Figure 2). Effort was highest in summer and winter close Tobermory and in the Sound of Mull. Visual and acoustic effort was lowest at the boundaries of the research area west of Uist, the north Minch and Stanton Bank. 

In areas where it was not possible to tow the hydrophone, for example, the Inner Sound of Raasay, Kyle Rhea and Sound of Eriskay, visual only data were collected. 

HWDT conducted visual and acoustic surveys in each of the three MPAs designated for mobile species on the west coast of Scotland; the Sea of the Hebrides NCMPA, North-east Lewis NCMPA and the Inner Hebrides and Minches SAC (Table 2). 77% percent of the total effort was conducted within the Inner Hebrides and the Minches SAC. 39% percent of the total effort was conducted in the Sea of the Hebrides NCMPA and 2% percent in the North-east Lewis NCMPA. 

Table 2. Acoustic and visual survey effort within and out with mobile species protected areas (MPA and SAC) off the west coast of Scotland in summer and winter from April 2021 to March 2024.

Summer

Winter

All effort

Visual Effort

Visual effort surveying was carried out across the research area, between Islay to the south, the Flannan Isles in the west and around the Butt of Lewis in the north totalling 16,596 km. Comparable visual effort was conducted in the summers of 2022 and 2023, with a very limited amount of visual effort conducted in 2021 due to COVID-19 restrictions (Figure 3). 

Visual survey effort in the summers of 2022 and 2023 provided a good coverage of the research area. In both years, few transects west of the Outer Hebrides were conducted due to limited periods of extended settled weather which are required to survey these exposed areas. Despite this, limited but comparable coverage was completed in each year. No visual effort was conducted in the study period in the furthest south-west part of the research area, around Stanton Bank due to limited continuous settled weather opportunities. The Sea of the Hebrides, Minches and south of Islay and Jura were well surveyed. 

Winter surveys in 2022/23 and 2023/24 were conducted with full visual and acoustic effort. Winter surveys in 2021/22 were impacted by COVID-19 restrictions with one survey in November 2021 conducted as a crew-only acoustic survey and all sightings recorded opportunistically. Two winter surveys were delivered in the 2023/24 winter survey period to allow essential maintenance of the vessel to take place ahead of the summer season, so the effort for the final winter period is less than 2022/23 where three surveys took place. 

Coverage in winter data collection periods is comparable across all years, with good coverage in the Sound of Mull in all years. The Small Isles, Coll, Sound of Sleat, Loch Linnhe and the Sound of Luing were surveyed as conditions allowed and were not completed every year. Winter data collection was limited to areas close to HWDT’s base in Tobermory. Winter effort was carried out inside the Inner Hebrides and Minches SAC and Sea of the Hebrides NCMPA. No effort was carried out in the North-east Lewis NCMPA in winter. 

All Sightings

A total of 3,089 sightings were recorded between April 2021 and March 2024, 2,927 of which occurred on summer surveys and 162 on winter surveys (Table 3). Sightings in the summer of 2021 (n=437) were lower than in 2022 (n=1,269) and 2023 (n=1,221) as a reduced number of surveys operated during this time due to COVID-19 restrictions. Sightings are lower (n=47) for the winter of 2023/24 as 10 days of survey were conducted in this period compared to 15 days of survey in 2021/22 and 2022/23 as explained above. 

Table 3. Marine animal sightings recorded from Silurian between April 2021 and March 2024. 3089 sightings of 12 species were recorded.

Ten species of mammal were recorded; harbour porpoise (Phocoena phocoena), grey seal (Halichoerus grypus), common dolphin (Delphinus delphis), minke whale (Balaenoptera acutorostrata), common seal (Phoca vitulina), Risso’s dolphin (Grampus griseus), bottlenose dolphin (Tursiops truncatus), white-beaked dolphin (Lagenorhynchus albirostris), killer whale (Orcinus orca), otter (Lutra lutra), and two species of fish; basking shark (Cetorhinus maximus) and oceanic sunfish (Mola mola). The most commonly recorded species was the harbour porpoise (n=801), followed by grey seal (n=706) and common dolphin (n=587). 

Priority Marine Features

Sightings for harbour porpoise were recorded year-round across the survey area. Highest sightings rates in summer occurred around the Isle of Mull, Coll and Tiree, the Small Isles, Sound of Rasaay and east of Tiumpan Head on the Isle of Lewis (Figure 4). Harbour porpoise encounter rates were higher in deep, coastal areas and in strong tidal races. 

In winter the highest sightings rates occurred at the entrances to the Sound of Mull, and the Treshnish Isles. 

Sightings of harbour porpoise largely occurred in the Inner Hebrides and the Minches SAC for this species, where the majority of the effort took place. High sighting rates occurred south of the SAC boundary, around Islay, Jura and Barra. All winter data collection, and therefore sightings, occurred within the boundary of the SAC. 

Minke whale sightings showed a clear seasonal pattern. In summer, sightings were distributed across the survey area with the highest sighting rates in the northern Minch, northern Sea of the Hebrides and south of Mull (Figure 5). In contrast, there was only one sighting in the winter data collection period, in November 2021, west of Ardnamurchan Point. Sightings occurred throughout the Sea of the Hebrides NCMPA for the species, however, minke whale sighting rates were also high to the north of the NCMPA boundary, particularly around Tiumpan Head, Isle of Lewis. 

A high number of minke whale sightings were recorded during the summers of 2022 (n=123) and 2023 (n=167), a marked increase on the previously documented figures reported by the programme (HWDT, 2018; MacLennan et al., 2021).

Sighting rates in the summers of 2022 (1.08 sightings per 100 km) and 2023 (1.57 sightings per 100 km) were the highest recorded for minke whales in the 20 years the programme has been running (Figure 6).

Risso’s dolphins were encountered on 11 occasions during summer data collection periods. No sightings of Risso’s dolphin occurred in winter data collection periods. The highest number of encounters occurred in 2022 (n=8). 

Risso’s dolphins were distributed across the research area most often in deep water close to the shore (Figure 7). One sighting occurred in the North-east Lewis NCMPA for the species, however, effort within the NCMPA was low. 

Sightings of basking sharks occurred only in the summer season and were predominantly in the Sea of the Hebrides NCMPA for the species, close to Mull and Barra (Figure 8). Two sightings occurred north of the NCMPA boundary, east of Tarbet, Isle of Harris and in Gairloch.

These data show a clear seasonal pattern as no sightings of basking sharks occurred in the winter. 

Low numbers of basking sharks were encountered during the study period (n=16), with just 7 sightings recorded in each of the summers of 2022 and 2023 (Table 3). Basking shark sightings rates for both 2022 and 2023 (0.07 sightings per 100 km) were the lowest recorded in the 20 years the programme has been running (Figure 9). 

Bottlenose dolphins were recorded on 12 occasions, predominantly in coastal waters (Figure 10). While most sightings of the species occurred around Barra and the surrounding islands, bottlenose dolphins were encountered across the survey area most often in coastal areas.

Bottlenose dolphin sightings occurred in both summer (n=11) and winter seasons (n=1). One notable sighting of bottlenose dolphins occurred in 2021 west of the Isle of Lewis in open water. Photographs of these individuals were obtained, and further investigation is required to identify whether these animals belong to the Inner Hebridean population or a poorly characterised offshore population.

Common dolphins were widespread across the survey area in summer with the highest sightings rates in the Little Minch and Sea of the Hebrides (Figure 11). Winter sightings were limited to the waters between Ardnamurchan and the Cairns of Coll, the Firth of Lorn and the Sound of Sleat. 

Common dolphins were recorded during summer and winter in all years of the study period (Table 3). The winter data collection period of 2021/22 had the highest number of sightings of this species (n=11). For summer, the number of common dolphin sightings increased to a peak of 250 encounters and a sighting rate of 2.86 common dolphins per 100 km in 2023 (Figure 12). 

White-beaked dolphins were recorded in summer (n=18) and winter seasons (n=1) (Table 3). The number of sightings was highest in summer of 2022 (n=8). In summer, white-beaked dolphin sightings occurred predominantly in the north of the survey area in the north Minch and west of the Isle of Harris (Figure 13). One sighting occurred in open water north of Tiree. 

One sighting of white-beaked dolphins occurred in February 2024 in the winter data collection period in inshore coastal waters near Oban (Figure 13). 

Killer whales (Orcinus orca) were encountered on five occasions in the survey period. All sightings occurred in the summer around the Ardnamurchan Peninsula (n=4) and South Uist (n=1) and were of the same two individuals from the West Coast Community population, W01 ‘John Coe’ and W08 ‘Aquarius’ (Figure 14).

Acoustic Effort

Acoustic effort was conducted in all survey periods with a total 20,601 km of acoustic survey effort carried out between April 2021 and March 2024. Effort was lower in 2021 due to Covid-19 restrictions. 

Acoustic coverage of the research area is comparable for summer 2022 and 2023, with reduced coverage in the summer of 2021 towards the north of the survey area, west of the Outer Hebrides, and the waters between Barra and Tiree and Coll (Figure 15). Limited coverage of the area south of Mull and west of Tiree was carried out in the summer of 2021 as a result of fewer surveys. However, acoustic data collection within the SAC and NCMPAs was good across all summer seasons.

Winter acoustic data collection resulted in good coverage of the Sound of Mull and Small Isles in all years with variable coverage of Coll, Tiree and Loch Linnhe due to limitations of the weather. There was reduced acoustic effort in the winter of 2023/24 as only two surveys were delivered, compared to three surveys in 2022/2023, to allow essential maintenance of the vessel to take place ahead of the summer season. 

Harbour porpoise acoustic detection rates were higher than sightings rates. There were 2,777 acoustic detection events, compared to 801 sightings in the reporting period. The highest number of acoustic detection events occurred in summer in 2023 (n=933) and winter in 2021 (n=165). Harbour porpoise acoustic detections were widespread across the survey area (Figure 16).

Acoustic detections of harbour porpoise in summer occurred predominantly in the Inner Hebrides and the Minches SAC. The highest detection rates occurred in the Sound of Mull, around the Small Isles, Skye and east side of the Outer Hebrides. Some high detection rates were also recorded out with the boundary around Islay, Jura and south of Barra. Few detections occurred west of the Outer Hebrides. 

Harbour porpoise acoustic detections in winter occurred around the coast of Mull, with highest detection rates were recorded at both entrances to the Sound of Mull, Loch Linnhe and around Ardnamurchan. No harbour porpoise detection events occurred in the waters to the south of Coll and few detections were recorded around the Smal Isles during the winter, where they had been detected in high numbers in the summer surveys. This is likely to be an effect of reduced effort in the winter, as the acoustic effort in these areas was generally limited to a single transect. 

Additional observations

Photo-identification images of minke whales, basking sharks, bottlenose dolphins, Risso’s dolphins and white-beaked dolphins were taken where possible on all surveys. Additional images of common dolphins and harbour porpoise were taken where individuals had distinct identifying features or signs of entanglement. Work to match these images to the HWDT Hebridean Minke Whale Catalogue (Hebridean Whale and Dolphin Trust, 2023) and HWDT Bottlenose Dolphin Catalogue is ongoing. High quality images of other species will be made available for colleagues working on other projects, for example, Whale and

Dolphin Conservation’s (WDC) long-term study of Risso’s dolphin in the area. 

Mother calf records. A mother calf pair of minke whales were sighted south of the Isle of Eigg on 25/05/23 (Figure 17). Although the adult animal has a distinctive fin, it was not matched to any minke whales in the photo-identification catalogue at the time. HWDT’s minke whale photo-identification catalogue has since been published up to 2020 (Hebridean Whale and Dolphin Trust, 2023), with work ongoing to ensure all individuals within this reporting period have been catalogued.  

A large group of Risso’s dolphins (n=20) were encountered west of the Isle of Lewis in 2023 with two young calves in the group (Figure 18). Images will be shared with WDC for comparison and inclusion in the Scottish Risso’s dolphin catalogue.

Killer whales were sighted five times in the reporting period. Each sighting was of the same two individuals, W01 ‘John Coe’ and W08 ‘Aquarius’ of the West Coast Community of killer whales. No other individuals from the group were seen. Four of the sightings occurred in the same area west of Ardnamurchan Point. The animals' surface behaviour involved regular occurrences of both animals surfacing very close to one another (Figure 19), in many instances appearing to be touching. They exhibited diving and resting behaviours at Ardnamurchan point.

Acoustic deterrent devices (ADDs). Acoustic aural assessment of the underwater soundscape was carried out every 15 minutes while conducting acoustic surveys to create an assessment of biological and anthropogenic noise. ADDs were present in 7.76% of all acoustic listening stations between 2006 and 2022 (Hampson et al., 2021). During this reporting period, there was a significant decline and apparent cessation in the use of ADDs around salmon farms in the research area. 

Acoustic deterrent devices were identified in 3% of all listening stations in 2021, compared to a high of 13.74% of listening stations in 2017 (Hampson et al., 2021). No ADDs were identified in listening stations in the summer or winter data collection periods of 2022-2024.

Discussion

In this report, we have provided an overview of the visual and acoustic data collected by the Hebridean Whale and Dolphin Trust’s Cetacean Research Programme between April 2021 and March 2024. During this period, the programme has recorded the highest number of sightings and sightings rates for minke whales and common dolphins and the lowest number of sightings and sightings rates for basking sharks in the 20 years the programme has been running. 

Common dolphin

Sightings rates of common dolphins have increased from 0.05 sightings per 100 km surveyed in 2003 to 2.86 sightings per 100 km in 2023. Common dolphins were infrequently recorded when the programme first started and no sightings were documented in 2004 (Hebridean Whale and Dolphin Trust, 2018). Since then, sightings of common dolphins have increased, and they are now one of the most frequently recorded species with record-breaking numbers of sightings recorded consecutively in 2022 (n=169) and 2023 (n=208). 

Common dolphins were recorded in large numbers throughout the survey area, including towards the northern reaches of the research area, where once sightings of this species were less common (Hebridean Whale and Dolphin Trust, 2018). It is important to note that in the early years of the programme (2003-2006) survey effort was lower and focussed mainly on the waters around Mull, Coll, Tiree, the Small Isles, Islay, Jura and the Kintyre Peninsula so effort in the northern reaches of the survey area was limited. However, since then survey effort has increased and the waters towards the north of the survey area are now surveyed annually. 

More detailed analyses have also demonstrated geographical range shifts in common dolphins showing a northward extension in their range, noting the increasing abundance of common dolphins in the waters off western Scotland, particularly in the northern Minch (MacLeod et al., 2005; Evans and Waggitt, 2020). 

Minke whale

Minke whales have been regularly recorded throughout the programme’s history. Sightings rates for minke whales varied between years reaching a low of 0.26 in 2005 and have increased year on year since 2017, reaching a peak of 1.57 sightings per 100 km surveyed in 2023.

During this study period, minke whales occurred throughout the region, with the highest sighting rates within the Sea of the Hebrides NCMPA but high sighting rates were also recorded outside the NCMPA, particularly in the Minch around Tiumpan Head, Isle of Lewis.

Minke whale sightings demonstrate a clear seasonal pattern in occurrence with all sightings occurring between April and November. Data collected through community sightings networks, like HWDT’s Whale Track, WDC’s Shorewatch programme and the Sea Watch Foundation (Anderwald and Evans, 2007), show that some minke whales are present in the region during the winter months. Future studies should analyse these data alongside the Silurian dataset to assess the year-round presence of minke whales in the region.

Basking shark

Sightings rates for basking sharks were the lowest ever recorded by the programme in 2022 and 2023 with 0.07 sightings per 100 km surveyed recorded for both years. Sightings rates were higher in the early years of the programme reaching peaks of 2.24 and 2.31 in 2006 and 2010 respectively. The Sea of the Hebrides, particularly the waters around Mull, Coll, Tiree and up to the Small Isles are a known hotspot for this species (Witt et al., 2012; Hebridean Whale and Dolphin Trust 2018). As noted above, the early years of the programme focussed on surveying these areas and since then total survey effort has increased and the coverage of the region has become more widespread. Survey effort however still remains high in the waters around Mull, Coll, Tiree and the Small Isles (Figure 2). More detailed analysis of these data are required to properly account for any potential changes in effort.   

The results presented here indicate that when sightings rates of basking shark are high, minke whale sightings rates were low, and vice versa. For example, 2023 saw a record high for minke whales and record low for basking sharks, and 2005/2006 showed the opposite trend. Future studies could consider the relationship between these species and their environment. 

The sightings rates presented in this report provide a basic analysis of the data, which do not account for any other influences on the data such as changes in the distribution of survey effort or provide any indication of what is causing these changes. Further analyses of the minke whale, basking shark and common dolphin data are required to account for these changes and assess drivers of distribution and abundance for these species. 

Harbour porpoise

Sightings and acoustic detections of harbour porpoise were widespread across the research area. These data highlight the strength of acoustic monitoring for this species, as it provided a more detailed representation of harbour porpoise distribution identifying harbour porpoise in areas where there were no visual observations. However, for the purposes of this report, all harbour porpoise acoustic events were included the detection per unit effort plots. We did not account for spatial or temporal autocorrelation between detections, which will have affected the detection rates as some detections will have been counted multiple times. 

Further work should be conducted to account for autocorrelation of these data in the next reporting period to provide a more accurate representation of these data. It is essential that ongoing monitoring continues to prioritise both data collection methods to provide accurate density estimates. 

Risso’s dolphin 

During this reporting period, Risso’s dolphins were encountered on 11 occasions across the research area, consistent with previous years surveys as documented in HWDT’s Marine Mammal Atlas (Hebridean Whale and Dolphin Trust, 2018). 

One sighting of Risso’s dolphin was recorded in the North-east Lewis NCMPA for the species during this data collection period. Survey effort in the NCMPA was low (n=2%) due to the distance of the area from survey departure points, weather conditions and the exposure of the area to north-easterly winds. The North-east Lewis NCMPA for Risso’s dolphins was designated in 2020 following the collection of extensive evidence for the importance of the site for the species (Weir et al., 2019), where WDC’s long-term study of the genetically distinct population indicated probable nursey grounds around the Eye Peninsula. Prior to 2010, HWDT’s coverage of the northern part of the survey area was limited, as surveys were focused closer to HWDT’s base on the Isle of Mull. HWDT’s Cetacean Research Programe now conducts three surveys per year departing from Ullapool to increase the potential of coverage in the northern and western part of the research area. 

White-beaked dolphin

Sightings of white-beaked dolphins in the research period in summer were predominantly in deep, open water or in areas of relatively deep water close to shore in the north and western extents of HWDT’s research area. Sightings occurred in both summer and winter seasons. The only encounter in a winter data collection period occurred furthest south of any recorded from Silurian and in very coastal waters, near Oban. Further analysis of HWDT’s visual and acoustic data on white-beaked dolphin presence throughout the duration of the programme, in addition to effort-corrected and opportunistic sightings from HWDT’s Whale Track are necessary to assess the abundance and distribution of the species and investigate any seasonal changes. 

It is vital to continue direct long-term monitoring of these species to ensure that MPAs and conservation measures adequately protect the species across their full range and in all areas of high density. These data need further investigation to identify potential drivers of the variation in sightings rates and will be shared with research associates looking to increase our understanding of minke whale and basking shark distribution, abundance and behaviour in the Hebrides. 

Acoustic deterrent devices (ADDs)

HWDT recorded a decline in ADD detections in 2021 and no ADDs were detected in the study area from March 2022 indicating a cessation in the use of ADDs around salmon farms in the research area. This follows a clarification of legislation by Marine Scotland in 2021. A report by the Scottish Government (2021) confirmed that a European Protected Species License was required by any fish farm that deployed ADDs, with the aim of preventing disturbance to European Protected Species, including the harbour porpoise. It is important that HWDT’s long-term monitoring continues to monitor for ADD use in Scotland, and any new devices that may be used in future, as it provides an essential tool to quantify the impacts of underwater noise.

Data sharing

HWDT’s Cetacean Research Programme collects data on a variety of anthropogenic pressures in the research area. These data are shared with researchers to build a long-term picture of cetacean populations and distributions and understand the threats they face. Here are some examples of how other data recorded during the surveys are currently being used.

Vessel presence and type are recorded within 30-minute watches, to supplement Automatic Information System (AIS) data which is continually logged. Together these data capture a true picture of vessel activity, since some water users may not be legally required to transmit information. This includes vessels under 15 m in length and recreational craft. These data are currently being analysed by PhD researcher Emily Hague from Heriot-Watt University as part of the Scottish Vessel Project, which aims to assess the impact of vessel activity on cetaceans in Scotland.  

Presence of creel fleets are also recorded to provide an estimate of fishing effort. These data have been important to identify areas of high risk of entanglement to cetaceans and basking sharks (Northridge et al., 2010; MacLennan et al., 2021). These data are currently being analysed by PhD researcher Tim Awbery from the Scottish Association of Marine Science. Following on from Northridge et al., 2010 and MacLennan et al., 2021, work is also ongoing to assess the proportion of live minke whales, in HWDT’s Minke Whale Catalogue (Hebridean Whale and Dolphin Trust, 2023), with signs of past or current entanglement. This methodology could also be utilised to assess photo-identification images of all species for signs of entanglement, in particular those known to be susceptible to entanglement such as humpback whales and basking sharks. A similar methodology could be developed to assess photographs of species susceptible to by-catch such as common dolphin and harbour porpoise.

Aural assessments of the marine soundscape were carried out every 15 minutes while conducting acoustic effort. Listening station data, in addition to harbour porpoise acoustic detection data, have been provided to a new MSc research project at SAMS investigating whether there has been a change in the distribution of harbour porpoises detected in the Sound of Mull following the cessation of ADD use in the area. This project builds on the long-term collaboration between HWDT and SAMS researchers to investigate the impact of ADD use on harbour porpoises off the west coast of Scotland (Findlay et al., 2018; Findlay, 2022; Findlay et al., 2024). 

These are just some of the projects we are currently supporting, but there is a significant potential to utilise the data set collected by the programme to address a range of research questions and knowledge gaps. 

Conclusion

In 2023, HWDT’s Cetacean Research Programme recorded the highest annual sightings rates and number of sightings for minke whales and common dolphins since monitoring began from Silurian. In contrast, the lowest annual sightings rates and numbers of basking sharks were recorded. These results suggest that the distribution of these highly mobile species is changing. Continued long-term monitoring is essential to detect and monitor population trends like these over time. Analysis of the behavioural, environmental, and demographic data associated with these sightings in combination with previous years would provide further insight into fine and broad scale distribution patterns for these priority species. Further analysis is required to investigate these trends in more detail and investigate the impact of any potential drivers such as climate change. 

Harbour porpoise, minke whale, basking shark and Risso’s dolphin were recorded within their designated protected area (NCMPA or SAC). The low level of survey effort carried out in the North-east Lewis NCMPA limits our ability to make meaningful comparisons within and out with that designation in this report.

Acoustic detections of harbour porpoise from Silurian remains a powerful tool for assessing their distribution providing a more detailed view when compared to visual survey data. Acoustic assessments of the marine soundscape from Silurian are also an effective method of monitoring acoustic pressures on cetaceans. Data collected by the programme has recently been used to demonstrate that harbour porpoise on the west coast of Scotland respond to repeated and chronic and underwater noise from ADDs and are either displaced from habitats and/or echolocate less when ADDs are in use (Findlay et al., 2024).

Cetaceans and basking sharks are long-lived, highly mobile species, and long-term monitoring by programmes such as HWDT’s Cetacean Research Programme are essential to understand patterns of presence and distribution, to assess whether populations are stable, increasing, or declining, and to identify drivers of change and monitor threats. 

Citizen scientists are also crucial to the long-term monitoring conducted by HWDT’s Cetacean Research Programme. During this study period, 294 volunteers were trained in the programme’s data collection protocols, supervised by five experienced marine mammal observers. These volunteers are not only essential for the effective long-term monitoring of cetaceans and basking sharks by HWDTs Cetacean Research Programme, but importantly also go on to become advocates for marine conservation. 

It is vital that HWDT’s Cetacean Research Programme continues to receive support to ensure the data collection programme can continue. These data are powerful to detect long-term changes and trends in cetacean and basking shark distribution on the west coast of Scotland and monitor and identify pressures that may be impacting their survival.

References

Anderwald, P. and Evans, P.G.H. 2007. Minke whale populations in the North-Atlantic – an overview with special reference to UK Waters. In: an integrated approach to non-lethal research on minke whales in European waters (Editors K.P. Robinson, P.T. Stevick and C.D. MacLeod). European Cetacean Society Special Publication Series, 47, 8-13.

Birch, C.P.D., Oom, S.P. and Beecham, J.A., 2007. Rectangular and hexagonal grids used for observation, experiment and simulation in ecology Ecological Modelling, 206 (3–4), 347–359.

Buckland, S.T., Anderson, D.R., Burnham, K.P., Laake, J.L., Borchers, D.L. and Thomas, L., 2001. Introduction to distance sampling: estimating abundance of biological populations. Oxford: Oxford University Press.

Evans, P.G.H. and Hammond, P.S., 2004. Monitoring cetaceans in European waters Mammal Review, 34, 131–156.

Evans, P.G.H and Waggitt, J.J. 2020. Impacts of climate change on marine mammals, relevant to the coastal and marine environment around the UK MCCIP Science Review 2020, 420–454.

Findlay, C., Ripple, H., Coomber, F., Froud, K., Harries, O., van Geel, C. F., Calderan, S., Benjamins, S., Risch, D., & Wilson, B. 2018. Mapping widespread and increasing underwater noise pollution from acoustic deterrent devices Marine Pollution Bulletin, 135, 1042-1050. 

Findlay, C.R., 2022. Predicting Risks from Acoustic Deterrents to Marine Mammals Doctoral Thesis, University of the Highlands and Islands.

Findlay, C.R., Coomber, F.G., Dudley, R., Bland, L., Calderan, S.V., Hartny-Mills, L., Leaper, R., Tougaard, J., Merchant, N.D., Risch, D. and Wilson, B., 2024. Harbour porpoises respond to chronic acoustic deterrent device noise from aquaculture Biological Conservation, 293, p.110569.

Gailey G. and Karczmarski L., 2012. DISCOVERY: Photo-identification data-management system for individually recognizable animals

Gillespie, D., Mellinger, D.K., Gordon, J., McLaren, D., Redmond, P., McHugh, R., Trinder, P., Deng, X.Y. and Thode, A., 2009. PAMGUARD: Semiautomated, open source software for real‐time acoustic detection and localization of cetaceans The Journal of the Acoustical Society of America, 125(4_Supplement), pp.2547-2547.

Hampson, J., Findlay, C.R., Ripple, H.D., Risch, D., Lomax, A., Hartny-Mills, L., 2023. Policy change leads to reduction in use of acoustic deterrent devices on the west coast of Scotland In: Proceedings of the 34th Annual Conference of the European Cetacean Society Conference, O’Grove, Spain, pp. 263. 

Hebridean Whale and Dolphin Trust, 2018. Hebridean Marine Mammal Atlas. Part 1: Silurian, 15 years of marine mammal monitoring in the Hebrides A Hebridean Whale and Dolphin Trust Report, Scotland, UK. 60 pp.

Hebridean Whale and Dolphin Trust, 2023. Minke Whale Photo-ID Catalogue for the west coast of Scotland, (1990-2020) A Hebridean Whale and Dolphin Trust Report, Scotland, UK. 58 pp.

Leaper, R., MacLennan, E., Brownlow, A., Calderan, S.V., Dyke, K., Evans, P.G., Hartny-Mills, L., Jarvis, D., McWhinnie, L., Philp, A. and Read, F.L., 2022. Estimates of humpback and minke whale entanglements in the Scottish static pot (creel) fishery Endangered Species Research, 49, pp.217-232.

MacLeod, C., Bannon, S., Pierce, G., Schweder, C., Learmonth, J., Herman, J. and Reid, R. 2005. Climate change and the cetacean community of north-west Scotland Biological Conservation, 124, 477–483.

MacLennan, E., Hartny-Mills, L., Read, F.L., Dolman, S.J., Philp, A., Dearing, K.E., Jarvis, D. and Brownlow, A.C., 2021. Scottish Entanglement Alliance (SEA) - understanding the scale and impacts of marine animal entanglement in the Scottish creel fishery NatureScot Research Report 1268. 

Northridge, S., Cargill, A., Coram, A., Mandleberg, L., Calderan, S. and Reid, R., 2010. Entanglement of minke whales in Scottish waters: an investigation into occurrence, causes and mitigation Contract Report. Final Report to Scottish Government CR/2007/49, 57.

Scottish Government, 2017. Scottish Marine Protected Areas (MPA) Monitoring Strategy A Marine Scotland Report, 27 pp.

Scottish Government, 2021. Acoustic Deterrent Device (ADD) use in the Aquaculture sector A Scottish Government Parliamentary Report, 106 pp.

Speedie, C.D., Johnson, L.A. and Witt, M.J., 2009. Basking shark hotspots on the west coast of Scotland: Key sites, threats and implications for conservation of the species Scottish Natural Heritage Commissioned Report No. 339.

Teilmann, J., 2003. Influence of sea state on density estimates of harbour porpoises (Phocoena phocoena) Journal of Cetacean Research and Management, 5 (1), 85–92.

Weir, C.R., Hodgins, N.K., Dolman, S.J. and Walters, A.E., 2019. Risso's dolphins (Grampus griseus) in a proposed Marine Protected Area off east Lewis (Scotland, UK), 2010–2017 Journal of the Marine Biological Association of the United Kingdom, 99(3), pp.703-714.

Witt, M.J., Hardy, T., Johnson, L., McClellan, C.M., Pikesley, S.K., Ranger, S., Richardson, P.B., Solandt, J., Speedie, C., Willaims, R. and Godley, B.J., 2012. Basking sharks in the Northeast Atlantic: spatio-temporal trends from sightings in UK waters Marine Ecology Progress Series, 459, 121-134.

Annexes

Annex 1

Harbour Porpoise click train event types and descriptions taken from PAMTech training material.