Tides of Change: Monitoring Hebridean Seas
Whales and dolphins are long-lived and well-travelled, with some species crossing oceans on migration. Understanding these magnificent creatures and safeguarding their futures relies on long-term monitoring, offering us a deeper glimpse into their lives and challenges.
Data collected over the long-term is used to understand patterns of species presence and distribution, to assess whether populations are stable, increasing, or declining, and to identify drivers of change and detect and monitor threats.
The visual, acoustic, and photo-identification data collected during our research expeditions provides the baseline evidence for cetaceans (the collective for whales, dolphins and porpoises) in the Hebrides. Thanks to everyone who has participated in an expedition over the past two decades, an incredible amount of data has been gathered. A report published by Nature Scot, details the results collated from the data collected on board our research vessel, Silurian, over the past three years.
PEOPLE POWER
Map of the west coast of Scotland with survey transect lines shown in blue. Data plotted are the tracks recorded during visual and/or acoustic survey effort between April 2021 and March 2024.
Between April 2021 and March 2024, 294 people joined our crew on board Silurian and were trained as citizen scientists in marine mammal survey protocols and set to work gathering data. The vital data collected over the past 22 years has been sustained by everyone who has participated in an expedition.
16,596 km of visual survey effort and 20,601 km of acoustic survey effort was undertaken, 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.
A total of 3,089 sightings were recorded: 2,927 of which occurred on summer surveys and 162 during winter survey work.
10 species of mammal were recorded, alongside two species of fish: the basking shark and sunfish. The gallery below shows the species recorded during these expeditions ordered by highest to lowest sightings rate.
PORPOISE MONITORING
Monitoring porpoises can be tricky, given how inconspicuous the species can be; sightings rates drop very rapidly as sea state rises. Acoustic monitoring therefore provides an effective method of detecting porpoises in all conditions. On board Silurian, citizen scientists have gathered acoustic data on porpoises since our monitoring began in 2003. Between 2021 and 2024, the highest sightings rates in summer occurred around the Isle of Mull, Coll and Tiree, the Small Isles, Sound of Raasay and east of Tiumpan Head on the Isle of Lewis. Harbour porpoise encounter rates were higher in deep, coastal areas and in strong tidal races.
Maps of the west coast of Scotland with the sightings rates (sightings per 1 km of effort) for harbour porpoise (Phocoena Phocoena) in sea states (Beaufort scale) less than or equal to 3, presented on a 50 km2 hexagonal grid for summer 2021-2023 (left) and winter 2021-2024 (right). Dark red cells represent the highest sightings rates, and yellow cells show lower sightings rates. Blue cells represent visual effort with no sightings.
Acoustic data is also a great tool for monitoring how human activity is impacting the marine soundscape over time. Using expedition data, it was revealed that Acoustic Deterrent Devices (ADDs) deployed by fish farm sites posed a significant and chronic source of underwater noise, which has been steadily increasing over the past decade . Expedition data also showed that this noise was disturbing and displacing porpoises from important areas of habitat. These findings were used to advocate for quieter seas for cetaceans. As a result of a change in policy by the Scottish Government implemented in 2021,the use of ADDs at marine aquaculture sites now requires a license.
During this reporting period, there was a significant decline and apparent cessation in the use of ADDs around salmon farms in the research area. ADDs were detected in just 3% of all listening stations in 2021, down from 14% in 2017. No Acoustic Deterrent Devices were detected in any HWDT surveys between 2022 and 2024.
WHALES V’s SHARKS
During expeditions in 2023, the highest sighting rate and number of minke whales was recorded in the Hebrides over the past two decades of monitoring from Silurian, this was juxtaposed with the lowest known for basking sharks. These findings suggest changes in sightings rates for minke whales and basking sharks, and a possible association between these two highly mobile and long-lived species.
In the face of the nature and climate emergencies, and with marine life at risk from human activities including entanglement, pollution, underwater noise and habitat degradation, gaining new insights and understanding into what is happening in Scotland’s seas is vital. Long-term monitoring like this is crucial to improve understanding of the impacts, and how best to protect and restore biodiversity.
COMMONS CONTINUE ON THE UP
Long-term monitoring is essential to decipher whether an observation is a trend or a blip - sightings rates for species do vary year-on-year. However, data collected during our research expeditions has documented an increase in common dolphin sightings rates off the west coast of Scotland.
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 research expeditions first started, and no sightings were documented over the 2004 field season. 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 Hebrides including towards the northern reaches of the research area, where once sightings of this species were less common.
As we continue down this path of warming seas, we may well see the distribution of warmer water species, like the common dolphin, shift even further north increasing their range.
Thanks to Nature Scot for supporting the monitoring work undertaken on board Silurian, helping safeguard this critical monitoring work over the past two decades.
HOW CAN I GET INVOLVED?
HWDT’s long-term research and monitoring depends on people joining an expedition. As more people participate in expeditions, the knowledge of the cetacean species and individual animals found in Scottish seas increases, alongside the evidence needed to protect them.
Anyone can take part in our research expeditions on board Silurian. Full training and support will be provided by our passionate, knowledgeable and dedicated crew - from domestic duties to survey protocol, we have you covered.
Join an expedition in 2025; explore the Hebrides, have an unforgettable experience and make a positive impact on our blue planet.
A massive thank you to everyone who has participated in a research expedition, without you this vital monitoring work wouldn’t have been undertaken.