A year in the life of Scottish cetaceans heard for the first time

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Marine scientists have for the first time captured the vast soundscape of whales and dolphins off Scotland’s west coastover the course of an entire year.

A research team led by the Oban-based Scottish Association for Marine Science (SAMS) deployed underwater microphones between the Hebrides and the continental shelf edge between September 2020 and August 2021. Known as passive acoustic monitoring, the microphones detected sounds from a range of marine mammals, including dolphins and fin, minke, humpback and sei whales.

The data collection, funded by the European Marine Fisheries Fund (EMFF) via the Marine Scotland directorate of the Scottish Government, will help to explain when migrating species arrive in Scottish waters, where they are spending their time and how long they stay. This information is crucial to regulators and advisors, such as Marine Scotland, as it will inform placement and mitigation strategies for renewable energy developments and other human activity at sea. It will also allow an assessment of how well populations are recovering since the end of whaling in the 1950s. As sentinel species, cetacean movements can also show how climate change is affecting the ocean ecosystem.

The findings have been published today (Wednesday) in the journal Frontiers in Remote Sensing: Frontiers | Monitoring cetacean occurrence and variability in ambient sound in Scottish offshore waters (frontiersin.org)

Dr Nienke van Geel, a marine mammal expert at SAMS and lead author on the publication, said: “At the moment, most of our predictive models on species distribution and population sizes are based on occasional sightings at sea. Visual surveys typically take place inshore and during daylight hours in the summer months, so can’t provide the full picture.

“We therefore have large seasonal and night-time data gaps for marine mammals in Scottish waters and particularly in that offshore area from the Hebrides out to the continental shelf edge.

“This year-round acoustic data collection of what’s happening and when is extremely exciting and gives the best indication yet of which species we have in our own waters.

“For example, we detected a lot of dolphin activity – their sounds were detected almost daily throughout the year at some sites. There was also more sei and humpback whale songs than we had expected.”

The research team included scientists from Marine Scotland and the University of Plymouth, and was carried out with assistance from SAMS Enterprise and boat operator Seatrek Marine Ltd. To gather the more than 12 terrabytes of data, the team deployed 10 underwater microphones at depths of between 60-175 metres, covering an area that stretched from Lewis and Barra to the west of St Kilda.

Researchers then used a combination of manual analysis of sound files and automated detection software to detect species of interest. They also analysed and compared overall sound levels of the recordings and identified sources of natural sound from wind, rain and tides, as well as human made noise, including the noise from ships, echosounders and military sonar.

The collected sound files will provide key baseline data for developments at sea such as windfarms but will also feed into longer term marine mammal distribution models, under various climate change scenarios.

SAMS marine mammal ecologist and publication author Dr Denise Risch said continued funding for such acoustic monitoring would be needed to provide crucial long-term data to help measure changes in the environment.

“As the ocean warms, more species that are adapted to warmer water come north. For example, in recent years we’ve seen more common dolphins follow their prey as they migrate northwards. It will only take a couple more decades for there to be a different species composition in Scottish waters.

“This kind of data is valuable in explaining how a warming ocean is affecting the movement of cetaceans and their prey, but it is also the best way to find out whether certain species are recovering from the devastating effects of whaling and how we can protect them from current threats, such as entanglement in fishing gear and ocean noise.”

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