Lending an ear
Unlike the better-known common and bottlenose dolphin, the Atlantic white-sided dolphin prefers the deep ocean to coastal waters and is rarely seen by humans. Marine scientists are now delving into the secret world of this elusive dolphin species by examining their unique sounds for the first time.
The Atlantic white-sided dolphin (Lagenorhynchus acutus) is endemic to the North Atlantic where it lives in cold-temperate and sup-polar waters. It is not only one of the most heavily exploited cetacean species in the North Atlantic, it is also one of the most poorly understood, according to Susannah Calderan, a Scottish Association for Marine Science (SAMS) fellow and marine mammal expert. She explains: “In common with most small cetaceans, Atlantic white-sided dolphins are subject to a range of anthropogenic impacts including fisheries bycatch, accumulation of contaminants, underwater noise, and prey depletion. They are also thought to be particularly vulnerable to climate change.”
Currently classified as a species of Least Concern by the IUCN Red List of Threatened Species, researchers argue that this is due to insufficient research into the species. As these dolphins tend to primarily inhabit offshore areas that are out of sight of humans, impacts on the species are not well-documented. Furthermore, the species is so poorly understood in science that there are no accurate estimates on population size.
“The primary conservation concern is the drive hunt for Atlantic white-sided dolphins in the Faroe Islands. Between 1992 to 2021, 7603 Atlantic white-sided dolphins were caught and killed. In 2021, 1428 were killed in a single drive hunt,” says Calderan who recently compiled a report on the species that concluded there were no reliable population estimates available for the species, leaving them potentially vulnerable. Because Atlantic white sided dolphins are primarily an offshore species, they don’t fall under the various protections of individual countries – another reason no major research studies have been undertaken.
Calderan explains: “For Atlantic white sided dolphins it can be a case of ‘out of sight, out of mind’. It is more straightforward to study bottlenose dolphins because they are a coastal species; we have more of an idea where they are going to be. White sided dolphins are usually on the continental shelf or further out in the ocean. When we do find them, they don’t often ‘play’ with the boats and, because of their speed, are very hard to photograph. We have recordings of white-sided dolphins – they have an unusual whistle that sounds like a space alien – but no-one has documented the acoustic repertoire in detail before.”
To find out more about the elusive dolphin species, scientists at SAMS in Oban now plan to examine recordings of Atlantic white-sided dolphins to describe its vocal behaviour or acoustic repertoire. By documenting the dolphin’s unique sounds, the team will provide a crucial baseline for further studies to learn more about this secretive cetacean. The project, funded by the Royal Society of Edinburgh, will document sounds such as the dolphins’ echolocation clicks and the duration and pitch of their whistles.
“Sound recordings are a valuable survey tool which can increase understanding of Atlantic white-sided dolphin distribution and habitat use. In areas which are difficult to survey, passive acoustic monitoring can provide long-term datasets to provide information on where dolphins are, and when, which is necessary in order to assess what threats they are subject to, and how those threats might be mitigated,” explains Calderan.
For the project, the team of scientists utilises hydrophones (underwater microphones) to collect the underwater sounds made by Atlantic white-sided dolphins. The hydrophones are towed behind survey vessels that have visual observers onboard to verify the identity of the sounds that are recorded. “Once we have used these recordings to describe the vocalisations, we will then be able to analyse recordings from unsupervised hydrophones. These are usually long-term recorders moored on the seabed,” adds Calderan.
Through this approach, the SAMS team and colleagues have already collated recordings from various sources from the eastern and western North Atlantic. By examining all these sounds, they can now start to build a blueprint, which can be used to train computer systems to detect these sounds automatically in long-term recordings. The system effectively ‘learns’ what a certain species sounds like and is able to filter these sounds from large data sets.
SAMS marine mammal ecologist Dr Denise Risch says: “It is remarkable that a marine mammal species we know about – although not often seen – has not really been acoustically described. We need to establish a baseline of its acoustic repertoire and then build machine learning systems to be used in monitoring the species. Establishing the size of the population is important but we also need to know where they are, given that many marine mammals are changing their distribution to follow prey, which themselves are changing habitats due to climate change. It is also important that we know more marine mammal movements when considering the siting of offshore windfarms.”
The Oban-based project hopes to build a full description of what sounds Atlantic white-sided dolphins make which, in turn, will enable it to build detectors and classifiers to automatically search through large datasets to find their sounds. “This will mean we can analyse recordings from moored hydrophones in the north Atlantic to find out where and when this dolphin species is detected, and understand more about their distribution, and how that may be changing,” explains Calderan.
And by examining the dolphin’s acoustic repertoire, the scientists hope to find out more about the species’ habitat, population numbers and how it may be affected by climate change. By laying this foundation, they might be able to learn more about this secretive cetaceans to help build a successful conservation framework.
Just like other dolphins, Atlantic white-sided dolphins use clicks and whistles to communicate. Their sounds are unique and high-pitched. Listen here to find out how they sound:
Photos by: Dr. Elliott Hazen NMFS/SWFSC/ERD | NOAA/MBARI 2006 | Ocean Image Bank – Hannes Klostermann
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