Ministry of Defence asked for naval sonar data after whales stranded
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The MoD is being asked whether its recent Nato war games event around the coast of Sutherland was responsible for a number of cetaceans washed ashore.
Marine experts concluded that deep-diving whales washed ashore around Scotland's coast have suffered from the bends.
The MoD is now being asked formally if naval sonar used in the recent Joint Warrior exercise was responsible.
It is thought that the sonar waves can frighten deep-diving whales, forcing them to surface too quickly and leading to symptoms similar to decompression sickness, also known as the bends, in humans.
Investigators from the Scottish Marine Animal Strandings Scheme (SMASS) said the recent spate of beaked whale strandings was unusual.
But some had "an unusually high number and distribution of gas bubbles throughout the tissues" – suggestion decompression sickness.
"Given this was an obvious cluster of unusual cases, we were keen to gain what information we could from them," said a report posted by SMASS.
"The first two cases were Sowerby’s beaked whales (Mesoplodon bidens), both live stranded on the Lothian coast. These were both sub-adult males with significant trauma from the stranding process, but no other clear underlying infection or disease and, thankfully, no indication of entanglement, boat strike or ingestion of marine debris.
"Neither animal had fed recently but both were in reasonable body condition, suggesting an acute cause of death. Notably, both animals showed an unusually high number and distribution of gas bubbles throughout the tissues – especially lung, liver and intestinal mesentery.
"Given how sensitive beaked whales are to underwater noise, specifically naval sonar, we have to consider noise-mediated DCS as a possible cause for these two strandings. We are therefore in the process of trying to find data on sources of noise in this region, including putting a request for activity logs to the MoD following the recent Joint Warrior naval exercises."
The report added: "The last three cases were northern bottlenose whale (Hyperoodon ampullatus) strandings; one in the Western Isles and two in the Clyde sea lochs.
"All were sub adult animals, showing indications of live stranding but no evidence for disease or other trauma, entanglement, boat strike, plastic ingestion and no clear indication of DCS. Unlike the Sowerby’s, all these cases were reasonably decomposed which limits what diagnostics we can run, and unfortunately pretty much rules out any assessment of noise overexposure.
"One of the animals stranded in the Clyde had been positively identified as one of animals seen around the Clyde for the past couple of months, and this animal in particular was not in great body condition, had not fed recently and was dehydrated.
"Northern bottlenose whales are deep-water specialists where they hunt in deep ocean canyons, predominantly for squid. Whilst some areas of the Firth of Clyde are over 1000 feet deep, these sea lochs are not their typical habitat and it is not clear how much prey these loch regions can provide. It’s a worrying possibility that, as their prey decrease into winter, unless the remaining animals make it back out to open sea, we may see more strandings.
"We’ve certainly had more than the usual number of beaked whale strandings over the past few years and it is becoming quite clear just how important our waters are for these species; species we still know remarkably little about other than they are particularly susceptible to human impact."
The MoD has already been asked if sonar was deployed during Joint Warrior which could have played a part in a 21ft bottlenose whale's death at Stornoway.
The Hebridean Whale and Dolphin Trust's (HWDT) research vessel Silurian – which was used in the BBC series Blue Planet – normally tracks the controversial massive Joint Warrior exercise using hydrophones and other equipment.
Most of the training takes place around Scotland's north, north-west and north-east coasts and includes live-firing at ranges such as Cape Wrath in Sutherland.
"The trust believe it is important to monitor cetacean presence and behaviour during these exercises. Unusual behaviour has been documented historically and we are committed to better understanding the impact of these activities on cetaceans," say HDWT.
Naval sonar is still being investigated as a cause to why more than 90 deep-diving whales mysteriously washed up along the Scottish, Irish and Icelandic coasts in 2018.
During previous surveys, which have coincided with the Joint Warrior exercises, HWDT has observed minke whales moving at high speed and leaping clear of the water, at the same time as military sonar was detected on hydrophones.
A pod of pilot whales stranded in July 2011 at the Kyle of Durness in what is believed to have been Scotland's largest ever such event. Some 19 of the 70 whales died.
Four large bombs exploded underwater by the Royal Navy were later blamed by government scientists for the mass stranding.
A long-delayed report by the Department for Environment Food and Rural Affairs said that the noise from the explosions could have damaged the hearing and navigational abilities of the whales, causing them to beach and die.
But a spokeswoman for the MoD has said the Navy does all it can to ensure sonar is not damaging marine life.
"The MoD takes its environmental responsibilities very seriously; environmental impacts are always considered in the planning of military exercises. During the planning of the exercise Environmental Impact Assessments have been produced and findings implemented where required, such as for the use of active sonar and live weapons," she said.