BECKYHITCHIN
The perils of deep diving
Becky Hitchin casts a scientific eye at the beaked whales , and the effect military sonar may have on their forays into the abyss
Beaked whales are a mysterious group of cetaceans . While there are over 24 species alive today , only a few are known with any degree of familiarity , even to the scientists who have spent their lives studying them . It may be surprising , therefore , that actually they are the second largest family of cetaceans , after dolphins .
This family of whales , as the name suggests , is grouped together by all the species having a ‘ beak ’ similar to those seen on some dolphins . They range from around 4m to 13m in length , and males can be identified from females by the presence of two teeth jutting out from the lower jaw , even sometimes protruding past the upper jaw . These are the only teeth they possess .
Of the beaked whales that we know about , the best known is Cuvier ’ s beaked whale , named after the father of palaeontology , Georges Cuvier ( 1769-1832 ), who originally identified the species . Blainville ’ s beaked whale , helpfully named after Cuvier ’ s student , and later academic rival , Henri de Blainville ( 1777-1850 ), could likely be considered the second best known .
So – what ’ s so special about these whales ? Well , at least two things . Firstly , they are incredible divers . Cuvier ’ s beaked whales are known to dive to depths of almost 3km , the deepest dives known by any mammal . At those depths , they can ’ t hold enough oxygen in their lungs for even half the dive , and the
Georges Cuvier
alveoli in their lungs completely collapse . Much of each dive relies on anaerobic respiration , especially when heading back to the surface .
Each species of beaked whale has a unique pattern of dives . Cuvier ’ s undertake one long , deep dive , usually as a hunting pack of three or four whales , followed by a series of ever shallower ‘ bounce ’ dives until they come to rest and recuperate at depths between 40-50m , getting rid of their accumulated lactic acid burden before the next deep dive . They dive so deeply to gain access to a rarely utilised prey resource – deep water squid , which they echolocate together before splitting apart to chase individual squid . Each deep dive needs to net them around 30 to 40 squid each . Way back in the days of Cuvier and Blainville , when decompression illness ( DCI ) was first being understood , it was thought that whales were not affected by decompression issues , and this remained the current thinking until 2003 , when gas emboli were found in the bodies of a pod of stranded Cuvier ’ s beaked whales . In a later stranding , similar gas bubbles were analysed , and this demonstrated that the gas came from nitrogen saturated tissues . Gas emboli – and the fat emboli also found - have been shown to cause nervous system , respiratory and cardiovascular distress and disorientation . In the inner ear , gas embolisms can also cause vestibular disorientation . But was this DCI ... or something else ?
All of which brings us onto the second reason why they are so special . They are extremely sensitive to noise at the frequency ( in the range of 1 – 10 kHz ), used for submarine detection by military sonar – and many of the strandings have been at the time of military exercises nearby using this particular sonar .
Is this a more likely cause for the gas bubbles found in stranded beaked whales ? Several mechanisms have been proposed to explain how sonar might instead lead to stranding and / or death of beaked whales . Maybe their dive profile is disturbed by the noise and that results in nitrogen accumulation and bubbles if they ascend unexpectedly , or too quickly . Maybe the sound actually causes damage to the tissues themselves . Maybe the noise from military active sonar frightens the whales , causing their heart rate to increase , causing DCS .
No-one is sure , but scientific consensus at the moment seems to be that the gas emboli are most likely being formed from either behavioural or physiological responses to sonar frequency sound , rather than DCI . Tellingly , mass strandings of Cuvier ’ s beaked whale ceased at least in the Canary Islands , once naval exercises where sonar was used were banned there .
We ’ ll come back to this issue next month , when I want to talk more widely about sound in the sea . Ships , construction activities for our green energy platforms , dredging , pretty much everything we do in the sea produces noise . Well , except for rebreather divers of course . You guys can be smugly satisfied for the next month or so – as long as you don ’ t hum tunes or sing songs during your dives . �
Cuvier ’ s beaked whale at the surface
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