Collaborating scientists from ten research and conservation centres (nine across the USA and one in Fife) found that mid-frequency sounds, such as those used for military sonar, induced disruption of feeding and displacement from high-quality prey areas, which could have significant and previously undocumented impacts on whale foraging ecology, individual fitness, and population health.
Mid-frequency military sonars have previously been associated with lethal mass strandings of deep-diving toothed whales, but the effects on endangered baleen whale species (including the blue whale) have been virtually unknown.
The conservationists carried out 17 controlled exposure experiments with simulated military sonar to measure behavioural responses of tagged blue whales in Southern Californian feeding areas. Despite using source levels orders of magnitude below some operational military systems, the results demonstrate that sonar can significantly affect blue whale behaviour, especially during deep feeding.
The behaviour or the whales was monitored for 30 minutes pre-exposure, during a 30 minute exposure period, and for 30 minutes post-exposure. When a response occurred, behavioural changes varied widely from cessation of deep feeding to increased swimming speed and travel away from the sound source depending on their initial behavioural state and the received sound level.
Whales rely on large aggregations of dense krill to sustain their extreme body size. They continuously dive and feed throughout the day when high-density prey patches are present. Therefore, the observed behavioural responses result in reduced foraging efficiency. The researchers calculated that based on an average observation, where, after the onset of sound exposure, the animal stopped foraging for a total of 62 minutes, this resulted in a loss of over one metric ton of krill. The energy content of this loss is equivalent to a blue whale’s minimum daily metabolic demands.
A further paper documenting this phenomenon, instead on Cuvier’s beaked whales, was published in another of the Royal Society’s journals, Biology Letters, today. The researchers, led by the University of St Andrews, found that the whales responded strongly to mid-frequency sounds at low received levels (89–127 dB): after ceasing normal fluking and echolocation, they swam rapidly, silently away, extending both dive duration and subsequent non-foraging interval. The observed responses occurred at levels well below current regulatory thresholds; equivalent responses to operational sonars could elevate stranding risk and reduce foraging efficiency.
Both of the papers are Open Access and so can be read free of charge on the Proceedings of the Royal Society B and Biology Letters websites.