Whales and sound - the full story
The controversy over high frequency whale communication being disturbed, as well as the animals themselves, by medium frequency noise, rages on. Measurements have usually been taken to try and find out how different toothed whales are affected by MFA military sonar. This is because these Ziphid whales are the commonest stranded whales. Baleen whales, despite being the largest ever living creatures, were omitted from most surveys.
Anthropogenic sounds of low frequency were thought to mask calling behaviour in the Ziphid toothed whales, using multiple stranding records. Blue whales, Balaenopterus musculus, which frequent the Southern Californian Bight, where the US Navy is very well-known.
The significant effect of quite inoffensive sounds on the deep-feeding behaviour of blue whales is a major finding. From simply stopping feeding to speed and direction of travel, the whales were literally deeply affected. Each individual was digitally tagged then followed remotely for a period of 30 minutes. Another 30 minutes of noise exposure followed, another monitoring ensured that behaviour change could be recorded for a second neutral period.
The sound used was either pseudo-random noise (PRN, used as a control) or simulated military MFA (as in the US Navy) signals, but very much lower in source level. The minimum range was 200m and the starting level was 160dB at one minute, increasing by 3db every 25 seconds until 210dB was reached. This was always the maximum level attempted. The similar level limits applied to PRN, control noise were 206dB. The whales' sensors conveyed data about their personal data as well as sound received. The three sets of these data comprised:
Of the 17 whales involved, individuals varied in response, as the surface feeding animals all maintained their previous behaviours. If the individual was non-feeding or deep-feeding, however, the effect was very significant. The context of each individual's circumstances as well as the sound variability affected the response a lot.
Brief avoidance responses were one way of translating their movements, but these responses only took place in certain conditions. Being on the surface obviously required less response, for example, from most individuals. The response of deep-diving whales in stopping their feeding could cause their population big problems, as the distance at which this sound affects them has not yet been investigated.
That means they could use a lot of energy in diving, then frequently have to return to the surface, losing valuable krill catches and potentially damaging their survival where such species are endangered. Confusion of the sounds with those employed by killer whales is likely to be another reason for some very high speed flight response. The other facets of the whales' behaviour, however, didn't indicate a true flight from harm like the known response to the killers' presence.
Generalised avoidance of danger is the only real behaviour that could be assumed. MFA sonar exercises are regularly carried out in the area. The whales would therefore have experience of them. As a first demonstration of baleen response to MFA, the experiments indicated complex behaviour, even at very low-received levels. We need to protect these giants. To do so may mean breaking a few naval hearts.
Jeremy A. Goldbogen and his many colleagues from the US and Scotland wrote the paper in the Proceedings of the Royal Society B: Biological Sciences, entitling it, "Blue whales respond to simulated mid-frequency military sonar."
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