Seismic testing on dolphins

Sound is important for the survival of marine mammals, as they use it for foraging, reproduction, communication, detection of threats and navigation. Weilgart (2007), Williams et al. (2015) and Erbe et al. (2018) reference marine mammals as being particularly sensitive to anthropogenic noise. Both lethal and sub-lethal physiological effects are possible when marine mammals are exposed to high intensityunderwater noises at close range. Potential effects include damage to body tissues (resembling decompression sickness in humans), damage to hearing and chronic stress. 

Gordon et al., (2003) and Gray and van Waerebeek (2011) reported a single pantropical spotted dolphin showing severe behavioural distress followed by ataxia near a seismic array. Mann et al. (2010) reported several incidences of permanent hearing loss in stranded odontocetes where exposure to high levels of anthropogenic noise cannot be dismissed.

There is very limited research on the impact of seismic blasts on dolphins.

A proposed control method for the management of acoustic disturbance was a shutdown zone for dolphins. Another proposal is the use of Marine Fauna Observers (MFO) to watch for marine fauna during the course of the survey. It is believed that having one observer on board a vessel is inadequate, as their ability to monitor the water around the entire vessel is impeded. There is no way for the observer to see behind the vessel and the observer’s view is diminished in the dark, making it almost impossible to see dolphins and whales.  

It must be recognised that dolphins and whales can swim considerable distances, so an aerial survey 7 days before blasting is impractical and ill-advised.

Marine fauna observers on board seismic testing vessels to look for dolhpins are inadequate. How can an observer on the bridge see what dolhpins are around behind them, beside them, they can only see what is in their peripheral vision, their vision further reduces at night as visibility decreases due to low light.  There was no place on the ship from which a marine fauna observer could monitor all sides of the ship at the same time to ensure that no dolphins are in the vicinity and safe from harm. In previous seismic survey proposals reported that small vessels were “not conductive to effective marine mammal monitoring” and are limited when the weather is poor and low visibility.

Passive Acoustic Monitoring (PAM) which involves surveying and monitoring marine animals and environments using sound recorded using acoustic sensors have been suggested as mitigation strategies to protect dolphins. PAM only works when dolphins are communicating, and are ineffective at determining the range and bearing of animals. 

Written by Tim Hawthorne

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