SEISMIC SURVEYS AND HARBOUR PORPOISES: THE FACTS

There are heated debates on levels, frequencies, propagation distances of seismic exploration noise introduced into the water column, and effects on harbour porpoises (Phocoena phocoena) which has resulted in the promulgation of both factual and inaccurate information, depending on source consulted. It is agreed generally, that most seismic noise is lower frequency than other anthropogenic sources such as Mid-Frequency Active Sonar (MFAS), or sonars used in some site surveys (e.g. multibeam).

Without entering any debate, this page expands upon information already introduced in www.porpoisedetector.com about scientific facts on seismic exploration and harbour porpoises. For more information on seismic noise and cetaceans in general, see www.marinemammalseismic.co.uk or www.marinemammalseismic.com.

THE STONE & TASKER STUDY

To assess effects of seismic operations on cetaceans, Stone and Tasker (2006) examined visual Marine Mammal Observation (MMO) data from seismic surveys that occurred in UK and adjacent waters between 1997 and 2000 (for information on the role of an MMO, see www.marinemammalobserver.co.uk). Of the 201 surveys examined, 110 surveys utilised large airgun arrays (volumes above 1,300 cubic inches) and the remaining 91 were conducted using lower powered arrays (820 cubic inches or less).

Sighting rates of all species declined significantly during operations with large volume airgun arrays, but only sighting rates of small odontocetes, including harbour porpoises, reduced significantly during operations with low power arrays. No instances of harbour porpoises swimming towards seismic vessels were recorded when airguns were firing on either large or small arrays. The observers were also of the impression that harbour porpoises and other small odontocetes swam away from the vessel at a faster rate when airguns were active as opposed to inactive, but the observers had no way of measuring this, and therefore no evidence to support their theory.

The data from Stone and Tasker (2006) suggest harbour porpoises, and small odontocetes in general, display avoidance responses to active seismic sources.

Seismic survey vessel transiting passed an offshore platform. © OSC 2015.

Seismic survey vessel transiting passed an offshore platform. © OSC 2015.

THE PIROTTA STUDY: C-PODS

Pirotta et al. (2014) performed an investigation to see if activity patterns of harbour porpoises that remained in the area during a seismic survey were affected or not. They used an array of passive acoustic loggers (C-PODs, see www.cpodclickdetector.co.uk for general information on C-PODs) during August and September 2011, to record porpoise echolocation clicks (http://en.wikipedia.org/wiki/Animal_echolocation). Their results indicated that porpoises remaining in the area during the seismic survey reduced their buzzing activity by 15%. Also, as distance from sound source increased, so did number of clicks detected, and probability of detecting clicks also increased.

These data suggest that the seismic survey impacted the porpoise’s activity negatively; however, this behavioural modification was only temporary and returned to baseline levels after the seismic activity had ended.

THE LUCKE STUDY: TEMPORARY THRESHOLD SHIFT

A study on the hearing threshold of a captive harbour porpoise following exposure to a seismic airgun was conducted by Lucke et al. (2009). Baseline data for the single experimental animal’s hearing threshold was obtained during the first part of the study, with the second part focusing on Temporary Threshold Shift (TTS). Temporary Threshold Shift occurs when an animal’s sensory hair cells are damaged by high sound levels, causing an upward shift in threshold of hearing. To test for TTS, the authors exposed the porpoise to a seismic impulse and then immediately measured the hearing threshold; a reduction in hearing sensitivity was regarded as evidence of a threshold shift. Hearing was tested at three frequencies 4 kHz, 32 kHz, and 100 kHz, with only one frequency being tested after each exposure. If no evidence of Threshold Shift (TS) for any of the three frequencies was detected, exposure level of the airgun was increased and the procedure repeated until a TTS was observed. Subsequent measurements of hearing sensitivity were then performed to determine when TTS ended, and to provide information regarding recovery. When hearing levels returned to normal, the session was terminated to prevent any long-lasting damage to the porpoise.

Temporary Threshold Shift was observed at 4 kHz after exposure to impulses with Sound Exposure Levels (SEL) of 164.5, 165.5, and 165.8 dB re 1 μPa2 s. Elevation of hearing threshold was observed during 32 kHz tests, but was not significant and did not meet the criteria for TTS. No change in hearing sensitivity was observed for 100 kHz tests. Sound Exposure Levels above 145 dB re 1 μPa2 s elicited avoidance responses. It should be noted that harbour porpoises have a peak hearing frequency of 130 kHz and therefore utilise much higher frequencies than that which TTS was observed (4 kHz). With this in mind, it is unclear how much TTS at 4 kHz will affect harbour porpoises in the wild.

THE THOMPSON STUDY

The Thompson et al. (2013) study is described in detail in www.porpoisedetector.com, and is the only study to date (2015) that assesses long-term avoidance behaviour of seismic surveys by harbour porpoises. The three studies listed above (Stone & Tasker 2006; Lucke et al. 2009; Pirotta et al. 2014) observed avoidance reactions, but due to their nature were unable to assess long-term displacement.

It’s not clear if there is an underlying reason for harbour porpoises to keep returning to the site in the Thompson et al. (2013) study; it could be a high-quality feeding ground and necessary for their survival. Alternatively, as seismic noise is predominantly low frequency (<1 kHz) and outside harbour porpoise peak hearing range (ca. 130 kHz), it is possible effects were small, and habituation occurred over time. It is clear that more research is needed to determine the effects of seismic surveys on harbour porpoises. Increased knowledge could in turn lead to modifications of mitigation zones (www.marinemammalriskmitigation.co.uk) for seismic surveys, and offshore construction, which are more accurate and beneficial for the animals and potentially stakeholders.

REFERENCES

Lucke K., Siebert U., Lepper P.A. & Blanchet M.A. (2009) Temporary shift in masked hearing thresholds in a harbour porpoise
(Phocoena phocoena) after exposure to seismic airgun stimuli. Journal of the Acoustical Society of America 125, 4060-70.
Pirotta E., Brookes K.L., Graham I.M. & Thompson P.M. (2014) Variation in harbour porpoise activity in response to seismic survey noise.
Biology Letters 10.
Stone C.J. & Tasker M.L. (2006) The effects of seismic airguns on cetaceans in UK waters. Journal of Cetaean Research and
Management 8, 255-63.
Thompson P.M., Brookes K.L., Graham I.M., Barton T.R., Needham K., Bradbury G. & Merchant N.D. (2013) Short-term
disturbance by a commercial two-dimensional seismic survey does not lead to long-term displacement of harbour porpoises. Proceedings of the Royal Society of London Series B-Biological Sciences 280.