How do whales and dolphins react to seismic surveys?

Contrary to the claims of many environmental Non-Government Organisations (eNGOs), any changes to normal behavioural patterns are either non-existent or minimal (such as slight course changes in order to avoid the vessel).  We know this by the extensive monitoring that is carried out during seismic surveys.  This monitoring always includes Marine Mammal Observers (MMOs) maintaining watch on seismic vessels and can also include aerial surveys, vessel-based monitoring from support vessels and even acoustic monitoring using either autonomous sea-bed recorders (loggers) or towed hydrophones.  Analysis of MMO reports provides a variety of observations including:

  1. dolphins often come in close to an operational seismic vessel, often riding the bow-wave while the source is active and, in some cases, even swimming in the bubbles generated by the array;
  2. pilot whales and minke whales are also often seen in close proximity to the vessel even if the seismic source has been operating for some time;
  3. numerous “first sightings” of whales are seen well within the designated exclusion or power-down zone (distance at which the sound source is reduced to the lowest possible level, ie 2km in Australia and 500m in UK and US) showing that they’ve been in the so called “danger zone” for some time before being detected; and
  4. many whales are first seen well outside the 2km exclusion or power-down zone but continue into the zone despite the source being operational.

In addition to MMO reports, aerial surveys have sometimes coincided with whales being in the vicinity of an operating seismic vessel and have provided the opportunity to closely monitor any behavioural responses of the whales relative to the seismic operations.

A report on the aerial surveys conducted while a seismic survey was being carried out to the West of Kangaroo Island (KI), South Australia, showed that blue whales continued to feed in the area, with no discernible behavioural reactions (such as avoidance of the operating vessel) to within about 2.5km of the active source before it was shutdown.  Fig 1 shows one such occasion:

Seismic vessel and blue whale spatial interrelationship over time

Fig 1. Seismic vessel and blue whale spatial interrelationship over time

The linked dots show the locations of the whale that was being monitored over a period of time. The linked crosses show the position of the source (ie the vessel), which, in this case, was shut down at 1401hrs when the distance between the blue whale and the active source was 2.5km.  The colour coding of the linked dots and linked crosses represent the movement of the blue whale relative to the movement of the source during the same time periods.  The key points that can be derived from this analysis are:

  1. the blue whale did not move away from the oncoming vessel – it continued to feed on the very small scattered krill swarms present just off the edge of the shelf, even though there were much larger krill swarms, where other whales were feeding, on the shelf to the north.
  2. the shortest distance between the blue whale and the active source was 2.5km. Given the precautionary power-down distance of 3km used for this survey, it is not known at what distance this whale would have responded to the approaching source.
  3. when the soft start recommenced as a result of the whale being more than 6km away from the source (ie. beyond the tail-buoy of the 6km streamer), it’s interesting to note that it foraged in the general direction of the source.

Additional evidence of the lack of reaction of whales to seismic surveys can be extracted from recordings of seabed acoustic loggers deployed before, during and after seismic surveys.  The following example from the Southern Margins of Australia shows that blue whales (and even fish!) continue to vocalise in an area where a seismic survey is being carried out:

5-day playback from acoustic logger showing seismic signatures, fish chorus and blue whale calls

Fig 2. 5-day playback from acoustic logger showing seismic signatures, fish chorus and blue whale calls

Fig 2 is a 5-day playback of the sound recorded at a sea-bed acoustic logger just before and during the first few days of a seismic survey conducted in the Otway Basin offshore Portland in Victoria, Australia, during November 2004. Key parameters for this plot are:

i)               Horizontal scale is time in hours/days, with every 12 hours annotated from mid-day on 9 November through to midday on 14 November 2004.

ii)             Vertical scale is frequency (ie pitch).

iii)            Colour coding is instantaneous sound energy level at each 1Hz of frequency

Key observations that can be made about this plot are:

1. the vertical asymmetrical features are seismic traverses being recorded towards and away from the logger. The asymmetry is due to the more rapid attenuation relative to time (distance) when the sound is heading shore-wards compared to ocean-wards;

Individual Fish "Pop"

Fig 3. Individual Fish “Pop”

2. the light blue sounds seen at about 1000Hz is a diurnal fish chorus which starts up at dusk, is most intense in the evening and continues through to dawn. An individual fish “pop”, which contributes to these events on the 5-day plot, is displayed in Fig 3;

3. blue whale calls can be seen in the 10-100Hz range as resonant harmonics.

Blue whale call

Fig 4. Blue whale call

The event at about 7pm on 9 November is most likely a series of blue whale calls (before the survey started) but similar signatures can be seen throughout the plot as the survey progressed. A single blue whale call from another logger is shown in Fig 4; and 4. there appears to be no diminution in calling during the survey.

Fig 5 below shows the unmistakable signature of a blue whale call recorded at the same time as the seismic pulses (which are the vertical lines every 10 seconds).

Blue whale call coinciding with seismic pulses

Fig 5. Playback of a seabed acoustic recorder (logger) showing seismic pulses and the last 85 seconds of a typical 120 second blue whale call. Vertical scale is 10-100Hz, horizontal scale is time with approximately 10 seconds between seismic pulses and colour coding is signal intensity.

All these observations demonstrate that the reactions of whales in the vicinity of seismic surveys are either non-existent or barely detectable in that they continue to feed, continue to occupy an area or continue on their travels without showing any behavioural changes.

Advertisements

Trackbacks

  1. […] science, such as other sounds in the ocean, the sounds of breaching whales, Antarctic ice sounds, the reaction of cetaceans to seismic sounds, attenuation of sound in water, etc, is covered in articles on this website. Perhaps this is why […]

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: