Subject: Sound: Sonar vs. whale strandings, cont'd (fwd)

Mike Williamson (pita@www1.wheelock.edu)
Thu, 19 Mar 1998 12:28:24 -0500 (EST)

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---------- Forwarded message ----------
Date: Sun, 15 Mar 1998 05:18:59 -0800
From: MARMAM Editors <marmamed@UVic.CA>
Reply-To: Marine Mammals Research and Conservation Discussion
     <MARMAM@UVVM.UVIC.CA>
To: MARMAM@UVVM.UVIC.CA
Subject: Re: Sonar vs. whale strandings, cont'd (fwd)

From: "Gordon, Jonathan" <jgordon@ifaw.org>

In response to Bill Rossiter's posting.  I have some information on a
sonar source operating at similar frequencies in the same geographical
area.

In the summer of 1992 "Song of the Whale", the research boat of the
International Fund for Animal Welfare, was working in the Liguarian Sea
Cetacean Sanctuary (between the French Italian Riviera and Corsica)
conducting acoustic surveys of dolphins.  On many occasions through the
summer, while the boat was between northwest Corsica and the coast of
Italy, Monaco and France, sonar signals were heard.  These consisted of
1/6 sec pulses that swept up from 4 to 5kHz and were either emitted as
single blips or back to back in repeated sequences as "screams".  These
components were repeated in a regular pattern consisting of 3 blips
separated by 7 seconds, followed after 10 seconds by a 2-second scream,
with the whole cycle repeated every 42 seconds.  On some occasions, these
sounds were so loud they woke up people sleeping below decks.  Eventually,
we stumbled upon the source of the sounds, an Italian Naval ship towing a
large fish-based sonar.

On one occasion "Song of the Whale" had a long encounter with pilot whales
while the sonar could be heard on the hydrophones.  Analysis of recordings
made at the time (by my colleague Luke Rendell) showed short-term
responses by the pilot whales to the sonar.  Whistle rates were higher
during and immediately after those part of the cycle with pulses.  (These
findings are being published as a note in Marine Mammal Science soon.)  No
possible source vessels could be seen during this encounters leading us to
assume that the sonar was at least 15 miles away.  "Song of the Whale" had
been hearing sonar pulses for five hours before the encounter, during
which we had steamed some 25 miles, and continued to hear it for 8 hours
after the encounter.  Presumably then, the pilot whales had been exposed
to these pulses for many hours and had had plenty of time to habituate.

An earlier version of this note, and also a paper that explores the
theoretical effects of this source on cetaceans (Nascetti,P., Perazzi A.,
and Hastrup, O.  1996.  An investigation of the Interaction between active
sonar operations and marine mammals.)  Appeared in European Research on
Cetaceans -10, proceedings of the tenth annual conference of the European
Cetacean Society, Lisbon, 11-13 March 1996.  Peter Evans ed.)

Nascetti et. al. modeled propagation of a source at a position in the
Ligurian sea (which, intriguingly, was the same location at which "Song of
the Whale" had encountered the source boat) for both summer and winter
conditions.  They present a table giving the characteristics of three main
types of military sonar sources used by the Italian Navy.

Type         Depth               Frequency(kHz)     Vertical Aperture
Source level

Bow L.F      5                        3.5                             15
230
Hull M.F.     5                        7.5                             15
220
VDS            150                    7.5                             15
220

VDS = variable depth sonar, i.e. towed fish.


They then assumed a danger level of 210dB, a "safe" level (no physical
damage) of 170dB and a non-interference level of 150dB.

(One might comment that the non-interference level of 170db was based on
experiments with grey whales using seismic pulses that showed 50%
"interference" not the complete lack of it.  For sounds of longer
duration, such as the screams we recorded, the grey whale experiments
suggest a threshold of 120dB for 50% interference would be more
appropriate.)

Based on these assumptions, Nascetti et al calculated that there was a
risk of physical injury within 100m of the bow sonar and ranges could not
be considered safe until they reached 800-1500m.

The author's do not give "non-interference" ranges and the propagation
loss diagrams are not reproduced well enough for these to be inferred from
the diagrams, However, they do state that they may exceed 20km.


Consider now, that in operational use these vessels are transiting the
world's oceans at speeds up to 20 knots and it soon adds up to a very
large surface area that is potentially being affected.


The Italian-based research and conservation group, Tethys, subsequently
contacted the Italian Navy and were assured that at least these tests
would not be continued in the Ligurian sea in future.  (I have had
unconfirmed reports that these sounds have been heard there since then
however.)  Although the Ligurian sea is an are of particular importance
for some cetaceans in the Mediterranean whales and dolphins are widespread
there and simply moving the trials out of this area would at best have
only reduced the risk of disturbing cetaceans.

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