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Cetaceans, bioindicators of noise pollution: understanding the changes of the marine environment
In January 2010, a report concerning the Descriptor of
Good Environmental Status under the EU’s Marine Strategy
Framework Directive (MSFD) for inputs of energy and noise
was released (Table 1). The main output of the report
concentrated in the definition of three indicators that are
presented in the following table:
Cetacean Acoustic Signals
As was explained before, the choice of cetaceans
as bio- indicators of oceanic acoustic pollution is not
coincidental. The marine environment, as with all envi -
ronments, is organized on the basis of the balance of
organisms inhabiting them; each one is positioned on
a specific trophic level that allows the development of
higher levels. Disruption in any of these levels unbalan-
ces the chain, in both senses. Faced with a problem
of conservation, the challenge of scientists is to find
an organism, sufficiently representative, that ’s to say,
whose balance and development may have an influence
on the balance and development of the rest of the food
chain, and use it as a bio- indicator against a contami -
nating source. Cetaceans, for their vital dependence
and almost exclusive relationship with sound informa-
tion, represent, up until now, the best bio- indicators of
marine acoustic pollution.
The auditory system of cetaceans is characterized by a
series of unique morphological adaptations: one of the
most interesting ones is the capacity to select frequen-
cies in order to distinguish acoustic images across audi-
tory channels, which act as frequency filters.
In a healthy organism, this frequency selectivity of the
ear (and of the acoustic signals which are produced and
received therein) is evolvable and directly in relation with
the specific use of its habitat, and as such, characteri-
zes each cetacean species. On the other hand, within this
frequency selectivity, the sensitivity of the ear in some
species allows the measurement of the physiological or
pathological condition of the auditory system in a prede-
termined individual, and to estimate its auditory capacity
to use its habitat.
Each of the 80 species of cetaceans relies on a complex
acoustic repertoire (see Table 2). This diversity of acoustic
signals, intra and interspecies, complicates any analysis
we make and considerably limits our capacity to adequa-
tely estimate the effects of a polluting sound source.
Each of the species that make up the order of cetaceans
offers a unique acoustic repertoire in direct relation with
the habitat where it has evolved over millions of years (Table
2). It is understood, that in order to detect prey, a coastal
species will need to extract precise short distance details
of the surrounding relief, while the absence of such relief
will require pelagic cetaceans (those living in the open
sea) to obtain information over medium and long distan-
ces to the presence of fish shoals or plankton blooms.
Notwithstanding, all toothed cetaceans share the same
acoustic production mechanism, which includes the projec-
tion of air across nasal air ducts and its exit by vocal lips,
situated on the top of the head. Throughout immersions
or dives, this air is recycled and permits them to vocalize,
with the aim of echolocation or communication depending
on the social context at that time.
TG11 Energy
to assess the descriptor
to be measured
Underwater noise - Low and mid-frequency
impulsive sound
High amplitude impulsive anthropogenic sound
within a frequency band between 10Hz and 10
kHz, assessed using either sound energy over
time (Sound Exposure Level SEL) or peak sound
level of the sound source. Sound thresholds set
following review of received levels likely to cause
effects on dolphins; these levels unlikely to be
appropriate for all marine biota. The indicator
addresses time and spatial extent of these
The proportion of days within a calendar
year, over areas of 15’N x 15’E/W in
which anthropogenic sound sources
exceed either of two levels, 183 dB
re 1µPa2.s (i.e. measured as Sound
Exposure Level, SEL) or 224 dB re
1µPapeak (i.e. measured as peak sound
pressure level) when extrapolated to one
metre, measured over the frequency band
10 Hz to 10 kHz
Underwater noise – high frequency impulsive
Sounds from sonar sources below 200 KHz
that potentially have adverse effects, mostly on
marine mammals, appears to be increasing. This
indicator would enable trends to be followed.
The total number of vessels that are
equipped with sonar systems generating
sonar pulses below 200 kHz should
decrease by at least x% per year starting
in [2012].
Underwater noise – low frequency continuous
Background noise without distinguishable sources
can lead to masking of biological relevant
signals, alter communication signals of marine
mammals, and through chronic exposure, may
permanently impair important biological functions.
Anthropogenic input to this background noise has
been increasing. This indicator requires a set of
sound observatories and would enable trends in
anthropogenic background noise to be followed.
The ambient noise level measured
by a statistical representative sets of
observation stations in Regional Seas
where noise within the 1/3 octave bands
63 and 125 Hz (centre frequency) should
not exceed the baseline values of year
[2012] or 100 dB (re 1µPa rms; average
noise level in these octave bands over
a year).
Table 1 : Noise Descriptor of Good Environmental Status under the EU’s Marine Strategy Framework Directive (MSFD)