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The ocial journal of the
ISBE
International Society for Behavioral Ecology
Behavioral Ecology (2014), 25(5), 1022–1030. doi:10.1093/beheco/aru029
Behavioral
Ecology
The ocial journal of the
ISBE
International Society for Behavioral Ecology
Behavioral
Ecology
Invited Review
Acoustic communication in a noisy world: can
fish compete with anthropogenicnoise?
Andrew N.Radford,a EmmaKerridge,a and Stephen D.Simpsonb
aSchool of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK and bBiosciences,
College of Life and Environmental Sciences, University of Exeter, Stocker Road, Exeter EX4 4QD, UK
Received 15 September 2013; revised 24 January 2014; accepted 12 February 2014; Advance Access publication 11 March 2014.
Anthropogenic (man-made) noise has changed the acoustic environment both on land and underwater and is now recognized as a
pollutant of international concern. Increasing numbers of studies are assessing how noise pollution affects animals across a range
of scales, from individuals to communities, but the topic receiving the most research attention has been acoustic communication.
Although there is now an extensive literature on how signalers might avoid potential masking from anthropogenic noise, the vast
majority of the work has been conducted on birds and marine mammals. Fish represent more than half of all vertebrate species, are
a valuable and increasingly utilized model taxa for understanding behavior, and provide the primary source of protein for >1 billion
people and the principal livelihoods for hunderds of millions. Assessing the impacts of noise on fish is therefore of clear biological,
ecological, and societal importance. Here, we begin by indicating why acoustic communication in fish is likely to be impacted by
anthropogenic noise. We then use studies from other taxa to outline 5 main ways in which animals can alter their acoustic signaling
behavior when there is potential masking due to anthropogenic noise and assess evidence of evolutionary adaptation and behavioral
plasticity in response to abiotic and biotic noise sources to consider whether such changes are feasible in fish. Finally, we suggest
directions for future study of fish acoustic behavior in this context and highlight why such research may allow important advances in
our general understanding of the impact of this global pollutant.
Key words: acoustic signaling, adaptation, anthropogenic noise, behavioral plasticity, fitness benefits, hearing, masking,
pollution.
INTRODUCTION
Anthropogenic (man-made) noise has changed the acoustic land-
scape of many areas around the globe and is now recognized as
a pollutant of international concern (e.g., inclusion in the US
National Environment Policy Act and the European Commission
Marine Strategy Framework Directive, and as a permanent item on
the International Maritime Organization Marine Environmental
Protection Committee agenda). Noise-generating human activities
in aquatic environments, such as commercial shipping, recreational
boating, pile-driving, seismic exploration, and energy production,
are widespread and occur with increasing frequency (McDonald
et al. 2006; Normandeau Associates 2012). In terrestrial environ-
ments, the prevalence of transportation networks, resource extrac-
tion, and urban development, for example, is similarly greater now
than ever before (Watts etal. 2007; Barber etal. 2009).
In addition to increasing the amount of noise, human activities
often generate sounds that are very dierent from those arising
from natural sources (Hildebrand 2009; Popper and Hastings 2009;
Normandeau Associates 2012). Common and consistent ambient
noises can exert a strong selective influence on the frequencies used
by species to communicate acoustically; adaptation to the acous-
tic environment can include utilization of available “windows” in
the background frequency range (see Brumm and Slabbekoorn
2005; Lugli 2010). Anthropogenic noises often have prominent
frequencies within those naturally occurring windows (McDonald
etal. 2006; Barber et al. 2009) and thus have the potential to dis-
rupt communication eciency. More generally, anthropogenic
noises may dier from abiotic or biotic sounds in such acoustic
characteristics as constancy, rise time, duty cycle, and impulsive-
ness (Hildebrand 2009; Popper and Hastings 2009; Normandeau
Associates 2012). For instance, pile-driving generates high-energy
impulsive sounds, which are characterized by a rapid rise time to
a maximal pressure value followed by a decay period during which
there is gradual reduction in the oscillating maximal and mini-
mal pressure fluctuations. Underwater transmission of explosions
includes an initial shock pulse followed by a succession of oscillat-
ing bubble pulses. Anthropogenic noise, therefore, presents a very
real, and often novel, challenge to animals.
It is well established in humans that anthropogenic noise can
cause physiological, neurological, and endocrinological problems;
cognitive impairment; sleep disruption; and an increased risk of
Address correspondence to A.N. Radford. E-mail: andy.radford@bristol.ac.uk.
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Radford etal. • Acoustic communication in a noisy world
coronary disease (World Health Organization 2011; Le Prell et al.
2012). In the last decade, there has also been a burgeoning research
interest in the potential impacts of noise on nonhuman animals;
a recent survey of the peer-reviewed literature showed that more
than 30% of studies published by the end of 2011 appeared within
that last year alone (Radford et al. 2012; see also Morley et al.
2014). Eects have been demonstrated in a variety of taxonomic
groups across a range of scales, from the physiology and behav-
ior of individuals to changes at the population and community
level (see Tyack 2008; Barber etal. 2009; Slabbekoorn et al. 2010;
Kight and Swaddle 2011 for reviews). Despite the overall breadth
of considered impacts, one topic—acoustic communication—has
dominated research attention; nearly 60% of terrestrial studies, for
example, have considered this aspect of behavior (Radford et al.
2012; Morley etal. 2014).
The most obvious way in which anthropogenic noise can dis-
rupt acoustic communication is through masking, whereby there
is an increase in the threshold for detection or discrimination of
one sound as a consequence of another (Fay and Megela-Simmons
1999; Brumm and Slabbekoorn 2005). Masking can be complete,
when the signal is not detected at all, or partial, when the signal
is detectable by the listener but the content is hard to understand
(Clark et al. 2009). Communication gets more dicult as back-
ground sounds increase for all vertebrates that have been studied,
including birds, marine mammals, fish, and amphibians (see Fay
and Megela-Simmons 1999; Brumm and Slabbekoorn 2005; Clark
etal. 2009; Dooling etal. 2009). Individual fitness can consequently
be compromised, either through eects on survival, if say signals
indicating a predation threat are unheard or altered (Lowry et al.
2012), or as a result of impacts on reproductive success, arising
from incorrect assessment of the quality of rivals or potential mates
(Halfwerk etal. 2011) or from disrupted communication between
parents and ospring (Leonard and Horn 2012). Numerous studies
have, therefore, investigated how eective acoustic communication
can be maintained despite rising levels of anthropogenic noise; in
particular, research has focused on how signalers may enhance the
likelihood of being heard and of conveying their intended message
accurately.
The vast majority of the work examining how anthropogenic
noise aects acoustic signaling behavior has been on birds and
marine mammals (e.g., Miller et al. 2000; Slabbekoorn and Peet
2003; but see Sun and Narins 2005; Rabin et al. 2006; Lampe
et al. 2012 for exceptions). Although it has become increasingly
apparent in recent decades that many fish species also communi-
cate acoustically (Ladich etal. 2006) and that noise has the poten-
tial to alter the likelihood of detection of these signals (Amoser
et al. 2004; Vasconcelos et al. 2007), the ways in which the
acoustic behavior of this taxonomic group is aected by anthro-
pogenic noise has received virtually no direct empirical attention
(see Picciulin et al. 2012 for an exception). Fishes represent more
than half of all vertebrate species, possess a broad range of hear-
ing and sound-production mechanisms, and exhibit a diverse array
of vocal, reproductive, and social traits (Bone and Moore 2008).
Consequently, fish are a valuable, and increasingly utilized, model
taxa for understanding behavior. Additionally, fish provide the pri-
mary source of protein for >1 billion people and the principal live-
lihoods for hunderds of millions (FAO 2012). Because the majority
of fish species live in coastal or freshwater environments, they are
exposed to many forms of anthropogenic noise. Therefore, assess-
ing the impacts of noise on fish is of clear biological, ecological,
and societal importance.
Here, we begin with brief overviews of acoustic communication
in fish and of the evidence that fish can indeed be aected by noise
generated from human activities (full reviews of these topics are
provided elsewhere). Combined, these bodies of work suggest that
fish acoustic communication is likely to be disrupted by anthropo-
genic noise. We then use studies on other taxa to outline 5 main
ways in which animals can alter their acoustic signaling behavior
when there is potential masking due to anthropogenic noise and
use evidence of evolutionary adaptation and behavioral plasticity
in response to abiotic and biotic noise sources to consider whether
such changes are feasible in fish. Finally, we suggest directions for
future study of fish acoustic behavior in this context and highlight
why such research may allow important advances in our general
understanding of the impact of this global pollutant.
WHY MIGHT THE IMPACTS OF
ANTHROPOGENIC NOISE ON ACOUSTIC
COMMUNICATION BE OF RELEVANCE TO
FISH?
Acoustic communication infish
More than 800 species of fish from over 100 families have been
documented to produce sounds, and many more are likely to do
so (detailed reviews in Tavolga 1971; Myrberg 1981; Hawkins and
Myrberg 1983; Ladich et al. 2006; Bass and Ladich 2008). As in
other taxa, acoustic characteristics of the sounds produced can
vary considerably between species and populations, in relation
to gender and size, and with fluctuations in motivation (Hawkins
and Rasmussen 1978; Myrberg et al. 1993; Parmentier et al.
2005; Verzijden etal. 2010). Sounds generated by fishes, therefore,
provide valuable information in a variety of dierent contexts,
including during territorial disputes and competition for food, pred-
atory attacks, courtship interactions, and spawning aggregations
(Myrberg et al. 1986; Hawkins and Amorim 2000; Amorim and
Neves 2008). Consequently, there is mounting evidence that acous-
tic communication can aect the survival and reproductive success
of fish (Rowe etal. 2008; Verzijden etal. 2010).
Fishes produce sounds in many varied ways, but they can be
broadly divided into those that arise incidentally from another
activity and those generated, often by specialized organs or struc-
tures, for communication (Tavolga 1971; Bass and Ladich 2008).
Unspecialized sounds include those resulting from feeding, move-
ment, or respiration. It is unlikely that these incidental sounds are
either under selection pressure or can be controlled flexibly by the
individual and thus they probably do not adapt across evolution-
ary time or exhibit plasticity in response to noise. Actively produced
acoustic signals include stridulation, drumming, and stringing.
Stridulation involves the rubbing together of mobile bony elements
such as teeth, jaws, fin rays, and vertebrae. For example, when
damselfish and clownfish open and close their mouths, bringing
into contact their pharyngeal teeth, sounds described as pops and
chirps are the result (Parmentier et al. 2007). Drumming sounds
arise from the high-frequency contraction and relaxation of sonic
muscles, which induces vibrations of the swim bladder wall. Cod
(Gadus morhua) and haddock (Melanogrammus aeglefinus), for instance,
use this mechanism, with their sounds described as knocks and
grunts (Hawkins and Rasmussen 1978). Some species, such as
croaking gouramis (genus Trichopsis), also generate sound through
vibrations of tendons within the pectoral fins (Henglmuller and
Ladich 1999). It is more plausible to expect that anthropogenic
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Behavioral Ecology
noise might result in changes in these specialized types of sounds
generated for communicatory purposes.
In general, sounds produced by fish for communication are
made up of pulses (Winn 1964; Myrberg and Spires 1972; Bass
and Ladich 2008). The wide interspecific diversity in sound char-
acteristics will influence the likelihood of a given species being
able to respond to noise created by human activities (see Francis
et al. 2011). Moreover, the type and extent of intraspecific varia-
tion will determine the capacity to minimize masking arising as a
consequence of anthropogenic noise. Although fish sounds most
commonly vary in temporal patterning, with receivers usually
extracting information from pulse number, duration, and rep-
etition rate, interindividual dierences in fundamental and domi-
nant frequency, bandwidth, harmonic structure, and amplitude
ratio have also been documented in various species (Hawkins and
Rasmussen 1978; Myrberg et al. 1993; Bass and Ladich 2008).
Thus, there is certainly the basis for adaptation across evolutionary
time. Intraindividual variation in acoustic structure is also known
to occur in a range of fishes, with fluctuations in relation to season,
time of day, context, level of competition, and motivation (see Bass
and Ladich 2008). Although some of this variation is due to dier-
ences in parameters such as temperature and circulating androgen
levels, which are not under the control of the individual, examples
of behavioral plasticity in sound production are becoming apparent
(Parmentier etal. 2010; Amorim etal. 2011).
Known impacts of anthropogenic noise onfish
Fish species dier greatly in their hearing abilities as a consequence
of dierences in ear structure and other anatomical features,
such as the presence of a swim bladder (Popper and Fay 1999;
Bone and Moore 2008; Popper and Schilt 2008). The most valu-
able measurements of fish hearing have considered both particle
motion and sound pressure (although all fishes can detect the for-
mer, only some are sensitive to the latter) and have been carried out
in the free field or at specialized acoustic facilities (e.g., Hawkins
and Chapman 1975; Popper et al. 2007; Halvorsen et al. 2012).
Many other studies have created audiograms (plots of the low-
est sound levels detectable at dierent frequencies) using auditory
evoked potential methods in standard tank conditions, which is an
approach that is now questioned (see Fay and Popper 2012 for a
full discussion). Accurate assessments of precise hearing abilities are
therefore relatively rare, but it is clear that although there are some
species that can hear above 100 kHz, and many with capabilities
above 3 kHz, the majority of fishes are likely to be able to detect
sounds from below 50 Hz up to at least 500–1500 Hz (Popper and
Fay 1999; Normandeau Associates 2012). Because most anthropo-
genic activities generate considerable noise at frequencies below
1 kHz (Normandeau Associates 2012), the potential for an impact
is readily apparent.
Conclusive empirical evidence for a negative impact of anthro-
pogenic noise on fish is rarer than for other taxa, most notably
birds. This is partly due to a smaller research eort to date, partly
because observational studies in natural conditions are often di-
cult to interpret, and partly because experimental work conducted
in captivity can indicate an eect of increased noise but lacks eco-
logical validity (see Normandeau Associates 2012; Slabbekoorn
2014 for a full discussion). However, there is sucient information
to suggest that the same range of impacts as found in other taxo-
nomic groups—behavioral responses furthest from the source, with
an increasing likelihood of physiological impacts, hearing dam-
age, injury, and death with increasing proximity (Dooling et al.
2009)—may be apparent in fish (see Popper and Hastings 2009;
Slabbekoorn etal. 2010; Normandeau Associates 2012 for reviews).
It is important to note, though, that dierent noises can have dif-
ferent eects, and the same or similar noise sources may not aect
all species in the same way; intraspecific dierences are also likely
(see Radford et al. 2014). For example, some fish species but not
others may suer injury or even death when very close to certain,
particularly impulsive, sound sources (Keevin and Hempen 1997;
Halvorsen et al. 2012). Likewise, some sources of anthropogenic
noise have been shown to cause, for instance, temporary threshold
shifts (transient reductions in hearing sensitivity) and stress, but only
in some of the tested species (Popper et al. 2005, 2007; Wysocki
et al. 2006, 2007). Catch rates, as indicators of movement away
from a sound source, have also produced contrasting results in dif-
ferent studies (Engås etal. 1996; Løkkeborg etal. 2012).
In general, because they can be caused by lower sound intensities
than other potential eects, the most important impacts might be
those on behavior, including acoustic communication. The acoustic
signals produced by many fish fall within a frequency band between
100 Hz and 1 kHz, making them vulnerable to anthropogenic noise
from a variety of sources (Ladich etal. 2006; Bass and Ladich 2008;
Normandeau Associates 2012). Several studies have suggested that
noise from boat trac, for example, could reduce the eective
range of communication signals and therefore the signaling e-
ciency between individuals (Amoser etal. 2004; Vasconcelos etal.
2007; Codarin et al. 2009). This is because detection distances are
reduced through masking (Codarin etal. 2009) and/or the auditory
sensitivities of receivers are diminished (Vasconcelos et al. 2007).
Only one study, however, has directly examined how noise might
impact fish acoustic behavior. Picciulin et al. (2012) found that the
mean pulse rate of brown meagres (Sciaena umbra) was higher follow-
ing repeated, though not single, boat passes compared with during
ambient conditions (it was assumed that the noise generated by the
boats was the causal eect, although this was not tested directly).
The observed increase in vocal activity could have arisen either
from an increased density of callers or from an increased acoustic
output by those individuals already calling (Picciulin etal. 2012).
HOW MIGHT FISH ACOUSTIC SIGNALERS
RESPOND TO ANTHROPOGENICNOISE?
Fish have not evolved in a quiet environment; as with most animals
that communicate acoustically, they face the problem of naturally
occurring, potentially masking, noise arising from abiotic sources
including wind, rain, and waves and biotic noise from chorus-
ing conspecifics or heterospecifics (see Luther and Gentry 2013).
Solutions to ensure the audibility of signals over background noise
could be manifested over 3 dierent time frames (see Brumm and
Slabbekoorn 2005). First, acoustic signals might be shaped by nat-
ural selection across evolutionary time. Long-term adaptations in
response to natural noise have been demonstrated in some fishes
(Lugli etal. 2003; Lugli 2010), with interspecific and interpopula-
tion dierences in frequency range recorded in a number of species
(Hawkins and Rasmussen 1978; Parmentier et al. 2005). Second,
an animal could potentially reduce the masking eects of habitat-
specific noise by making adjustments to signal properties during
its lifetime. Such ontogenetic changes are feasible in species that
exhibit vocal learning (e.g., passerine birds; Catchpole and Slater
2008) but are perhaps less likely in fish; although there is evidence
of changes in the acoustic characteristics of some fish signals with
age, vocal flexibility in this regard has not been documented (see
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Radford etal. • Acoustic communication in a noisy world
Vasconcelos and Ladich 2008). Third, animals may exhibit behav-
ioral plasticity to temporary changes in background noise. Some
fishes are known to be capable of such acoustic flexibility in certain
contexts (Luczkovich etal. 2000; Parmentier et al. 2010; Amorim
et al. 2011) although this may be less common than evolutionary
adaptation.
Studies of the impact of anthropogenic noise on acoustic signal-
ing in other taxa, primarily in relation to mate choice and territory
defense (e.g., Miller etal. 2000; Slabbekoorn and Peet 2003; Luther
and Baptista 2010; Bermudez-Cuamatzin etal. 2011), but also par-
ent–ospring communication (Leonard and Horn 2012) and alarm
calling (Lowry etal. 2012), suggest 5 main ways in which fish might
alter their acoustic behavior. These are not necessarily mutually
exclusive—multiple adaptations for communication in noisy condi-
tions could occur (Brumm etal. 2004)—and there may be a trade-
os between them—for example, increasing amplitude may be
energetically costly and so result in a compromise in terms of sig-
nal duration (Fernandez-Juricic etal. 2005). Research investigating
responses to nonanthropogenic noise sources allows an assessment
of whether these putative changes in signaler behavior might be
feasible in fish. However, it is important to note that relevant studies
have been conducted on only a small minority of the >32 000 spe-
cies of known fish; given their great diversity, generalizations should
be avoided.
Avoidance ofnoise
The simplest means of avoiding the potential impacts of anthropo-
genic noise is to move away from the source. However, this is not
always possible if the source dominates certain frequencies, as is the
case with low-frequency shipping noise (Wright etal. 2007), or if
an entire area is aected, as might occur in harbors and estuaries
subjected to large amounts of shipping. Also, if a species is depen-
dent on a particular area because of crucial resources, such as food
or nesting sites, or is restricted by the geography of the region, then
there may be no option but to remain despite the noise. An alter-
native way to maximize signal transmission is through temporal
adjustments in communication, taking advantage of inherent gaps
or fluctuations in competing noise to enhance the likelihood of sig-
nal detection and discrimination. A number of diurnal bird spe-
cies have been shown to sing more at night when there is greater
daytime competition from similar sounding species (see La 2012)
and that adjustment in timing might also be selected for in response
to sources of anthropogenic noise that are variable over time. For
example, by singing at night in areas where there are high levels of
daily urban noise, European robins (Erithacus rubecula) may benefit
from minimizing acoustic competition or from an increase in the
clarity of their signal (Fuller etal. 2007).
A single study of silver perch (Bairdiella chrysoura) provides the only
suggestion to date that adaptive suppression of calling in response to
external stimuli might occur in fish. Following natural occurrences
or playbacks of whistles from bottlenose dolphins (Tursiops trunca-
tus), a major predator of the perch, sound levels recorded from the
chorusing fish were significantly reduced (Luczkovich etal. 2000).
This reduction is most likely due to the cessation of calling by indi-
viduals close to the sound source, although it is also possible that
fish in the vicinity moved away from the potential danger, causing
a decrease in the amplitude of the chorus (Luczkovich etal. 2000).
Either response would also aid in minimizing acoustic overlap with
a nonthreatening but potentially masking influence (i.e., anthropo-
genic noise) although that remains to be tested directly. Moreover,
it is currently unknown if the ability to adjust calling flexibly in this
way exists in other fish species.
Temporal adjustments
Perceptual studies have shown that the detectability of brief acous-
tic signals is considerably enhanced by increasing their duration, as
a consequence of the temporal summation of signal energy in the
peripheral auditory system of receivers (see Brumm and Slabbekoorn
2005). Some animals, such as common marmosets (Callithrix jacchus)
and killer whales (Orcinus orca), appear to take advantage of this eect
by extending the duration of their calls in response to temporarily
elevated, man-made noise (Brumm etal. 2004; Foote etal. 2004). For
longer acoustic signals, an increase in duration cannot be explained
by exploitation of temporal summation but could increase the proba-
bility that some of the signal is given during a quieter period in terms
of the background noise; this could be the case with some whale
vocalizations (e.g., Fristrup et al. 2003; Di Iorio and Clark 2010).
The likelihood of detection can also be enhanced through increased
redundancy, achieved either by repeating the signal or through an
increase in the rate of calling. Various whale, frog, and bird species
have been shown to respond in this way to either playback or natu-
ral sources of anthropogenic noise (Lesage et al. 1999; Kaiser and
Hammers 2009; Diaz etal. 2011).
The potential for evolutionary changes in the temporal structure
of fish acoustic signaling is evidenced by the geographic variation
observed in the skunk clownfish Amphiprion akallopisos, with popula-
tions in Indonesia and Madagascar having dierent call character-
istics (Parmentier etal. 2005). The pulse duration of “short pops,”
1 of 3 call types produced, is longer in the Madagascan popula-
tion, but the number of peaks per pulse is higher in the Indonesian
population. For “long pops,” pulse period is longer, but the number
of peaks is lower in the Indonesian population compared with the
Madagascar population. It is not known whether the call param-
eters are dierent because of adaptation to dierent local abiotic or
biotic sources of noise or as a consequence of genetic drift in repro-
ductively isolated populations (Parmentier et al. 2005). However,
there clearly exists the capacity for changes in the duration and rate
of calling over evolutionary timescales, at least in this species.
There is also evidence that some fishes can respond flexibly
in terms of the temporal structure of their calling. For example,
certain damselfish (Pomacentridae) produce acoustic signals with
dierent pulse rates depending on whether they are interacting
agonistically with conspecifics or heterospecifics (Mann and Lobel
1998; Parmentier etal. 2010). Other species, such as the gulf toad-
fish (Opsanus beta), reduce their call rate when a predator is nearby
(Remage-Healey etal. 2006). Male gulf toadfish, which produce a
characteristic boatwhistle advertisement call to attract females, have
also been shown to increase their call rate to compete acoustically
with nearby rivals (Fine and Thorson 2008). Amore recent study
on Lusitanian toadfish (Halobatrachus didactylus) has demonstrated
that males reduce their call duration and pulse period at low tide
(Amorim etal. 2011), potentially because low-frequency sound rap-
idly attenuates in shallow water, so any calls produced would not
be detected by distant females (Mann 2006). Behavioral plasticity
in response to anthropogenic noise might, therefore, be feasible in
terms of such acoustic characteristics.
Amplitudeshifts
Animals experiencing elevated noise levels may increase the signal-
to-noise ratio during communication by raising the amplitude of
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Behavioral Ecology
their vocalizations, a response known as the “Lombard Eect”
(Brumm and Zollinger 2011). To date, the Lombard Eect in
response to anthropogenic noise has been demonstrated in a vari-
ety of species, including beluga whales (Lesage et al. 1999), killer
whales (Holt etal. 2009), common marmosets (Brumm etal. 2004),
domestic fowl (Gallus gallus domesticus: Brumm et al. 2009), and
nightingales (Luscinia megarhynchos: Brumm and Todt 2002; Brumm
2004). Nightingales in noisier territories were neither bigger nor
heavier than those in quieter territories, eliminating the possibil-
ity that the ability to sing at higher amplitudes is only exhibited by
individuals that are big enough to enable them to do so (Brumm
2004). Brumm and Todt (2002) also demonstrated that nightingales
do not just sing at maximum amplitude but regulate vocal intensity
depending on the level of masking noise and whether it is within
the spectral region of their ownsongs.
To date, there is little evidence that fish adjust the amplitude of
their acoustic signaling in response to background noise. Whether
they have the capacity to do so is likely to be constrained by body
size as well as by the energetic costs of producing louder sounds
(see Oberweger and Goller 2001). It is notable that all the exist-
ing examples of the Lombard eect are from birds and mammals.
Although anurans, for example, are capable of varying the ampli-
tude of their calls, there is no evidence that they do so in response
to elevated noise levels (see Love and Bee 2010). It is also likely that
noise-dependent regulation of signal amplitude does not occur in
insects because there seems to be strong selection for increased
loudness in this group, meaning they are often signaling close to
their power capabilities anyway (Gerhardt and Huber 2002). As
with many insects and frogs, fishes often call in aggregations (e.g.,
Luczkovich et al. 2000; Amorim et al. 2011) where a Lombard
eect would quickly escalate and lead to all males signaling at max-
imum levels (Brumm and Zollinger 2011). Another potential reason
for a lack of control over the amplitude of acoustic output would
be if auditory feedback does not play a role in sound production;
this is known to be the case with stridulating insects (reviewed in
Gerhardt and Huber 2002), but whether it is also the case in fish
requires further research before conclusions can be drawn.
Frequencyshifts
Laboratory studies have shown that sounds with a greater band-
width and a higher rate of frequency modulation are harder to
detect from noise (Lohr etal. 2003), and animals in habitats with
high levels of natural noise converge on vocalizations with primar-
ily pure tones (e.g., Dubois and Martens 1984). To date, only one
study has documented such a change in response to anthropogenic
noise, with red-winged blackbird (Agelaius phoeniceus) songs exhibit-
ing increased signal tonality due to an emphasis on lower frequen-
cies (Hanna et al. 2011). More commonly, birds and cetaceans
produce songs or calls with a higher average minimum or funda-
mental frequency at times or in areas with more low-frequency
noise from sources such as trac and seismic surveys (Lesage etal.
1999; Slabbekoorn and Peet 2003; Fernandez-Juricic etal. 2005;
Parks etal. 2007). There are also examples of animals adjusting the
relative amplitude of dierent frequency components: California
ground squirrels (Spermophilus beecheyi), for instance, shift the peak
energy of their calls from lower to higher harmonics when there
is low-frequency noise from wind turbines (Rabin et al. 2006).
Frequency shifts in response to noise could arise through evolution-
ary adaptation, as evidenced by a 30-year study of white-crowned
sparrow (Zonotrichia leucophrys) songs: there was an increase in mini-
mum frequency as urban noise increased across time (Luther and
Baptista 2010). Afrequency change could also come about through
behavioral plasticity if individuals prioritize the use of higher fre-
quencies within their existing repertoire. Such acoustic flexibility
has been demonstrated experimentally in birds, with individual
house finches (Carpodacus mexicanus) exposed to playback of urban
noise increasing the minimum frequency of their song elements
and also decreasing the frequency when lower levels of anthropo-
genic noise were transmitted (Bermudez-Cuamatzin etal. 2011).
Two Italian freshwater gobies (Padogobius martensii and Gobius
nigricans) provide evidence that some fishes can adapt their call fre-
quencies in response to abiotic noise sources. The gobies inhabit
environments where waterfalls produce low-frequency noise but
with a quiet window in the noise spectrum around 100 Hz (Lugli
etal. 2003). Both species exhibit the highest sensitivity to sound at
100 Hz and produce sounds with a fundamental frequency from
73 to 200 Hz (Lugli et al. 2003). Frequency dierences between
dierent populations of the same species of skunk clownfish
(A. akallopisos) (Parmentier et al. 2005) also suggest the possibility
of adjustments over evolutionary timescales. Although it was ini-
tially considered unlikely that fish could flexibly modulate their call
frequencies (see Bass and Ladich 2008), a recent study has shown
that male Lusitanian toadfish calling at low tide increase their fun-
damental frequency; as low-frequency sound rapidly attenuates in
shallow waters, this is likely to enhance the transmission of their
calls to females (Amorim et al. 2011). There is thus at least the
potential for such adjustments in response to anthropogenic noise.
Species that characteristically have higher frequencies within their
repertoire may be better able to respond to anthropogenic noise in
such fashion (see Francis etal. 2011). Dierent fish species produce
acoustic signals in dierent frequency ranges although they are
often structurally much simpler than, for example, birdsong, which
may provide reduced opportunities for immediate flexibility.
Change in signaling modality
Animals may also increase the ecacy of communication amid
noise by shifting emphasis to another modality (Brumm and
Slabbekoorn 2005; van der Sluijs etal. 2011). The “redundant sig-
naling” hypothesis proposes that animals have multiple signals so
that if one modality fails to transmit to the receiver, other signals
will successfully convey the message (Johnstone 1996). Many ani-
mal displays include various signal components, often in 2 or more
sensory modalities (e.g., Candolin 2003). However, to the best of
our knowledge, empirical testing of this possibility in response to
anthropogenic noise has yet to be undertaken in anytaxa.
Some fish species are certainly capable of shifting signaling
modalities depending on present conditions. For example, the rel-
ative importance of visual and olfactory cues in determining the
mate preference of female three-spined sticklebacks (Gasterosteus acu-
leatus) diers in clear versus turbid water (Heuschele et al. 2009).
The potential for anthropogenic noise to alter the relative impor-
tance of dierent sensory modalities, therefore, seems likely in at
least some species. Mate selection in Lake Malawi rock-dwelling
cichlids, for instance, depends on visual, chemical, and acoustic
cues (Plenderleith etal. 2005; Amorim etal. 2008). If boat trac
noise were to increase the masking of the latter, then a shift toward
one or other modality might be expected. It is worth bearing in
mind that signals in dierent modalities have dierent advantages
and disadvantages. In most terrestrial habitats, for example, visual
displays act at a much shorter range than acoustic signals and
thus long-range aspects of acoustic cues may not be readily com-
pensated for by visual alternatives. However, the more intense the
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Radford etal. • Acoustic communication in a noisy world
background noise (and, as a result, the shorter the communication
range of acoustic signals), the more important short-range visual
signals will become. For fishes, the trade-o may be somewhat dif-
ferent, as acoustic communication tends to be over relatively short
distances compared with, say, birdsong.
MOVING FORWARD
Although there is undoubtedly a rapidly increasing research interest
in the impacts of anthropogenic noise in general, and on acoustic
communication in particular, there are a number of steps that we
suggest would enhance considerably our current understanding.
Realistic masking experiments
Much early research examining the potential impact of anthropo-
genic noise used correlative, observational data and thus did not
provide suitable controls for potential confounding factors (see
Radford et al. 2012; Morley et al. 2014). If the eects of noise
at dierent natural sites are to be compared, ideally these should
be matched for other variables (see Francis et al. 2009, 2011).
Ultimately, however, it is carefully controlled experimental manip-
ulations (e.g., Ber mudez-Cuamatzin et al. 2011; Halfwerk et al.
2011) that are crucial to tease out the direct eect of noise and thus
provide the strongest possible conclusions.
High-quality noise-related experiments require accurate and
suitable characterization of the sound source (see Schaub et al.
2009; Morley et al. 2014). Underwater sound includes 2 compo-
nents: pressure (as detected by humans) and particle motion, which
results from the oscillatory displacement of particles back and forth
within a propagating sound wave. Although some fish species are
sensitive to sound pressure, all detect particle motion and thus mea-
suring this element of a sound source and determining the masking
impact are critical (see Normandeau Associates 2012). Moreover,
variation in characteristics beyond just absolute amplitude should
be considered; for instance, dierences in temporal patterns and
fluctuations in periodicity, frequency, and amplitude may well result
in dierent impacts. Relatively loud, continuous noise might be
expected to lead to long-term acoustic adjustments, whereas fluctu-
ating noise levels might result in acoustic plasticity (see Luther and
Gentry 2013 and references therein).
As with all experimental research, there are likely to be advan-
tages and disadvantages to dierent approaches (Slabbekoorn
2014). Captive studies often allow behaviors to be examined in
greater detail than those conducted in the wild, as well as oer-
ing potentially more control over the conditions and contexts of
the focal animals. However, aquatic tank setups result in complex
sound fields (e.g., Parvulescu 1964); although it is possible to deter-
mine the potential for an eect of additional noise, assessments of
absolute values for masking and the scale of impact are not fea-
sible. Moreover, captive animals are usually more constrained than
in the wild and may not exhibit their full behavioral repertoire.
Studies in the wild do not suer from these issues, thus providing
ecological validity, but they can be logistically much more challeng-
ing and do not normally allow the same level of control, provid-
ing more limited information in terms of subtle responses. Given
the current dearth of detailed knowledge relating to the impact of
anthropogenic noise in general, and on acoustic signaling in fish in
particular, we would advocate a complementary approach, includ-
ing full consideration of the relevant limitations where appropri-
ate (see also Slabbekoorn 2014). In general, masking experiments
should include readily controllable noise sources; the potential to
manipulate and measure the magnitudes, direction, and spatial
characteristics of both particle motion and sound pressure; and the
ability to collect and analyze response variables of most relevance.
Receiver’s perspective
Acoustic communication of course entails not only the actions of
the signaler, as considered in this review, but also signal perception
and discrimination on the part of the receiver. Sensory adaptations
and perceptual flexibility to enhance extraction of signals from
background noise include selective attention and auditory stream
segregation, mechanisms to counteract constraints on discrimina-
tion, and improvements in detection thresholds (see Brumm and
Slabbekoorn 2005). Such evolutionary adaptations and behavioral
plasticity are relevant not only to acoustic communication but also
to the use of natural sounds for habitat selection, settlement, preda-
tor avoidance, and prey detection (e.g., Simpson etal. 2005, 2011;
Schaub etal. 2009). However, although examples of these receiver
abilities in relation to natural noise sources are readily apparent in
a variety of taxa, including fish (Fay and Edds-Walton 1997; Lugli
etal. 2003), far less research attention has focused on how anthro-
pogenic noise aects receivers compared with signalers. Moreover,
assessments of fish hearing have rarely been conducted in acousti-
cal conditions that allow accurate measurement of the true sound
field in terms of both sound pressure and particle motion (Fay and
Popper 2012) and thus precise hearing thresholds for most species
are not yet known (see above). Use of current hearing–assessment
methods do potentially allow comparative research, as long as all
measurements are conducted in the same laboratory using the same
equipment, and thus provide an important tool when considering
the possibility of temporary or permanent threshold shifts; if the
hearing ability of the receiver is detrimentally aected by anthro-
pogenic noise, then acoustic communication will be compromised
(Vasconcelos etal. 2007).
Nonmasking effects ofnoise
In addition to masking signals, anthropogenic noise might also
aect acoustic communication in various indirect ways (Naguib
2013). For instance, studies have shown that noise can cause stress
in a range of taxa, with some preliminary indications in fish (e.g.,
Wysocki etal. 2006, but see Wysocki et al. 2007), and stress can
aect individual performance and decision making, with conse-
quences for any behavior, including communication (Kight and
Swaddle 2011). Moreover, noise may be distracting, shifting atten-
tion away from signals of relevance as well as impairing cognitive
performance; animals have a finite capacity to attend simultane-
ously to multiple stimuli (Chan and Blumstein 2011). Work in non-
communicative contexts has demonstrated that additional noise
in the environment can potentially distract fish and detrimentally
aect behavioral performance (Purser and Radford 2011). Noise
can also influence habitat choice, individual spacing, and popula-
tion density (Francis etal. 2009; Simpson etal. 2010); although less
is known about such eects in aquatic environments compared with
terrestrial species, there is evidence that anthropogenic noise can at
least temporarily aect space use by fish (Engås etal. 1996; Holles
et al. 2013). Alterations in the way conspecifics are distributed in
the environment could have consequences for the likelihood of
detecting particular signals, the number of rivals or potential mates
that can be assessed, and the eort and resources needed to acquire
that information; noise could influence not just individual com-
municative interactions but information flow through conspecific
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Behavioral Ecology
networks and whole communities (Naguib 2013). As yet, there is
hardly any empirical work assessing these indirect consequences of
anthropogenic noise on acoustic communication, and certainly not
in fish.
Fitness consequences
The vast majority of experimental studies investigating anthropo-
genic noise have, to date, considered relatively short-term eects
(see Radford etal. 2012; Morley etal. 2014 and references therein).
However, both for fundamental scientific progress in this field and
for successful policymaking and mitigation, assessments of the likely
impacts on individual survival and reproductive success are vital.
Some short-term eects (e.g., increased predation risk) can be trans-
lated relatively easily into ultimate fitness consequences; others,
including acoustic communication, need more careful consideration
because animals may be able to compensate in some way and thus
there may be no direct link between short- and long-term impacts
(Bejder etal. 2006). Although the masking of communication sig-
nals elicits demonstrable behavioral changes in many passerine bird
species and marine mammals, for instance, there is little evidence
to support resultant changes in fitness. That is not to say changes
in fitness do not arise, but rather that the experiments required to
determine them have rarely been conducted (but see Halfwerk etal.
2011; Kight et al. 2012). Moreover, adjustments in acoustic signal-
ing may result in greater energetic needs, an increased likelihood
of detection by predators, or the loss of vital information, but these
costs are rarely considered (see Read etal. 2014). Among aquatic
organisms, fish oer a more feasible opportunity than marine mam-
mals for a direct examination of the benefits and costs of acoustic
alterations made in response to anthropogenic noise and thus to
determine the impact on individual fitness and population viability.
CONCLUSIONS
The human population is projected to increase by 2.3 billion
between 2011 and 2050 (United Nations 2011) and thus noise
pollution is not just a pressing issue, but one of growing concern.
There is increasing recognition that sublethal impacts of anthro-
pogenic noise are perhaps the most important considerations for
populations of animals. Potentially, our greatest knowledge in
this regard currently relates to acoustic communication and par-
ticularly the way signalers minimize the risk of masking; certainly,
this is the research topic that has so far received the most atten-
tion. But, there is a strong taxonomic bias in those studies that have
been conducted; the majority focus on birds and marine mammals.
Although the range of sounds made by fishes is not as diverse as in
those taxa, their acoustic communication is also likely to be aected
by anthropogenic noise and their biological and societal value
mean that research into potential negative impacts is important.
The evidence available from other fields suggests that some spe-
cies of fish have the potential to compete with anthropogenic noise;
there are at least some of the same capabilities as birds and mam-
mals in terms of adaptation or flexible adjustment in their acoustic
signaling. Perhaps the most likely responses to anthropogenic noise
are changes in temporal patterning and signal modality although
alterations in frequency parameters and when to call may also be
feasible in some cases. The likelihood that a given species will be
able to respond in suitable fashion to a particular noise source, and
thus maintain ecient acoustic communication, will depend on the
mechanism of its sound production (not something that varies so
considerably in birds and mammals), the types of sound produced,
and the intraspecific and intraindividual levels of variation shown.
It is worth emphasizing that the body of relevant empirical work
is currently very limited, and the huge diversity among the more
than 32 000 species of fish means that extrapolations should be
treated with extreme care. The same noise may aect species dif-
ferently, dierent impacts can be expected at dierent spatial scales,
and there may also be variation between individuals of the same
species and even dierences by the same individual depending on
condition, state, and motivation. Clearly what is needed now are
experimental tests of these ideas, both to enhance our understand-
ing of how fish respond to anthropogenic noise and because studies
of this taxonomic group present an ideal opportunity to take the
whole field forward in exploring the ultimate eects of this global
pollutant on individual fitness, population viability, and community
structure.
FUNDING
A.N.R. and S.D.S. were partly supported by Department for
Environment, Food and Rural Aairs (DEFRA) contract ME5207;
S.D.S. was also supported by Natural Environment Research
Council (NE/J500616/2).
We are grateful to J.Purser, R.Bruintjes, E. Morley, I. Voellmy, S.Holles,
M.Wale, K.Everley, and T.Hawkins for fruitful discussions and to 2 anony-
mous referees for helpful comments on the manuscript.
Forum editor: Sue Healy
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