cMB FUTURE SCIENCE BRIEF 
"here are some general messages emerging from the multiple 
investigations on behavioural response undertaken since 2008: 
The likelihood and intensity of the response depends 
an the physical properties of the received sound. Sound 
aressure level, frequency and duration (i.e. acoustic dose) 
are important factors that influence responses, but there 
are other properties that may be influential too (Southall 
et al., 2007; Hawkins et al., 2015); 
Reactions to the same sound input can be extremely 
variable within and across species, as well as within 
and between individuals, and seem to depend on 
additional contextual variables such as behavioural and 
ohysiological state, food availability, prior exposure, age, 
sex, season, time of day and many more (Ellison et al, 
2011; Hawkins et al., 2015; Harris et al.‚2018)}; 
The US National Research Council (NRC) developed a framework 
for investigating the Population Consequences of Acoustic 
Disturbance (PCAD;NRC, 2005), laterdefinedas PCoD,i.e. Population 
Consequences of Disturbance (Pirotta et al., 2018). Originally the 
work on PCAD focused on marine mammals, but recently PCAD / 
PCoD studies have also included fishes. The PCAD / PCoD model 
involves several steps describing how behavioural effects could 
cause further effects on life functions (e.g. feeding) which in turn 
can affect vital rates (e.g. survival and reproduction). Ultimately, 
this cascade can lead to effects at the population-level. One of 
:he challenges with PCoD is that the understanding of how 
disturbance can affect life functions and vital rates is extremely 
limited and more empirical data are needed (Pirotta et al., 2018). 
In Europe, a limited number of studies have been able to apply 
the PCoD framework. These include for example, investigations 
an the effects of offshore wind farm construction in the North 
Sea on harbour porpoises (King et al, 2015) and population 
zonsequences of acoustic exposure in cod (Mortensen et al, 
2021). 
Hearing impairment 
In the past decade there have been advances in our understanding of 
hearing impairment in marine mammals (e.g. sea lions and bottlenose 
dolphins; for a review, see Finneran, 2015) and to a lesser extent in 
fishes (see Popper et al, 2014). For both marine mammals and fishes, 
the nature and intensity of the effects depend on the sensitivity of 
the animal in question and the received dose of noise. In principle, 
multiple pulses (e.g. from pile-driving) have a larger effect than a 
single pulse as they increase the dose (Finneran et al, 2015; Popper 
et al., 2014, 2019). The recovery time from Temporary Threshold Shift 
{TTS) is a function of its severity. The larger the TTS, the longer it takes 
for the hearing to recover (Finneran, 2015; Breitzler et al., 2020). There 
is uncertainty about recovery from TTS for multiple pulses. This is 
yet to be considered in standard impact assessments. As pointed 
out before, there is now evidence that some marine mammals may 
also have evolved mechanisms of self-mitigation when exposed to 
potentially injurious noise. These include behavioural reactions that 
indicate anticipation and avoidance (Finneran, 2015) and reduction 
in hearing sensitivity when a loud sound was preceded by a faint 
warning sound (Nachtigall et al., 2014). The many unknowns in the 
Both fishes and marine mammals react to certain 
impulsive and continuous sound sources such as pile- 
driving, airguns, sonar and acoustic deterrent devices at 
relatively long distances of several kilometres (Morton 
& Symonds, 2002; Brandt et al., 2011; Thomsen et al., 
2012; Hawkins et al., 2014; Miller et al., 2014; Dunlop et 
al., 2018). Most of these effects are of short duration, 
but there have been cases where displacement was long 
term (e.g. Morton & Symonds 2002). Studying such long 
term changes in distribution due to noise is challenging 
due to the lack of adequate long-term species and noise 
monitoring programmes (Thomsen et al., 2011) but also 
due to potentially confounding factors such as habitat 
changes as a function of other human activities, 
field of impaired hearing in mammals arise partly because Permanent 
Threshold Shift (PTS) is always extrapolated and never intentionally 
tested for reasons of animal welfare. In the case of fishes, there is no 
evidence for PTS. Indeed PTS might not occur since hearing cells can 
regrow (Popper et al., 2019). 
Physical and physiological effects 
Physical and physiological effects have also become better 
understood in the last 13 years. In Boyd et al, (2008), studies 
on strandings of cetaceans due to military mid-frequency sonar 
were a high priority, reflecting the significant discussions within 
the scientific community at that time. Since 2008, much effort 
has been made to further understand the physiological causes 
and especially the behavioural mechanisms behind the stranding 
events, and our understanding is much improved. The most 
widely accepted explanation for the cause of strandings is that 
the received sonar pulses trigger an extreme behavioural reaction 
resulting in rapid dives and surfacing which lead to decompression 
sickness effects, similar to what happens to humans when getting 
the bends’, which in case of the affected whales can lead to fatal 
stranding (see Bernaldo de Quirös et al., 2019). Several controlled 
exposure experiments have shown that responses vary greatly 
between individuals and with behavioural state (Southall et al, 
2016). Strandings of marine mammals have also been reported 
concurrent with other activities, such as hydrographic surveys using 
multibeam echosounders (Southall et al., 2014). 
Concerning fishes, studies show that Barotrauma (= the physical 
damage to tissue caused by noise) and even mortality was found 
in response to high intensity impulsive sounds such as from pile- 
driving and explosions. As in the case of hearing impairment, the 
magnitude of injury was dependent on the received dose (Popper 
et al., 2014, 2019). 
For invertebrates, very few studies have been undertaken. Injury of 
tissue due to exposure to noise was found in molluscs in experiments 
ın tanks (Andre et al., 2011) and subsequently also in the wild (Sole 
et al., 2017). There is also evidence that noise from airguns causes 
mortality in zooplankton (McCauley et al., 2017). Wale et al., (2019) 
found evidence of shipping noise induced changes at multiple levels