cCMB FUTURE SCIENCE BRIEF 
Figure 10. Marine Mammal Observers (MMO) can be used on board vessels to monitor the area for the presence of marine mammals,and call a halt to the 
noise generating activity when they are ahserven 
ı'he application of a mitigation zone around the sound source is a 
well-established strategy used across multiple industries. Here the 
aim is to ensure, as best as possible, that no animals of a certain 
species are present before starting or continuing the operations. 
Real-time monitoring of marine mammal presence using marine 
mammal observers (MMO), who survey the area and can halt to the 
aperation if they are detected, is one way of doing that. There are, 
however, limitations to this, since marine mammals are difficult to 
detect, especially in poor conditions for visual or acoustic detection. 
Guidelines for minimising the risk of injury have been developed 
and are applied around the world (e.g. JNCC, 2017). 
Acoustic deterrent devices (ADD), which emit noise either 
constantly or when approached, are another option to reduce the 
risk of injury by clearing the mitigation zone of certain species, 
especially for noise sources like explosions and pile-driving which 
zan be particularly damaging. Originally developed for use in fish 
Farms and fisheries, many new devices have come to market in the 
last 12 years (McGarry et al., 2020). However, recent studies have 
highlighted concerns that in some cases the resulting spatial and 
temporal footprint of disturbance could be larger than needed 
(Brandt et al., 2013), and therefore, ADD use should be optimized to 
achieve a defined deterrence range and avoid unnecessary far-field 
disturbance (Thompson et al., 2020). 
Operational measures to reduce sound at source can be used for 
most noise sources. For pile-driving for example, these include 
’amp-up procedures (or soft-start) ofimpact hammer energy where 
che hammer is operated at a lower energy initially, constraints on 
maximum impact hammer energy to the level required to achieve 
embedded depth, optimized pile-driving procedure, and duration 
for driving of single piles (OSPAR Commission, 2014). Real-time noise 
monitoring can be used in some instances to adjust parameters like 
hammer energy whilst operation is ongoing. 
Ramp-up (or soft-start) procedures, are also widely used for seismic 
surveys, military sonar and some sub-bottom profilers, either by 
switching on different components one by one until full power, 
or gradually increasing the sound levels. However geophysical 
surveys in the future are likely to be increasingly undertaken from 
autonomous vehicles, and those that hover just above the seabed 
would reduce noise propagation in the water column (Duarte et al, 
2021) and hence address some of the challenges posed by current 
seismic surveys.