Far-field effects of impulsive noise on coastal bottlenose dolphins

Increasing levels of anthropogenic underwater noise have caused concern
over their potential impacts on marine life. Offshore renewable energy
developments and seismic exploration can produce impulsive noise which is
especially hazardous for marine mammals because it can induce auditory
damage at shorter distances and behavioural disturbance at longer
distances. However, far-field effects of impulsive noise remain poorly
understood, causing a high level of uncertainty when predicting the
impacts of offshore energy developments on marine mammal populations. Here
we used a 10-year dataset on the occurrence of coastal bottlenose dolphins
over the period 2009-2019 to investigate far-field effects of impulsive
noise from offshore activities undertaken in three different years.
Activities included a 2D seismic survey and the pile installation at two
offshore wind farms, 20-75 km from coastal waters known to be frequented
by dolphins. We collected passive acoustic data in key coastal areas and
used a Before-After Control-Impact design to investigate variation in
dolphin detections in areas exposed to different levels of impulsive noise
from these offshore activities. We compared dolphin detections at two
temporal scales, comparing years and days with and without impulsive
noise. Passive acoustic data confirmed that dolphins continued to use the
impact area throughout each offshore activity period, but also provided
evidence of short-term behavioural responses in this area. Unexpectedly,
and only at the smallest temporal scale, a consistent increase in dolphin
detections was observed at the impact sites during activities generating
impulsive noise. We suggest that this increase in dolphin detections could
be explained by changes in vocalization behaviour. Marine mammal
protection policies focus on the near-field effects of impulsive noise;
however, our results emphasize the importance of investigating the
far-field effects of anthropogenic disturbances to better understand the
impacts of human activities on marine mammal populations.