Auditory impairment from acoustic seal deterrents predicted for harbour porpoises in a marine protected area

1. Management interventions to reduce human-wildlife conflict can have
unintended consequences for non-target species. Acoustic deterrent devices
(ADDs) are used globally by the aquaculture sector. However, the potential
for these sound emissions to impact non-target species, such as cetaceans,
has not yet been quantified at population relevant spatial scales. 2. To
better understand the extent of potential impacts on cetaceans, such as
harbour porpoises, we used acoustic modelling to investigate levels of ADD
noise throughout the west coast of Scotland and across a Special Area of
Conservation (SAC) for this species. 3. Using an energy-flux acoustic
propagation model and data on aquaculture sites known to be using ADDs, we
predicted the spatial extent of ADD noise on the Scottish west coast from
the 1st February 2017 to 31st January 2018. Noise maps were produced to
determine the risk of auditory impairment for harbour porpoises under a
range of scenarios which assumed single or multiple ADDs and simultaneous
use across all sites. 4. The acoustic propagation model performed well
when tested against field measurements up to 5 km, with 98% of sound
exposure level (SEL) predictions within ±10% of the measurements.
Predictions of SELs over a 24-hour period suggested extensive temporary
hearing loss zones (median radius: ~28 km) for harbour porpoises around
aquaculture sites. Assuming a single device at each site, 23% of the
harbour porpoise SAC was predicted to be exposed to ADD noise sufficient
to induce a temporary threshold shift, and under the worst-case scenario
(multiple, continuously running devices per site with an aggregate duty
cycle of 100%), levels exceeding permanent threshold shift could reach
0.9% of the SAC. 5. Policy implications. This study highlights the
potential for ‘collateral damage’ from interventions such as acoustic
deterrent devices (ADDs) which are intended to reduce human-wildlife
conflicts with pinnipeds but may affect the long-term health and habitat
use of non-target species. This is especially true for harbour porpoises
which are protected under the EU and UK Habitats Regulations. The
aquaculture industry, policymakers and regulators in countries where ADDs
are used should consider these findings when attempting to mitigate
pinniped depredation.