Behavioral response of delphinids to sonar - supporting acoustic data

Oceanic delphinids around naval operations are regularly exposed to
intense military sonar broadcast within the frequency range of their
hearing. However, empirically measuring the impact of sonar on the
behavior of highly social, free-ranging dolphins is challenging.
Additionally, baseline variability or the frequency of vocal
state-switching among social oceanic dolphins during undisturbed
conditions is lacking, making it difficult to attribute changes in vocal
behavior to anthropogenic disturbance. Using a network of drifting
acoustic buoys in controlled exposure experiments, we investigated the
effects of mid-frequency (3-4 kHz) active sonar (MFAS) on whistle
production in short-beaked (Delphinus delphis delphis) and long-beaked
common dolphins (Delphinus delphis bairdii) in southern California. Given
the complexity of acoustic behavior exhibited by these group-living
animals, we conducted our response analysis over varying temporal windows
(10 min – 5 s) to describe both longer-term and instantaneous changes in
sound production. We found that common dolphins exhibited acute and
pronounced changes in whistle rate in the 5 s following exposure to
simulated Navy MFAS. This response was sustained throughout sequential
MFAS exposures within experiments simulating operational conditions,
suggesting that dolphins may not habituate to this disturbance. These
results indicate that common dolphins exhibit brief yet clearly detectable
acoustic responses to MFAS. They also highlight how variable temporal
analysis windows – tuned to key aspects of baseline vocal behavior as well
as experimental parameters related to MFAS exposure – enable the detection
of behavioral responses. We suggest future work with oceanic delphinids
explore baseline vocal rates a-priori and use information on the rate of
change in vocal behavior to inform the analysis time window over which
behavioral responses are measured.