Behavioral responses of common dolphins to naval sonar

Despite strong interest in how noise affects marine mammals, little is
known about the most abundant and commonly exposed taxa. Social delphinids
occur in groups of hundreds of individuals that travel quickly, change
behavior ephemerally, and are not amenable to conventional tagging
methods, posing challenges in quantifying noise impacts. We integrated
drone-based photogrammetry, strategically-placed acoustic recorders, and
broad-scale visual observations to provide complimentary measurements of
different aspects of behavior for short- and long-beaked common dolphins.
We measured behavioral responses during controlled exposure experiments
(CEEs) of military mid-frequency (3-4 kHz) active sonar (MFAS) using
simulated and actual Navy sonar sources. We used latent-state Bayesian
models to evaluate response probability and persistence in exposure and
post-exposure phases. Changes in sub-group movement and aggregation
parameters were commonly detected during different phases of MFAS CEEs but
not control CEEs. Responses were more evident in short-beaked common
dolphins (n=14 CEEs), and a direct relationship between response
probability and received level was observed. Long-beaked common dolphins
(n=20) showed less consistent responses, although contextual differences
may have limited which movement responses could be detected. These are the
first experimental behavioral response data for these abundant dolphins to
directly inform impact assessments for military sonars.