Baleen whales (Mysticeti) communicate using low-frequency acoustic signals. These
long-wavelength sounds can be detected over hundreds of kilometres, potentially allowing
contact over large distances. Low-frequency noise from large ships (20-200 Hz) overlaps
acoustic signals used by baleen whales, and increased levels of underwater noise have
been documented in areas with high shipping traffic. Reported responses of whales
to increased noise include: habitat displacement, behavioural changes and alterations
in the intensity, frequency and intervals of calls. However, it has been unclear whether
exposure to noise results in physiological responses that may lead to significant
consequences for individuals or populations. Here, we show that reduced ship traffic
in the Bay of Fundy, Canada, following the events of 11 September 2001, resulted in
a 6 dB decrease in underwater noise with a significant reduction below 150 Hz. This
noise reduction was associated with decreased baseline levels of stress-related faecal
hormone metabolites (glucocorticoids) in North Atlantic right whales (Eubalaena glacialis).
This is the first evidence that exposure to low-frequency ship noise may be associated
with chronic stress in whales, and has implications for all baleen whales in heavy
ship traffic areas, and for recovery of this endangered right whale population.