Low-frequency noise from shipping is causing chronic stress in whales, a study has found.
The period of reduced ship traffic in Canada's Bay of Fundy following the terrorist attacks of September 11 allowed scientists to examine the effects on right whales' stress hormones, which can be detected from their feces.
"Essentially, the animals' stress levels dropped when the underwater ship noises did," says Professor Douglas P Nowacek of Duke University.
"There was a six-decibel decrease in underwater noise in the bay following 9/11, with an especially significant reduction in the low-frequency ranges below 150 hertz. This correlated to reduced baseline levels of stress-related hormone metabolites in samples collected from whales later that fall," he explains.
In subsequent years, he says, ship traffic – and noise – were higher, along with the whale's stress-hormone levels.
Most harmful, he says, are the sounds made by large ships, whose propellers and engines generate low-frequency noise that overlaps with the frequency band used by baleen whales for communication. These low frequencies travel very well through the ocean.
Whales have in the past been observed to react to increased noise with habitat displacement, behavioral changes, and alterations in the intensity, frequency and intervals of their calls.
However, this study marks the first evidence that exposure to the noise also results in physiological responses that could be harmful to the whales.