Scientists analyzed underwater noise levels in Canada's Bay of Fundy during a period of reduced ship traffic following the attacks and compared stress-related hormone metabolites in the fecal samples of right whales before and after the attacks.
The analysis showed a strong correlation between noise and stress, they said, with implications for all whales in areas with heavy ship traffic and for the recovery of the endangered North Atlantic right whale population.
"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," researcher Douglas P. Nowacek of Duke University said.
"This correlated to reduced baseline levels of stress-related hormone metabolites in samples collected from whales later that fall."
"Essentially, the animals' stress levels dropped when the underwater ship noises did," Nowacek said in a university release Wednesday.
Sounds made by the propellers and engines of large ships are in at low frequencies that travel very well through the ocean and overlap the frequency band used by whales for communication, he said.
Stress from increased noise can result in habitat displacement, behavioral changes and alterations in the intensity, frequency and intervals of whales' calls, researchers said.