Noise from ships stresses out the whales, new research has shown.

Experts based at the Duke University and the New England Aquarium in the U.S. found that noise generated by engines of large ships overlaps the frequency range that some whales use for communicating, thereby increasing their stress level.

During the experiment, the scientists had studied the right whale's stress levels when ship traffic in the U.S. was dropped because of the September 11 attacks. They found out that stress level of the whale was quite low compared to the normal days. This clearly shows how whales are getting affected by shipping noise.

A rise in noise generated by ships can result in habitat displacement, behavioural changes and alterations in the intensity, frequency and intervals of whales' calls, according to the UPI.

"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," Douglas P Nowacek, researcher from the Duke University, told the UPI.

"This correlated to reduced baseline levels of stress-related hormone metabolites in samples collected from whales later that fall," he said.

"Essentially, the animals' stress levels dropped when the underwater ship noises did," Nowacek said in a statement.