A decrease in ship traffic in the wake of the September 11, 2001 attacks lowered the levels of stress hormones in the feces of endangered North Atlantic right whales in the Bay of Fundy, a new study shows.

This is the first evidence that exposure to ship noise is associated with chronic stress in whales, and has implications for baleen whales in heavy ship traffic areas, according to the report, published in the Proceedings of Royal Society B in London.

The findings are the result of an unplanned experiment, the report states.

Two teams of scientists had been in the area, studying whale singing and whale health, when commercial transportation around the world was brought to a standstill to assess security measures following the attacks.

That included shipping traffic into the Bay of Fundy, which is home to the busy Saint John Port and the principal summer feeding waters for right whales.

The slowdown resulted in a significant decrease in underwater noise from large ships, which can overlap low-frequency acoustic signals whales use to communicate for feeding or mating.

Previous studies have shown that whales have responded to the noise from ships with habitat displacement, behavioural changes and alterations in the intensity, frequency and intervals of calls.

But it has been unclear whether exposure to noise results in physiological responses that may lead to serious consequences for individual whales or populations.

The researchers, from a team led by Boston's New England Aquarium, found that large whales, like people, can show physical signs of increased chronic stress when exposed to elevated noise levels over prolonged periods of time.

Right whales are the most endangered large whales in the Atlantic, with only about 450 of them left. Their low population is thought to be a product in part of multiple stressors in their environment.