This has been a popular story this week. After the September 11 attacks in 2001, sound in the ocean plummeted, due to the decrease in shipping traffic. Two studies of the North Atlantic right whale happened to be going on at that time, which of course had nothing to do with measuring whale response to lower levels of ocean noise.
One of the studies collected whale fecal samples for five years (using former drug sniffing dogs to lead them to the samples, nonetheless), and the other study collected acoustic data for a couple days before and after the attacks. It was not until 2009 that one of the researchers realized that they had inadvertently collected data showing (1) how much quieter the ocean became after the attacks, and (2) what the whale stress hormone levels were for a five year period including the period after the attacks.
The fecal samples show stress hormone levels in the whales was much lower during the period after the attacks, which corresponds with the drop in sound levels, than at any other time during the five year period. This, of course, is an uncontrolled experiment, so it could just be a coincidence. The researcher does point out, however, these animals weigh fifty tons and are difficult (i.e., impossible) to study under controlled conditions.
It is not necessarily surprising that noise increases whale stress levels, but it is good to have supporting data.
There’s a lot of sources, but here are a couple to get you started: