Previously it had been thought that the phenomenon could be a
response to noise pollution caused by increased human marine traffic, but
the new study ruled that out because it used data collected from the
southern Indian Ocean which has grown quieter in recent years and does
not have extensive shipping traffic. 30------- the researchers think that the
change was a response to a recovering whale population or to changes in
the ocean due to climate change. The louder a whale tries to make its
calls, the higher the pitch gets. Therefore the drop in pitch would mean
that whales are not straining so hard to be heard.