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Part of my job in the Navy has been to hunt submarines from the air. There are two ways to track a sub, passively (you try and listen for tell-tale sounds with sensors in the water) or you use active methods, basically emit energy in the water and hope for a return echo from the submarine (in very simple terms, it is like radar in the water.)
As submarines have become increasingly more quiet (US subs are amazingly quiet) active sonar tecnologies have been improved, usually by increasing the power out of the sensor (more power increases range of detection) or moving to low frequency spectrums, which travel farther in water than higher frequencies.
The impact of these sensors on marine mammals and fish has been an issue for years...when I was doing flight test, we had to get specific environmental clearances to test our airborne, low-frequency sonars we were developing for the H-60. In addition, employment of the sensors are often very restricted.
There are some sub guys on the board who are no doubt experts, but this has been, and will continue to be an important issue.
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1996 FJ80.
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