The systems described in this article are "passive". They are simply listening to the sounds around them. This is much more common for submarines than "active" sonar - which ensonifies the water around you and then you listen for echos. It is those loud pings which can threaten or frighten wildlife. However they are also like holding up a big lit sign that says "there is a submarine here!" - and the whole point of submarines is stealth. They say that if you hear a ping from a submarine, it is too late - there is already a torpedo heading your way.

source - used to be a sonar tech