Reported mass strandings of certain whale species have risen worldwide since the military started using sonar half a century ago. Biologists also are concerned that marine mammals may suffer stress from changes in diving, feeding and communication.
Studies so far have involved a small group of tagged animals, and it's unclear how changes in behaviour would affect populations.
For the first time, researchers coordinating with the Navy are conducting experiments using mid-frequency active sonar transmissions from ships. Last summer, the team tagged six whales and dolphins off the Southern California coast. Results are still being analysed.