The Navy said that the studies focused on waters off the East Coast, the Gulf of Mexico, Southern California and Hawaii from 2014 through 2019, the main areas that the service branch tests equipment and trains sailors.
The studies were done ahead of the Navy applying to the National Marine Fisheries Service for permits for its activities. The Navy said that it if hadn't done so and was later found to have harmed marine mammals, it would be found in violation of federal environmental law and have to stop its training and testing.
Most of the deaths would come from explosives, though some might come from testing sonar or animals being hit by ships.
Rear Adm. Kevin Slates, the Navy's energy and environmental readiness division director, told reporters this week the Navy uses simulators where possible but sailors must test and train in real-life conditions.
"Without this realistic testing and training, our sailors can't develop or maintain the critical skills they need or ensure the new technologies can be operated effectively," Slates said in a conference call with reporters on Wednesday.
According to the reports, computer models show it may kill 186 whales and dolphins off the East Coast and 155 off Hawaii and Southern California.
The Navy said it developed the estimates by totaling the hours it will test and practice with sonar, torpedoes, missiles, explosives and other equipment over five years. Experts then combine the data with what's known about the marine mammals and then use computer modeling.
Off the East Coast, there could be 11,267 serious injuries and 1.89 million minor injuries like temporary hearing loss. The reports said the testing and training might also cause marine mammals to change their behavior
- such as swimming in a different direction -- in 20 million instances.