The US Navy can build a US$100 million (NZ$123m) submarine training range off the coast of Southern Georgia and Northern Florida, a federal judge ruled, dismissing a lawsuit by environmental groups claiming the project would harm the already endangered North Atlantic right whale.

Judge Lisa Wood in Savannah held the navy complied with federal environmental law in analysing the impact of the range on the whale and its only known calving grounds.

Although North American right whales have been protected from commercial whaling since 1935, their badly depleted population has never recovered and could now number as few as 313, Wood wrote.

Right whales are among the rarest of all marine mammal species, according to the National Oceanic and Atmospheric Administration (NOAA), and are also protected under the Endangered Species Conservation Act of 1969.

Adults, which can grow to 55 feet (17m) in length and weigh up to 70 tons, favour shallow coastal waters. They were given their name by early whalers who believed their high blubber content, buoyancy when killed and coastal tendencies made them the "right" whale to hunt.

Environmentalists say the navy's proposed training ground is too close to the waters between Savannah, Georgia and Cape Canaveral, Florida, an area to which adult females and their young calves have historically migrated during the winter months.

The navy studied potential harm from ship strikes, sonar and entanglement of the whales in debris, the judge said. According to the navy, in 60 years of training in the South Georgia-North Florida area, there has not been a single case of a naval vessel hitting a whale, the judge wrote.

"There are no reported instances of sonar causing marine mammals to surface and collide with ships," the judge stated.

She noted the navy has developed procedures to mitigate harm to the whales, including requiring ships to slow down during calving season and when practical, to travel near critical habitat only during daylight and when visibility is good. Also, the navy has developed early warning systems to alert ships to the locations of whales.

Kathleen Sullivan, spokeswoman for the Southern Environmental Law Center in Charlottesville, Virginia, which is representing the plaintiffs in the lawsuit, said it is currently "reviewing the decision and reviewing our options with our clients".

- Reuters