California's population of blue whales has apparently recovered from past times of whaling, though it may be the world's only group to have done so.

According to BBC News, a team of researchers, whose study is published in the journal Marine Mammal Science, said California's blue whale population is once again at a sustainable level. Now, the main threat they face is colliding with passing ships.

Conservationists have already tried to address this issue by working with international shipping companies to reroute their ships. The move may be too costly, but would certainly put the whales at less risk.

"The recovery of California blue whales from whaling demonstrates the ability of blue whale populations to rebuild under careful management and conservation measures," study lead author Cole Monnahan, a doctoral student in quantitative ecology and resource management at the University of Washington (UW), said in a press release.

While whaling has historically been worse in the frigid waters of the Antarctic, it still took its toll on the eastern Pacific blue whales, BBC News reported. Poachers killed about 3,400 whales between 1905 and 1971, though whaling was banned in 1966.

The study authors said 11 blue whales collide with ships every year on average and that rate is expected only to increase. Such a trend would mean the chances of the blue whales' population sinking back to "depleted" levels would be 50-50.

"Even accepting our results that the current level of ship strikes is not going to cause overall population declines, there is still going to be ongoing concern that we don't want these whales killed by ships," Trevor Branch, an assistant professor of aquatic and fishery sciences at UW, said in the release. "We think the California population has reached the capacity of what the system can take as far as blue whales."