Decades of conservation and countless "Save The Whales" bumper stickers have paid off.

California blue whale populations have recovered to near pre-whaling levels, according to a new study.

The researchers also found that cargo ships are killing at least 11 blue whales a year, but that number is not enough to significantly harm the overall numbers.

Related: Blue whales at risk of being struck by ships

California blue whales were hunted for more than half a century before the International Whaling Commission banned practice in 1966.

At the time, these massive sea mammals that live along the west coast between the equator and Alaska were on the brink of extinction.

After decades of conservation, the California blue whale population hit about 2000 and then leveled off, explained University of Washington doctoral student Cole Monnahan.

"What wasn’t known was why are they stagnating, that was the big question," he said.

Related: Blue Whales Croon A New Tune

Monnahan and his colleagues set out to find an answer.

They started by trying to estimate the number of whales in the region before 1905, when blue whale hunting took off.

By looking at historic records of catches and comparing them to natural population growth, the researchers determined there were probably about 2,200 blue whales in the area before the 1900s.

That's about the same as the number of them alive today, said Monnahah.

"Which suggests that the population has recovered... this is the first time anyone has been able to show a blue whale population recovering from whaling."

Trevor Branch, UW assistant professor and co-author of the study thinks the California blue whales have reached the habitat limit, which explains why the population seems to have leveled off.

"We think the California population has reached the capacity of what the system can take as far as blue whales," he said in a press release.

While the study notes that strikes from cargo ships are a real threat to some number of blue whales, the annual deaths aren't enough to cause an overall decline in the population.

Monnahan stressed though, just because populations have rebounded doesn't mean protection for Earth's largest animals should be revoked.

"It just shows if we stop harming the ocean and give it time to recover it can bounce back," he explained.

The research appeared in the journal Marine Mammal Science.