The California blue whale population has rebounded to near historical levels, proof that conservation measures are working, says a study released Friday by the University of Washington.

The whales now number about 2,200 on the eastern side of the Pacific Ocean, according to monitoring by other research groups, and their habitat has nearly reached its capacity to support the huge marine mammals.

Blue whales as a species were hunted to near extinction before receiving worldwide protection in 1966, when the International Whaling Commission declared a ban on blue whale hunting.

While the number of California blue whales now being reported may seem low, the figure needs to be considered against how many were killed during the whaling era, said Cole Monnahan, lead author of the paper published in the journal Marine Mammal Science.

According to new data Monnahan and his colleagues published earlier this summer in the scientific journal PLoS One, about 3,400 California blue whales were killed between 1905 and 1971.

"Considering the 3,400 caught in comparison to the 346,000 caught near Antarctica gives an idea how much smaller the population of California blue whales was likely to have been," Branch said.

The California variety of the creatures has a roaming range from the equator up into the Gulf of Alaska, but they are at their most visible while at feeding grounds 30 to 50 kilometres off the California coast.

This is the only population of blue whales known to have recovered from whaling — and while the number struck by ships is likely above allowable U.S. limits, the study says such strikes do not immediately threaten that recovery.

'It's a conservation success story.' -—Doctoral student Cole Monnahan

There are likely at least 11 blue whales struck each year along the U.S. West Coast, which is above the "potential biological removal" of 3.1 whales per year allowed by the U.S. Marine Mammal Protection Act.

"Without ship strikes as a big factor holding the population back — and no other readily apparent human-caused reason (although noise, chemical pollution and interactions with fisheries may impact them) — it is even more likely that the population is growing more slowly because whale numbers are reaching the habitat limit, something called the carrying capacity," the study said.

"We think the California population has reached the capacity of what the system can take as far as blue whales," Branch said.

"Our findings aren't meant to deprive California blue whales of protections that they need going forward," Monnahan said.

"California blue whales are recovering because we took actions to stop catches and start monitoring. If we hadn't, the population might have been pushed to near extinction — an unfortunate fate suffered by other blue whale populations."

"It's a conservation success story," Monnahan said.