California blue whales, the largest animals on Earth once driven to near extinction by whaling, have made a remarkable comeback to near historic, 19th-century levels, according to a University of Washington study released on Friday.

The recovery makes California blue whales - which study authors say now number about 2,200, or 97 per cent of historical levels - the only population of blue whales known to have recovered from whaling.

"The recovery of California blue whales from whaling demonstrates the ability of blue whale populations to rebuild under careful management and conservation measures," said Cole Monnahan, a University of Washington doctoral student and lead author of the study.

Despite the resurgence, the study's authors said the whales - which as adults can reach nearly 30 metres in length and weigh 190 tonnes, twice as much as the largest known dinosaur - are still being struck by ships off the California coast at numbers above allowable US limits.

Conservation groups say at least 11 blue whales are struck each year along the US West Coast, nearly four times the "potential biological removal" level of 3.1 permitted under the US Marine Mammal Protection Act.

"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," University of Washington assistant professor of aquatic and fishery sciences Tim Branch said.

According to the University of Washington paper and a separate paper published earlier this year, some 3,400 blue whales were caught between 1905 and 1971, a number determined in part by examining once-secret Russian whaling archives.

The study's authors say that the population of California blue whales is now growing more slowly, partly due to ship strikes and also because numbers are reaching the habitat limit.

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

Reuters