Image copyright UWNews Image caption A blue whale estimated to be around 20m long, swimming off Baja California 
 
 Researchers believe that California blue whales have recovered in numbers and the population has returned to sustainable levels. 
 
 Scientists say this is the only population of blue whales to have rebounded from the ravages of whaling. 
 
 The research team estimate that there are now 2,200 of these giant creatures on the eastern side of the Pacific Ocean. 
 
 But concerns remain about their vulnerability to being struck by ships. 
 
 At up to 33m in length and weighing in at up to 190 tonnes, blue whales are the largest animals on the planet. 
 
 The California variety is often seen feeding close to the coast of the state, but they are found all the way from the Gulf of Alaska down to Costa Rica. 
 
 Soviet secrecy 
 
 Writing in the journal, Marine Mammal Science, researchers from the University of Washington say the California blue whales are now at 97% of their historical levels. 
 
 The real key finding here is that they are close to recovery which is a bit of a surprise Dr Tevor Branch, University of Washington 
 
 Working out that this species is now back at its traditional numbers required some dogged scientific sleuthing. 
 
 Whaling nations concentrated their hunting efforts on the colder waters of the Antarctic and until the practice was banned in 1966 some 346,000 of the animals were killed by harpoon. 
 
 The numbers of blue whales caught in the Pacific was much lower, approximately 3,400 between 1905 and 1971. 
 
 Much of this hunting was carried out by Russian fleets. 
 
 However most of the data on the catches was kept secret under the Soviet regime. Scientists have only recently been able to get access to this information in the archives. 
 
 However the location and size of the catches didn't give any clues to the types of blue whales that had been caught. There are two distinct populations, the California group and others that live near Japan and Russia. 
 
 To figure out which whales were which, the scientists turned to song. 
 
 "We were trying to separate the catches into east and west, but we didn't know the boundary between the two," said Dr Trevor Branch from the University of Washington. 
 
 Image copyright UWNEWS Image caption A cow and calf in the waters off the California Channel Islands 
 
 "So we used the current locations of where they sing to figure out the dividing line. Their repetitive calls are different." 
 
 By being able to accurately work out the numbers lost to whaling, the research team was able to calculate a historic population. 
 
 Now back at 97% of their past numbers, the team believes that a rise in population has slowed down as these whales have reached the capacity of what the ocean system can support. 
 
 One concern for the scientists at present are ship strikes. 
 
 Most of these happen off the coast of California, and so worried are the authorities that they are now paying merchant shipping to slow down. 
 
 "Our perspective is that we'd rather there were no ship strikes at all, and they are over the legal limit," said Dr Branch. 
 
 "They have to do something to stop it, but 11 per year is so much lower than historic catches." 
 
 This new data suggests that there could be an 11-fold increase in ships before there is a 50% chance that the population will drop below what is considered "depleted" by regulators. 
 
 "My impression is that they are fairly robust," said Cole Monnahan, also from the University of Washington and the lead author on the paper. 
 
 "If you can whale them pretty extensively for 50-70 years and they are able to recover I think that says a lot about moving forward. 
 
 "In terms of things like climate change, it is hard to predict but I don't think we would expect a precipitous drop off." 
 
 While applauding the success of the conservation efforts in the California region, the scientists are well aware that not all whale populations have managed to rebound. In Antarctica, blue whales are at approximately 1% of their historic numbers. 
 
 "California blue whales are recovering because we took actions to stop catches and start monitoring," said Cole Monnahan, 
 
 "If we hadn't, the population might have been pushed to near extinction - an unfortunate fate suffered by other blue whale populations." 
 
 Follow Matt on Twitter @mattmcgrathbbc. ||||| Much has been made about efforts to reduce the number of endangered whales killed by collisions with ships off the California coast. While those efforts are commendable, a new report says, such ship strikes do not immediately threaten the recovery of the California blue whale, the largest animal on Earth. University of Washington researchers say the California blue whale population, which had been decimated by decades of hunting, has rebounded to near historical levels. The whales number about 2,200 today, up from a low of 951 in 1931, according to Trevor Branch, UW assistant professor of aquatic and fishery sciences. “Whaling caused the drop, and was then banned in 1966. Illegal Soviet whaling continued until 1971 when all whaling ceased" for blue whales, Branch told NBC News via email. "Since then the population has been recovering steadily.” 
 
 Other groups have estimated that at least 11 blue whales are struck a year along the U.S. West Coast. The UW researchers calculate that there could be an eleven-fold increase in vessels before there is a 50 percent chance that the blue whale population will drop below what is considered "depleted" by regulators. "The recovery of California blue whales from whaling demonstrates the ability of blue whale populations to rebuild under careful management and conservation measures," Cole Monnahan, a UW doctoral student in quantitative ecology and resource management, said in a statement. 
 
 Cole is the lead author of the paper published online Friday by the journal Marine Mammal Science. Branch and André Punt, a UW professor of aquatic and fisheries sciences, are co-authors. 
 
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 SOCIAL 
 
 A blue whale calf already weighs around 3 tons at birth and can drink about 100 gallons of milk per day. pic.twitter.com/iGffK8ToQ8 — Factual Animal Facts (@CredibleCritter) August 31, 2014 
 
 — James Eng
– After decades of deadly whaling, the California blue whale population has come back with a vengeance. A study in the Marine Mammal Science journal says that the once-endangered mammal, which can be found from Alaska all the way down to Costa Rica, has bounced back to "sustainable levels," with around 2,200 of the creatures now swimming in the Pacific surf right off the West Coast, reports the BBC. That growth spurt is due to meticulous conservation efforts—but also because the Russians aren't harpooning enormous numbers of them anymore. An estimated 346,000 California blue whales were harpooned before the practice was banned in 1966; between 1905 and 1971, about 3,400 were killed in the Pacific, mostly by Russian whalers. Despite the ban, Russians kept at it illegally until 1971—and as soon as they stopped, "the population has been recovering steadily," a University of Washington researcher tells NBC News via email. One major issue that still plagues blue whales: They keep getting smashed by ships, especially near California. Some groups estimate about 11 are hit each year, although scientists believe there would have to be an "elevenfold increase in vessels" before there's even a chance of the blue-whale population dipping to dangerously low levels. Meanwhile, they remain the largest animals on Earth, notes National Geographic. (Sorry, Dreadnoughtus.) Read how many calories a blue whale consumes in one gulp.