Several great whale species were once far more abundant than records suggest, geneticists say.
By Alex Kirby
BBC News Online environment correspondent
They think there were about 10 times more fin and humpback whales in the North Atlantic than modern estimates claim.
Humpbacks' songs are legendary
But minke whale numbers appear historically to have been much closer to current populations.
The findings could radically change the terms of the debate on a possible resumption of whaling.
The geneticists, from the universities of Stanford and Harvard, US, report their findings in the magazine Science.
To estimate the size of whale populations before commercial hunting began, they studied genetic variation in the maternally-inherited mitochondrial DNA of North Atlantic fins, humpbacks and minkes.
They found a "surprisingly high" degree of variation, implying a correspondingly large population size for those species in the past.
Stephen Palumbi, professor of biological sciences at Stanford, is co-author of the report.
He said: "The genetics of populations has within it information about the past. If you can read the amount of genetic variation - the difference in DNA from one individual whale to another - and calibrate that, then you can estimate the historic size of the population.
"A small population tends to weed out all of its genetic differences through inbreeding. A large population, by contrast, should have a lot more genetic variation."
Estimates of historic population size are made by the International Whaling Commission (IWC) on the basis of whaling records dating back to the mid-19th century.
But Professor Palumbi and the report's lead author, Joe Roman of Harvard, write: "Reliable estimates of former whale abundances are elusive.
"Whaling logbooks provide clues, but may be incomplete, intentionally under-reported, or fail to consider hunting loss."
Their own study of fin whales, second in size only to the 30-metre blue whale, concludes that the IWC's historic estimates are far too low.
Fins: IWC suggests 40,000, authors 360,000
Humpbacks: IWC 20,000, authors 240,000
Minkes: IWC 130,000, authors 265,000
The commission thinks there were once about 40,000 North Atlantic fins, and that they have recovered to an all-time high of about 56,000.
But the authors' genetic comparison of 235 fins shows a pre-whaling population of probably about 360,000 animals.
They reach a similar conclusion with humpbacks, which the IWC estimates now number about 10,000, half the apparent historic high of 20,000 whales.
Roman and Palumbi compared DNA samples from 188 humpbacks to reach their own historic estimate of about 240,000 animals.
Professor Palumbi judges from this that the global humpback population could have been as high as 1.5 million whales, against an IWC figure of 100,000.
The immediate significance of what Roman and Palumbi are saying lies in the debate over the resumption of commercial whaling, suspended since 1986 under an IWC moratorium. There is pressure, led by Japan, Norway and Iceland, to lift the moratorium.
Minkes are relatively abundant
The IWC says there can be no whaling until populations have returned to at least 54% of their historic levels.
Other arguments aside, that could mean North Atlantic humpbacks being killed again some time within the next decade.
But the genetic analysis suggests whaling should not start for something like 70 or 100 years.
On minke whales, Roman and Palumbi conclude the original North Atlantic population was about 265,000, roughly twice the number the IWC thinks are there now.
Hunting decisions about the minkes, the authors say, "must be based on other data".
Professor James Estes, of the US Geological Survey, said the loss of more than 800,000 fins, humpbacks and minkes from the North Atlantic would not have been an isolated event and was likely to have altered the entire web of life.
Dr Doug Butterworth, a mathematician specialising in fisheries population modelling, from the University of Cape Town, is sceptical about the research.
Iceland wants to hunt fin whales
He told the BBC: "If the genetics is right, then either our estimates of past catches are very wrong or there's been some major ecological shift in the oceans. Where is the stuff all these whales were eating?
"There may be questions about the accuracy of our figures. They may be two or three times out, but even that's pushing it. They're not five to ten times out."
Humpback and fin whale images courtesy of the US National Oceanic and Atmospheric Administration.