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THE oceans were once awash with great whales, large predatory fish and resplendent schools of smaller prey. Then human hunters arrived and raped the oceans of these riches, causing so much damage that the seas may never be the same again.

Fact or fiction? Last week, new research stoked the debate by suggesting we have wreaked more havoc than we ever thought, a realisation that has left marine scientists questioning our basic assumptions about the health and resilience of the oceans and how they should be conserved.

In the journal Science, two researchers in the US have published a genetic survey of three species of north Atlantic whale that suggests that the oceans were once filled with 10 times as many great whales as previously supposed. That requires “a fundamental revision of our conception of the natural state of the oceans,” they say.

Some marine experts challenge the veracity of this latest claim. But it comes just two months after a paper in Nature concluded that 90 per cent of large predatory and bottom-dwelling fish have been lost from most fishing regions (91av, 17 May, p 4).

Both findings reinforce a plea made just weeks ago by marine ecologists for a fundamental rethink on marine conservation. At a conference in Los Cabos in Mexico in June, called “Defying Ocean’s End”, the world’s leading marine ecology experts gathered to launch an attack on the oceans’ custodians. Policy is too often based on pervasive “myths”, they said, with the most prevalent being that marine species are “inherently less vulnerable to extinction and depletion than their terrestrial counterparts”. But the latest scientific research is showing that just isn’t so.

The new study of fin, minke and North Atlantic humpback whales is the first to use genetic techniques to assess past populations. Stephen Palumbi of Stanford University in California and Joe Roman of Harvard University looked at the genetic diversity of mitochondrial DNA, which is inherited from the mother, in samples of DNA from almost a thousand animals.

Then, based on estimates of the likely rate of genetic mutation, they calculated how many females would have been needed in the past to accumulate the diversity found in each species. The researchers then estimated the size of historic populations.

Palumbi and Roman calculate that numbers of humpback, fin and minke whales were much higher before commercial whaling began in the mid-19th century than previously thought. They suggest that 240,000 humpback whales once swam the North Atlantic, 12 times as many as previous estimates, and 24 times present numbers. And there were 360,000 fin whales, more than nine times previous estimates and six times present numbers. Minke numbers reached 265,000 compared to 149,000 today (Science, vol 301, p 508). The researchers claim past estimates based on combining whale census data with whaling records are hopelessly wrong. “Whaling logbooks may be incomplete, intentionally under-reported, or fail to consider whales that were struck and lost,” they say.

The new estimates suggest that calls to lift the International Whaling Commission’s 17-year-old moratorium on commercial whaling are based on a misreading of history. IWC rules allow whaling to be considered if populations exceed half the ocean’s “carrying capacity” – its theoretical ability to support a given number of whales.

Current estimates of past populations suggest humpbacks may be hovering around this threshold, while minkes and fin whales have already exceeded it. The new findings call this into question where fin and humpback whales are concerned.

But other marine experts dispute Palumbi and Roman’s conclusions. Fisheries statistician Doug Butterworth of the University of Cape Town in South Africa says there are three possible reasons for the discrepancy in population estimates. Past catches may have been underestimated, as Palumbi suggests. Or some environmental shift may have recently reduced the whales’ food supply. Or, as he thinks most likely, the genetic technique used by Palumbi and Roman is flawed. “Standard population analysis methods are fundamentally simple,” he says. But the mutation rates assumed using the genetic technique are hard to test.

Other critics point out that Palumbi and Roman have no idea if whale populations peaked just before commercial whaling began, or tens of thousands of years ago. That will determine if past populations are, as Palumbi suggests, the right benchmark for conservation, and whether human intervention is the reason that populations are dwindling.

Another problem is that changes in the Atlantic ecosystem, including those caused by commercial fishing, could have reduced the carrying capacity of the ocean, limiting the number of whales it can sustain.

Either way, fisheries managers act “as if marine species and habitats were invulnerable,” says Carl Gustaf Lundin, head of the marine programme at the World Conservation Union (IUCN), which helped organise the Defying Ocean’s End meeting. The conference called for a global network of protected marine areas and a moratorium on unregulated fishing in marine ecosystem hot spots in international waters. “We have to shatter the myth that marine species are resilient to extinction,” says Lundin.