Three hundred years ago the view from the cliffs of Cornwall would have been very different. Rather than today’s lonely fishing trawlers, the scene was dominated by the glistening bulk of blue whales. Huge schools of harbour porpoise chased shoals of fish so thick they darkened the water, while common dolphins filled the inland waters. Eighteen-foot orca menaced the mammals, while schools of blue sharks harassed fishermen who ventured out to dip their nets.

The vision may seem like the stuff of legend, but by delving into ship’s logs, ancient manuscripts, tax records, legal documents and even the devoted labours of monks living in Russia’s frozen north, an international team of researchers — part of the ten-year Census of Marine Life — has revealed the teeming abundance of life that once filled the seas not just off Britain but around the world. “We hope to be able to use this data to reverse the trend that we’ve been seeing,” Poul Holm, global chairman of the History of Marine Animal Population project, told The Times. “Lots of fish management is done using only 25-30 years of data. Using the timescales gives a much more realistic picture.”

The project’s findings reveal not only how much has been lost, but how far the sea can be expected to rebound, if given a chance. It is hoped that, by providing an accurate vision of Cornwall’s past productivity, the public and policymakers can be motivated to help its recovery. “History is a powerful motivator,” Professor Holm said.

The results of other investigations — which will be shared at a conference in Vancouver this week — show that rampant whaling left the waters off New Zealand with an estimated 25 southern right whales by 1925.

Although populations have risen to more than 1,000 whales, in the early 1800s those waters supported about 27,000. “Eighty-five to ninety per cent of the biomass has been removed. These are significant changes with big implications for ecosystem functioning,” Professor Holm said.

Size as well as abundance has been affected. Using records of trophy catches in Key West, Florida, researchers from the Scripps Institution of Oceanography showed that between 1956 and 2007 the average weight of fish fell from about 20kg to 2.3kg and that their average length shrank from nearly 2m to 35cm.

“The worst part of these ‘shifting environmental baselines’ is that we have come to accept the degraded condition of the sea as normal,” Callum Roberts, author of The Unnatural History of the Sea, said. “We cannot return the oceans to some primordial condition absent of human influence, but it is in everyone’s interest to recover some of the lost abundance of creatures in the sea.”

Experts are encouraged by the knowledge that the sea can return to former levels of productivity. The connectivity of the oceans means that is difficult to drive marine animals to extinction. Several rebounds are recorded, most notably during the world wars when it was too dangerous for fishing fleets to operate in the North Sea. Between 1914 and 1918 stocks trebled. A similar rebound occurred after the Second World War, although the bounty was quickly retaken.

“We now have long-term evidence that catches can be three or four times their current level,” Ron O’Dor, chief scientist of the Census of Marine Life, said. “The oceans will sustain that.” The time that the ocean would require to recover depends on how much protection is afforded. “It’s not about shutting down fisheries but about letting the ecosystem, with its shelter, structure and nurseries, rebound,” Dr O’Dor said.

Humans appear to have had an impact on marine life as far back as the Middle Stone Age — between 300,000 and 30,000 years ago, to judge from archaeological evidence. Investigations into the history of some populations also show periods when climate, not fishing, was the main driver of fish stocks.

“We have an astonishing record of 400 years of salmon runs in the Bering and White Seas in Russia,” Professor Holm said. Monks living in frozen monasteries of the far north were obliged to measure the length and weight of every salmon caught. The records show that as the climate varied, so did the size and quantity of salmon. Just as one hot summer does not mean that the world is warming, Professor Holm says: “We need a long timescale to see if variation is significant or if it’s just a blip.”

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