Call it the myth of industrial sin. It seems fish stocks were declining due to human exploitation long before the arrival of giant trawlers and factory ships, according to marine scientists at a conference being held this week in Canada.

"We are discovering that human pressure on marine life was much earlier, much larger and much more significant than previously thought," says Poul Holm, an environmental historian at Trinity College Dublin in Ireland. "We now know that there was major commercial exploitation of fisheries, doing huge damage to fish populations, back in medieval times and even before. The idea that it is only modern fishing technology that has done damage turns out to be completely wrong."

The Oceans Past II conference in Vancouver, Canada, is part of the decade-long Census of Marine Life, a global effort to understand the past, present and future of ocean life. The census is due to be completed next year.

To reconstruct the state of the oceans centuries and even millennia ago, researchers are combining population modelling techniques with historical records, such as ships' logs, restaurant menus, paintings, diaries, legal documents and even tax returns. For example, in 1153 a Moroccan geographer called al-Idris wrote that the north Atlantic Ocean contained "animals of such great size that the inhabitants of islands use the bones and vertebrae in place of wood to build houses".

Much of the work presented at the conference concludes that fish stocks were already depleted before the industrial exploitation of the 20th century made the situation even worse. "We used to think that if we could get fish stocks back up to the levels of the 1970s we would be well on the way to recovery," says Holm. This now seems to be an optimistic idea.

James Barrett and Jen Harland of the University of Cambridge reported at the conference that freshwater fisheries in much of Europe were already in decline 1000 years ago, causing fishers to switch to marine fishing. By 1500, says Maria Lucia De Nicolo of the University of Bologna, Italy, coastal fish stocks were disappearing and deep-sea fishing began, with trawling starting in the mid-1600s. By the early 1800s, the once super abundant European herring fishery had collapsed.

Other studies show that whale numbers were also plummeting at this time. By the early 1800s, wind-powered whaling ships had virtually wiped out a population of nearly a million bowhead whales in the eastern Arctic Ocean. Meanwhile, in the 18th century there were an estimated 27,000 southern right whales off New Zealand, but by 1925, before the introduction of factory ships, they had been reduced to about 25 reproducing females, according to Emma Carroll of the University of Auckland.

What does this say about the future prospects for marine life? In part the news is bad, as it is now clear how far numbers of marine fish and mammals have crashed as a result of human hunting. "Few marine species have gone extinct, but entire marine ecosystems may have been depleted beyond recovery," Holm says.

But conference chair Andy Rosenberg, from the University of New Hampshire in Durham, is more optimistic. If stocks can crash, he argues, they can also recover. He believes the census shows that the fruits of recreating pristine marine ecosystems through conservation could be far greater than previously imagined. "The oceans were once even more productive than we supposed, so they could be again."