HALIFAX–Almost 30,000 right whales cruised the coasts of New Zealand while sharks were so plentiful they darkened waters off England 200 years ago, according to an ambitious project that has traced the history and worrisome decline of several marine species.

For the first time, scientists from around the world have developed a picture of what oceans looked like centuries ago, showing how most species started to decline in abundance as fisheries took hold.

The research, which took 10 years and is part of the Census of Marine Life's animal population project, gives a distressing look at how fish stocks began dropping off when fishing pressures intensified as early as 1000 AD and moved offshore.

Where 27,000 southern right whales plied New Zealand waters before whaling began in the early 1800s, only 25 females were left in 1925.

"Whenever we look at the large fish – the whales – we get a picture that 300 years ago they would have been 10 times as abundant as they are today," Poul Holm, a marine scientist at Dublin's Trinity College, said in an interview.

"In some regions they've been totally fished out, so we're heading down a really blind alley if we continue that way."

Using an eclectic mix of data sources, including fisheries logbooks, tax accounts, mounted trophies and even restaurant menus, researchers chronicled different species' location, size and abundance.

They found that as fish stocks thinned out closer to shore in the 11th century, fishermen looked further out to sea, developing new boats and equipment in the 1500s that allowed them to move to deeper waters.

Things changed dramatically a century later when pairs of boats began dragging a net behind them to scoop up as much as possible.

By the middle ages, Europeans were eating deep-sea species caught with a hook and line.

"The impact of fisheries were felt much earlier than anyone had thought previously," said Holm, whose findings are being presented this week at a convention in Vancouver.

"This was before the onset of modern technology."

The scientists claim they found Greek and Latin verse from the second century AD that suggests Romans began trawling with nets, while they can date fishing to the Middle Stone Age – 10 times earlier than once thought.

They also traced periwinkle snails and rockweed in ballast on ships headed from England to Nova Scotia in the mid-1800s, making it one of the first invasive species in North America.

And trophy reef fish caught in Key West, Fla., shrank dramatically over 50 years because of intense fishing pressure, going from 20 kilograms in 1956 to 2.3 kilograms in 2007.

Marine biologists are particularly concerned about fishing's relentless reach as boats move further offshore in search of fish, leaving only one per cent of the world's oceans virtually untouched.

"We've run out of geography," Ron O'Dor, of the Census of Marine Life, said in Halifax. "We've really exploited every place of the world and we can't really go out anywhere else."

"We just have to figure out how to manage the resources effectively."

Most fishing activity used to take place in the northern hemisphere, but has extended to the southern region as stocks are depleted and recovery is slow, if at all.

Using what Holm called a "gold mining" approach to fishing where you clean out one area and move on to the next, he says nine-tenths of the top marine food web has been fished out."

But the research also found signs of recovery, with some species of seals and otters restoring their populations to healthy levels after protective measures have been introduced.

In California, hunting restrictions have helped bring the population up to 2,000 sea otters after it was reduced to 54.

Heike Lotze, a researcher at Dalhousie University in Halifax, said bans on harvesting and restrictions on fishing areas have helped bring some back from the brink of extinction.

"It just needs a really concerted effort and often legal action, and you really need to reduce exploitation and protect habitats," she said.