It hasn't been generally understood how much has been lost and what might be regained

Jeremy Jackson, report co-author

So says a report by 14 universities and international scientific bodies, which takes a historical perspective on the declining health of the sea.

It paints a picture of coasts once teeming with herds of walrus-like mammals, tens of millions of sea turtles and shoals of giant cod.

Today, whales, manatees, sea cows, monk seals and many other large animals have disappeared altogether in many waters.

Changing oceans

The key cause, the authors say, is human exploitation of marine bounty from prehistoric times until the present day.

"Virtually all coastal ocean communities were dominated by very large populations of very large animals," said Jeremy Jackson, lead author of the report. "What you would have seen around the coast would have been dramatically different from what you see today."

Chesapeake Bay: The whales and large sharks have gone

It is the virtual disappearance of key marine creatures such as cod, oysters and sea turtles that has led to the recent collapse of coastal ecosystems, he told BBC News Online.

Professor Jackson, of the Scripps Institution of Oceanography, University of California, San Diego, US, brought together a team of historians, palaeontologists, archaeologists and biologists to create a historical view of the oceans.

They investigated fossil records, historical archives, archaeological data and ecological studies from the past century to track ecological changes.

Giant turtles

The study suggests that there were once 40 million giant green turtles in the Caribbean alone, while estuaries such as Chesapeake Bay in North America would have been teeming with whales and large sharks.

Shellfish were in such abundance that they interfered with navigation and there were enough oyster reefs in Chesapeake Bay to filter all the water in three days.

People are not aware of how much has been lost, say researchers

"In Maine, we have evidence from Indian middens dating back 5,000 years that our coastal zone was dominated by large predatory fish such as cod," said co-author Robert Steneck from the University of Maine Darling Marine Center, Walpole, US.

"The average size of cod for thousands of years was about a metre long," he added, "which is impressive considering the fishers used crude hooks made of deer bone with line made of deer intestines."

The researchers say that overfishing precedes pollution, destruction of habitats, disease, and human-induced climate change. This historical perspective could suggest new goals for coastal management and restoration, they add.

"It hasn't been generally understood how much has been lost and what might be regained," said Professor Jackson.

The research is published in the journal Science.