By Mark Kinver
Science and environment reporter, BBC News
Understanding past impacts can help today's conservation efforts
Medieval fishermen first took to the open seas in about AD1,000 as a result of a sharp decline in large freshwater fish, scientists have suggested.
They say the decline was probably the result of rising population and pollution levels.
The study forms part of a series that examines the impact of humans on life beneath the waves throughout history.
The findings will be presented at a Census of Marine Life (CoML) conference in Canada, which begins on Tuesday.
"Fish bones are found in archaeological sites... all around the north-western part of Europe," said co-author James Barrett, from Cambridge University's McDonald Institute for Archaeological Research.
"What we have done is to start to piece together some of the information that has been gathered."
This involved looking at the fish bones to determine what species they came from, and from what time period.
One of the straightforward hypotheses is that freshwater fish were no longer sufficient to satisfy demand
Dr James Barratt,
University of Cambridge
Dr Barrett observed: "At the end of the first millennium AD there is this wholesale shift in emphasis from reliance on freshwater fish towards marine species."
"It is not rocket science, it is just literally looking at the proportion of species that are obligatory freshwater ones, such as pike... and which ones are obligatory sea fish, such as cod and herring."
As for understanding what caused the shift, Dr Barratt said that it would be inappropriate to attempt to identify a single cause.
"But when you look very carefully at the freshwater fish bones from the York site, where a big collection was gathered, you can see that the length of the fish are decreasing through time," he told BBC News.
"Certainly, one of the straightforward hypotheses is that freshwater fish were no longer sufficient to satisfy demand.
"This was likely to have been for two reasons: one was because there had been a reduction in the availability of freshwater fish as a result of overfishing, or from things such as people building dams for water mills.
"The second thing would have been that there would have simply been more people."
Dr Barrett added that around this period there was a rapid expansion of towns and cities in north-western Europe.
"So this meant that there was an increased pressure on freshwater fish, and there was an increase in demand that probably could not have been satisfied even if the supply had remained stable."
Improved fishing methods meant human activities changed the marine ecology beyond recognition
Dr Barrett's team's study will be one of a number of research projects that form part of the CoML's History of Marine Animal Populations (HMAP).
The project aims to address a number of questions, including how the diversity and distribution of marine animals have changed over the past 2,000 years, and what factors forced or influenced these changes.
Professor Poul Holm, the global chairman of the HMAP project, said that the history of marine animals had been one of the great unknowns.
But recent scientific advances was allowing researchers to gain a better understanding, he added.
"We now know that the distribution and abundance of marine animal populations change dramatically over time," he explained.
"Climate and humanity force changes and while few marine species have gone extinct, entire marine ecosystems have been depleted beyond recovery.
"Understanding historical patterns of resources exploitation and identifying what has actually been lost in the habitat is essential to develop and implement recovery plans for depleted marine ecosystems."
Many of the findings by HMAP researchers will be presented at the Oceans Past II Conference, which is begins on Tuesday at the University of British Columbia, Vancouver, Canada.
COML, which began back in 2000, is an international research programme involving thousands of scientists from around the world.
The goal of the decade-long endeavour is to assess and explain the diversity, distribution and abundance of marine life in the world's seas and oceans.
The publication of the first complete global Census of Marine Life is scheduled for October 2010.