A sea change: How freshwater fish slump forced medieval men to brave the oceans

Medieval fishermen first went to sea 1,000 years ago because of a steep decline in freshwater fish, according to scientists.
Researchers from Cambridge and York University studied and dated fish bones found in archaeological sites around the north-western part of Europe and compared historical records.
They found there were fewer species of freshwater fish, which became smaller over time. Both over-fishing and pollution are thought to be to blame for the decline which caused a major change in the diet of our ancestors.

One of the earliest depictions of trawling from the 5th century, Tunisia. Historical records reveal how fishing has changed over time

'At the end of the first millennium AD there is this wholesale shift in emphasis from reliance on freshwater fish towards marine species,' study co-author Dr Barrett told the BBC.
'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.'