Open Sea Fishing: Starts 1000 CE in North Europe?

Experts examining excavated fish bones from around north Europe have concluded that fishermen first regularly travelled out to the open seas to fish around 1000 CE. Before then the remains of freshwater fish were dominant, but after this time marine species increase in number. Doctor James Barret, from Cambridge University's McDonald Institute for Archaeological Research, explained to 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."

Why would this change take place? Dr. Barret goes on to say: “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."