Apparently during the first millennium we caught only fresh water fish. The middens that archaeologists spend their lives delving into demonstrate that towards the end of that millennium, such fish became smaller and fewer; so that we had to diversify into sea fish.
This provides the background of something in Bede that has always puzzled me. He says that when S Wilfrid (who was responsible for giving the English Church its admirable Romanita) decided to evangelise the South Saxons, he found them so afflicted by famine that they were lining up to jump off Beachy Head. There was fish galore, but the only fishing they were familiar with was catching eels. So he showd them how to use their eel nets to supplement their diet.
It used to mystify me that people could have been starving who had the English Channel in which to fish and no restrictive European quotas to hamper them. Now I understand.