Interestingly, before the reformation, when fish was eaten every Friday. Every monastery and Abbey had a stew pond where carp were kept and grown.
Also beaver was classed as a fish and beaver tail was a delicacy. Barnacle geese were also classed as fish as they were believed to be the mature form of the goose barnacle and so the high ups could eat them.
Also related to the development of the cod fishery off Newfoundland, which produced masses of salt fish for the European market.