As has been mentioned,.. a good OS map, a bit of diplomacy and legwork can get a result, worked well for my dad on both pond and rivers.
Worth remembering that fishing rights don’t always form part of riparian land ownership, and farmers don’t always own the land.
Some estates have fishing rights on both banks but may only have riparian land ownership of one.
I’ve experienced unscrupulous land owners attempting to sell their land with fishing rights,.. even when said rights are owned and registered to another party!
There have been a number of occasions when land adjacent to a river has changed hands with the new owners oblivious to the fact that the fishing rights are owned by a club and there is an existing legal easement for potentially over a 1000 members to walk the bank. Most of these new owners aren’t short of a few bob so you would have thought that their legal team would have carried out a thorough search of land registry,... but not the case. I’ve also known of riverside land changing hands over a card game,... usually very dodgy and a nightmare for those who own fishing rights and have to spend money on solicitors letters to protect what is legally theirs!
Land registry is always a good start for establishing who owns what,.. as fishing rights can be a separate riparian lot. Not all are registered,.. foolishly.
Sorry to digress a bit,... but being a club sect for some years opened my eyes to riparian rights.