I kid you not, only a few days ago I counted 30+ Magpies congregated.Our apiary site at catherton common has fish pools, he does much the same thing as you, grows the fry on then sells them, he has carp, roach, perch, koa carp among others..
When I go to inspect my hives out in the field, I have to disconnect the electric fence that runs all around the perimeter of his ground this is to stop beavers.
Do you have such problems with beavers.. He was telling me that they can pick a sent up 2 miles or more down stream, all from the water.
He's also had corvid ( crows etc) taking the young ducklings this year there a meniss and don't just take ducklings.. Owlets as well, I've seen one this year with a baby owl.
If there was one species of bird that needs culling it's them.