Twitcher, first things first, contact your local EA fisheries people and see if they can you to acquire a licence to cull some birds. Don't forget that the fisheries boys also hate the birds.
After that the best way is to build fish refuges under water, but that does tend to make the fishery and bit "swimmy", ie if you're not near a refuge - no bites.
Another idea on fisheries with lots of trees around the edge is to cut some down in between the swims. That slows the birds down, but the fishery looks a bit of a mess.
Lilies too are a good idea, but plant native ones, preferably nymphoides alba (the white one). DO NOT plant small leaved lilies as they will choke the lake and nuphar lutium (the yellow one sometimes called cabbages on the river) grows a tangle of roots above the soil level, making fishing a pain in the neck when hooked fish go underneath them. Mind you, in winter when the cormorants are mostly inland, the lillies have died back, but they do still help and make a nicer fishery.
