It's likely to constitute trespass and poaching. Do you know who owns the land? If it was, me I'd ask around or perhaps even ask the EA. Funnily enough I have some land surrounded by an EA flood scheme and have a small bit of river running through it. We don't have any notices up, but it is private land and private fishing nonetheless. We don't get trouble with poaching touch wood, probably due to the location, but we do get know it all village townie dog walkers though!