Yup all a bit chicken and egg, pardon the pun. I think farming and river systems have been at odds with each other for years, much to the detriment of our native wild fish stocks.
For the record I don't think the chicken farms are liable for cleaning up the rivers but the environment agency needs a more robust, holistic approach to tackling the threat to our freshwater river systems. Chicken farms and other intensive farming practices are contributing to the problem and its not just the NPK rich byproduct that concerns me its other stuff we put in the diet of that livestock that also ends up in the river.
Fish farms are also having a devastating effect on our migratory stocks and along with the climate changes and river pollution some of our stocks of migratory fish are on the brink of extinction.
You might find this an interesting slant on the Environment Agency and how it's failing in it's guardianship of our freshwater rivers and estuaries. In the stuff I read, which is admittedly leaning towards angling, you'll understand why reports on our rivers by the EA are often taken with a pinch of salt. When you allow the polluters to self regulate and monitor run off you may as well throw the towel in.
Farmers are intrinsically interested in their land and seldom concerned with the biodiversity of the rivers that run through their farmland. Not all of them, but a disproportionate number are polluting rivers without any meaningful deterrent There certainly needs to be a shift in that situation.