Stephen...Personally speaking i would say, considering the EA have been re-stocking coarse fish in virtually every river in the UK, is that the eggs or young of some or most species are not surviving even if they do spawn. There are a combination of other things relating to pollution and predation etc, but heres one of the main reasons i think. Whist helping out on another issue regarding Powick Weir and Twaite Shad, i came across this little gem article and quote from an EA Calverton Fish Farm official which says it all. This is the very first time i have ever heard an EA official come clean and admit that something is very very wrong with our rivers regarding natural recruitment....even though we ourselves knew it all along for over 20+ years. Here is the relevant quote/article. See what you think! There is a link to the full article at the bottom and is worth a read.
....Here’s the Quote: "One of the first things Ian made clear was that this was 'farming' - everything relating to the complete development of the fish until release into the natural environment was carefully, meticulously controlled to ensure the best product. THE WATER HAD TO BE THE BEST -- RIVER OR STREAM WATER FOR REARING PURPOSES WILL NOT DO AS UNFORTUNATELY IT IS ALREADY POLLUTED WITH HUMAN AND ANIMAL HORMONES, WHICH SURVIVE SEWAGE TREATMENT, AND THESE GENETICALLY DISTORT THE YOUNG FISH."
http://www.calvertonvillage.com/fish-farm