Is it just the catch results that are indicating that?
I know of a chap that smashed it to pieces over winter, he caught loads. I fished it myself on a social last year and had something silly like 27 takes, there were certainly no signs of poor stocks.
The place was a circus, always rammed, there is no way anyone could have been nicking fish without being spotted.
I hear they've made it season ticket now? Assuming angler numbers have dropped, could it not just be that the fish are avoiding the lines (where they had no choice previously)? The place is too big to be netted.
Tbh otters could hammer that place for 5 years and hardly dent the stock, there's thousands in there.