I was a member of the club in the 80's and Oxfold Beck and Costa Beck were full of rainbows of all sizes which was probably because the water authority fish farm bordered Oxfolds. The small fish were pristine fin perfect little beauties but I doubt they were bred in the beck but you could have been fooled. I caught rainbows upto 2.5 lbs and they certainly seemed to thrive in the clear spring fed water. I did also catch the odd brook trout. The two becks were astonishingly good fishing and as you say the grayling were special. The best brownie I heard caught was 8lb and I know a 20lb pike was caught. Sadly I believe both becks have been destroyed by drainage works.When did the Diploid / Triploid stocking ruling come in ?
I certainly know of a small stocking of Costa beck in N yorks, by the club, as I watched Bob Leng fishing for them, I was early teens so 45+years ago.
They believe they came from a local fish farm, which was renown for leaking fish into Pickering beck, and used to rear its own fish so they were viable.
One year, due to a bank collapse, they lost all their breeding stock into the river. My mate, and I, were catching them up to 3 miles below Pickering, as none seemed to go upstream...well we never caught, or heard of any being caught.
They were all fish of around 4/5lb mark, so would be surprised if all were caught / became Pike fodder ?
As I had seen them being stripped they were definitely viable, problem was there were so many fish leaked....it would be hard to tell if they bred ?