I wonder if any of you can recall? ....I have in my mind a memory of reading about a non-native and rare fish stocked somewhere in a Southern stillwater, possibly Sussex.
American Smallmouth Black Bass? They went into a few waters many many years ago, even bred and survived in one or two, legend has it. Was told their whereabouts umpteen years ago, but filed the information under "Don't wish to do" and quickly forgot them.
Large mouth bass definitely in Dorset (all gone now), possibly Surrey (Leney was the supplier in the mid 1930s). Pumpkinseed in many waters including some in Sussex. Rock bass maybe in Sussex.
Our lake used to be alive with them, it's taken us nearly 20 years to control them. Apparently a problem across the Sussex Ouse catchment. A well known commercial I believe was the source. About 3" long and devour anything that moves.