As a very young chap, and I'm talking early 60's, I fished some ' pits ' known as Gunville Ponds....They are ( or were! ) located, more or less central on the island so a OS map will show them. They contained wild carp of sorts which went like stink, but that's all I can tell you. In those days they were hardly fished as most islanders concentrated on sea fishing.
ps I have Googled them and they still exist,............Day ticket as well as being old!!