sure it's an antenna? first thought is someone put it there just for kicks 
It's a tag. I caught a steely on Cowlitz with one like that once. It was tagged in the Sea of Japan or something like that if I remember correct.
Why would a planted trout in a youth pond be tagged 
They are giving prizes to kids that catch the tagged fish. Idk if thats still going on or if that was just the other weekend when they had the kids fishing day down there.