Swimming upstream is what brown trouts do all the time. If hunter gatherers did it 7000 years ago, should be possible to suspect it, but modern people do this

Swimming upstream is what brown trouts do all the time. If hunter gatherers did it 7000 years ago, should be possible to suspect it, but modern people do this things all the time. It should be easier to prove at least that they not were released in, lets say, 1780.