Source: https://www.newscientist.com/article/2580119-hunter-gatherers-introduced-fish-to-a-mountain-lake-7000-years-ago/ From the post: >>Hunter-gatherers transported brown trout into the mountains of Norway at least 7000 years ago so they could be harvested during summer foraging trips into the highlands. That is the only possible explanation for how fish could have populated isolated mountain lakes that had been covered by a kilometres-thick ice sheet for tens of thousands of years, says Axel Mjærum at the University of Oslo, Norway.