Jan Kristensen has fished all over the world. He grew up in Denmark but moved to Australia after earning his PhD in biotechnology. Ten years ago, he and his friend Daniel Goz set out to film one of the most unique projects ever undertaken in fly fishing:

This week we’re talking about a spectacle of nature that happens every year in the Rockies in June—the salmonfly hatch. This is a giant stonefly that excites big trout and fly fishers—but it’s difficult to plan for and not as easy to fish as...