Why does Thoreau say he goes "a-fishing" in time?

"The circle and the line make wonderful metaphors, and Walden Pond is the circle, or the dot, if you like. And the river is the line. And when you think about that magical moment of your youth, Walden Pond is a great place — it's a moment in time, that you may go back to over and over again. But when you think about your whole life, it's a line. There's these eddies and these rapids and these currents and these sluggish zones, it's a wonderful metaphor for life. And Henry Thoreau lived it his whole life. In fact, the episode at Walden Pond was a deviation away from a life spent on the river before, and on the river after. In fact, there are big pieces of 'Walden' that come straight from the river, but he's inventing a mythology here, so he doesn't want to tell you that outright.