The “central artery” of Gauja National Park is the River Gauja with many tributaries. Many of these are rapid and swift, with clean, unpolluted water enriched with water coming from many springs, and therefore they are suitable for spawning of salmon-like fish — the Atlantic salmon (Salmo salar), brown trout (Salmo trutta), and stream trout (Salmo trutta fario). After a longer trip, the salmon-like fish travel in autumn to their birth places, and thus this is the time best suitable not only to watch the fish, but also enjoy what is happening on the riverside. The best time to go is late October, beginning of November. The local guides on staff of the Nature Conservation Agency take visitors to the best salmon watching spots.
http://www.entergauja.com/en/enter-gauja/routes/spawning-of-salmon-like-fish-in-gauja-national-park