Q: How many flies am I allowed to use at the Two-Fly Challenge? A: Next question. Yep – this event, entering its 15th year, lets anglers use just two flies as they try to land the biggest fish in this competition. They do it on the West Branch of the Ausable River, a legendary trout-fishing destination that attracts fly-fishers from around the world. The river is special for a few reasons. The water holds a high amount of oxygen because it’s mercilessly churned over the rocky riverbed; those rocks offer shelter and places for the trout to eat, and the oxygen helps fish survive during warmer winter months.