Swimming home

"Some fish, like the river herring called alewives, hatch in a lake or big pond, and when they are a little older, swim down a river and out to the ocean. They return home to their lake when it is time to spawn, but swimming upstream can be an adventure full of danger and challenges. After their long swim home, what will happen if the entrance to the lake is now blocked with a new road or a dam? Can the fish still go home?"--