The dam itself is a barrier to the movement of fish upstream. A fish ladder was in place to allow fish to move past the dam but only the largest fish were able to make use of the ladder.
The new Fish Bypass Channel was built in 2015 to allow more salmon and trout living in Lake Ontario to travel upstream to spawn. The fish move up the ladder through a series of pools after entering the channel through an opening below the dam. If you look, you will see two rubber baffles that keep the fish from jumping into the concrete walls of the dam as they try to enter the passage.
You may wonder why the first jump is so high for the fish to get into the channel. The channel was designed to prevent access to invasive Sea Lamprey and so it acts as a barrier preventing lamprey from moving upstream. It may take the salmon and trout a few tries to get in but eventually, they do.