This would be crazy if fish start to understand the cannon. Use it on their own and then continue on to find their destination (rather than be totally lost and confused)
Unless I'm mistaken, the fish head upstream to die. None of them will encounter the cannon twice, unless the same river has multiple dams on it.
Similar to how the salmon "learned" to swim back to where they were born, the increased survival frequency of salmon who can efficiently negotiate such a cannon would be expected to change the population.
But Epigenetics.
Or unless they jump from the damn just to be able to ride the tube again. And again. Wheee!