Looking at the PFBC stocking lists(the same one BeenThereDoneThat posted) they last stocked the Beaver River with hybrid striped bass back in 2001.
So.....
The fish are either spilling out of the lake, or making the very long journey(above 5 dams) up from the Ohio River. I have no doubt that fish can make it over the dam upstream of Sharon during very high water. I'm not sure of the dam near Farrel, I haven't seen it in person, but I take your word for it mike55
But, can they make it over the 3 dams in Beaver Falls and New Brighton??? I doubt it. Those are much bigger than the two on the Shenango.
If they're jumping over the dams in Beaver falls, then why aren't they jumping over the dam in Orangeville?? They're not, unless the dam is underwater of course(which it sometimes is when the lake is very high). Still doesn't explain why they chose to swim all the way up to Shenango, and not the Mahoning, Connoquesssing, Slippery Rock, Mosquito Creek, Neshannock, etc., etc....
And even this year when it's "slow" hybrids are still being caught at the Shenango Outflow. But not below Pymatuning(MUCH easier trek from Shenango Lake to Pymatuning, compared to the long rough journey from the Ohio River to Shenango Dam). Plenty of shad there too.
I'm still not convinced that hybrids are swimming up to Shenango from the Ohio River. But, ya know, anything can happen I guess.........