Cool!
Well, LMBs, Carp, Green Sunfish all sneak way up that drainage. One key thing to consider, and it's part of the challenge, is simply that these fish travel around a LOT. I have found them at the lake and other times nowhere except miles upstream. It's always work even when it is good. Temps and flows are a huge factor. Oh and it is very possible that some of the fish you and your father experienced were stockers. They do migrate. That said, they have not planted lake Piru in a very long time. But they most likely did when you and your father went there.
You're gonna love these (I hope that you can view them):
https://www.instagram.com/p/CBZsqL6DrHI/
https://www.instagram.com/p/CYmx2TaPiB4/
https://www.instagram.com/p/CCjpKSHjYrh/
I would normally be more discrete but *, it's so tough sorting through So Cal wild fisheries that, well, all the power to those who venture out and find anything; even a minnow!
B.