Over 40 years ago the CA DF&G planted these small Theadfin Shad for forage food for the game fish in most of our valley reservoirs.
In the fall, actually right now (October on average), is the prime time for top water activity for mostly Rainbow trout in thse big impoundments to be "busting" these 1 to 2 inch long chrome horderves.
Seems like back in the 1970s-80s many more fly fishers where chasing these wild surface boils in the fall. Most used round dognut float tubes but other use boats from 8 feet prams to 16 foot outboard skiffs.
Up north on Shasta Lake this time of year Bob Quigley and the late Andy Puyans use to traditionally chaser the boils up there.
Merle Collins or Collins Lake northeast of Sacramento has been a consistent lake for this activity.
Other Threadfin lakes are Folsom, Berryessa, Oroville, Amador, New Hogan (Stripers).....probably hundreds of lakes now.
Also these fish washed down the rivers all the way to the delta where the first places we heard of them living was the Port of Stockton and the Port of Sacramento.
They like shallow lakey areas like Franks Tract and Mildred Island in the Delta too.
On a calm day go to one of these reservoirs or the Delta with some binoculars and see if you can spot some Rainbow trout or Stripers or LMB busty on schools of small Threadfin Shad.