Look at the shad. If it has a "nose" it's a gizzard shad. Gizzard shad have a subterminal mouth, just below their "face." Threadfin shad have a terminal mouth, at the very end of their face.
Watch the shad...if they are gizzard (likely), they tend to congregate around docks when the water becomes cold. If and when they do that, a cast net works well.
A full grown threadfin is about 7 inches long. A full grown gizzard is two pounds, maybe more.

Teach a man to grow fish...
He can teach to catch fish...