There will be a very short period of time that the fish can eat the dead shad. just in a few days they'll decompose on the bottom, which will benefit
the catfish. I doubt a crappie or bass would eat a decomposed fish. It also depends if the lake is filled by creeks vs rivers. I believe a river filled lake will recover allot quicker vs a creek filled lake. With less shad the fish would definitely bite better and more agressively on artificial bait. Just my opinion.