http://en.wikipedia.org/wiki/American_gizzard_shad

Gizzard shad are filter feeders and don't eat other fish or everything in sight. And I don't think that they spawn ever month during a full moon. They are prolific at spawning but I think someone is exaggerating above.

They eat the plankton and zoo plankton floating and swimming in the open waters of the lake and therefore make it harder for other fish to have enough to eat. Which is probably why the bluegill's size has decreased at Patoka Lake.

Gizzard Shad of a smaller size are eaten by larger bass but the smaller bass can't eat the larger Gizzard Shad. So they really don't help feed the entire bass population. Just a very small portion of Bass over 5 lbs are big enough to eat the larger Gizzard Shad.