Here's what I've seen with threadfin shad in the private waters we manage. When there's an adequate bloom, threadfins thrive. What most people don't really think about, or understand, is the dynamics imposed by habitat, or the lack of habitat. Where we have bass-crowded lakes, there's typically not enough of the proper habitat for the support species. In other words, there's not enough escape cover for small fish, not enough spawning areas for bluegill, or skewed amounts of habitat that favor small bass. In those lakes, we focus on appropriate habitat, plus a good food chain for bluegills. Keep in mind bluegills, especially in the south, are spawning before threadfin shad. So, that first hatch of bluegills has a decent advantage in the food chain, if they have strong escape cover in the form of plants or something else that's dense. When threadfins spawn, they lay their eggs on grassy substrate, coming in waves before daylight, until just after the sun comes up, then they head to open water. When those babies are hatched, they head toward open water as well, but will see the "safety" of docks and other places where they migrate in and out. I haven't seen a managed lake where the population density of shads suggests a compromise with baby bluegills. What we have seen is an increase in relative weights of intermediate sized bass where threadfins are thriving.
Lots more to this, but there's a few thoughts.
One of the 'problems' with a study to figure this out is just how different every pond is. One of my favorite phrases is "As goes the habitat, so goes what lives in it". I totally believe that. When habitat favors a certain size class of a certain species, that one will thrive...especially if some of that habitat favors what that size class of that species eats. That's why, with my little pond management company, we focus on habitat, first.
