One study in the U.S. indicates that the elimination of sharks resulted in the destruction of the shellfish industry in waters off the mid-Atlantic states of the United States, due to the unchecked population growth of cow-nose rays, whose mainstay is scallops.
Other studies in Belize have shown reef systems falling into extreme decline when the sharks have been over-fished, destroying an entire ecosystem. It was a chain reaction: the spike in grouper population (thanks to the elimination of sharks) resulted in a decimation of the parrotfish population, who could no longer perform their important role: keeping the coral algae-free. Corals rely on sunlight for photosynthesis, and die when encrusted in algae. Parrotfish are also important!