Ecological impacts of eliminating top predators, including mesopredator release and trophic cascades, can be far-reaching. In the oceans, impacts of chronic overfishing are evident in population depletions worldwide, yet indirect ecosystem effects, induced by predator removal, remain unpredictable. While some argue that in complex marine food webs with many interacting species top-down effects may attenuate, fundamental constraints on studying oceanic food webs and apex predators may alternatively hinder detection of such effects. Here, we meet this challenge using a unique compilation of time series data and predator exclusion experiments, to investigate ecosystem consequences of functionally eliminating apex predatory sharks. We provide evidence that as abundances of all 11 great sharks in the northwest Atlantic Ocean that consume elasmobranchs (rays, skates, small sharks) plummeted over the past 35 years, 12 of 14 of their elasmobranch prey species increased significantly in coastal ecosystems. Effects of this community restructuring have cascaded downwards: order-of-magnitude enhancement of one mesopredator, cownose ray, led to sufficiently enhanced predation on its bay scallop prey sufficiently to terminate a century-long scallop fishery and may now also be inhibiting restoration of other bivalves. Analogous top-down effects may be a predictable consequence of eliminating entire functional groups of predators.