Apex predators can control community structure by preying on strongly interacting species at lower trophic levels. Fishing of apex predators in the marine realm often results in herbivore dominated systems. In the Gulf of Maine, coastal subtidal communities became dominated by grazing green sea urchins (Strongylocentrotus droehachiensis) following the extirpation of large, predatory groundfish from coastal zones. Subsequent depletion of sea urchins since the late 1980s functionally eliminated this dominant herbivore from vast regions. Sea urchin recruitment is low or nonexistent in communities dominated by fleshy algae that have developed since the decline of sea urchin populations. We hypothesized that sea urchin populations would be restored if grazing pressure resumed. We moved adult sea urchins to a site where they had been abundant but were virtually absent by the late 1990s. During a two year study, 5 1,000 urchins were relocated to the shallow subtidal zone at Cape Elizabeth, ME (3000 urchins (35 - 45 mm test diameter) to 8 replicate plots in 2000, and 3000 urchins (50 - 71 mm test diameter) to 9 replicate plots in 2001). We monitored population changes in fleshy algae, urchins and urchin predators. Urchin grazing denuded fleshy algae from May through July in 200 1, while crab predator (Cancer spp.) abundances remained low. In August and September, predation by migratory populations of large Jonah crabs (C. borealis) decimated relocated urchin populations and restored fleshy-algal dominance at these locations. In laboratory experiments, we confirmed that sea urchin grazing decreases algal biomass and that Jonah crabs are stronger sea urchin predators than rock crabs (C. irroratus). Historical and present-day evidence describes Atlantic cod (Gadus morhua) and other groundfish as important Jonah crab predators. NMFS trawl data showed a 4-fold increase in Jonah crab abundance in 2000 and 2001 in the Gulf of Maine which may be related to a continuing decline in Gulf-wide fish predator populations. We speculate that highly mobile Jonah crabs at high densities may have become apex predators since their release from predatory control by groundfish (e.g. cod) in some shallow subtidal zones of the Gulf of Maine
